Characterization of the β-Carbon Processing Reactions of the Mammalian Cytosolic Fatty Acid Synthase: Role of the Central Core

Publisher Website
Google Scholar
Abstract
The properties of the β-ketoacyl reductase, dehydrase, and enoyl reductase components of the animal fatty acid synthase responsible for the reduction of the β-ketoacyl moiety formed at each round of chain elongation have been studied by engineering and characterizing mutants defective in each of these three catalytic domains. These “β-carbon processing” mutants leak the stalled four-carbon intermediates by direct transfer to CoA. However, enoyl reductase mutants leak β-ketobutyryl, β-hydroxybutyryl, and crotonyl moieties, a finding explained, at least in part, by the observation that the equilibrium and rate constant for the dehydrase reaction favor the formation of β-hydroxy rather than enoyl moieties. In this regard, the type I animal fatty acid synthase resembles its type II counterpart in Escherichia coli in that both systems rely on the enoyl reductase to pull the β-carbon processing reactions to completion. Kinetic and nucleotide binding measurements on fatty acid synthases mutated in either of the two nucleotide binding domains revealed that the NADPH binding sites are nonidentical, the enoyl reductase exhibiting higher affinity. Surprisingly, NADPH binding is also completely compromised by certain deletions and mutations in the central core region distant from the nucleotide binding sites. Comparable central core sequences are present in the structurally related modular polyketide synthases, except in those modules that lack all three β-carbon processing enzymes. These findings suggest that the central core region of fatty acid and polyketide synthases plays an important role in facilitating the β-carbon processing reactions.

This publication has 15 references indexed in Scilit: