Define the following as related to proteins :
(i) Peptide linkage
(ii) Primary structure
(iii) Denaturation

Explanation

(i) Peptide bond. Proteins are condensation polymers of `alpha`-amino acids in which the same or different `alpha`-amino acids are connected by peptide bonds. Chemically, a peptide bond is an amide linkage formed between `-COOH` group of one `alpha`-amino acid and `-NH(2)` group of the other `alpha`-amino acid by loss of a molecule of water. For example,

(ii) Primary strcuture : Proteins may contain one or more polypeptide chains. Each polypeptide chain has a large number of `alpha` - amino acids which are linked to one another in a specific sequence. The specific sequencein which the various `alpha` -amino acids present in a protein are linked to one another is called is primary structure. Any change in the sequence of `alpha`-amino acids creates a different protein. The gerneral primary structure of a protein is given.
(iii) Denaturation : Each proteins in the biological system has a unique three - dimensional structure and has specific biological activity. This is called native form of a protein. When a protein in its native form is subjected to physical changes in temperature, p H etc .hydrogen bonds are broken. Due to cleavage of hydrogen bonds, unfolding of protein molecule occurs and the protein loses its biological activity. This loss of biological activity is called denaturation. As a result of denaturation, globular proteins (soluble in `H_2O`) are converted into fibrous proteins (insoluble in `H_2O`). In other words , denaturation leads to coagulation. That is why coagulated proteins are also called denatureated proteins.
The most common example of denaturation of proteins is the coagulation of albumin present in the while of an egg. When the egg is boiled hard , the soluble globular protein present in it is denatured and is converted into insoluble fibrous protein. Another example is curding of milk which is caused due to formation of lactic acid by bacteria present in milk.