Ketoacyl synthase

Ketoacyl synthases (KSs) catalyze the condensation reaction of acyl-CoA or acyl-acyl ACP with malonyl-CoA to form 3-ketoacyl-CoA or with malonyl-ACP to form 3-ketoacyl-ACP. This reaction is a key step in the fatty acid synthesis cycle, as the resulting acyl chain is two carbon atoms longer than before. KSs exist as individual enzymes, as they do in type II fatty acid synthesis and type II polyketide synthesis, or as domains in large multidomain enzymes, such as type I fatty acid synthases (FASs) and polyketide synthases (PKSs). KSs are divided into five families: KS1, KS2, KS3, KS4, and KS5.

Multidomain enzyme systems

Fatty acid synthase

Fatty acid synthase (FAS) is the enzyme system involved in de novo fatty acid synthesis. FAS is an iterative multienzyme consisting of several component enzymes, one of which is ketoacyl synthase. There are two types of FASs: type I and type II. Type I FASs are highly integrated multidomain enzymes. They contain discrete functional domains responsible for specific catalytic activities of the reaction sequence, either on a single polypeptide chain or on two different multifunctional proteins. Type II FASs are dissociated systems, meaning the component enzymes are independent proteins encoded by a series of separate genes.

Polyketide synthase

Polyketide synthases (PKS) are structurally and functionally related to FAS's, both which are enzymes that catalyze the condensation of activated primary metabolites such as acetyl-CoA and malonyl-CoA.

The main reaction they catalyze is:

CO2-CH2-CO-S-CoA + CH3-CO-S-PKS → CH3-CO-CH2-CO-S-PKS + CoA-H + CO2

Like FASs, PKSs will use a β-ketoacylsynthase (KS), an optional (malonyl) acyl transferase (MAT/AT), and a phosphopantethienylated acyl carrier protein (ACP) or coenzymeA (CoA). They also both used a ketoreductase, dehydratase, and enoyl reductase to create a fully saturated acyl backbone.