PRIMARY STRUCTURE AND ENZYMATIC PROPERTIES OF CARBOXYPEPTIDASE II FROM WHEAT BRAN by

Wheat carboxypeptidase II has been isolated from wheat bran by affinity chromatography and its enzymatic properties and amino acid sequence has been determined. The enzyme is a dimer of molecular weight around 110,000 with each monomer composed of two peptide chains, an A-chain and a B-chain, linked by disulfide bridges. The A-chain exists in two forms, one, the A'-chain, being three amino acids shorter at the N-terminus, and consequently two different subunits, i.e. A-B and A'-B, and three different dimers, i.e. A'-B/A'-B, A'-B/A-B and A-B/A-B, are found. These different forms could be separated by ion exchange chromatography due to the Aand A'-chains differing by a single charge. Fragments of the A'and B-chains, obtained by chemical cleavages with either cyanogen bromide or hydroxylamine and by enzymatic cleavages with either trypsin or S. aureus V8 protease, were sequenced and aligned to give the complete sequences of the two chains. The A'and B-chains contain 260 and 160 amino acid residues, respectively. Three glycosylated asparagines are found in each chain. Alignment of the Aand B-chains with the corresponding chains of the known carboxypeptidases from germinated barley (malt) showed 95% homology with carboxypeptidase II and 37% homology with carboxypeptidase I. Similarly, the A-chain of wheat carboxypeptidase II exhibits 29% homology with the N-terminal part ofcarboxypeptidase Y and the B-chain exhibits 21% homology with the C-terminal part of carboxypeptidase Y. Like other serine carboxypeptidases wheat carboxypeptidase II exhibits peptidase activity with an acidic pH-optimum and esterase, amidase and peptidyl amino acid amide hydrolase activities with a basic pH optimum. The three different forms of the enzyme hydrolyse peptides with slightly different Km and k~at values but identical kcat/Km values. The specificity of the enzyme is identical to that of carboxypeptidase II from germinated barley: it exhibits a preference for basic and hydrophobic amino acid residues in the Pj and/or P/positions.