The structure and mechanism of action of papain

Abstract

The cysteine proteinases form a group of enzymes which depend for their enzymic activity on the thiol group of a cysteine residue. Several which occur in plants have been investigated extensively and include papain, ficin and stem bromelain (Smith & Kimmel i960). Although the term papain, introduced last century to describe the proteolytic principle in papaya latex (Wurtz & Bouchut 1879) is still used to describe crude dried latex, the crystalline enzyme is readily obtained (Kimmel & Smith 1954). Ficin is known to consist of several closely related enzymes which have been resolved (Sgarbieri, Gupte, Kramer & Whitaker 1964), but for most structural and mechanistic studies the unresolved mixture of enzymes has been used. Stem bromelain also appears to be a mixture of at least two proteolytic enzymes which have not yet been resolved (Ota, Moore & Stein 1962; Murachi 1964). In spite of the recognized heterogeneity of ficin and stem bromelain, it does seem that both structurally and mechanistically they are similar to papain. Only one bacterial cysteine proteinase has received a detailed study, namely, streptococcal proteinase, and it appears to have little or no relation in its amino acid sequence with the plant enzymes (Liu, Stein, Moore & Elliott 1965). The functional groups involved in the catalytic mechanism are apparently the same as in the plant proteinases (Gerwin, Stein & Moore 1966; Liu 1967; Husain & Lowe 1968 a , c ), but the mechanism of action has not been extensively studied. It may well be however that the plant and bacterial cysteine proteinases have converged onto a similar mechanism of action by two independent evolutionary pathways, as now seems apparent for the animal and bacterial serine proteinases (Alden, Wright & Kraut, this volume, p. 119). Because the tertiary crystal structure of papain (Drenth, Jansonius, Koekoek, Swen & Wolthers 1968; see also the preceding paper, p. 231) is now known, a critical survey of this enzyme is apposite.