Coenzyme specificity in aldehyde dehydrogenase.

Influences on coenzyme preference are explored. Lysine 137 (192 in class 1/2 ALDH) lies close to the adenine ribose, directly interacting with the adenine ribose in NAD-specific ALDHs and the 2'-phosphate of NADP in NADP-specific ALDHs. Lys-137 in class 3 ALDH interacts with the adenine ribose indirectly through an intervening water molecule. However, this residue is present in all ALDHs and, as a result, is unlikely to directly influence coenzyme specificity. Glutamate 140 (195) coordinates the 2'- and 3'-hydroxyls of the adenine ribose of NAD in the class 3 tertiary structure. Thus, it appeared that this residue would influence coenzyme specificity. Mutation to aspartate, asparagine, glutamine or threonine shifts the coenzyme specificity towards NADP, but did not completely change the specificity. Still, the mutants show the 2'-phosphate of NADP is repelled by Glu-140 (195). Although Glu-140 (195) has a major influence on coenzyme specificity, it is not the only influence since class 3 ALDHs, can use both coenzymes, and class 2 ALDHs, which are NAD-specific, have a glutamate at this position. One explanation may be that the larger space between Lys-137 (192) and the adenine ribose hydroxyls in the class 3 ALDH:NAD binary structure may provide space to accommodate the 2'-phosphate of NADP. Also, a structural shift upon binding NADP may also occur in class 3 ALDHs to help accommodate the 2'-phosphate of NADP.