Characterization of a structure close to the coenzyme-binding site of liver aldehyde dehydrogenase.

NAD analogues with the nicotinamide moiety exchanged for acetylpyridino-pentyl or acetylpyridino-butyl groups function as coenzymes in the enzymatic reaction with liver aldehyde dehydrogenase. The corresponding bromoacetyl derivatives bind to the coenzyme-binding site of the enzyme and inactive the protein by covalent modification of single residues close to the active site. Protection by coenzymes and substrate against the inactivation differs slightly for the two coenzyme analogues, suggesting the presence of more than one reactive residue. This is consistent with the results of differential carboxymethylation of cysteine residues of the basic isozyme in the presence and absence of the inhibitor disulfiram. The amino acid sequence around one reactive cysteine residue close to the active site of the acidic isozyme was determined after labeling with the butyl coenzyme analogue. This structure bears no extensive homology to corresponding known structures of dehydrogenases working on other types of aldehyde substrates.