Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Biosynthesis of Monomers
Decarboxylation as a Driving Force in Biosynthesis

Let us examine the relationship between well-known biosynthetic intermediates:

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We have seen that acetyl-CoA can be utilized for the synthesis of long-chain Fatty acids, a process achieved through its carboxylation to malonyl-CoA. The malonyl group can be viewed as a β-carboxylated acetyl group. During fatty acid synthesis, the carboxyl group is cleaved off, so that ultimately only the acetyl group is incorporated into the fatty acid chain. Similarly, Pyruvate can be regarded as an α-carboxylated acetaldehyde, and oxaloacetate as both an α- and β-dicarboxylated acetaldehyde. In biosynthetic pathways, these three- and four-carbon compounds very frequently undergo decarboxylation. Thus, both types of compounds can be considered as "activated acetaldehyde units." Phosphoenolpyruvate represents the α-carboxylated phosphoenol form of acetaldehyde; prior to the incorporation of the two-carbon unit into the final product, it undergoes both decarboxylation and dephosphorylation.

It is often instructive to compare the two chain-elongation processes that result in the joining of two-carbon units. In FATTY ACID Biosynthesis, acetyl units are reduced and, following Condensation, form a straight hydrocarbon chain. In the keto acid chain-elongation mechanism, incorporation of the acetyl unit is followed by decarboxylation. Consequently, the chain is extended by a single carbon atom at each such stage.

These two mechanisms are of paramount importance for chain elongation in biosynthesis. However, other mechanisms also exist. For example, Glycine (carboxylated methylamine), in the presence of Pyridoxal phosphate, can undergo a condensation reaction with compounds such as succinyl-CoA [Equation (8-20)] accompanied by decarboxylation. This results in the elongation of the carbon chain and the simultaneous Introduction of an amino group. Analogously, Serine (carboxylated ethanolamine) condenses with palmitoyl-CoA during sphingosine biosynthesis [Equation (8-21)]. Phosphatidylserine is decarboxylated to phosphatidylethanolamine in The final stage of the synthesis of this phospholipid (Fig. 12-8).