An homology model of Candidamethylica formate dehydrogenase (cmFDH) was constructed based on the Pseudomonas sp. 101 formate dehydrogenase (psFDH) structure. In wild type cmFDH, Thr169 and Thr226 can form hydrogen bonds with each other. We measured the interaction energy between the two threonines independent of other interactions in the proteins by using a so-called double mutant cycle and assessing the protein stability from the concentration of guanidine hydrochloride needed to denature 50% of the molecules. We conclude that the hydrogen bonds stabilize the wild type protein by -4 kcal mol(-1).