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Transcript
26.6 Synthesisof Amino Acids 797 dized to carbon dioxide in the citric acid cycle. Moreover, by forming oxaloacetate and a-ketoglutarate from glucogenic amino acids, cells can replenish or increase the concentrations of intermediates of the citric acid cycle. An increase in these intermediates enables cells to step up energy production. You may recall that oxaloacetate for the citric acid cycle can come from severalplaces in metabolism. We have seen that the carboxylation of pyruvatein gluconeogenesisis one of those places.Nowwe seethat amino acid metabolism is another. Certainemergenciessuch as diabetesor starvationresult in a reduction in the amount of acetyl CoA in the liver. Liver cells respond by using acetyl CoA produced in amino acid metabolism to make ketone bodies. The ketone bodies are transported to other tissues,where they are oxidized for energyproduction. S5mthesis of glycogen and triglycerldes From our study of glucose metabolism, we know that glucose is formed from pyruvate by gluconeogenesis,in which oxaloacetateis an intermediate (seeSec.24.8).The cell does not recognizewhether the pyruvate has come from glucose or from amino acid metabolism. Once glucose has been synthesized,it can be assembledinto glycogenand stored in muscle or liver cells.Oxaloacetatefrom amino acid metabolism also can be converted to glucosein gluconeogenesis. Fatty acids, as we have seen, are synthesized from acetyl CoA. The acetyl CoA can come from glucose metabolism, from fatty acid metabolism, or from amino acid metabolism. Newly synthesized fatty acids are either used immediately for energy production or converted to triglycerides or membrane lipids. The triglycerides are stored in adipose tissue as an energy reserve. Humans cannot synthesize glucose from acetyl CoA, since people lack the enzyme that converts acetyl CoA to pyruvate. 26,6 Synthesis of omino ocids AIM: Toshow the relotionship betweenthe citric ocid cycleond the synthesisof nonessentialomino ocids. Slmthesis of 7 of the 12 nonessential amino acids is simple. Most animal proteins have a higher nutritional value than vegetableproteins becausethey have more essentialamino acids. Our bodies need to synthesizenonessential amino acids (see Sec.21.6) becausetheir proportions in our diet seldom match our bodies' needs.The main starting materials for this qmthesis are pyruvate and two intermediates of the citric acid cycle: a-ketoglutarate and oxaloacetate.As we have seen, cr-ketoglutarateaccepts amino groups from other amino acids in transamination to give glutamic acid, and arginine is formed in the urea cycle. TWo other nonessential amino acids-aspartic acid and alaninemay be qmthesized directly from cr-ketoacidsbecausethe reactions catalyzed by the transaminases are reversible. Reversalsof transamination reactions form alanine from pyruvic acid and aspartic acid from 794 CHAPTER 26 Metabolismof NitrogenCompounds oxaloacetic acid. oo iltl o II + Glutamic acid ;---- CH3CH-C-OH CH3-C-C-OH * a-Ketoglutarate I NHz Pyruvic acid o il oo illl Alanine + Glutamic acid ;- HO-C-CH2-C-C-OH oo il + d-Ketoglutarate HO-C-CH2CH-C-OH I NH, Asparticacid Oxaloacetic acid Glutamineand asparagineareformed from glutamic acid and aspartic acid by reaction of the side-chain carboxyl groups with ammonia. oooo ilililil HO-C-CH2CH-C-OH rl + NH3 -------HzN-C-CH2CH-C-OH NHz NHz Aspartic acid o Asparagine o HO-C-CH2CH2CH-C-OH + HzO o + NH3 + o II + H2O HrN-C-CH2CH2CH-C-OH I NHz NHz Glutamicacid Glutamine Tyrosine,the only nonessential amino acid with an aromatic side chain, is produced from the essential amino acid phenylalanine. The conversion requires a single oxidation step catalyzed by the enzyme phenylalanine hydroxylase. Phenylalanine ( }.crt \\//t-"\\//l -cu-co2H -!try: to{ fcu,-ctt-co2H NH, Phenylalanine NH, Tyrosine So far we have seen how 7 of the 12 nonessential amino acids are synthesized. The syntheses of the remaining S-cysteine, histidine, glycine, proline, and serine-are more complex. Their synthesiswill not be considered here.The qmtheses of 4 of the nonessential amino acids from oxaloacetate and a-ketoglutarate, two intermediates in the citric acid cycle, demonstrate an important metabolic principle: Besidesbeing a pathway of catabolism, the citric acid rycle is a pathway of anabolism-a metabolic switch-hitter.