Polyketides complex, cyclization

The biosynthesis of polyketides (including chain initiation, elongation, and termination processes) is catalyzed by large multi-enzyme complexes called polyketide synthases (PKSs). The polyketides are synthesized from starter units such as acetyl-CoA, propionyl-CoA, and other acyl-CoA units. Extender units such as malonyl-CoA and methylmalonyl-CoA are repetitively added via a decarboxylative process to a growing carbon chain. Ultimately, the polyketide chain is released from the PKS by cleavage of the thioester, usually accompanied by chain cyclization [49]. 

Polyketide and fatty acid biosyntheses begin with condensation of the coenzyme A thioester of a short-chain carboxylic acid starter unit such as acetate or propionate with the coenzyme A thioester of a dicarboxylic acid extender unit such as malonate or methyl malonate. The driving force for the condensation is provided by the decarboxylation of the extender unit. In the case of fetty acid synthesis, the resulting -carbonyl is completely reduced to a methylene however, during the synthesis of complex poly-ketides, the -carbonyl may be left untouched or variably reduced to alcohol, olefinic, or methylene functionalities depending on the position that the extender unit will occupy in the final product. This cycle is repeated, and the number of elongation cycles is a characteristic of the enzyme catalyst. In polyketide biosynthesis, the full-length polyketide chain cyclizes in a specific manner, and is tailored by the action of additional enzymes in the pathway. 

The purification of bacterial aromatic PKSs is likely to result in elucidation of at least some of the structural features of these enzymes. For example, knowledge of the stoichiometry of the protein components in the active complex could suggest possible physical mechanisms for the chain elongation steps and perhaps imply a mechanism for the role of the CLF in control of the chain lengths of polyketides synthesized by these enzymes. Towards this end, several AGP proteins have been overexpressed and purified to homogeneity [111,112]. Likewise, purification of KR, ARO, and CYC components and reconstitution of their activities with the act minimal PKS will be useful in answering questions regarding their precise functions and the temporal sequence of the reduction and cyclization reactions, which may or may not occur before completion of the synthesis of the polyketide backbone. 

Type II PKS complexes are comprised at a minimum of four types of subunits encoded by discrete open reading frames acyl carrier protein, ketosynthase a, ketosynthase p (also referred to as chain length factor ), and a malonyl CoA acyltransferase responsible for loading acyl-CoA extender units on to the acyl carrier protein subunit (34 Fig. 4). Additional subunits containing ketoreductase, cyclase, or aromatase activity may also occur in more complex type II synthases. Typically, the four core subunits (acyl carrier protein, ketosynthase a, ketosynthase p, and malonyl-CoA acyltransferase) participate in the iterative series of condensation reactions until a specified polyketide chain length is achieved, then folding and cyclization reactions yielding the final... 

The type III plant and bacterial synthases feature the least complex architecture among the three PKS types, occurring as comparatively small homodimers possessing subunits between 40-45 kDa in size. As in the case for type II enzymes, type III PKSs catalyze iterative decarboxylative condensation reactions typically using malonyl-CoA extender units, however in contrast to type II synthases, the subsequent cyclization and aromatization of the nascent polyketide chains occurs within the same enzyme active site (25). Also unique to this family of PKSs, free CoA thioesters are used directly as substrates (both starter an extender units) without the involvement of acyl carrier proteins. 

Figure 7 Type I PKS biosynthesis in myxobacteria (b). Biosynthesis of stigmatellin A (14) in Stigmatella aurantiaca Sg a15. The KR domain from module 8 (marked with an asterisk) is most likely inactive. The hydroxy group generated by module 2 by reduction of the first extender unit is assumed to be dehydrated by the module 7 DH domain. StiH and StiJ incorporate three malonyl-CoA extender units in total. Thus, one of these modules appears to function iteratively. The polyketide chain is released and cyclized by the terminal Cyc domain, most likely via intermediate 22, and further decorated in post-PKS biosynthetic steps catalyzed by StiK and StiL. The stereochemistry of the linear intermediates bound to the enzyme complex was assigned based on the absolute configuration of 14.

A mixture of ionophoric polyether antibiotics produced by Streptomyces lasaliensis, from which the components A to E have been separated. L. exert antibacterial and antiviral (HIV) activities, LD50 (mouse p.o.) 146 mg/kg. L. A mp. 110-114°C, (aJo -7.5° (CH3OH), which preferentially forms complexes with divalent cations, is formed biosynthetically by the polyketide pathway from five acetate units, four propionate units, and three butanoate units, the benzene ring arises through cyclization. L. A (Bovatec ) in the form of its sodium salt (Avatec ) is used in fowl breeding as a coccidostatic. 

C42H70O11, Mr 751.01, cryst., mp. 112.5-113.5 °C, [a]o -63 (C2H5OH), an ionophoric polyether antibiotic with a dispiroketal structure produced by Strep-tomyces albus it preferentially forms complexes with monovalent cations (Na ). S. is formed biosyntheti-cally on the polyketide pathway, cyclization in the linear carboxylic acid proceeds on the diene/diepox-ide pathway. The 20-deoxy derivative is also formed. S. is used against coccidiosis in poultry breeding and also has antiviral activity (HIV). LD50 (mouse p.o.) 50 mg/kg. 

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