Egg White Avidin

Abstract

The complete amino acid sequence of chicken egg white avidin has been elucidated from sequence studies on 4 cyanogen bromide peptides and 15 tryptic peptides obtained from the protein. Avidin consists of four essentially identical polypeptide chains, each containing 128 amino acid residues with a carbohydrate moiety attached at asparaginyl Residue 17. Each chain has NH₂-terminal alanine, COOH-terminal glutamic acid, and an intrachain disulfide bond between Residues 4 and 83, but approximately half of the chains have isoleucine at Residue 34, and the other half have threonine at this position. No other evidence of heterogeneity in the sequence was found. The cyanogen bromide reaction with avidin represents another example of partial cleavage at Met-Ser or Met-Thr bonds, despite quantitative conversion of the methionyl residue to a homoseryl residue.