Proteins- Definition, Properties, Structure, Classification, Functions

Proteins are the most abundant and versatile biological macromolecules, occurring in all living cells and playing a crucial role in many vital processes. Proteins are composed of smaller units called amino acids, which are linked together by peptide bonds to form long chains. The sequence of amino acids in a protein determines its unique shape and function.

There are 20 different kinds of amino acids that can be found in proteins, each with a specific chemical structure and properties. Some amino acids can be synthesized by the body, while others must be obtained from the diet. These are called essential amino acids and they are important for growth and repair.

Proteins have different levels of structural organization, from the primary structure (the linear sequence of amino acids) to the quaternary structure (the interaction of multiple polypeptide chains). The secondary and tertiary structures are formed by various types of interactions between the amino acid residues, such as hydrogen bonds, hydrophobic interactions, electrostatic interactions, and disulfide bonds. These interactions result in different folding patterns, such as alpha-helices, beta-sheets, and loops.

The structure of a protein determines its function, which can be very diverse and specific. Some examples of protein functions are:

• Enzymes: Proteins that catalyze chemical reactions in the body, such as digestion, metabolism, and synthesis of biomolecules.
• Structural proteins: Proteins that provide support and shape to cells and tissues, such as collagen, keratin, and elastin.
• Hormones: Proteins that act as chemical messengers and regulate various physiological processes, such as insulin, growth hormone, and adrenaline.
• Antibodies: Proteins that recognize and bind to foreign substances (antigens) and help the immune system to fight infections.
• Transport proteins: Proteins that carry substances across membranes or in the blood, such as hemoglobin, albumin, and glucose transporter.
• Receptors: Proteins that bind to specific molecules (ligands) and trigger cellular responses, such as nerve impulses, gene expression, and cell growth.
• Transcription factors: Proteins that bind to DNA and regulate gene expression, such as p53, NF-kB, and STATs.

Proteins are species-specific and organ-specific, meaning that they differ from one species to another and from one organ to another within the same organism. Proteins are also dynamic molecules that can undergo various modifications after their synthesis on the ribosome. These modifications can affect their activity, stability, localization, and interactions with other molecules.

Proteins are essential for life and health, but they can also be involved in diseases when they are defective or dysfunctional. For example, mutations in protein-coding genes can cause genetic disorders such as cystic fibrosis, sickle cell anemia, and Huntington`s disease. Misfolding or aggregation of proteins can cause neurodegenerative diseases such as Alzheimer`s disease, Parkinson`s disease, and prion diseases. Overexpression or underexpression of proteins can cause cancer or autoimmune diseases.

In this article, we will explore the properties, structure, classification, and functions of proteins in more detail. We will also discuss how proteins can be studied using various techniques and tools.

Proteins are the most abundant and versatile biological macromolecules, with many different functions and properties. Some of the properties of proteins are as follows:

• Colour and Taste: Proteins are colourless and usually tasteless. However, some proteins may have a characteristic colour or taste due to the presence of certain pigments or flavour compounds in their structure. For example, hemoglobin is a red protein that transports oxygen in blood, and casein is a white protein that gives milk its flavour.
• Shape and Size: Proteins have various shapes and sizes, ranging from simple spherical structures to long fibrillar structures. The shape and size of a protein depend on its amino acid sequence and the interactions between its amino acid residues. The shape and size of a protein determine its solubility, stability, and function.
• Molecular Weight: Proteins have large molecular weights, typically between 5 × 10^3^ and 1 × 10^6^ Daltons (Da). The molecular weight of a protein is calculated by multiplying the number of amino acids in its sequence by the average molecular weight of an amino acid, which is about 110 Da. Different proteins have different molecular weights because they have different amino acid compositions and sequences.
• Solubility in Water: The solubility of proteins in water depends on their shape, size, charge, and polarity. Generally, globular proteins are more soluble than fibrous proteins, because they have more hydrophilic (water-loving) amino acid residues on their surface that can interact with water molecules. The solubility of proteins also depends on the pH and temperature of the solution, as these factors can affect the charge and conformation of the proteins.
• Denaturation and Renaturation: Proteins can undergo denaturation, which is the loss of their native structure and function due to external factors such as heat, pH, chemicals, or mechanical forces. Denaturation breaks the non-covalent bonds that maintain the secondary and tertiary structures of proteins, but does not affect the primary structure (the peptide bonds). Some proteins can regain their native structure and function after denaturation by removing the denaturing agent, a process called renaturation. However, some proteins may not be able to renature completely or at all, depending on the extent and type of denaturation.
• Coagulation: Coagulation is a special type of denaturation that involves the formation of insoluble aggregates of proteins due to heat or chemicals. Coagulation is irreversible and results in the loss of protein function. For example, when egg white (which contains albumin protein) is heated, it coagulates into a solid mass that cannot be dissolved in water again.
• Isoelectric Point: The isoelectric point (pI) is the pH at which a protein has no net charge and is electrically neutral. At this point, the protein does not move in an electric field and has minimum solubility in water. The pI of a protein depends on its amino acid composition and the ionization state of its side chains. The pI can be used to separate or purify proteins based on their charge differences.

