Amino-acid sequence and glycan structures of cysteine proteases with proline specificity from ginger rhizome Zingiber officinale

The ginger proteases (GP-I and GP-II), isolated from the ginger rhizome Zingiber officinale, have an unusual substrate specificity preference for cleaving peptides with a proline residue at the P₂ position. The complete amino-acid sequence of GP-II, a glycoprotein containing 221 amino acids, and about 98% that of GP-I have been determined. Both proteases, which are 82% similar, have cysteine residues at positions 27 and histidines at position 161, corresponding to the essential cysteine-histidine diads found in the papain family of cysteine proteases, and six corresponding cysteine residues that form the three invariant disulfide linkages seen in this family of proteins. The sequence homology with other members (papain, bromelain, actinidin, protease omega, etc.) of this family is ≃ 50%. GP-II has two predicted glycosylation sites at Asn99 and Asn156. Analysis by electrospray and collision-induced dissociation MS showed that both sites were occupied by the glycans (Man)₃(Xyl)₁(Fuc)₁(GlcNAc)₂ and (Man)₃(Xyl)₁(Fuc)₁(GlcNAc)₃, in a ratio of ≃ 7: 1. Both glycans are xylose containing biantennary complex types that share the common core structural unit, Man1→6(Man1→3) (Xy11→2)Man1→4GlcNAc1→4(Fuc1→3)GlcNAc for the major form, with an additional N-acetylglucosamine residue being linked, in the minor form, to one of the terminal mannose units of the core structure.