Actinidain

 ( Ec 3.4.22 ) 

Actinidain is similar to papain in size, shape, active site location and conformation, as well as in kinetic studies, which is especially interesting as they only share 48% amino acid similarity. Electron density mapping shows similar α-helices and overall polypeptide folding. While the electron density map indicates 218 amino acids, further sequencing work suggests 220 amino acids with the extra two being found at the C-terminus. The active site includes cysteine and histidine residues that are conserved across several other proteins in the fruit peptidase family. Electron density mapping indicates a double crossover with domain 1 being made up of AA 19-115 and 214-218 and domain II composing of AA 1-18 and 116-213, with both the N-terminal and the C-terminal ends crossing over into both domains. Domain 1 has several α-helices whereas domain 2 is primarily made up of one anti-parallel β-sheet. Actinidain comprises up to 50% of the soluble protein content in kiwifruits at harvest. Actinidain is active over a wide range of pH, including very acidic conditions, with a pH optimum from 5-7. At least ten different isoforms that all have the same molecular weight and cysteine protease activity as actinidain have been identified but they vary in isoelectric point from acidic (pI 3.9) to basic (pI 9.3).