• C H A P T E R • 16 • AMINO ACID METABOLISM • Nonessential Amino Acid Synthesis Essential Amino Acids Amino Acid Degradation Generalities of Amino Acid Catabolism Products of Amino Acid Degradation • • • • • • • • • • • • NONESSENTIAL AMINO ACID SYNTHESIS AMINO ACID SYNTHETIC ROUTE Ala From Pyruvate by transamination Glu From -Ketoglutarate by transamination Asp From Oxaloacetate by transamination Gln Glu NH 4 ATP Gln Asn Asp Gln ATP Asn AMP PPi Glu Ser Glucose hydroxypyruvate Ser Glucose phosphohydroxypyruvate Ser Gly Ser THfolate Gly CH2-THfolate Arg Glu Glu-semialdehyde ornithine Arg Pro Glu Glu-semialdehyde Pro Tyr Phe Tyr (phenylalanine hydroxylase, biopterin cofactor) Cys Met homoCys Ser cystathionine Cys 186 16 Amino Acid Metabolism • 187 • The other nine amino acids are essential and must be taken from the diet. Notice that some of the amino acids require other amino acids for their synthesis. Exam questions usually center on whether or not an amino acid is essential and the metabolites that serve as precursors for specific amino acids. ESSENTIAL AMINO ACIDS His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val AMINO ACID DEGRADATION Ketogenic: Leu, Lys Degraded to acetyl-CoA. Glucose cannot be made from these. Glucogenic and Ketogenic: Ile, Phe, Tyr, Trp Goes both ways. Glucogenic: Everything else Degraded to pyruvate or a member of the TCA cycle Glucose can be made from these. The complete catabolic pathways of the individual amino acids are a complex set of pathways that are probably not worth remembering in detail (this is obviously opinion). This doesn’t mean they’re not impor- tant. In fact, there are diseases that are caused by inherited defects in most of the pathways. The preceding table is a general guide that shows where the amino acids go and points out significant intermediates. GENERALITIES OF AMINO ACID CATABOLISM If a vitamin or cofactor is involved in amino acid metabolism, it’s most likely pyridoxal phosphate (B6), unless it involves serine, and then it’s B6 and folic acid. Nitrogen is dumped into the urea cycle by transamination to make Asp or Glu or by deamination to make ammonia. • 188 • Basic Concepts in Biochemistry PRODUCTS OF AMINO ACID DEGRADATION Ala to pyruvate by transamination Arg to urea and glutamate Asp to oxaloacetate by transamination or to fumarate via urea cycle Asn to Asp Cys carbon to pyruvate, sulfur to sulfate Glu to -ketoglutarate by transamination, then to glucose Gln to glutamate by hydrolysis Gly to glyoxylate or serine His to glutamate and one-carbon pool Met to propionyl-CoA via homocysteine cystathionine ketobutyrate Pro to glutamate Ser to glycine and CH2THfolate Thr to propionyl-CoA through ketobutyrate Val to propionyl-CoA through transamination, decarboxylation, and a bunch of rearrangements Leu to acetoacetate and acetyl-CoA through transamination, decar- boxylation, and a bunch of rearrangements Ile to propionyl-CoA through transamination, decarboxylation, and a bunch of rearrangements Phe to Tyr, then to acetoacetate and fumarate Tyr to acetoacetate and fumarate Try to acetyl-CoA via ring oxidation and cleavage to ketoadipate Lys to acetyl-CoA via transamination and deamination to ketoadi- pate The nitrogen contained in the amino acids is usually disposed of through the urea cycle. One of the early, if not the first, steps in amino acid catabolism involves a transamination using oxaloacetate or - ketoglutarate as the amino-group acceptor. This converts the amino acid into a 2-keto acid, which can then be metabolized further. R—CH(NH 3)CO 2 oxaloacetate ∆ R—(C“O)CO 2 Asp These enzymes invariably involve a cofactor, pyridoxal phosphate (vitamin B6). In addition, pyridoxal phosphate is also required for most decarboxylations, racemizations, or elimination reactions in which an amino acid is a substrate. Pyridoxal phosphate is not involved in decar- boxylations in which the substrate is not an amino acid. So if a question 16 Amino Acid Metabolism • 189 • asks something about an amino acid and a vitamin, the answer will most likely be pyridoxal phosphate. There are a couple of exceptions in which pyridoxal phosphate may not be the answer to a vitamins-amino acid question. If the amino acid is serine, then the answer might also include folic acid (the reaction here is the conversion of serine to glycine with the formation of methylene tetrahydrofolic acid–see the section in Chap. 21 on one-carbon metabolism). The other place you might see a vitamin other than pyridoxal phosphate is in the metabolism of propionyl-CoA, a product of the catabolism of some amino acids. In this case, the vita- min may be B12 (the conversion of methylmalonyl-CoA to succinyl- CoA—see “ -Oxidation Odd-Chain-Fatty Acids” in Chap. 13). The nitrogen from the amino groups of most amino acids is transam- inated into glutamate or aspartate at some point in the degradative scheme. This nitrogen enters the urea cycle as glutamate, which is reduc- tively deaminated by glutamate dehydrogenase to yield ammonia or by the reaction of aspartate with citrulline to give argininosuccinate (urea cycle).
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