Carbohydrates, enzymic synthesis from fatty acids

As we began this chapter, we saw that photosynthesis traditionally is equated with the process of COg fixation, that is, the net synthesis of carbohydrate from COg. Indeed, the capacity to perform net accumulation of carbohydrate from COg distinguishes the phototrophic (and autotrophic) organisms from het-erotrophs. Although animals possess enzymes capable of linking COg to organic acceptors, they cannot achieve a net accumulation of organic material by these reactions. For example, fatty acid biosynthesis is primed by covalent attachment of COg to acetyl-CoA to form malonyl-CoA (Chapter 25). Nevertheless, this fixed COg is liberated in the very next reaction, so no net COg incorporation occurs. 

The addition of water to carbon-carbon double bonds is very common to biology, and a large variety of enzymes from different sources representing almost a hundred different hydro-lyase types have been characterized biochemically. Hydro-lyases are for instance involved in the metabolism of a variety of carbohydrates and play a prominent role in fatty acid synthesis and degradation as well. Despite the abundant presence of hydro-lyases in nature, however, applications of these enzymes in organic chemical synthesis are not as widespread. This is mainly due to the limited availability of these enzymes and the fact that many of the enzymes cannot easily be stably maintained during catalysis. 

Biotin serves as the prosthetic group of several enzymes that catalyse the transfer of carbon dioxide from one substrate to another. In animals there are three biotin-dependent enzymes of particular importance pyruvate carboxylase (carbohydrate synthesis from lactate), acetyl coenzyme A carboxylase (fatty acid synthesis) and propionyl coenzyme A carboxylase (the pathway of conversion of propionate to succinyl-CoA). The specific role of these enzymes in metabolism is discussed in Chapter 9. 

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