An 8-year-old child is taken for a physical examination because of pressure from the boy's teachers that something is "not right" and that the child is underperforming in class. The boy is remarkable for being tall and thin, with long limbs, and a small curvature to his spine (scoliosis). He displays pectus carinatum and a dislocation of the lens in one eye. He has also had a number of recent blood clots that required treatment. After examination, and the running of some tests, the boy was placed on pharmacologic doses of vitamin B6, and this treatment appeared to alleviate some of the symptoms. The enzyme that is most likely to be defective in this child is which one of the following?

Some amino acids (isoleucine, tryptophan, phenylalanine, and tyrosine) are both glucogenic and ketogenic because their degradation pathway forms multiple products.a. Amino acids that are converted to pyruvate (see Figure I).(1). The amino acids that are synthesized from the intermediates of glycolysis (serine, glycine, cysteine, and alanine) are degraded to form pyruvate.(a). Serine is converted to 2-phosphoglycerate, an intermediate of glycolysis, or directly to pyruvate and NH4+ by serine dehydratase, an enzyme that requires PLP.(b). Glycine, in a reversal of the reaction utilized for its synthesis, reacts with methylene-FH4 to form serine.1. Glycine also reacts with FH4 and NAD+ to produce CO2 and NH4+ (glycine cleavage enzyme).2. Glycine can be converted to glyoxylate, which can be oxidized to CO2 and H2 O or converted to oxalate.(c). Cysteine forms pyruvate. Its sulfur, which was derived from methionine, is converted to H2 SO4 , which is excreted by the kidneys.(d). Alanine can be transaminated to pyruvate.b. Amino acids that are converted to intermediates of the TCA cycle (see Figure II).(1). The carbons from the four groups of amino acids form the TCA cycle intermediates: a-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate.(a). Amino acids that form a-ketoglutarate (see Figure III).1. Glutamate can be deaminated by glutamate dehydrogenase or transaminated to form a- ketoglutarate.2. Glutamine is converted by glutaminase to glutamate with the release of its amide nitrogen as NH4+ 3. Proline is oxidized so that its ring opens, forming glutamate semialdehyde, which is oxidized to glutamate.4. Arginine is cleaved by arginase in the liver to form urea and ornithine. Ornithine is transaminated to glutamate semialdehyde, which is oxidized to glutamate.5. Histidine is converted to formiminoglutamate (FIGLU). The formimino group is transferred to FH4, and the remaining five carbons form glutamate.(b). Amino acids that form succinyl-CoA (see Figure IV)1. Four amino acids are converted to propionyl-CoA, which is carboxylated in a biotin requiring reaction to form methylmalonyl-CoA, which is rearranged to form succinyl-CoA in a reaction that requires vitamin B12 (seen previously in the metabolism of odd-chain number fatty acids).a). Threonine is converted by a dehydratase to NH4+ and a-ketobutyrate, which is oxidatively decarboxylated to propionyl-CoA.In a different set of reactions, threonine is converted to glycine and acetyl-CoA.b). Methionine provides methyl groups for the synthesis of various compounds; its sulfur is incorporated into cysteine; and the remaining carbons form succinyl-CoA.i. Methionine and ATP form S-adenosylmethionine (SAM), which donates a methyl group and forms homocysteine.ii. Homocysteine is reconverted to methionine by accepting a methyl group from the FH4 pool via vitamin B12 .iii. Homocysteine can also react with serine to form cystathionine. The cleavage of cystathionine produces cysteine, NH4+ , and a-ketobutyrate, which is converted to propionyl-CoA.c). Valine and isoleucine, two of the three branched-chain amino acids, form succinyl-CoA (see Figure IV).i. The degradation of all the three branched-chain amino acids begins with a transamination followed by an oxidative decarboxylation catalyzed by the branched-chain a-keto acid dehydrogenase complex (Figure V). This enzyme, like pyruvate dehydrogenase and a-ketoglutarate dehydrogenase, requires thiamine pyrophosphate, lipoic acid, CoA, flavin adenine dinucleotide (FAD), and NAD+.ii. Valine is eventually converted to succinyl-CoA via propionyl-CoA and methylmalonyl-CoA.iii. Isoleucine also forms succinyl-CoA after two of its carbons are released as acetyl-CoA.