Glutamate dehydrogenases (GDHs; EC 188.8.131.52 –4) catalyse the oxidative deamination of l-glutamate to -ketoglutarate, using NAD(+) and/or NADP(+) as a cofactor. Subunits of homo-hexameric bacterial enzymes comprise a substrate-binding domainI followed by a nucleotide-binding domainII. The reaction occurs in a catalytic cleft between the two domains. Although conserved residues in the nucleotide-binding domains of various dehydrogenases have been linked to cofactor preferences, the structural basis for specificity in the GDH family remains poorly understood. Here, the refined crystal structure of Escherichiacoli GDH in the absence of reactants is described at 2.5-angstrom resolution. Modelling of NADP(+) in domainII reveals the potential contribution of positively charged residues from a neighbouring -helical hairpin to phosphate recognition. In addition, a serine that follows the P7 aspartate is presumed to form a hydrogen bond with the 2-phosphate. Mutagenesis and kinetic analysis confirms the importance of these residues in NADP(+) recognition. Surprisingly, one of the positively charged residues is conserved in all sequences of NAD(+)-dependent enzymes, but the conformations adopted by the corresponding regions in proteins whose structure has been solved preclude their contribution to the coordination of the 2-ribose phosphate of NADP(+). These studies clarify the sequence-structure relationships in bacterial GDHs, revealing that identical residues may specify different coenzyme preferences, depending on the structural context. Primary sequence alone is therefore not a reliable guide for predicting coenzyme specificity. We also consider how it is possible for a single sequence to accommodate both coenzymes in the dual-specificity GDHs of animals.