A HIGHLY-ACTIVE DECARBOXYLATING DEHYDROGENASE WITH RATIONALLY INVERTED COENZYME SPECIFICITY

The isocitrate dehydrogenase of Escherichia coli, which lacks the Ross mann fold common to other dehydrogenases, displays a 7000-fold prefere nce for NADP over NAD (calculated as the ratio of k(cat)/K-m). Guided by x-ray crystal structures and molecular modeling, site-directed muta genesis has been used to introduce six substitutions in the adenosine binding pocket that systematically shift coenzyme preference toward NA D, The engineered enzyme displays an 850-fold preference for NAD over NADP, which exceeds the 140-fold preference displayed by a homologous NAD-dependent enzyme, Of the six mutations introduced, only one is ide ntical in all related NAD-dependent enzyme sequences-strict adherence to homology as a criterion for replacing these amino acids impairs fun ction, Two additional mutations at remote sites improve performance fu rther, resulting in a final mutant enzyme with kinetic characteristics and coenzyme preference comparable to naturally occurring homologous NAD-dependent enzymes.