Glycine synthase

Glycine. The glycine synthase complex of Ever mitochondria sphts glycine to COj and NH4+ and forms A, A -methylene tetrahydrofolate (Figure 30-5). 

Figure 30-6. Reversible cleavage of glycine by the mitochondrial glycine synthase complex. (PLP, pyri-doxal phosphate.)...

GLYCINE SYNTHASE GLYCINE DECARBOXYLASE GLYCINE FORMIMINOTRANSFERASE [ N]Glycine kinetics,... 

GLYCINE REDUCTASE GLYCINE SYNTHASE Glycogen debranching enzyme, DEBRANCHING ENZYME Glycogenolysis tracer kinetics,... 

FIGURE 22-12 Biosynthesis of serine from 3-phosphoglycerate and of glycine from serine in all organisms. Glycine is also made from C02 and NH( by the action of glycine synthase, with N5,N10-methy-lenetetrahydrofolate as methyl group donor (see text). 

These tetrahydrofolate derivatives serve as donors of one-carbon units in a variety of biosyntheses. Methionine is regenerated from homocysteine by transfer of the methyl group ofF -methyltetrahydrofolate, as will be discussed shortly. We shall see in Chapter 25 that some of the carhon atoms of purines are acquired from derivatives of N lO-formyltetrahydrofolate. The methyl group of thymine, a pyrimidine, comes from N, N lO-methylenetetrahydrofolate. This tetrahydrofolate derivative can also donate a one-carhon unit in an alternative synthesis of glycine that starts with CO2 and NH4 +, a reaction catalyzed by glycine synthase (called the glycine cleavage enzyme when it operates in the reverse direction). 

The one-carbon carrier N, N -methylenetetrahydrofolate is derived from reactions of the one-carbon pool (Chapter 27). [The term one-carbon pool refers to all single-carbon-containing metabolites (e.g., -CH3, -CHO, NH=C-, etc.) that can be utilized in biosynthetic reactions such as formation of purine and pyrimidine.] These reactions include oxidation of glycine by glycine cleavage enzyme complex (glycine synthase) ... 

Pyruvate is an intermediate in the conversion of these amino acids to acetyl-CoA. Note that glycine is also degraded by glycine synthase to form C02, NHJ, and N5,N10-methylene-THF in an NAD+-requiring reaction. In primates most threonine molecules are degraded to propionyl-CoA. 

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