- - - - more number of amino acids linked this way you get a polypeptide. If the number of amino acids are less it is called as a polypeptide, if it has large number of amino acids (and preferably has a function) then it is called a protein. H2N — CH2 — COOH + H2N — CH — COOH H2N — CH2 — C — N —CH — COOH CH3 -H2O Glycine Alanine Peptide Bond Glycyl alanine - Dipeptide — — — — CH3 — The amino end of the peptide is known as N-terminal or amino terminal while the carboxy end is called C-terminal or carboxy terminal. In general protein sequences are written from N-Terminal to C-Terminal.
The atoms other than the side chains (R-groups) are called main chain or the back bone of the polypeptide. . . Classification of proteins Proteins are classified based on their structure (overall shape) into two major types.
They are fibrous proteins and globular proteins. Fibrous proteins Fibrous proteins are linear molecules similar to fibres. These are generally insoluble in water and are held together by disulphide bridges and weak intermolecular hydrogen bonds. The proteins are often used as structural proteins.
Example: Keratin, Collagen etc… Globular proteins Globular proteins have an overall spherical shape. The polypeptide chain is folded into a spherical shape. These proteins are usually soluble in water and have many functions including catalysis (enzyme). Example: myoglobin, insuline Figure .
(a) Structure of fibrous proteins Figure . (b) Structure of globular proteins XII XII - - - - . . Structure of proteins Proteins are polymers of amino acids.
Their three dimensional structure depends mainly on the sequence of amino acids (residues). The protein structure can be described at four hierarchal levels called primary, secondary, tertiary and quaternary structures as shown in the figure . . Primary structure of proteins: Proteins are polypeptide chains, made up of amino acids are connected through peptide bonds.
The relative arrangement of the amino acids in the polypeptide chain is called the primary structure of the protein. Knowledge of this