Pyruvic acid, ionization

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. 

For convenience, we show pyruvate here in its un ionized form, pyruvic acid, although at physiological pH it would be largely dissociated.) All four electrons and two of the four protons are transferred to two molecules of the oxidized form of the electron carrier nicotinamide adenine dinucleotide (NAD ) to produce the reduced form, NADH (see Figure 2-26) ... 

Pyruvate dehydrogenases (PDH) are multienzyme complexes responsible for the conversion of pyruvate of acetyl-CoA. The decarboxylation step requires thiamin pyrophosphate (TP). The mechanism involves ionization of thiamine to produce an yielde which adds to the carbonyl of pyruvate forming a covalent adduct. This adduct has properties which permit CO2, to leave, an event that could not occur in pyruvic acid. 

Thiamine pyrophosphate (13) is the co-factor for a number of enzymes that can be described as stabilizing hypothetical acyl anion intermediates. For instance, it is the coenzyme for the enzyme carboxylase that catalyses the conversion of pyruvic acid to acetaldehyde. We had early shown that this mechanism involves a thiazolium anion (14) whose second resonance form (15) is a carbene. Ionization of the C-2 proton of the thiazolium ring generates this species that can add nucleophilically to carbonyl groups such as that in pyruvic acid, forming an intermediate whose decarboxylation generates a stabilized anion. 

Show that the conversion of glucose to two molecules of pyruvate is an oxidation. (H/nt That it is an oxidation is easiest to see if you take the product to be pyruvic acid recognize, of course, that, under the pH conditions at which this reaction takes place in cells, pyruvic acid is ionized to pyruvate.)... 

Mizuhara [65] developed a more adequate model by demonstrating that thiamine catalyzes the decarboxylation of pyruvic acid in basic aqueous solution (pH 8.8). Acetoin is the final product of the reaction. Breslow [66] later showed that the hydrogen in position 2 of the thiazole ring of the coenzyme is exchanged with deuterium when deuterium oxide (D2O) is added to the incubation system. Thus, carbon 2 of the thiamine appears to react in this chemical process. The hydrogen in position 2 is acidic and is thus readily ionized to an anion in basic media (see Fig. 4-7). On the basis of these findings, researchers have proposed the following sequence of reactions to explain the catalytic action of carboxylase. The carbon 2 of the thiazo-... 

Solution (a) The acid ionization constant for pyruvic acid should be somewhat greater than that of acetic acid because the carbonyl function on the o-carbon atom exerts an electron-withdrawing effect on the carboxylic acid group. In the C—O—H bond system the electrons are shifted from hydrogen, facilitating loss of the hydrogen as a proton. (Section 16.10)... 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

Decarboxylation of an a-keto acid like pyruvate is a difficult reaction for the same reason as are the ketol condensations (see fig. 12.33) Both kinds of reactions require the participation of an intermediate in which the carbonyl carbon carries a negative charge. In all such reactions that occur in metabolism, the intermediate is stabilized by prior condensation of the carbonyl group with thiamine pyrophosphate. In figure 13.5 thiamine pyrophosphate and its hydroxyethyl derivative are written in the doubly ionized ylid form rather than the neutral form because this is the form that actually participates in the reaction even though it is present in much smaller amounts. 

In the first step of the conversion catalyzed by pyruvate decarboxylase, a carbon atom from thiamine pyrophosphate adds to the carbonyl carbon of pyruvate. Decarboxylation produces the key reactive intermediate, hydroxyethyl thiamine pyrophosphate (HETPP). As shown in figure 13.5, the ionized ylid form of HETPP is resonance-stabilized by the existence of a form without charge separation. The next enzyme, dihydrolipoyltransacetylase, catalyzes the transfer of the two-carbon moiety to lipoic acid. A nucleophilic attack by HETPP on the sulfur atom attached to carbon 8 of oxidized lipoic acid displaces the electrons of the disulfide bond to the sulfur atom attached to carbon 6. The sulfur then picks up a proton from the environment as shown in figure 13.5. This simple displacement reaction is also an oxidation-reduction reaction, in which the attacking carbon atom is oxidized from the aldehyde level in HETPP to the carboxyl level in the lipoic acid derivative. The oxidized (disulfide) form of lipoic acid is converted to the reduced (mer-capto) form. The fact that the two-carbon moiety has become an acyl group is shown more clearly after dissocia-... 

PDC increases the rate of decarboxylation of pyruvate by thiamine alone by a factor of 3 x 1012 at pH 6.2 and 30 °C [52], The capacity of ThDP to catalyse the decarboxylation of a-keto acids depends mainly on two properties of the thiazolium ring of ThDP (a) its capacity to ionize to form a nucleophilic anion and thus bind to the a-carbonyl group of pyruvate, and (b) its ability to stabilize the negative charge upon cleavage of carbon dioxide. 

The plCaS for ionization of several biologically important carbon acids are summarized in Scheme 1.5. The pfCaS of 17 for pyruvate 2 [36] and 18 for dihydroxy-acetone phosphate 3 [24] are close to the pKa of 19 for the parent ketone acetone 4 [37]. The a-protons of carboxylate anions are much less acidic than those of the... 

Bartok, M., Balazsik, K., Szollosi, G., Bartok, T. (2002), Electrospray ionization-mass spectrometry in the enantioselective hydrogenation of ethyl pyruvate catalysed by dihydrocinchonidine modified Pt/Al203 in acetic acid, J. Catal. 205, 168-176. 

The higher the ionization degree of the xanthan molecule, the stronger the interactions between chains and ions as calcium. In the present case, the ionization degree of xanthans was dependent only on their pyruvate content since glucuronic acid and acetate contents of the different samples were identical. Xanthan C had the the lowest pyruvate content and consequently the smallest ionization degree of all the samples. Thus, its special behavior cannot be explained by this factor. 

Autocatalytic enzyme reactions are frequent, and in many or all of those known, the effect is due to production of an acid, such as phosphoric, lactic, pyruvic, or oleic acid. Here the rate of reaction changes with pH, since ionization and destruction of the enzyme at different pH values affect the amount of the active form of the enzyme. Hence no general rules can be set up, and it is not surprising that reaction rates do not follow changes in reactant concentration. For example, the degree of buffering of the reaction mixture will affect the pH changes produced by the autocatalytic acid, and hence the rate of reaction. 

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