• Chemical Properties: Proteins can react with various chemicals to form new compounds or modify their structure and function. Some common chemical reactions involving proteins are:

  • Biuret test: This test detects the presence of peptide bonds in proteins by reacting them with copper sulfate (CuSO4) in alkaline solution. A violet colour indicates a positive result.
  • Ninhydrin test: This test detects the presence of free amino groups in proteins by reacting them with ninhydrin solution. A purple colour indicates a positive result.
  • Phosphorylation: This is the addition of phosphate groups to certain amino acid residues (usually serine, threonine, or tyrosine) by enzymes called kinases. Phosphorylation can alter the activity, stability, or interactions of proteins.
  • Glycosylation: This is the addition of carbohydrate chains to certain amino acid residues (usually asparagine, serine, or threonine) by enzymes called glycosyltransferases. Glycosylation can affect the folding, solubility, recognition, or protection of proteins.
  • Hydroxylation: This is the addition of hydroxyl groups (-OH) to certain amino acid residues (usually proline or lysine) by enzymes called hydroxylases. Hydroxylation can enhance the stability or flexibility of proteins.
  • Methylation: This is the addition of methyl groups (-CH3) to certain amino acid residues (usually lysine or arginine) by enzymes called methyltransferases. Methylation can influence the expression, localization, or interactions of proteins.

The solubility of a protein in water depends on its three-dimensional shape and the interactions between its amino acid residues and water molecules. Generally, globular proteins are more soluble than fibrous proteins because they have more hydrophilic residues on their surface that can form hydrogen bonds with water. Fibrous proteins, on the other hand, have more hydrophobic residues that tend to aggregate and exclude water.

The solubility of a protein also depends on the pH and ionic strength of the solution. Proteins have different charges at different pH values depending on the ionization state of their acidic and basic residues. The charge affects the electrostatic interactions between protein molecules and water molecules, as well as between protein molecules themselves. At low or high pH values, proteins tend to have more net charge and repel each other, increasing their solubility. However, at a certain pH value called the isoelectric point (pI), proteins have no net charge and tend to aggregate and precipitate, decreasing their solubility.

The ionic strength of the solution affects the solubility of proteins by influencing the screening effect of salt ions. Salt ions can reduce the electrostatic interactions between protein molecules by surrounding them with a layer of opposite charges. This can increase the solubility of proteins by preventing aggregation. However, if the salt concentration is too high, it can also reduce the interactions between protein molecules and water molecules by competing for hydrogen bonds. This can decrease the solubility of proteins by causing dehydration.

Some proteins can be denatured by agents such as heat, pH extremes, organic solvents, detergents, or urea that disrupt their secondary and tertiary structures. Denaturation can affect the solubility of proteins in different ways depending on the nature of the denaturing agent and the protein. Some denatured proteins may become more soluble because they expose more hydrophilic residues to water. Others may become less soluble because they expose more hydrophobic residues that aggregate and form insoluble clumps. Some denatured proteins may regain their native structure and solubility when the denaturing agent is removed, while others may not.

Denaturation is the term used for any change in the three-dimensional structure of a protein that renders it incapable of performing its assigned function. A denatured protein cannot do its job. Denaturation can be caused by various agents and conditions, such as heat, organic solvents, pH changes, heavy metal ions, and alkaloid reagents . For example, when an egg is fried, the clear egg white turns opaque as the albumin protein denatures and coagulates.

Renaturation is the term used for the process that can restore the protein to its original form and function after denaturation. Renaturation can occur if the denaturing agent is removed and the optimal conditions for the protein are restored . However, renaturation is not always possible or complete, depending on the extent and nature of the denaturation. Some proteins may lose their biological activity permanently after denaturation.

