Pre-steady-state kinetics of Bacillus licheniformis 1,3-1,4-β-glucanase: Evidence for a regulatory binding site

In a previous paper, we reported the first stopped-flow experiments on a Bacillus licheniformis 1,3-1,4-β-glucanase [Abel, Planas and Christensen (2001) Biochem. J. 357, 195-202]. It was shown that the pre-steady-state kinetics of the 1,3-1,4-β-glucanase using the substrate 4-methylumbelliferyl 3-O-β-cellobiosyl-β-D-glucoside may be explained by a reaction scheme involving an induced fit and the binding of two substrates as well as a second enzymic conformational change, whereas the results definitely could not be explained in terms of the simple double-displacement scheme. In the present study, we report further stopped-flow kinetic results on the glucanase using a series of low-molecular-mass substrates with various leaving groups and varying chain length. The analysis of the resulting data leads to the conclusion that the free enzyme exists in two conformations, one of which binds the substrates rather strongly in a regulatory site, before any productive interactions can take place. This corresponds to an allosteric activation mechanism. With these substrates, however, the productive enzyme-substrate species are also able to change into less active or inactive forms. This may be seen as a feedback inhibitory mechanism.