Residues that influence coenzyme preference in the aldehyde dehydrogenases

To find out the residues that influence the coenzyme preference of aldehyde dehydrogenases (ALDHs), we reviewed, analyzed and correlated data from their known crystal structures and amino-acid sequences with their published kinetic parameters for NAD(P)+. We found that the conformation of the Rossmann-fold loops participating in binding the adenosine ribose is very conserved among ALDHs, so that coenzyme specificity is mainly determined by the nature of the residue at position 195 (human ALDH2 numbering). Enzymes with glutamate or proline at 195 prefer NAD+ because the side-chains of these residues electrostatically and/or sterically repel the 2′-phosphate group of NADP+. But contrary to the conformational rigidity of proline, the conformational flexibility of glutamate may allow NADP+-binding in some enzymes by moving the carboxyl group away from the 2′-phosphate group, which is possible if a small neutral residue is located at position 224, and favored if the residue at position 53 interacts with Glu195 in a NADP+-compatible conformation. Of the residues found at position 195, only glutamate interacts with the NAD+-adenosine ribose; glutamine and histidine cannot since their side-chain points are opposite to the ribose, probably because the absence of the electrostatic attraction by the conserved nearby Lys192, or its electrostatic repulsion, respectively. The shorter side-chains of other residues—aspartate, serine, threonine, alanine, valine, leucine, or isoleucine—are distant from the ribose but leave room for binding the 2′-phosphate group. Generally, enzymes having a residue different from Glu bind NAD+ with less affinity, but they can also bind NADP+ even sometimes with higher affinity than NAD+, as do enzymes containing Thr/Ser/Gln195. Coenzyme preference is a variable feature within many ALDH families, consistent with being mainly dependent on a single residue that apparently has no other structural or functional roles, and therefore can easily be changed through evolution and selected in response to physiological needs.