The role of Ala198 in the stability and coenzyme specificity of bacterial formate dehydrogenases

Abstract

It has been shown by an X-ray structural analysis that the amino acid residues Ala198, which are located in the coenzyme-binding domain of NAD+-dependent formate dehydrogenases (EC 1.2.1.2., FDH) from bacteria Pseudomonas sp.101 and Moraxella sp. C-1 (PseFDH and MorFDH, respectively), have non-optimal values of the angles ψ and ϕ. These residues were replaced with Gly by site-directed mutagenesis. The mutants PseFDH A198G and MorFDH A198G were expressed in E.coli cells and obtained in active and soluble forms with more than 95% purity. The study of thermal inactivation kinetics showed that the mutation A198G results in a 2.5-fold increase in stability compared to one for the wild-type enzymes. Kinetic experiments indicate that A198G replacement reduces the KMNAD+ value from 60 to 35 and from 80 to 45 μM for PseFDH and MorFDH, respectively, while the KMHCOO- value remains practically unchanged. Amino acid replacement A198G was also added to the mutant PseFDH D221S with the coenzyme specificity changed from NAD+ to NADP+. In this case, an increase in thermal stability was also observed, but the influence of the mutation on the kinetic parameters was opposite: KM increased from 190 to 280 μM and from 43 to 89 mM for NADP+ and formate, respectively. According to the data obtained, inference could be drawn that earlier formate dehydrogenase from bacterium Pseudomonas sp. 101 was specific to NADP+, but not to NAD+.