The primary structure of a protein, which is the sequence of amino acids in the polypeptide chain, is usually not affected by denaturation. However, the secondary, tertiary, and quaternary structures, which are responsible for the folding and interactions of the polypeptide chains, are disrupted by denaturation. These structures are stabilized by various types of non-covalent bonds, such as hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces. These bonds are sensitive to changes in temperature, pH, solvent polarity, and ionic strength. When these factors are altered by a denaturing agent, the bonds are broken and the protein loses its native conformation.

Renaturation occurs when the non-covalent bonds are reformed between the amino acid residues in the polypeptide chains. This requires that the primary structure of the protein is intact and that the amino acid residues are able to find their correct positions in the three-dimensional structure. Renaturation can be facilitated by factors such as low temperature, neutral pH, low ionic strength, and mild agitation. Renaturation can completely or partially restore the protein function lost because of denaturation. For example, ribonuclease A, an enzyme that catalyzes the hydrolysis of RNA molecules, can be renatured after being denatured by urea and β-mercaptoethanol.

Denaturation and renaturation of proteins are important processes that affect the structure and function of proteins in living systems. They also have applications in biotechnology and medicine. For instance, denaturation can be used to purify proteins by separating them from other molecules based on their solubility or charge. Renaturation can be used to refold recombinant proteins that are produced in bacteria or yeast cells but are inactive due to improper folding.

Coagulation is the process of changing the structure of proteins from a liquid state to a solid or a thicker liquid form due to the influence of external factors, such as heat, acids, alcohols, enzymes, and other agents . The two main steps of coagulation are denaturation and precipitation.

• Denaturation is the loss of the secondary and tertiary structures of proteins without breaking the peptide bonds. It involves the disruption of the hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges that stabilize the three-dimensional shape of proteins. Denaturation can be caused by heat, pH changes, organic solvents, detergents, urea, and other agents.
• Precipitation is the aggregation of denatured proteins into insoluble clumps or coagulum. It occurs when the hydrophobic interactions between the exposed amino acid residues become stronger than the electrostatic repulsion between the charged groups. Precipitation can be enhanced by adding salt, which reduces the solvation of proteins by water molecules.

Coagulation is an irreversible process that affects the functional properties of proteins, such as solubility, viscosity, water-holding capacity, emulsifying ability, and gel formation. Coagulation is important for various food applications, such as cheese making, egg cooking, meat tenderization, yogurt production, and tofu preparation. Coagulation is also involved in blood clotting, which is a complex process that involves numerous coagulation factors that are produced by the liver and blood vessels. Coagulation tests measure the function or the amount of these factors in the blood to assess the risk of excessive bleeding or clotting.

The isoelectric point (pI) is the pH at which a protein molecule has no net charge or is electrically neutral. The net charge of a protein depends on the number and type of ionizable groups it contains, such as the amino and carboxyl groups of the amino acids and any other acidic or basic side chains. At a pH below the pI, a protein has a positive net charge; at a pH above the pI, it has a negative net charge. The pI of a protein can be calculated from the average of the pKa values of its ionizable groups .

The pI of a protein is an important characteristic that affects its solubility, stability, interactions, and separation. Proteins have minimum solubility at their pI because they do not interact with water or salt ions. They also tend to aggregate and precipitate out of solution at their pI due to electrostatic attraction between oppositely charged molecules. The pI of a protein also influences its folding and conformation, as different pH values can alter the hydrogen bonding and electrostatic interactions between amino acid residues. Furthermore, the pI of a protein determines its behavior in electrophoresis and chromatography, two common techniques for protein separation and purification. In electrophoresis, proteins migrate towards the electrode with opposite charge to their net charge; therefore, proteins do not move at their pI . In chromatography, proteins bind to a stationary phase with opposite charge to their net charge; therefore, proteins can be eluted by changing the pH to their pI.

The pI of a protein varies widely depending on its amino acid composition and sequence. Proteins that are rich in acidic amino acids (aspartic acid and glutamic acid) have low pI values, while proteins that are rich in basic amino acids (lysine, arginine, and histidine) have high pI values. For example, hemoglobin has a pI of 6.8, while lysozyme has a pI of 10.7. The pI of a protein can also be affected by post-translational modifications that add or remove ionizable groups, such as phosphorylation, glycosylation, acetylation, etc.. The pI of a protein can be experimentally determined by methods such as isoelectric focusing, titration curve analysis, or mass spectrometry. Alternatively, the pI of a protein can be predicted from its amino acid sequence using computational tools.

