Pyruvate formation from alanine

Figure 23.22. Pyruvate Formation from Amino Acids. Pymvate is the point of entry for alanine, serine, cysteine, glycine, threonine, and tryptophan.

Pyruvate is derived from phosphoenolpyruvate may be inter-converted into lactate, alanine, oxaloacetate. Formation ofacetyl-CoA from pyruvate is essentially irreversible... 

The activating enzyme will also generate radicals from short peptides such as Arg-Val-Ser-Gly-Tyr-Ala-Val, which corresponds to residues 731-737 of the pyruvate formate-lyase active site. If Gly 734 is replaced by L-alanine, no radical is formed, but radical is formed if D-alanine is in this position. This suggests that the pro-S proton of Gly 734 is removed by the activating... 

Cysteine and cystine are unstable towards hot base. The reaction rate and the decomposition products are very much dependent on the presence or absence of oxygen. Presumably, cysteine is more stable than cystine but as with the stability in acid this fact is of minor importance when considering the reaction of cysteine in plant extracts. The production of alanine from cysteine has been demonstrated (Wieland and Wirth, 1949), but again this formation is probably due to secondary transamination with pyruvic acid formed initially. The formation of ammonia, hydrogen sulfide, and pyruvic acid from both cysteine and cystine has been demonstrated. With cysteine the decomposition probably follows a route parallel to that for serine described above ... 

A pH-dependent chemoselective catalytic reductive amination of a-keto acids, affording a-amino acids with HCOONH4 in water, was achieved using the complex 31 or its precursor 28 as the catalyst [51]. The formation rates of alanine and lactic acid from pyruvic acid exhibited a maximum value around pH 5 and pH 3, respectively, and therefore, alanine was obtained quite selectively (96%) with a small amount of lactic acid (4%) at pH 5 (Scheme 5.18). A variety of nonpolar, uncharged polar and charged polar amino acids were also synthesized in high yields. 

Experimental support for the mechanism of Eq. 15-26 has been obtained using D-chloroalanine as a substrate for D-amino acid oxidase.252-254 Chloro-pyruvate is the expected product, but under anaerobic conditions pyruvate was formed. Kinetic data obtained with a-2H and a-3H substrates suggested a common intermediate for formation of both pyruvate and chloro-pyruvate. This intermediate could be an anion formed by loss of H+ either from alanine or from a C-4a adduct. The anion could eliminate chloride ion as indicated by the dashed arrows in the following structure. This would lead to formation of pyruvate without reduction of the flavin. Alternatively, the electrons from the carbanion could flow into the flavin (green arrows), reducing it as in Eq. 15-26. A similar mechanism has been suggested for other flavoenzymes 249/255 Objections to the carbanion mechanism are the expected... 

NAD0X + formate + HzO = NADred + C02tot NAD0X + malate + H20 = NADred + C02tot + pyruvate NAD0X + ethanol = NADred + acetaldehyde NAD0X + alanine + H20 = NADred + pyruvate + ammonia NADox + malate + acetylcoA 4- H20 = NADrcd + citrate + coA (See Problem 9.5.) [With permission from R. A. Alberty, Arch. Biochem. Biophys. 389, 94-109 (2001). Copyright Academic Press.]... 

Fig. 7.6 Mechanism of -replacement and /3-elimination reactions with L-serine (X =OH-) or /S-chloro-L-alanine (X- = Cl-). Formation of the Schiff base intermediate with the amino acid ES 1 is followed by removal of the a-proton (H ) and of the leaving group (X-) to form the Schiff base of amino acrylate ES III, the key intermediate in both types of reaction. ES III can be hydrolyzed to pyruvate and NH3 (/3-elimination) or can add the indole cosubstrate (RH) to form the Schiff base of the quinonoid of L-tryptophan ES IV (/3-replacement). Protonation of ES IV leads to release of L-tryptophan. ES IV can also be formed in the reverse direction from L-tryptophan.

