Among pyridine-nucleotide-dependent oxidoreductases, the class 3 family of aldehyde dehydrogenases (ALDHs) is unusual in its ability to function with either NAD or NADP. This is all the more surprising because an acidic residue, Glu140, coordinates the adenine ribose 2 H hydroxyl. In many NAD-dependent dehydrogenases a similarly placed carboxylate is thought to be responsible for exclusion of NADP. The corresponding residue in most (< 71%) sequences in the ALDH extended family is also Glu, and most of these are NAD-specific enzymes. Site-directed mutagenesis was performed on this residue in rat class 3 ALDH. Our results indicate that this residue contributes to tighter binding of NAD in the native enzyme, but suggest that additional factors must contribute to the ability to utilize NADP. Mutagenesis of an adjacent basic residue (Lys137) indicates that it is even more essential for binding both coenzymes, consistent with its conservation in nearly all ALDHs (. 98%).

Crystals of recombinant obelin, the Ca 2+ -regulated photoprotein from the marine hydroid Obelia longissima, have been grown from sodium citrate solutions. Crystals grow as hexagonal light-yellow rods (0.1 Â 0.1 Â 1.0 mm) which diffract to beyond 1.8 A Ê with synchrotron radiation of 1.0 A Ê wavelength. The crystals have a primitive hexagonal lattice with unit-cell parameters a = 81.55, c = 86.95 A Ê . The asymmetric unit contains two molecules. This represents the successful preparation of single crystals of a photoprotein obelin which have promising diffraction properties.