• The molecular weight of a protein is the sum of the atomic weights of all the atoms in the protein molecule.
• The molecular weight of a protein can be estimated by multiplying the number of amino acids in the protein by 110, which is the average molecular weight of an amino acid residue.
• The molecular weight of a protein can also be determined by various experimental methods, such as gel electrophoresis, mass spectrometry, and ultracentrifugation.
• The molecular weights of proteins vary widely, depending on their amino acid composition, length, and structure.
• Some proteins are composed of multiple subunits, each with its own molecular weight. The total molecular weight of a protein is then the sum of the molecular weights of its subunits.
• The molecular weights of proteins range from 5000 to 10^9^ Daltons. A Dalton is a unit of mass equal to one-twelfth of the mass of a carbon-12 atom.
• Most proteins have molecular weights between 10,000 and 100,000 Daltons. Some examples of proteins and their molecular weights are:

Posttranslational modifications (PTMs) are covalent and generally enzymatic changes that occur in some proteins after their biosynthesis. This process often occurs in the endoplasmic reticulum and the golgi apparatus. PTMs can alter the properties of a protein by adding or removing a functional group, such as phosphate, acetyl, methyl, glycosyl, etc., to one or more amino acids. PTMs can also involve proteolytic cleavage of a protein to produce smaller fragments.

PTMs are important for regulating the structure, function, and interactions of proteins. They can affect the stability, folding, localization, activity, and interactions of proteins. They can also serve as molecular switches that turn on or off the function of a protein in response to different signals. PTMs are involved in many biological processes, such as cell signaling, transcription, translation, metabolism, cell cycle, apoptosis, immunity, etc. Disruption of PTMs can lead to various diseases, such as cancer, diabetes, neurodegeneration, etc.

There are more than 400 different types of PTMs that have been identified so far. Some of the most common and well-studied PTMs are:

• Phosphorylation: The addition of a phosphate group to serine, threonine, or tyrosine residues by kinases. Phosphorylation can modulate the activity, interactions, and localization of proteins. It is one of the most prevalent and reversible PTMs that regulate many cellular processes.
• Acetylation: The addition of an acetyl group to lysine residues by acetyltransferases. Acetylation can affect the charge and structure of proteins and influence their interactions with other molecules. It is mainly involved in regulating gene expression by modifying histones and transcription factors.
• Methylation: The addition of a methyl group to lysine or arginine residues by methyltransferases. Methylation can alter the charge and structure of proteins and influence their interactions with other molecules. It is also mainly involved in regulating gene expression by modifying histones and transcription factors.
• Glycosylation: The addition of a carbohydrate chain to asparagine or serine/threonine residues by glycosyltransferases. Glycosylation can affect the folding, stability, recognition, and interactions of proteins. It is essential for protein quality control and secretion. It is also involved in cell-cell communication and immune response.
• Ubiquitination: The addition of one or more ubiquitin molecules to lysine residues by ubiquitin ligases. Ubiquitination can mark proteins for degradation by the proteasome or alter their activity, localization, or interactions. It is involved in many cellular processes, such as protein quality control, cell cycle regulation, DNA repair, signal transduction, etc.

There are many online databases and tools that provide information and prediction methods for PTMs. Some of the major ones are:

• UniProt: A comprehensive database of protein sequences and annotations that includes information on PTMs for each protein entry.
• dbPTM: A database that integrates experimentally verified PTMs from several databases and provides functional analysis and prediction tools for PTMs.
• PhosphoSitePlus: A database that focuses on phosphorylation and other PTMs that regulate signaling pathways. It provides information on substrates, kinases, pathways, diseases, and drugs related to PTMs.
• GlyConnect: A database that focuses on glycosylation and provides information on glycoproteins, glycans, glycosites, enzymes, pathways, and diseases related to glycosylation.
• UbiProt: A database that focuses on ubiquitination and provides information on ubiquitinated proteins, ubiquitin ligases, ubiquitin chains, pathways, and diseases related to ubiquitination.

Proteins are large, complex molecules that have a variety of chemical properties depending on their amino acid composition and structure. Some of the common chemical properties of proteins are:

• Biuret test: This is a test to detect the presence of peptide bonds in proteins. When a protein solution is treated with sodium hydroxide (NaOH) and copper sulfate (CuSO4), a violet color is formed due to the complexation of copper ions with the nitrogen atoms of the peptide bonds. This test is named after biuret, a compound that gives a similar reaction.

• Ninhydrin test: This is a test to detect the presence of free amino groups in proteins. When a protein solution is heated with ninhydrin, a purple color is formed due to the reaction of ninhydrin with the alpha-amino acids. This test is also used to identify amino acids by their different colors with ninhydrin.

