The NAD(+)-dependent formate dehydrogenase (FDH; EC 126.96.36.199) fromCandida boidinii(CboFDH) has been extensively used in NAD(H)-dependent industrial biocatalysis as well as in the production of renewable fuels and chemicals from carbon dioxide. In the present work, the effect of amino acid residues Phe285, Gln287, and His311 on structural stability was investigated by site-directed mutagenesis. The wild-type and mutant enzymes (Gln287Glu, His311Gln, and Phe285Thr/His311Gln) were cloned and expressed inEscherichia coli.Circular dichroism (CD) spectroscopy was used to determine the effect of each mutation on thermostability. The results showed the decisive roles of Phe285, Gln287, and His311 on enhancing the enzyme's thermostability. The melting temperatures for the wild-type and the mutant enzymes Gln287Glu, His311Gln, and Phe285Thr/His311Gln were 64, 70, 77, and 73 degrees C, respectively. The effects of pH and temperature on catalytic activity of the wild-type and mutant enzymes were also investigated. Interestingly, the mutant enzyme His311Gln exhibits a large shift of pH optimum at the basic pH range (1 pH unit) and substantial increase of the optimum temperature (25 degrees C). The present work supports the multifunctional role of the conserved residues Phe285, Gln287, and His311 and further underlines their pivotal roles as targets in protein engineering studies.