β-ketoacyl-ACP synthase (EC 2.3.1.41) (KAS) [1] is the enzyme that catalyzes the condensation of malonyl-ACP with the growing fatty acid chain. It is found as a component of the following enzymatic systems:

• Fatty acid synthase (FAS), which catalyzes the formation of long-chain fatty acids from acetyl-CoA, malonyl-CoA and NADPH. Bacterial and plant chloroplast FAS are composed of eight separate subunits which correspond to different enzymatic activities; β-ketoacyl synthase is one of these polypeptides. Fungal FAS consists of two multifunctional proteins, FAS1 and FAS2; the β-ketoacyl synthase domain is located in the C-terminal section of FAS2. Vertebrate FAS consists of a single multifunctional chain; the β-ketoacyl synthase domain is located in the N-terminal section [2].
• The multifunctional 6-methysalicylic acid synthase (MSAS) from Penicillium patulum [3]. This is a multifunctional enzyme involved in the biosynthesis of a polyketide antibiotic and which has a KAS domain in its N-terminal section.
• Polyketide antibiotic synthase enzyme systems. Polyketides are secondary metabolites produced by microorganisms and plants from simple fatty acids. KAS is one of the components involved in the biosynthesis of the Streptomyces polyketide antibiotics granatacin [4], tetracenomycin C [5] and erythromycin.
• Emericella nidulans multifunctional protein Wa. Wa is involved in the biosynthesis of conidial green pigment. Wa is protein of 216 Kd that contains a KAS domain.
• Rhizobium nodulation protein nodE, which probably acts as a β-ketoacyl synthase in the synthesis of the nodulation Nod factor fatty acyl chain.
• Yeast mitochondrial protein CEM1.

The condensation reaction is a two step process: the acyl component of an activated acyl primer is transferred to a cysteine residue of the enzyme and is then condensed with an activated malonyl donor with the concomitant release of carbon dioxide. The sequence around the active site cysteine is well conserved and can be used as a signature pattern.