STRUCTURE OF MITOCHONDRIAL ALDEHYDE DEHYDROGENASE - THE GENETIC COMPONENT OF ETHANOL AVERSION

Background: The single genetic factor most strongly correlated with re duced alcohol consumption and incidence of alcoholism is a naturally o ccurring variant of mitochondrial aldehyde dehydrogenase (ALDH2). This Variant contains a glutamate to lysine substitution at position 487 ( E487K). The E487K variant of ALDH2 is found in approximately 50% of th e Asian population, and is associated with a phenotypic loss of ALDH2 activity in both heterozygotes and homozygotes. ALDH2-deficient indivi duals exhibit an averse response to ethanol consumption, which is prob ably caused by elevated levels of blood acetaldehyde. The structure of ALDH2 is important for the elucidation of its catalytic mechanism, to gain a clear understanding of the contribution of ALDH2 to the geneti c component of alcoholism and for the development of specific ALDH2 in hibitors as potential drugs for use in the treatment of alcoholism. Re sults: The X-ray structure of bovine ALDH2 has been solved to 2.65 Ang strom in its free form and to 2.75 Angstrom in a complex with NAD(+). The enzyme structure contains three domains: two dinucleotide-binding domains and a small three-stranded beta-sheet domain, which is involve d in subunit interactions in this tetrameric enzyme. The E487K mutatio n occurs in this small oligomerization domain and is located at a key interface between subunits immediately below the active site of anothe r monomer. The active site of ALDH2 is divided into two halves by the nicotinamide ring of NAD(+). Adjacent to the A-side (Pro-R) of the nic otinamide ring is a cluster of three cysteines (Cys301, Cys302 and Cys 303) and adjacent to the B-side (Pro-S) are Thr244, Glu268, Glu476 and an ordered water molecule bound to Thr244 and Glu476. Conclusions: Al though there is a recognizable Rossmann-type fold, the coenzyme-bindin g region of ALDH2 binds NAD(+) in a manner not seen in other NAD(+)-bi nding enzymes. The positions of the residues near the nicotinamide rin g of NAD(+) suggest a chemical mechanism whereby Glu268 functions as a general base through a bound water molecule. The sidechain amide nitr ogen of Asn169 and the peptide nitrogen of Cys302 are in position to s tabilize the oxyanion present in the tetrahedral transition state prio r to hydride transfer. The functional importance of residue Glu487 now appears to be due to indirect interactions of this residue with the s ubstrate-binding site via Arg264 and Arg475.