Acyl-CoA carboxylation pathways

In contrast to the above-mentioned pathways for C02-fixation, which have been known for decades, several autotrophic C02-assimilation pathways occurring in anaerobic autotrophs have been elucidated only recently. If the 

3- hydroxypropionate/4-hydroxybutyrate pathway can be listed as the fifth pathway of C02-assimilation [11], it has been considered together with the other acyl-CoA carboxylation pathways because all the carboxylation steps show a common feature they all take place on a (CoA-activated) carboxylic acid substrate. In the 3-hydroxypropionate/malyl-CoA cycle and the 3-hydroxypropionate/ 

For this reason (similar approach of conversion), it was assumed that both enzymes are identical [38]. In both cases a C2-unit is produced the acetyl-CoA in the 3-hydroxypropionate/malyl-CoA cycle and the glyoxylate in the 3-hydroxypropionate/4-hydroxybutyrate pathway. 

The phosphoenol pyruvate so formed is carboxylated by phosphoenol pyruvate oxaloacetate lyase and oxaloacetate is obtained. The latter is transformed into 2 equiv. of acetyl-CoA, 1 equiv. of which re-enters the cycle. 

Interesting is the carboxylation of crotonyl-CoA to afford ethylmalonyl-CoA. The mechanism suggests that the p-carbon of crotonyl-CoA accepts a hydride from NADPH, then the a-carbon in the presence of CO2 is carboxylated to yield 

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