Kinetics of the coupled reaction catalysed by a fusion protein of beta-galactosidase and galactose dehydrogenase

The mechanistic implications of the kinetic behaviour of a fusion protein o f beta -galactosidase and galactose dehydrogenase have been analysed in vie w of predictions based on experimentally determined kinetic parameter value s for the galactosidase and dehydrogenase activities of the protein. The re sults show that the time course of galactonolactone formation from lactose in the coupled reaction catalysed by the fusion protein can be most satisfa ctorily accounted for in terms of a free-diffusion mechanism when considera tion is given to the mutarotation of the reaction intermediate galactose. I t is concluded that no tenable kinetic evidence is available to support the proposal that the fusion protein catalyses galactonolactone formation from lactose by a mechanism involving channelling of galactose. (C) 2001 Publis hed by Elsevier Science B.V.