Disulfide bonds are formed by pairs of which aminoacid?

Ans. is 'd' i.e. Cysteine* Disulfide bridges are R-S-S-R'* These bridges are formed by oxidation of Sulfhydryl groups of cysteine residues of proteins.* These disulfide bridges in a protein are found to stabilize quartenary structure of a protein.* In other words, disulfide bridges keep individual subunits of a protein together.* As these disulfide bridges are labile, they get easily reduced in the present of antioxidants like mercaptoethanol.* This is the basis or reducing type of SDS - PAGE. SDS PAGE is used to find the molecular weight of an unknown protein. Following treatment with mercaptoethanol, the disulfide bridges get reduced to form sulfhydryl groups and the individual subunits become free. This helps us to understand the number of subunits present in a protein. Hence reducing type of SDS-PAGE is used for studying Quartenary structure of a protein.* Disulfide bridges are not formed by methionine, as methionine does not have sulfhydryl group - they have a thioether linkage