Apo and holo crystal structures of an NADP-dependent aldehyde dehydrogenase from Streptococcus mutans

The aldehyde dehydrogenases (ALDHs) are a superfamily of multimeric enzymes which catalyse the oxidation of a broad range of aldehydes into their corr esponding carboxylic acids with the reduction of their cofactor, NAD or NAD P, into NADH or NADPH. At present, the only known structures concern NAD-de pendent ALDHs. Three structures are available in the Protein Data Bank: two are tetrameric and the other is a dimer. We solved by molecular replacemen t the first structure of an NADP-dependent ALDH isolated from Streptococcus mutans, in its apo form and hole form in complex with NADP, at 1.8 and 2.6 Angstrom resolution, respectively. Although the protein sequence shares on ly approximately 30% identity with the other solved tetrameric ALDHs, the s tructures are very similar. However, a large local conformational change in the region surrounding the 2' phosphate group of the adenosine moiety is o bserved when the enzyme binds NADP, in contrast to the NAD-dependent ALDHs. Structure and sequence analyses reveal several properties. A small number of residues seem to determine the oligomeric state. Likewise, the nature (c harge and volume) of the residue at position 180 (Thr in ALDH from S. mutan s) determines the cofactor specificity in comparison with the structures of NAD-dependent ALDHs. The presence of a hydrogen bond network around the co factor not only allows it to bind to the enzyme but also directs the side-c hains in a correct orientation for the catalytic reaction to take place. Mo reover, a specific part of this network appears to be important in substrat e binding. Since the enzyme oxidises the same substrate, glyceraldehyde-3-p hosphate (G3P), as IVAD-dependent phosphorylating glyceraldehyde-3-phosphat e dehydrogenases (GAPDH), the active site of GAPDH was compared with that o f the S. mutans ALDH. It was found that Arg103, Arg283 and Asp440 might be key residues for substrate binding. (C) 1999 Academic Press.