• Solubility: The solubility of proteins in water depends on their polarity and charge. Nonpolar proteins, such as those with alkyl side chains, tend to be insoluble in water due to their hydrophobic interactions. Polar proteins, such as those with acidic or basic side chains, tend to be soluble in water due to their hydrophilic interactions and ionization. The solubility of proteins also depends on the pH and salt concentration of the solution, which affect the net charge and hydration of the protein molecules.

• Denaturation: The denaturation of proteins refers to the loss of their native three-dimensional structure and function due to various physical or chemical agents, such as heat, pH, organic solvents, heavy metal ions, etc. Denaturation disrupts the noncovalent interactions that stabilize the secondary and tertiary structures of proteins, such as hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges. However, denaturation does not affect the primary structure of proteins, which is determined by the covalent peptide bonds between amino acids. Some denatured proteins can regain their native structure and function when the denaturing agent is removed, which is called renaturation.

• Coagulation: The coagulation of proteins refers to the formation of insoluble aggregates or clumps of protein molecules due to denaturation. Coagulation usually occurs when proteins are heated above a certain temperature, which causes them to unfold and expose their hydrophobic regions to each other. These regions then interact and form intermolecular bonds that result in coagulation. An example of coagulation is the formation of egg white when an egg is boiled.

• Isoelectric point: The isoelectric point (pI) of a protein is the pH at which the protein has no net charge and is electrically neutral. At this point, the protein does not migrate in an electric field and has minimum solubility in water. The pI of a protein depends on the number and type of acidic and basic amino acids in its sequence. Proteins with more acidic amino acids have lower pI values than proteins with more basic amino acids. The pI of a protein can be used to separate and purify proteins by a technique called isoelectric focusing.

  Protein Structure

The structure of a protein determines its function and interactions with other molecules. The structure of a protein is composed of four levels of organization: primary, secondary, tertiary, and quaternary. Each level of structure adds more complexity and specificity to the protein.

Primary Structure

The primary structure of a protein is the linear sequence of amino acids that make up the polypeptide chain. The amino acids are joined by peptide bonds, which are formed by the condensation reaction between the carboxyl group of one amino acid and the amino group of another. The primary structure is determined by the genetic code, which specifies the order of nucleotides in the DNA or RNA that encodes the protein.

The primary structure of a protein is essential for its function, as it defines the identity and properties of each amino acid residue in the chain. The amino acid residues have different side chains that vary in size, shape, charge, polarity, and reactivity. These side chains affect the interactions between amino acids and other molecules, as well as the folding and stability of the protein.

The primary structure of a protein can be represented by a string of letters, each corresponding to an amino acid. For example, the primary structure of insulin A chain is:

GIVEQCCTSICSLYQLENYCN

where G stands for glycine, I for isoleucine, V for valine, and so on.

Secondary Structure

The secondary structure of a protein refers to the local folding patterns of the polypeptide chain, which are stabilized by hydrogen bonds between the backbone atoms. The backbone atoms are the atoms involved in the peptide bond: the nitrogen, hydrogen, carbon, and oxygen atoms.

The most common types of secondary structure are the alpha helix and the beta sheet. These structures are formed by repeating patterns of hydrogen bonds between the carbonyl oxygen of one peptide bond and the amide hydrogen of another peptide bond.

• An alpha helix is a right-handed coil of the polypeptide chain, where each turn of the helix contains 3.6 amino acids. The side chains of the amino acids point outward from the axis of the helix. The hydrogen bonds are parallel to the axis and link every fourth peptide bond.
• A beta sheet is a flat or pleated arrangement of two or more polypeptide chains, where each chain is called a beta strand. The side chains of the amino acids alternate above and below the plane of the sheet. The hydrogen bonds are perpendicular to the direction of the strands and link adjacent strands. The strands can be parallel or antiparallel, depending on whether they run in the same or opposite direction.

The secondary structure of a protein can be represented by symbols or diagrams that indicate the type and location of each element. For example, an alpha helix can be shown by a helical ribbon or a cylinder, while a beta sheet can be shown by an arrow or a flat ribbon.

Tertiary Structure

The tertiary structure of a protein refers to the overall three-dimensional shape of the polypeptide chain, which results from various interactions between the amino acid side chains. These interactions include:

• Hydrophobic interactions: The tendency of nonpolar side chains to cluster together in the interior of the protein, away from water.
• Electrostatic interactions: The attraction or repulsion between charged side chains.
• Hydrogen bonds: The formation of weak bonds between polar side chains or between side chains and water molecules.
• Van der Waals forces: The weak attraction between any two atoms that are close together.
• Disulfide bonds: The covalent linkage between two cysteine residues that form a sulfur-sulfur bond.