Thus we designed and synthesized a bicyclic pyridoxamine derivative carrying an oriented catalytic side arm (16) [11], Rates for conversion of the ketimine Schiff base into the aldimine, formed with 26 (below) and a-ketovaleric acid, indolepyruvic acid, or pyruvic acid, were enhanced 20-30 times relative to those carried out in the presence of the corresponding pyridoxamine derivatives without the catalytic side arm. With a-ketovaleric acid, 16 underwent transamination to afford D-norvaline with 90% ee. The formation of tryptophan and alanine from indolepyruvic acid and pyruvic acid, respectively, showed a similar preference. A control compound (17), with a propylthio group at the same stereochemical position as the aminothiol side arm in 16, produced a 1.5 1 excess of L-norvaline, in contrast to the large preference for D-amino acids with 16. Therefore, extremely preferential protonation seems to take place on the si face when the catalytic side arm is present as in 16. 

Answer Lactate and alanine are converted to pyruvate by their respective dehydrogenases, lactate dehydrogenase and alanine dehydrogenase, producing pyruvate and NADH + H+ and, in the case of alanine, NH. Complete oxidation of 1 mol of pyruvate to C02 and H20 produces 12.5 mol of ATP via the citric acid cycle and oxidative phosphorylation (see Table 16-1). In addition, the NADH from each dehydrogenase reaction produces 2.5 mol of ATP per mole of NADH reoxidized. Thus oxidation produces 15 mol of ATP per mole of lactate. Urea formation uses the equivalent of 4 mol of ATP per mole of urea formed (Fig. 18-10), or 2 mol of ATP per mol of NH4. Subtracting this value from the energy yield of alanine results in 13 mol of ATP per mole of alanine oxidized. 

Other pyruvate-containing enzymes include aspartate -decarboxylase from Escherichia coli, the enzyme that catalyzes the formation of -alanine for the synthesis of pantothenic acid (Section 12.2.4) proline reductase from Clostridium sticklandiv, phosphatidylserine decarboxylase from E. coli and phenylalanine aminotransferase from Pseudomonas fluorescens. Phospho-pantetheinoyl cysteine decarboxylase, involved in the synthesis of coenzyme A (Section 12.2.1), and S-adenosylmethionine decarboxylase seem to be the only mammalian pyruvoyl enzymes (Snell, 1990). 

Aspartate undergoes /3-decarboxylation to /S-alanine unlike most amino acid decarboxylases, aspartate decarboxylase is not pyridoxal phosphate-dependent, but has a catalytic pyruvate residue, derived by postsynthetic modification of a serine residue (Section 9.8.1). Pantothenic acid results from the formation of a peptide bond between /3-alanine and pantoic acid. 

The nonessential amino acids are synthesized by quite simple reactions, whereas the pathways for the formation of the essential amino acids are quite complex. For example, the nonessential amino acids alanine and aspartate are synthesized in a single step from pyruvate and oxaloacetate, respectively. In contrast, the pathways for the essential amino acids require from 5 to 16 steps (Figure 24.8). The sole exception to this pattern is arginine, inasmuch as the synthesis of this nonessential amino acid de novo requires 10 steps. Typically, though, it is made in only 3 steps from ornithine as part of the urea cycle. Tyrosine, classified as a nonessential amino acid because it can be synthesized in 1 step from phenylalanine, requires 10 steps to be synthesized from scratch and is essential if phenylalanine is not abundant. We begin with the biosynthesis of nonessential amino acids. 

The full structures of pyridoxamine and pyridoxal are on p. 1384. The incorporation of ammor. into a-keto-glutarate and the formation of glutamic acid by NADPH reduction of the imine is -p. 1386. The transamination from glutamic acid to pyridoxamine is on p. 1385. We start fr -pyridoxamine, whose structure we abbreviate, and pyruvate. Imine formation (fiiU mechanism pp. 348-50) followed by proton removal and replacement gives a new imine whose hydrolysis (f mechanism on pp. 350-1) gives alanine and pyridoxal. The alanine is a single enantiomer beca.s enzyme-directed protonation occurs on one face of the imine. Pyridoxal is recycled transamination with glutamic acid. 

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