Structure of beta-ketoacyl-[acyl carrier protein] reductase from Escherichia coli: Negative cooperativity and its structural basis

The structure of beta -ketoacyl-[acyl carder protein] reductase (FabG) from Escherichia coli was determined via the multiwavelength anomalous diffract ion technique using a selenomethionine-labeled crystal containing 88 seleni um sites in the asymmetric unit. The comparison of the E. coli FabG structu re with the homologous Brassica napus FabG.NADP(+) binary complex reveals t hat cofactor binding causes a substantial conformational. change in the pro tein. This conformational change puts all three active-site residues (Ser 1 38, Tyr 151, and Lys 155) into their active configurations and provides a s tructural mechanism for allosteric communication between the active sites i n the homotetramer. FabG exhibits negative cooperative binding of NADPH, an d this effect is enhanced by the presence of acyl carrier protein (ACP). NA DPH binding also increases the affinity and decreases the maximum binding o f ACP to FabG. Thus, unlike other members of the short-chain dehydrogenase/ reductase superfamily, FabG undergoes a substantial conformational change u pon cofactor binding that organizes the active-site triad and alters the af finity of the other substrate-binding sites in the tetrameric enzyme.