A shared binding site for NAD+ and coenzyme a in an acetaldehyde dehydrogenase involved in bacterial degradation of aromatic compounds

The meta-cleavage pathway for catechol is a central pathway for the bacterial dissimilation of a wide variety of aromatic compounds, including phenols, methylphenols, naphthalenes, and biphenyls. The last enzyme of the pathway is a bifunctional aldolase/dehydrogenase that converts 4-hydroxy-2-ketovalerate to pyruvate and acetyl-CoA via acetaldehyde. The structure of the NAD(+)/CoASH-dependent aldehyde dehydrogenase subunit is similar to that of glyceraldehyde-3-phosphate dehydrogenase, with a Rossmann fold-based NAD(+) binding site observed in the NAD(+)-enzyme complex [Manjasetty, B. A., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 6992-6997]. However, the location of the CoASH binding site was not determined. In this study, hydrogen-deuterium exchange experiments, coupled with peptic digest and mass spectrometry, were used to examine cofactor binding. The pattern of hydrogen-deuterium exchange in the presence of CoASH was almost identical to that observed with NAD(+), consistent with the two cofactors sharing a binding site. This is further supported by the observations that either CoASH or NAD(+) is able to elute the enzyme from an NAD(+) affinity column and that preincubation of the enzyme with NAD(+) protects against inactivation by CoASH. Consistent with these data, models of the CoASH complex generated using AUTODOCK showed that the docked conformation of CoASH can fully occupy the cavity containing the enzyme active site, superimposing with the NAD(+) cofactor observed in the X-ray crystal structure. Although CoASH binding Rossmann folds have been described previously, this is the first reported example of a Rossmann fold that can alternately bind CoASH or NAD(+) cofactors required for enzymatic catalysis.