Sequestration of the active site by interdomain shifting - Crystallographic and spectroscopic evidence for distinct conformations of L-3-hydroxyacyl-CoA dehydrogenase

L-3-Hydroxyacyl-CoA dehydrogenase reversibly catalyzes the conversion of L- 3-hydroxyacyl-CoA to 3-ketoacyl-CoA concomitant with the reduction of NAD() to NADH as part of the beta-oxidation spiral. In this report, crystal str uctures have been solved for the apoenzyme, binary complexes of the enzyme with reduced cofactor or 3-hydroxybutyryl-CoA substrate, and an abortive te rnary complex of the enzyme with NAD(+) and acetoacetyl-CoA The models illu strate positioning of cofactor and substrate within the active site of the enzyme. Comparison of these structures with the previous model of the enzym e-NAD(+) complex reveals that although significant shifting of the NAD(+)-b inding domain relative to the C-terminal domain occurs in the ternary and s ubstrate-bound complexes, there are few differences between the apoenzyme a nd cofactor-bound complexes. Analysis of these models clarifies the role of key amino acids implicated in catalysis and highlights additional critical residues. Furthermore, a novel charge transfer complex has been identified in the course of abortive ternary complex formation, and its characterizat ion provides additional insight into aspects of the catalytic mechanism of L-3-hydroxyacyl-CoA dehydrogenase.