Harpers

238 / CHAPTER 28O– O O–NH+ 3– O O–OO –NH 3+O –OOα-KetoglutarateOOL-GlutamatePyruvateOOAlanineNH 4+H 2 OGlu or Aspα-Ketoglutarate or oxaloacetateNAD(P)H + H + NAD(P) +Figure 28–1. The glutamate dehydrogenasereaction.Figure 28–3. Formation of alanine by transaminationof pyruvate. The amino donor may be glutamate oraspartate. The other product thus is α-ketoglutarate oroxaloacetate.to asparagine is catalyzed by asparagine synthetase (Figure28–4), which resembles glutamine synthetase (Figure28–2) except that glutamine, not ammonium ion,provides the nitrogen. Bacterial asparagine synthetasescan, however, also use ammonium ion. Coupled hydrolysisof PP i to P i by pyrophosphatase ensures thatthe reaction is strongly favored.Serine. Oxidation of the α-hydroxyl group of theglycolytic intermediate 3-phosphoglycerate converts itto an oxo acid, whose subsequent transamination anddephosphorylation leads to serine (Figure 28–5).Glycine. Glycine aminotransferases can catalyze thesynthesis of glycine from glyoxylate and glutamate oralanine. Unlike most aminotransferase reactions, thesestrongly favor glycine synthesis. Additional importantmammalian routes for glycine formation are fromcholine (Figure 28–6) and from serine (Figure 28–7).Proline. Proline is formed from glutamate by reversalof the reactions of proline catabolism (Figure 28–8).Cysteine. Cysteine, while not nutritionally essential,is formed from methionine, which is nutritionallyessential. Following conversion of methionine to ho-NH 3+– O O–OOL-GlutamateFigure 28–2.NH 4+Mg-ATPNH 3+H 2 N O –OOL-GlutamineMg-ADP + P iThe glutamine synthetase reaction.–OO NH 3+OL-AspartateGlnMg-ATPO –H 2 NO NH 3+OL-AsparagineGluMg-AMP + PP iFigure 28–4. The asparagine synthetase reaction.Note similarities to and differences from the glutaminesynthetase reaction (Figure 28–2).POHOOHNH 3+OOL-SerineO −O −D-3-PhosphoglycerateNADHNH +3P i H 2 OPPOOOOOO –O −Phosphohydroxypyruvateα-AAα-KAO −Phospho-L-serineFigure 28–5. Serine biosynthesis. (α-AA, α-aminoacids; α-KA, α-keto acids.)