The structure of actinidin at 5-5 A resolution.

The structure of actinidin, a sulphydryl protease obtained from the fruit of Actinidia chinensis, has been determined from X-ray crystallographic data to a resolution of 5.5 Å. Three isomorphous heavy atom derivatives, prepared with uranyl acetate, dichloroethylenediamine platinum(II) and potassium iodomercurate(II), were used in the phase calculation, giving a mean figure of merit of 0.88. The molecule can be described as an oblate ellipsoid with approximate dimensions 50 Å × 40 Å × 36 Å, and consists of two globular units separated by a shallow cleft. Binding studies with mercuric chloride reveal two sites of attachment, both within this cleft, and although both sites are of low occupancy it is probable that one or other marks the position of the active sulphydryl group. Although the folding of the polypeptide chain of actinidin cannot be followed with certainty, it appears to be closely similar to that of papain, suggesting that these are members of a family of homologous proteins.