(c). Amino acids that form fumarate1. Three amino acids (phenylalanine, tyrosine, and aspartate) are converted to fumarate (see Figure II).a). Phenylalanine is converted to tyrosine by phenylalanine hydroxylase in a reaction requiring tetrahydrobiopterin and O2 (Figure VI).b). Tyrosine, obtained from the diet or by hydroxylation of phenylalanine, is converted to homogentisic acid. The aromatic ring is opened and cleaved, forming fumarate and acetoacetate.c). Aspartate is converted to fumarate via reactions of the urea cycle and the purine nucleotide cycle.d). Aspartate reacts with IMP to form AMP and fumarate in the purine nucleotide cycle.(d). Amino acids that form oxaloacetate (see Figure II)1. Aspartate is transaminated to form oxaloacetate.2. Asparagine loses its amide nitrogen as NH4+ , forming aspartate in a reaction catalyzed by asparaginase.c. Amino acids that are converted to acetyl-CoA or acetoacetate (see Figure VI).(1). Four amino acids (lysine, threonine, isoleucine, and tryptophan) can form acetyl-CoA, and phenylalanine and tyrosine form acetoacetate. Leucine is degraded to form both acetyl-CoA and acetoacetate.The boy has the recessive disorder homocystinuria, in which homocystine accumulates (owing to homocysteine accumulation). Homocysteine is produced when S-adenosylhomocysteine (SAH) releases adenosine. Homocysteine is used in the synthesis of cystathionine, requiring the enzyme cystathionine b-synthase. Homocysteine can also accept a methyl group (from N5-methyltetrahydrofolate via vitamin B12 ) to form methionine. Classical homocystinuria is as a result of a defect in cystathionine b-synthase, which requires pyridoxal phosphate as a cofactor. Most often, the mutation in the enzyme leads to a reduction in affinity of the cofactor for the enzyme. Increasing cellular levels of pyridoxal phosphate, by taking pharmacologic doses of vitamin B6 , can overcome the reduction in enzyme activity and reduce homocysteine, and homocystine, levels. None of the other enzymes listed as potential answers would regain activity in the presence of large amounts of vitamin B6 . An inactivating mutation in S-adenosylhomocysteine hydrolase would reduce homocysteine levels. A defect in methionine synthase (which requires vitamin B12 ) would raise homocysteine levels but could not be reversed by B6 treatment. A defect in N5,N10-methylenetetrahydrofolate reductase would also raise homocysteine levels, but that enzyme requires NADH, and would not be rescued by elevated B6 levels. A defect in SAM synthetase would lead to elevated methionine levels and increasing the concentration of B6 would not overcome such a mutation.I. Amino acids derived from the intermediates of glycolysis. These amino acids can be synthesized from glucose and can be reconverted to glucose in the liver.II. The degradation of amino acids. A. Amino acids that produce pyruvate or the intermediates of the TCA cycle. These amino acids are considered glucogenic because their carbons can produce glucose in the liver. B. Amino acids that produce acetyl-CoA or ketone bodies. These amino acids are considered ketogenic. CoA, coenzyme A; HMG-CoA, hydroxymethylglutaryl-CoA; TCA, tricarboxylic acid.III. Amino acids related through glutamate. These amino acids contain carbons that can be converted to glutamate, which can be converted to glucose in the liver. All of these amino acids except histidine can be synthesized from glucose.IV. Amino acids that can be converted to succinyl-CoA. The amino acids methionine, threonine, isoleucine, and valine, which form succinyl-CoA via methylmalonyl-CoA, are all essential. Because succinyl-CoA can form glucose, these amino acids are glucogenic. The carbons of serine are converted to cysteine and do not form succinyl-CoA by this pathway. A defect in cystathionine synthase causes homocystinuria. A defect in cystathionase causes cystathioninuria. B12-CH3, methylcobalamin; N5-CH3-FH4, N5- methyltetrahydrofolate; PLP, pyridoxal phosphate; SAM, S-adenosylmethionine; TCA, tricarboxylic acid.V. The degradation of the branched-chain amino acids. Valine forms propionyl-CoA. Isoleucine forms propionyl-CoA and acetyl-CoA. Leucine forms acetoacetate and acetyl-CoA. CoA, coenzyme A; FAD, flavin adenine dinucleotide; HMG-CoA, hydroxymethylglutaryl-CoA.VI. A. Ketogenic amino acids. Some of these amino acids (tryptophan, phenylalanine, and tyrosine) also contain carbons that can form glucose. Leucine and lysine are strictly ketogenic; they do not form glucose. B. A deficiency in various steps leads to the diseases indicated. CoA, coenzyme A; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; PKU, phenylketonuria; PLP, pyridoxal phosphate; TCA, tricarboxylic acid cycle