Structure and Mechanisms of PKS Domains

PKSs harbour an array of catalytic domains which, mechanistically, are relatively well understood. Within this section the structural and catalytic features of core PKS domains will be discussed. 

In the case of the platesimycin-FabF complex, the active site Cys was mutated to a glutamine residue, effectively mimicking an acylated state. The crystal 

KS domains act as decarboxylases when the active site Cys is mutated to a Gin, or in an acylated state (Fig. 1.22b) [61]. It is believed that the co-ordination of the carbonyl in the oxyanion hole aids the organisation of residues in the active site for effective decarboxylation and elongation [64]. The by-product of the decarboxylation reaction is, however, bicarbonate not carbon dioxide as previously postulated. 

The next step involves the decarboxylation of the chain extender unit (malonyl or methylmalonyl), which is supplied by the upstream ACP. Upon binding, a water molecule is activated by one of the catalytic histidine residues, facilitating the attack at the malonyl C3 carboxylate. This releases HCOs , and the resulting eno-late is stabilised by a conserved histidine. 

The final step involves the attack on the acyl-thioester by the enolate, with the tetrahedral intermediate stabilised by the oxyanion hole. The fi-keto product is then formed following collapse of the tetrahedral intermediate. The elongated intermediate is now free to diffuse out of the active site, albeit thethered to the receiving ACP, and downstream for further modification. 

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