Kinetic analysis of the reaction catalyzed by chitinase A1 from Bacillus circulans WL-12 toward the novel substrates, partially N-deacetylated 4-methylumbelliferyl chitobiosides

The kinetic behavior of chitinase A1 from Bacillus circulans WL-12 was investigated using the novel fluorogenic substrates, N-deacetylated 4-methylumbelliferyl chitobiosides [GlcN-GlcNAc-UMB (2), GlcNAc-GlcN-UMB (3), and (GlcN)(2)-UMB (4)], and the results were compared with those obtained using 4-methylumbelliferyl N.N'-diacetylchitobiose [(GlcNAc)(2)-UMB (1)] as the substrate. The chitinase did not release the UR;IB moiety from compound 4, but successfully released UMB from the other substrates, k(cat)/K-m values determined from the releasing rate of the UMB moiety were: 145.3 for 1, 8.3 for 2, and 0.1 s(-1) M-1 for 3. The lack of an N-acetyl group at subsite (-1) reduced the activity to a level 0.1% of that obtained with compound 1, while the absence of the N-acetyl group at subsite (-2) reduced the relative activity to 5.7%, These observations strongly support the theory that chitinase A1 catalysis occurs via a 'substrate-assisted' mechanism, Using these novel fluorogenic substrates, we were able to quantitatively evaluate the recognition specificity of subsite (-2) toward the N-acetyl group of the substrate sugar residue, The (-2) subsite of chitinase A1 was found to specifically recognize an N-acetylated sugar residue, but this specificity was not as strict as that found in subsite(-1). (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.