Proteins blocking cystine residues

In the native state, all the proteins contain a half-cystine residue as the carboxyterminal amino acid, and most have the aminoterminal blocked with an acetylalanine (there are a few exceptions in the ITT A family). The components of IIIA and B2 families exhibit a characteristically cystine-rich repetitive pentapeptide (-cys-cys-X-pro-Y-), which is not found in the IIIB family. The X position of the pentapeptide is frequently occupied by glutamine, glutamic acid, or arginine residues, while serine or threonine usually occurs in the Y position. The above sequence pattern has been found to occupy the major portion of the B2 protein [25,224]. In this protein, an intrachain disulfide bond is thought to link cys-2 of the first pentapeptide to cys-1 of the second pentapeptide unit, which would result in a convoluted pattern in the protein backbone [30,31,148,232]. 

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