B. Schiott et al., THEORETICAL INVESTIGATION OF THE HYDRIDE TRANSFER FROM FORMATE TO NAD(+) AND THE IMPLICATIONS FOR THE CATALYTIC MECHANISM OF FORMATE DEHYDROGENASE, Journal of the American Chemical Society, 120(29), 1998, pp. 7192-7200
The hydride transfer reaction between formate and NAD(+) has been inve
stigated by using molecular orbital theory in combination with continu
um solvation models. The reaction in the gas phase is extremely exothe
rmic due to the instability of the charged reactant species. The calcu
lations reveal that during the hydride transfer the pyridine ring of N
AD(+) takes a quasi-boat conformation. The nitrogen atom of the pyridi
ne ring remains planar, which is in agreement with the experimentally
established N-15 kinetic isotope effect (1.004 +/- 0.001) of the forma
te dehydrogenase catalyzed oxidation of formate to carbon dioxide. The
computed value at the HF/6-31+G(d,p) level of theory for the N-15 kin
etic isotope effect is 1.0042. In solution, however, there is a potent
ial energy barrier for the hydride transfer. At the MP2/6-31G(d)//HF/6
-31G(d) level of theory the self-consistent reaction field approach gi
ves a barrier height of 9.0 kcal/mol in acetonitrile (epsilon = 35.9).
Direct nucleophilic addition of one of the carboxylate oxygens of for
mate to the pyridine ring of NAD(+) competes with hydride transfer, an
d this study reveals that this nucleophilic addition is likely to be p
referred over the hydride transfer in the gas phase. Thus, the NAD(+)-
dependent formate dehydrogenase must orient the substrate formate in t
he active site in such a fashion as to prevent this competing reaction
from occurring. According to the recently solved X-ray crystal struct
ure, it is clear that the Arg-284 and Asn-146 are the two critical ami
no acid residues that hold formate in the productive orientation for h
ydride transfer.