Crystallization and preliminary X-ray analysis of beta-hydroxydecanoyl thiol ester dehydrase from Escherichia coli

Roles of the FabA and FabZ beta-hydroxyacyl-acyl carrier protein dehydratases in Escherichia coli fatty acid biosynthesis

There are two genes, fabA and fabZ, encoding beta-hydroxyacyl-acyl carrier protein (ACP) dehydratases that function in the dissociated, type II fatty acid synthase system of Escherichia coli. We have investigated their roles in fatty acid synthesis by purifying the two proteins and reconstituting cycles of fatty acid synthesis in vitro using five other purified proteins. FabA and FabZ exhibited broad, overlapping chain length specificities. The FabZ dehydratase efficiently catalyzed the dehydration of short chain beta-hydroxyacyl-ACPs and long chain saturated and unsaturated beta-hydroxyacyl-ACPs. FabA was most active on intermediate chain length beta-hydroxyacyl-ACPs and also possessed significant activity toward both short and long chain saturated beta-hydroxyacyl-ACPs. Significantly, FabA was virtually inactive in the dehydration of long chain unsaturated beta-hydroxyacyl-ACP. The introduction of the double bond at the 10-carbon stage of fatty acid synthesis by FabA was only detected in the presence of beta-ketoacyl-ACP synthase I (FabB). A yeast two-hybrid analysis failed to detect an interaction between FabA and FabB, therefore the channeling of intermediates toward unsaturated fatty acid synthesis by FabB was attributed to the affinity of the condensing enzyme for cis-decenoyl-ACP. The broad substrate specificity of FabZ coupled with the inactivity of FabA toward a long chain unsaturated beta-hydroxyacyl-ACP provides a biochemical explanation for the phenotypes of cells with genetically altered levels of the two dehydratases.

Structure of a dehydratase-isomerase from the bacterial pathway for biosynthesis of unsaturated fatty acids: two catalytic activities in one active site

BACKGROUND

Escherichia coli beta-hydroxydecanoyl thiol ester dehydrase (dehydrase) is essential to the biosynthesis of unsaturated fatty acids, by shunting a 10-carbon intermediate from the saturated fatty acid pathway into the unsaturated fatty acid pathway. Dehydrase catalyzes reactions of dehydration and of double-bond isomerization on 10-carbon thiol esters of acyl carrier protein (ACP). The aim of this work is to elucidate mechanisms for the two enzymatic reactions, which occur in an unusual bifunctional active site, and to understand the specificity of the enzyme for substrates with 10-carbon fatty acyl chains.

RESULTS

Crystal structures at 2.0 A resolution for free dehydrase and for the enzyme modified by its classic, mechanism-based inactivator, 3-decynoyl-N-acetylcysteamine, have been determined. Dehydrase is a symmetric dimer with an unusual alpha+beta 'hot dog' fold. Each of the two independent active sites is located between the two subunits of the enzyme, and is a tunnel-shaped pocket completely isolated from the general solvent. Side chains of histidine from one subunit and aspartic acid from the other are the only potentially reactive protein groups in the active site.

CONCLUSION

A two-base mechanism by which the histidine and aspartic acid together catalyze dehydration and isomerization reactions is consistent with the active-site structure. The unique topology of the protein fold and the identification of the active-site components reveal features of predictive value for another enzyme, FabZ, which may be the non-specific dehydratase involved in elongation of fatty acyl chains. A positively charged area surrounding the entrance to the active site, which could interact with the negatively charged ACP, was also found.