Pyruvic acid equilibria

Yang, X., Orlova, G., Zhou, X.J., and Leung, K.T., A DFT study of the radical, monomer and dimer of a-keto pyruvic acid equilibrium structures and vibrational analysis of stable... 

In this scheme the reversible conversion of A to O is the reaction whose rate is to be studied, whereas the reduction of O to R is the electrode process. Scheme XIV can also represent a pseudo-first-order formation of O. A specific example is the acid-base equilibrium of pyruvic acid, shown in Scheme XV. 

When grown in a mineral medium containing KDO as the only source of carbon, cells of Aerobacter cloacae can be induced to produce an enzyme that catalyzes the cleavage of KDO to give D-arabinose and pyruvic acid.89 This enzyme was purified 60-fold by Ghalambor and Heath.154 It has a pH optimum of 7, a KM = 6 mM, and an equilibrium constant of 77 mM. The reversible nature of the enzyme reaction can be utilized to synthesize 14C-labelled KDO from D-arabinose plus 14C-pyruvic acid. Cleavage of KDO as catalyzed by KDO aldolase has... 

The same group have used the enzyme combination employed in the aspartate deracemization cited above to deracemize 2-naphthylalanine, hut have made use of an interesting innovation introduced by Helaine et al to pull over the poised equilibrium of the transamination reaction. Cysteine sulphinic acid was used as the amino donor in the transamination. The oxoacid product spontaneously decomposes in to pyruvic acid and SO2 (Scheme 3). 

Enols and enolization feature prominently in some of the basic biochemical pathways (see Chapter 15). Biochemists will be familiar with the terminology enol as part of the name phosphoenolpyruvate, a metabolite of the glycolytic pathway. We shall here consider it in non-ionized form, i.e. phosphoenolpyruvic acid. As we have already noted (see Section 10.1), in the enolization between pyruvic acid and enolpyruvic acid, the equilibrium is likely to favour the keto form pyruvic acid very much. However, in phosphoenolpyruvic acid the enol hydroxyl is esterified with phosphoric acid (see Section 7.13.2), effectively freezing the enol form and preventing tautomerism back to the keto form. 

This is another exampie of substrate-level phosphorylation, but differs from the earlier example that involved hydrolysis of a mixed anhydride. Here, we have merely the hydrolysis of an ester, and thus a much lower release of energy. In fact, with 1,3-diphosphoglycerate, we specifically noted the difference in reactivity between the anhydride and ester groups. So how can this reaction lead to ATP synthesis The answer lies in the stability of the hydrolysis product, enolpyruvic acid. Once formed, this enol is rapidly isomerized to its keto tautomer, pyruvic acid, with the equilibrium heavily favouring the keto tautomer (see Section 10.1). The driving force for the substrate-level phosphorylation reaction is actually the position of equilibrium in the subsequent tautomerization. 

Fig. 4. Study of the equilibrium between pyruvic acid and glycine. 5.6 x 10 4 M Sodium pyruvate, 0.005% gelatin, glycine buffer pH 9.2. Concentration of the form of glycine with the free amino group is given in the polarogram. Ionic strength kept constant by addition of sodium chloride. Curves starting at —0.8 V, S.C. E., 200 mV/absc.,

Several types of onium dications have been studied in which a single acyl cationic center has been part of 1,3-dicationic superelectrophiles. For example, pyruvic acid has been studied in FSOsH-SbFs solution at low temperatures.34 Initially the diprotonated species is observed in equilibrium with some of the monocation (eq 55). 

The pH of honey ranges between 3.4 and 6.1 with an average of 3.9 (Iurlina and Fritz, 2005). However, the pH is not directly related to acidity, due to the buffering action of acids and minerals found in honey (de Rodriguez et ah, 2004). Its acidity is due to the presence of organic acids, particularly gluconic acid, pyruvic acid, malic acid, and citric acid. These are in equilibrium with lactones or esters, as well as to inorganic ions, such as, phosphate or chloride (Anklam, 1998). 

Pyruvic acid is an important metabolite in its own right as we shall see shordy. It is the simplest a-keto-acid (2-oxopropanoic acid). Having the two carbonyl groups adjacent makes them more reactive the ketone is more electrophilic and enolizes more readily and the acid is stronger. Pyruvate is in equilibrium with the amino acid alanine by an aminotransferase reaction catalysed by pyridoxal (above). 

The hydrolysis of pyruvic acid to 2,2-dihydroxypropanoic acid is a reversible reaction giving rise to the following equilibrium in aqueous solution ... 

Khan, L, Brimblecombe, R, Clegg, S.L. (1995) Solubilities of pyruvic acid and the lower (Ci-C ) carboxylic acids. Experimental determination of equilibrium vapour pressure above pure aqueous and salt solutions. J. Atmos. Chem. 22, 285-302. 

In cells, enolpyruvic acid is in equilibrium with pyruvic acid. In the lab in acidic media, the first step of this conversion is a protonation of enolpyruvic acid to give a resonance delocalized cation intermediate the second step is a deprotonation to give pyruvic acid. Give a detailed mechanism for the conversion. Draw both resonance forms of the intermediate cation and decide which form is major. Use H30 to protonate and H2O to deprotonate. 

The yield of ester can be raised appreciably by continuous removal of the water produced. If neither of the reactants boils at a lower temperature than water, the latter can be removed merely by distillation, as in the preparation of triglycerides from glycerol and a relatively involatile higher fatty acid or in the preparation of pentyl pyruvate. According to Simon794 the latter ester is obtained in 99% yield by heating pyruvic acid and the equivalent amount of pentyl alcohol in a water-pump vacuum, provided that after equilibrium has become established the water formed is slowly distilled off. 

Schiff base formation between pyridoxal (140) and amino acids leads to complexes of type (141) which are in tautomeric equilibrium with (142). This tautomeric equilibrium leads to transamination, thus the same metal complexes can be obtained when either pyridoxal and alanine or pyridoxamine and pyruvic acid are allowed to react together in the presence of a metal ion. Hopgood" has studied the rates of transamination of 15 amino acids in the presence of zinc(II) and pyridoxal 5-phosphate (143). On mixing the reagents zinc(II)-aldimine complexes are rapidly formed (ca. 5 min) and these species subsequently transaminate in a slow second step. Ai" and Zn" systems have been particularly well studied.The role of the metal ion seems to involve both stabilization or trapping of the Schiff base, and in addition it also ensures the planarity of the conjugated ir-system. In the case of the aldimine tautomer, extensive H NMR studies have shown that formation of the ternary complex results in activation at the amino acid 2-carbon. At room temperature the reaction occurs without incorporation of into the aldehyde methine position indicating that the primary mechanism is carbanion formation rather than tautomerism. 

The absolute value of the pA"a is in good agreement with values reported before for the pA"a s of pyruvic acid (3.2-3.9) and other N-substituted-Gly (3.75. 07) [61-63], The deprotonation/protonation equilibrium for compounds 19 and 26 is shown in Scheme 5.14. Unlike in Fc-GluOH 19, the two carboxylic acid groups (a pA al and YpA a2) of Fc-[GlyOH]2 26 are completely distinguishable. pKa. values were determined from a least-square fit of the experimental data. Data for... 

Fig. 67. Equilibrium between pyruvic acid and glycine. Dependence of the wave-height of the more positive (ketimine) wave (t) expressed as the ratio ijid on the glycine concentration.

The biological standard potential of the redox couple pyruvic acid/lactic acid is -0.19 V and that of the fumaric acid/succinic acid couple is -tO.03 V at 298 K. What is the equilibrium constant at pH = 7 for the reaction... 

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