The tertiary structure determines the function and specificity of a protein, as it defines its surface features and binding sites for other molecules. For example, enzymes have active sites that fit their substrates like a lock and key.

The tertiary structure of a protein can be represented by various models that show different aspects of its shape. For example, a wireframe model shows all atoms and bonds in detail, while a space-filling model shows only atoms as spheres with realistic sizes.

Quaternary Structure

The quaternary structure of a protein refers to the association of two or more polypeptide chains into a functional unit. Each polypeptide chain is called a subunit. The subunits can be identical or different, depending on whether they are encoded by the same or different genes.

The quaternary structure is stabilized by the same types of interactions that hold together the tertiary structure: hydrophobic interactions, electrostatic interactions, hydrogen bonds, van der Waals forces, and disulfide bonds.

The quaternary structure allows proteins to have greater complexity and diversity than single polypeptide chains. For example, hemoglobin is composed of four subunits: two alpha chains and two beta chains. Each subunit can bind one molecule of oxygen, allowing hemoglobin to transport oxygen in blood.

The quaternary structure of a protein can be represented by models that show how subunits are arranged relative to each other. For example, a ribbon model shows only backbone atoms as ribbons with different colors for different subunits.

Classification of Proteins

Proteins are the most abundant and diverse biomolecules in living systems, with many different functions and properties. They can be classified based on different criteria, such as their composition, structure, shape, and solubility.

Based on Composition

Proteins can be divided into two main types based on their composition :

• Simple proteins: They are composed of only amino acid residues. On hydrolysis, these proteins yield only constituent amino acids. Examples of simple proteins are fibrous proteins and globular proteins.
• Conjugated proteins: They are composed of a protein moiety (the apoprotein) and a non-protein portion (the prosthetic group). The prosthetic group can be a metal, a lipid, a sugar, a phosphate, or another type of molecule. On hydrolysis, these proteins yield amino acids and the prosthetic group. Examples of conjugated proteins are nucleoproteins, phosphoproteins, lipoproteins, metalloproteins, etc.

Based on Structure

Proteins can also be classified based on their structure :

• Primary structure: It refers to the linear sequence of amino acids in a polypeptide chain. The primary structure is determined by the covalent peptide bonds between amino acids. The primary structure determines the further levels of organization of protein molecules.
• Secondary structure: It refers to the local conformations of the polypeptide chain that are stabilized by hydrogen bonds between backbone atoms. The secondary structure can be alpha-helix, beta-sheet, or other types of folding patterns. The secondary structure depends on the interaction of peptide bonds with water and the side chains of amino acids.
• Tertiary structure: It refers to the overall three-dimensional shape of a protein that is formed by interactions between amino acid residues that may be located far apart in the primary sequence. The tertiary structure is stabilized by various types of non-covalent interactions, such as hydrophobic interactions, electrostatic interactions, hydrogen bonds, and van der Waals forces. In some cases, covalent disulfide bonds may also contribute to the tertiary structure. The tertiary structure determines the function and specificity of a protein.
• Quaternary structure: It refers to the arrangement of two or more polypeptide chains (subunits) to form a functional protein. The quaternary structure is also stabilized by non-covalent interactions and sometimes by disulfide bonds. The quaternary structure allows for greater diversity and complexity of protein functions.

Based on Shape

Proteins can also be classified based on their shape :

• Fibrous proteins: They have long and narrow shapes that resemble fibers or sheets. They are usually insoluble in water and have high tensile strength. They are mainly involved in structural roles, such as forming tendons, bones, hair, nails, etc. Examples of fibrous proteins are keratin, elastin, collagen, etc.
• Globular proteins: They have compact and spherical shapes that fit into water-soluble environments. They are usually soluble in water and have diverse functions, such as catalyzing reactions, transporting molecules, regulating processes, defending against infections, etc. Examples of globular proteins are enzymes, antibodies, hormones, hemoglobin, etc.
• Membrane proteins: They are embedded in or associated with the lipid bilayer membranes of cells and organelles. They have various shapes and functions depending on their location and orientation in the membrane. They can act as channels for ions or molecules that cannot cross the membrane; as receptors for signal molecules that trigger cellular responses; as anchors for other molecules to the membrane; etc. Examples of membrane proteins are ion channels, G-protein coupled receptors (GPCRs), integrins, etc.

