Reaction Mechanism of PolyKetide Biosynthesis

The polyketide synthesis chemically and biochemically resembles that of fatty acids. The reaction of fatty acid synthesis is inhibited by the fungal product ceru-lenin [9]. It inhibits all known types of fatty acid synthases, both multifunctional enzyme complex and unassociated enzyme from different sources like that of some bacteria, yeast, plants, and mammalians [10]. Cerulenin also blocks synthesis of polyketides in a wide variety of organisms, including actinomycetes, fungi, and plants [11, 12]. The inhibition of fatty acid synthesis by cerulenin is based on binding to the cysteine residue in the condensation reaction domain [13]. Synthesis of both polyketide and fatty acids is initiated by a Claisen condensation reaction between a starter carboxylic acid and a dicarboxylic acid such as malonic or methylmalonic acid. An example of this type of synthesis is shown in Fig. 1. An acetate and malonate as enzyme-linked thioesters are used as starter and extender, respectively. The starter unit is linked through a thioester linkage to the cysteine residue in the active site of the enzymatic unit, p-ketoacyl ACP synthase (KS), which catalyzes the condensation reaction. On the other hand, the extender 

Formation of a fully reduced saturated carbon chain is a three-step process requiring three distinct enzymatic functions. The first step is ketoreduction (P-ketoacyl ACP reductase KR) to produce the secondary alcohol residue in that an electron is supplied by NADPH to the carbonyl group followed by protonation. The second step is dehydration (dehydratase DH) to lead to the a,P unsaturated acyl group. The final step is enoyl reduction (enoyl reductase ER), which employs NADPH as an electron donor and proton to result in the formation of a methylene function at the P-carbon. After the reduction steps are completed, the generated acyl chain enters the KS domain and is equivalent to the starter for the next cycle of the reaction to condense with the next extender unit. 

Two differences exist between fatty acid and complex polyketide syntheses (Fig. 2). First, in fatty acid synthesis, synthase uses only malonyl moieties as extender units to build an acyl chain. In general, acetate is used as the starter unit in vertebrate fatty acid synthase, but bacterial fatty acid synthase may use a branched-chain carboxylic acid as the starter unit because bacterial fatty acids sometimes contain branched-chain fatty acids. In contrast, polyketide synthesis in bacteria uses malonyl, methylmalonyl, and ethylmalonyl units as extenders. In the polyketide synthase, respective extender units are used at every step of the condensation. The polyketide synthase in fungi uses malonyl units as extenders and methyl groups at a positions are added by C-methylation using 5-adenosyl-L-methionine. 

Second, in polyketide synthesis, the full steps of p-carbonyl reduction (ketoreduction, dehydration, and enoylreduction) are not always involved after every condensation as in fatty acid synthesis. The keto group in the polyketide backbone results from the lack of ketoreduction after the condensation step. Similarly, the hydroxyl group at the P position results from ketoreduction and failure to undergo the subsequent dehydration step, the a,p unsaturated group results from ketoreduction and dehydration steps, and the methylene group results from the full steps of reduction after the condensation. After all steps of condensation and P-carbonyl reduction are completed, the resulting acyl residue is released from polyketide synthases by thioesterase (TE) and spontaneous lactonization proceeds. 

To carry out these processes, the fatty acid synthase required a set of catalytic activities that are responsible for the respective step of a set of the cycle reactions. Acyltransferase (AT) transfers both the acetyl starter unit and malonyl chain-extender units from the respective coenzyme A esters to the appropriate thiol group of the synthase. P-Ketoacyl synthase and acyl carrier protein are responsible for chain elongation. P-Ketoreductase, dehydratase, and enoyl reductase are for 

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