Proteins are organic compounds of living organisms making 55 – 85% of dry weight of the cell.
Protein molecule has many different levels of structure and may be coiled and folded or it may interact with other protein molecules to form unique three dimensional structure.
Different kinds of protein molecules have different shapes related to their particular functions in life processes.
They are as under:
(1) Primary Structure: It is the linear sequence of amino acids in polypeptide chains comprising the molecule.
(2) Secondary Structure: It is repeating pattern of bonds (often Hydrogen bonds) between amino acids and it commonly takes the shape of alpha relize or pleated sheet.
(3) Tertiary Structure: It results from the helix folding into three dimensional shape.
(4) Quaternary Structure: In some instances two Protein chains join to form larger protein which is called quaternary structure.
(1) Primary Structure of Proteins:
It is the linear structure of amino acids in polypeptide chains comprising the molecule. In primary structure in addition to peptide bonds, disulphide bonds (S – S) may link into peptide chains as in insulin where two amino acids chains are linked with one another. In ribonuclease single chain is folded and has disulphide bond – Single peptide chain is formed as a result of polymerization of 100 – 1000 or more amino acids in straight chain. This chain will always be having a carboxyl group on one end which is known as C-termed and an amino group on other end thus known as N-terminal. New amino acids can be added both at C or N terminals. New amino acid can be added both at C or N terminals. Proteins have specific primary structure due to
(i) Number of amino acids in Polypeptide chain
(ii) Types of amino acids present in polypeptide chain
(iii) Sequence of amino acids in polypeptide chain.
DNA template determines the specificity in number, size, type and arrangement of amino acids in Protein.
(2) Secondary Structure of Proteins:
Polypeptide chain that shows folding due to the formation of hydrogen bonds and forms a stabilized structure is called secondary structure of protein. It may be of following types:
(i) α-Helix: It is formed when a polypeptide chain is twisted in such a way that every amide and carboxyl carbon is involved in hydrogen bonding. α-helix look like a helical spring α-helix is controlled by number of amino acids residues per turn, the pitch of single residue. α-helix protein chain is the structural protein of hair, wood, nails, claws, beaks, feathers, horns and vertebrate skin.
Amino acids have the ability to form hydrogen bonds and participate in helix formation. Such amino acids are called helix formers like Glutamic acid, Alanine, Leucine, Histidine.
(ii) β-Pleated sheets: In some cases torsion angles in polypeptide chains are so irregular that hydro bonding does not take place in extended peptide chain. Two of three such extended chains interact with one another by hydrogen bonding A form β-Pleated sheets.
(3) Tertiary Structure of Proteins:
Secondary proteins on extensive coiling or folding form compact structure called tertiary structure of protein. Tertiary structure determines the shape protein. Folding is held together due to interaction of R-group of different amino acids present in peptide chains. These interactions include ionic, hydrogen, disulphide bonds and hydrophobic interaction.
(4) Quaternary Structure of Proteins: Sometimes more than two monomeric proteins are essential to make a structure or collectively perform a function; such bigger structures with more than two units form Quaternary structure of Protein. It is classified as monometric, dimeric, oligomeric depending upon the number of submits as one, two or many respectively. Different monomers are held together by ionic bonds, hydrogen bonds and hydrophobic interactions.
Functions of protein are
(1) Enzyme Catalysis: Chemical reaction occurring in enzymes, the specific proteins, controls the body.
(2) Coordinated Motion: Proteins like myosin, actin, troponin and tubulin are responsible for movement of animals, organs, chromosomes, flagella, cilia, cells etc.
(3) Mechanical Support: A fibrous protein, collagen provides mechanical support and strength to skin, bones, connective tissues, tendons and ligaments.
(4) Transport and storage: Haemoglobin and myoglobin present in RBC and muscles are responsible for transportation of oxygen to the cells and muscles. During starvation proteins present in body is used by process of autolysis for energy production.
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