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BACKGROUND Glutamate, phenylalanine and leucine dehydrogenases catalyze the NAD(P)(+)-linked oxidative deamination of L-amino acids to the corresponding 2-oxoacids, and sequence homology between these enzymes clearly indicates the existence of an enzyme superfamily related by divergent evolution. We have undertaken structural studies on a number of members of this family in order to investigate the molecular basis of their differential amino acid specificity. RESULTS We have solved the X-ray structure of the leucine dehydrogenase from Bacillus sphaericus to a resolution of 2.2 A. Each subunit of this octameric enzyme contains 364 amino acids and folds into two domains, separated by a deep cleft. The nicotinamide ring of the NAD+ cofactor binds deep in this cleft, which is thought to close during the hydride transfer step of the catalytic cycle. CONCLUSIONS Comparison of the structure of leucine dehydrogenase with a hexameric glutamate dehydrogenase has shown that these two enzymes share a related fold and possess a similar catalytic chemistry. A mechanism for the basis of the differential amino acid specificity between these enzymes involves point mutations in the amino acid side-chain specificity pocket and subtle changes in the shape of this pocket caused by the differences in quaternary structure.