A ROLE FOR QUATERNARY STRUCTURE IN THE SUBSTRATE-SPECIFICITY OF LEUCINE DEHYDROGENASE

Background: Glutamate, phenylalanine and leucine dehydrogenases cataly ze 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 di vergent evolution. We have undertaken structural studies on a number o f members of this family in order to investigate the molecular basis o f their differential amino acid specificity. Results: We have solved t he X-ray structure of the leucine dehydrogenase from Bacillus sphaeric us to a resolution of 2.2 Angstrom. Each subunit of this octameric enz yme 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 s tep of the catalytic cycle. Conclusions: Comparison of the structure o f leucine dehydrogenase with a hexameric glutamate dehydrogenase has s hown that these two enzymes share a related fold and possess a similar catalytic chemistry. A mechanism for the basis of the differential am ino acid specificity between these enzymes involves point mutations in the amino acid side-chain specificity pocket and subtle changes in th e shape of this pocket caused by the differences In quaternary structu re.