Proteolytic modification of a glucoamylase from a Rhizopus sp.

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Abstract
Three forms of glucoamylase [EC 3.2.1.3] were purified from a Rhizopus sp. and named Gluc1, Gluc2 and Gluc3 in order of content. Gluc1 (MW 74,000; specific activity 66 U/mg; N-terminal Ala; C-terminal-Ser-Ala .cntdot. OH) was converted by papain and chymotrypsin into 2 active derivatives named pap-Gluc and chymo-Gluc, respectively. Pap-Gluc was characterized by a MW of 57,000 and a specific activity of 88 U/mg, and chymo-Gluc by a MW of 64,000 anmd a specific activity of 78 U/mg. The C-terminal amino acid sequences of both modified enzymes were identical with that of Gluc1 but their N-terminal amino acids were different from that of Gluc1. These results, together with the results of amino acid and sugar analyses, indicate that papain and chymotrypsin liberated glycopeptide and peptide moieties, respectively, from the N-terminal side of Gluc1. The 2 modified enzymes had almost the same pH optimum, pH stability and heat stability as those of Gluc1. They differed from Gluc1 in the values of Km and Vmax for high-MW substrates, although they showed identical kinetic parameters for low-MW substrates. The close similarity between pap-Gluc and Gluc2 as well as between chymo-Gluc and Gluc3 is discussed.