EVIDENCE FOR AN ACTIVE-CENTER CYSTEINE IN MEDICAGO-SATIVA LEAF SERINEPROTEINASE

A controlled conformational change with a very Limited loss of the sec ondary structure has been induced in the alfalfa (Medicago sativa L.) leaf serine proteinase using synthetic lysine homopolymer. Under these experimental conditions the enzyme activity is preserved and the uniq ue cysteinyl residue becomes titrable; furthermore the enzyme is inhib ited by p-chloro mercury benzoic acid (pCMB), indicating the active-ce ntre localization of the cysteine residue. This enzyme, although monom eric, is cooperatively inhibited by both synthetic and naturally occur ring polycations. The dependence of the accessibility of the thiol rea ctant to the active site on the cooperatively induced conformational c hange provides a possible explanation of the unusual behaviour of this plant enzyme.