Hysteresis of plant cell-wall beta-glucosidase.

A transient-kinetic study of beta-glucosidase from soyabean cell walls was performed with the use of a stopped-flow apparatus. The progress curve of the reaction exhibits a 'slow' burst of about 1 s before the steady state is reached. In the time scale investigated this burst may be accounted for by only one exponential, whose time constant varies with the substrate concentration. As this concentration is increased the value of the time constant increases at first, then decreases. Premixing the enzyme with glucose, the last product of the reaction sequence, reverses the 'slow' burst into a 'slow' lag. Taken together, these results are only compatible with a model that involves the existence of a 'slow' conformational transition of the enzyme.