Acid-thiol ligases

The loading module comprises three domains. The first (CL) shows homology to ATP-dependent carboxylic acid-CoA ligases, the second is a putative enoyl reductase (ER) and the third an ACP. The probable sequence of operations starts with the enoic acid 74 derived from shikimic acid which is reduced by the ER domain. The first domain will activate the carboxylic acid to an active acyl derivative ready for transfer to the thiol residue of the ACP. The final saturated product will end up attached to the ACP as a thioester derivative ready for transfer to the KS domain of the first chain extension module. The timing of the reduction in this sequence of operations cannot be predicted. 

Similarly the fatty acids must be activated by conversion to their CoA derivatives before they can be metabolized. Formation of the fatty acyl-CoA derivatives is catalysed by various/affy acid thiokinases (fatty acid CoA ligases) whose activity is linked with the breakdown of ATP to AMP and pyrophosphate, the liberated energy being used in the formation of the thiol ester bond ... 

Under the catalysis of ligases of the acid-thiol type, 2-oxoacids are transformed into acyl-CoA esters or by the action of decarboxylases (carboxylyases) into aldehydes having one carbon atom less than the original amino acid. For example, decarboxylation of pyruvic acid yields acetaldehyde and decarboxylation of 2-oxobutanoic acid gives propanal. The same aldehydes also form by Strecker... 

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