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Transcript
26,5 Catabolismof Amino Acids Nitrogen 795 balance Normal adults generally maintain a nitrogen balance-the quantity of nitrogen excreteddaily equals the intake. About B0% of the nitrogen excretedin the urine is in the form of urea, and almost all the body'swaste nitrogen of metabolism is excretedin the urine; the nitrogen compounds in the feces come mainly from indigestible materials. Children have a positive nitrogen balance-rhe excretion of lessnitrogen than is consumed.The nitrogen balance is positive becausechildren are growing and their cells are making new proteins and other nitrogen compounds. Several conditions result in a negative nitrogen balance-the excretion of more nitrogen than is consumed.During starvation and certain diseases, the carbon skeleton of amino acids derived from the breakdor,rmof muscle proteins must be catabolized as an energy source. Since no new protein is available to eat, starving people excretemore nitrogen than they consume.The lack of even one essentialamino acid in the diet resultsin a negative nitrogen balance. With an essential amino acid missing, the other amino acids cannot be used to make complete proteins.Theseother amino acids are deaminated, and the nitrogen is excretedas urea. 26.5Cotobolismof smino ocids AIMS: To distinguish betweenglucogenicond ketogenicomino scids. To discusshow omino ocids con be usedfor energy production,gluconeogenesis, and the synthesisof fots. Focus The carbon skeletons of amino acids are converted to intermediates of glucose metabolism and fatty acid metabolism. Once u-ketoacidshave been formed from amino acid by transamination reactions, their carbon skeletons are subjected to further chemical changes.One set of amino acids is converted to pyruvate, oxaloacetate,or cr-ketoglutarate(Fig. 26.4).Amino acids that are conuertedto theseintermediates are called glucogenic, since thesecompounds are also important to glucosemetabolism. Pyruvate,formed at the end of glycolysis,and oxaloacetate are intermediates of the citric acid cycle.The remainder of the amino acids are converted to acetyl CoA, which is also a product of fatty acid metabolism. The amino acids that are conuerted to acetyl CoA are called ketogenic. The conversion of all the amino acids to intermediates of glucose or fatty acid metabolism demonstrates the highly organized character of metabolism and the economy of nature. By using a single, central pathway for the metabolism of sugars, fats, and amino acids, the cell greatly decreasesthe number of enzymes and chemical steps that otherwise might be required to accomplishthe sametask. Cells have priorities for the utilization of amino acids present in the amino acid pool. In a normal individual, well nourished with carbohydrates, fats, and proteins, the s1'nthesisof nonessential amino acids, proteins, and other nitrogen-containing compounds is at the top of the prior- 796 CHAPTER 26 Metabolismof NitrogenCompounds Gliie ' I Y oo iltl cH3-c- c\ oPyruvate I v o tl CHr-C-S-eoA Acetyl CoA ,ffi*ur"u oo iltl -cu"- C - C H 2 - C - S - C o A Acetoacetyl CoA Ketone bodies Fatty acids oo illt c-c.. ,c-cHr_eJJlc-Ketoglutarate Figure26.4 Amino acidsare degraded to pyruvate,acetyl CoA,or intermediatesof the citric acid cycle. q" tt----:----.*"*,o"1---.--------------@ ity list. Energy production comes second. The body does not store amino acids as such. Any amino acids that remain after synthetic and energy needs are met are converted to glucose and fatty acids. :.r,, PRACTTCE EXERCTSE 26.5 = Explain how transamination provides the link between amino acid metabolism and glucose metabolism or fat metabolism. Energyproducdon The carbons of amino acids are used for energy production when needed. Upon demand, pyruvate and acetyl CoA derivgd from amino acids are oxi- 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. Certain emergenciessuch as diabetes or starvation result in a reduction in the amount of aceryl 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 energy production. Slmthesis of glycogen and triglycerides 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 acids AIM: Toshow the relotionship betweenthe citric ocid cycleond the synthesisof nonessentiol omino ocids. Synthesis of 7 of the 12 nonessential amino acids is simple. Most animal proteins have a higher nutritional value than vegetable proteins 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 synthesis are pyruvate and two intermediates of the citric acid cycle: o-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 synthesized directly from cr-ketoacidsbecause the reactions catalyzed by the transaminases are reversible. Reversalsof transamination reactions form alanine from pyruvic acid and aspartic acid from