nctions of Proteins

Proteins are essential for the proper functioning of all living organisms. They perform a wide range of functions that are vital for growth, development, health and survival. Some of the major functions of proteins are:

• Growth and maintenance: Proteins are required for the synthesis and repair of tissues, organs and cells. They also provide structural support and strength to bones, muscles, skin, hair and nails. Proteins are especially important during periods of growth, such as childhood, adolescence, pregnancy and lactation .
• Biochemical reactions: Proteins act as enzymes, which are catalysts that speed up the rate of chemical reactions in the body. Enzymes are involved in various metabolic processes, such as digestion, energy production, blood clotting and muscle contraction . Enzymes often require cofactors, such as vitamins or minerals, to function properly.
• Hormones: Proteins act as hormones, which are chemical messengers that regulate various physiological functions, such as growth, metabolism, reproduction and stress response. Hormones are produced by endocrine glands and secreted into the bloodstream, where they bind to specific receptors on target cells . Some examples of protein hormones are insulin, growth hormone and thyroxine.
• Transport and storage: Proteins act as transporters and carriers of various substances in the body. They help move oxygen, carbon dioxide, nutrients, hormones, drugs and toxins across cell membranes or within the blood or lymph . Some examples of transport proteins are hemoglobin, albumin and lipoprotein. Proteins also act as storage molecules for certain nutrients or ions, such as iron, calcium and zinc . Some examples of storage proteins are ferritin, casein and myoglobin.
• Immunity: Proteins act as antibodies, which are part of the immune system that protect the body from foreign invaders, such as bacteria, viruses and parasites. Antibodies recognize and bind to specific antigens on the surface of pathogens or toxins, and mark them for destruction by other immune cells . Some examples of antibodies are immunoglobulin G (IgG), immunoglobulin A (IgA) and immunoglobulin E (IgE).
• Cell signaling: Proteins act as receptors, which are molecules that receive signals from outside the cell and initiate a response inside the cell. Receptors can be located on the cell membrane or inside the cell. They bind to specific ligands, such as hormones, neurotransmitters or growth factors . Some examples of receptor proteins are insulin receptor, G protein-coupled receptor and nuclear receptor.
• Gene expression: Proteins act as transcription factors, which are molecules that regulate the expression of genes by binding to specific DNA sequences. Transcription factors control when, where and how much of a gene is transcribed into messenger RNA (mRNA), which is then translated into protein . Some examples of transcription factors are p53, NF-kB and STAT.
• Movement: Proteins act as motor proteins, which are molecules that generate movement within cells or between cells. Motor proteins use energy from adenosine triphosphate (ATP) to change their shape and interact with other molecules or structures. They are involved in various cellular processes, such as cell division, muscle contraction and vesicle transport . Some examples of motor proteins are actin, myosin and kinesin.

These are some of the main functions of proteins in the body. However, proteins can also have other specialized or unique functions depending on their location, structure and interaction with other molecules.

Proteins are the most abundant and diverse biomolecules in living systems, with many different functions and properties. They can be classified based on different criteria, such as their composition, structure, shape, and solubility.

Based on Composition

Proteins can be divided into two main types based on their composition :

• Simple proteins: They are composed of only amino acid residues. On hydrolysis, these proteins yield only constituent amino acids. Examples of simple proteins are fibrous proteins and globular proteins.
• Conjugated proteins: They are composed of a protein moiety (the apoprotein) and a non-protein portion (the prosthetic group). The prosthetic group can be a metal, a lipid, a sugar, a phosphate, or another type of molecule. On hydrolysis, these proteins yield amino acids and the prosthetic group. Examples of conjugated proteins are nucleoproteins, phosphoproteins, lipoproteins, metalloproteins, etc.

Based on Structure

Proteins can also be classified based on their structure :

• Primary structure: It refers to the linear sequence of amino acids in a polypeptide chain. The primary structure is determined by the covalent peptide bonds between amino acids. The primary structure determines the further levels of organization of protein molecules.
• Secondary structure: It refers to the local conformations of the polypeptide chain that are stabilized by hydrogen bonds between backbone atoms. The secondary structure can be alpha-helix, beta-sheet, or other types of folding patterns. The secondary structure depends on the interaction of peptide bonds with water and the side chains of amino acids.
• Tertiary structure: It refers to the overall three-dimensional shape of a protein that is formed by interactions between amino acid residues that may be located far apart in the primary sequence. The tertiary structure is stabilized by various types of non-covalent interactions, such as hydrophobic interactions, electrostatic interactions, hydrogen bonds, and van der Waals forces. In some cases, covalent disulfide bonds may also contribute to the tertiary structure. The tertiary structure determines the function and specificity of a protein.
• Quaternary structure: It refers to the arrangement of two or more polypeptide chains (subunits) to form a functional protein. The quaternary structure is also stabilized by non-covalent interactions and sometimes by disulfide bonds. The quaternary structure allows for greater diversity and complexity of protein functions.

