Harpers

BIOSYNTHESIS OF THE NUTRITIONALLY NONESSENTIAL AMINO ACIDS / 239H 3 CCholineH 3 CHDimethylglycineCH 32HCH 3N + CH 3H N + 3 C CH 3+NOHCH 3OO −[CH 2 O]Betaine aldehyde[CH 3 ]H 3 CNAD +BetaineOCH3N + CH 3H + CH 3 [CH 2 O]NH +N3HO −Sarcosine OOGlycine OO −O −OO −– O O NH+ 3L-GlutamateONADHH 2 OOOO −L-Glutamateγ-semialdehydeH 2 ONH 3+O − NADHO −NH+ 2NH +L-Proline∆ 2 -Pyrrolidine-5-carboxylateFigure 28–8. Biosynthesis of proline from glutamateby reversal of reactions of proline catabolism.OFigure 28–6.Formation of glycine from choline.NH 3+mocysteine (see Chapter 30), homocysteine and serineform cysteine and homoserine (Figure 28–9).Tyrosine. Phenylalanine hydroxylase convertsphenylalanine to tyrosine (Figure 28–10). Providedthat the diet contains adequate nutritionally essentialphenylalanine, tyrosine is nutritionally nonessential.But since the reaction is irreversible, dietary tyrosinecannot replace phenylalanine. Catalysis by this mixedfunctionoxygenase incorporates one atom of O 2 intophenylalanine and reduces the other atom to water. Reducingpower, provided as tetrahydrobiopterin, derivesultimately from NADPH.HONH 3+OSerineO –H 4 folateMethyleneH 4 folateNH 3+OO –GlycineFigure 28–7. The serine hydroxymethyltransferasereaction. The reaction is freely reversible. (H 4 folate,tetrahydrofolate.)O −H 3 N +OL-HomocysteineH 3 N +OO −HO O+ L-SerineS HH 2 ONH+ 3H 2 OO −O −L-HomoserineNH 3+H 3 N +OO −HS OOHL-Cysteine+SCystathionineFigure 28–9. Conversion of homocysteine and serineto homoserine and cysteine. The sulfur of cysteinederives from methionine and the carbon skeleton fromserine.OO −