Aldehyde dehydrogenase enzyme ALDH3H1 from Arabidopsis thaliana: Identification of amino acid residues critical for cofactor specificity.

The cofactor-binding site of the NAD(+)-dependent Arabidopsis thaliana aldehyde dehydrogenase ALDH3H1 was analyzed to understand structural features determining cofactor-specificity. Homology modeling and mutant analysis elucidated important amino acid residues. Glu149 occupies a central position in the cofactor-binding cleft, and its carboxylate group coordinates the 2'- and 3'-hydroxyl groups of the adenosyl ribose ring of NAD(+) and repels the 2'-phosphate moiety of NADP(+). If Glu149 is mutated to Gln, Asp, Asn or Thr the binding of NAD(+) is altered and rendered the enzyme capable of using NADP(+). This change is attributed to a weaker steric hindrance and elimination of the electrostatic repulsion force of the 2'-phosphate of NADP(+). Simultaneous mutations of Glu149 and Ile200, which is situated opposite of the cofactor binding cleft, improved the enzyme efficiency with NADP(+). The double mutant ALDH3H1Glu149Thr/Ile200Val showed a good catalysis with NADP(+). Subsequently a triple mutation was generated by replacing Val178 by Arg in order to create a "closed" cofactor binding site. The cofactor specificity was shifted even further in favor of NADP(+), as the mutant ALDH3H1E149T/V178R/I200V uses NADP(+) with almost 7-fold higher catalytic efficiency compared to NAD(+). Our experiments suggest that residues occupying positions equivalent to 149, 178 and 200 constitute a group of amino acids in the ALDH3H1 protein determining cofactor affinity.

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