Based on Shape

Proteins can also be classified based on their shape :

• Fibrous proteins: They have long and narrow shapes that resemble fibers or sheets. They are usually insoluble in water and have high tensile strength. They are mainly involved in structural roles, such as forming tendons, bones, hair, nails, etc. Examples of fibrous proteins are keratin, elastin, collagen, etc.
• Globular proteins: They have compact and spherical shapes that fit into water-soluble environments. They are usually soluble in water and have diverse functions, such as catalyzing reactions, transporting molecules, regulating processes, defending against infections, etc. Examples of globular proteins are enzymes, antibodies, hormones, hemoglobin, etc.
• Membrane proteins: They are embedded in or associated with the lipid bilayer membranes of cells and organelles. They have various shapes and functions depending on their location and orientation in the membrane. They can act as channels for ions or molecules that cannot cross the membrane; as receptors for signal molecules that trigger cellular responses; as anchors for other molecules to the membrane; etc. Examples of membrane proteins are ion channels, G-protein coupled receptors (GPCRs), integrins, etc.

Proteins are essential for the proper functioning of all living organisms. They perform a wide range of functions that are vital for growth, development, health and survival. Some of the major functions of proteins are:

• Growth and maintenance: Proteins are required for the synthesis and repair of tissues, organs and cells. They also provide structural support and strength to bones, muscles, skin, hair and nails. Proteins are especially important during periods of growth, such as childhood, adolescence, pregnancy and lactation .
• Biochemical reactions: Proteins act as enzymes, which are catalysts that speed up the rate of chemical reactions in the body. Enzymes are involved in various metabolic processes, such as digestion, energy production, blood clotting and muscle contraction . Enzymes often require cofactors, such as vitamins or minerals, to function properly.
• Hormones: Proteins act as hormones, which are chemical messengers that regulate various physiological functions, such as growth, metabolism, reproduction and stress response. Hormones are produced by endocrine glands and secreted into the bloodstream, where they bind to specific receptors on target cells . Some examples of protein hormones are insulin, growth hormone and thyroxine.
• Transport and storage: Proteins act as transporters and carriers of various substances in the body. They help move oxygen, carbon dioxide, nutrients, hormones, drugs and toxins across cell membranes or within the blood or lymph . Some examples of transport proteins are hemoglobin, albumin and lipoprotein. Proteins also act as storage molecules for certain nutrients or ions, such as iron, calcium and zinc . Some examples of storage proteins are ferritin, casein and myoglobin.
• Immunity: Proteins act as antibodies, which are part of the immune system that protect the body from foreign invaders, such as bacteria, viruses and parasites. Antibodies recognize and bind to specific antigens on the surface of pathogens or toxins, and mark them for destruction by other immune cells . Some examples of antibodies are immunoglobulin G (IgG), immunoglobulin A (IgA) and immunoglobulin E (IgE).
• Cell signaling: Proteins act as receptors, which are molecules that receive signals from outside the cell and initiate a response inside the cell. Receptors can be located on the cell membrane or inside the cell. They bind to specific ligands, such as hormones, neurotransmitters or growth factors . Some examples of receptor proteins are insulin receptor, G protein-coupled receptor and nuclear receptor.
• Gene expression: Proteins act as transcription factors, which are molecules that regulate the expression of genes by binding to specific DNA sequences. Transcription factors control when, where and how much of a gene is transcribed into messenger RNA (mRNA), which is then translated into protein . Some examples of transcription factors are p53, NF-kB and STAT.
• Movement: Proteins act as motor proteins, which are molecules that generate movement within cells or between cells. Motor proteins use energy from adenosine triphosphate (ATP) to change their shape and interact with other molecules or structures. They are involved in various cellular processes, such as cell division, muscle contraction and vesicle transport . Some examples of motor proteins are actin, myosin and kinesin.

These are some of the main functions of proteins in the body. However, proteins can also have other specialized or unique functions depending on their location, structure and interaction with other molecules.

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