Pyruvate-processing Enzymes

Figure 4.2 Reactions of pyruvate-processing enzymes pyruvate decarboxylase (PDC), p5Tuvate oxidase (POX), pyruvate ferredoxin oxidoreductase (PFOR) and pyruvate dehydrogenase (PDH). Their reactions diverge after the decarboxylation step. Pyr and PP represent 2,5-dimethyl-4-amino-pyrimidine and the ethyl diphosphate tail, respectively.

The whole process is multi-step, and catalysed by the pyruvate dehydrogenase enzyme complex, which has three separate enzyme activities. Dnring the transformation, an acetyl group is effectively removed from pyruvate, and passed via carriers thiamine... 

Acetaldehyde is the primaiy compound produced during alcoholic fermentation the main biosynthesis is during the anabolic process by the pyruvate decarboxylase enzyme. Aldehydes are also produced during the maturation stage by oxidation of alcohols (Nykanen., 1983 Berry. 1987). 

An important biochemical process is the shortening of carbon atom chains, such as those in fatty acids, commonly by the elimination of CO2 from carboxylic acids. For example, pyruvate decarboxylase enzyme removes CO2 from pyruvic acid... 

Finally, citrate can be exported from the mitochondria and then broken down by ATP-citrate lyase to yield oxaloacetate and acetyl-CoA, a precursor of fatty acids (Figure 20.23). Oxaloacetate produced in this reaction is rapidly reduced to malate, which can then be processed in either of two ways it may be transported into mitochondria, where it is reoxidized to oxaloacetate, or it may be oxidatively decarboxylated to pyruvate by malic enzyme, with subse-... 

The conversion occurs through a multistep sequence of reactions catalyzed by a complex of enzymes and cofactors called the pyruvate dehydrogenase complex. The process occurs in three stages, each catalyzed by one of the enzymes in the complex, as outlined in Figure 29.11 on page 1152. Acetyl CoA, the ultimate product, then acts as fuel for the final stage of catabolism, the citric acid cycle. All the steps have laboratory analogies. 

Let us consider Figure 5.3 again. Both pyruvate kinase and dtrate synthase (enzymes III and V) are inhibited by elevated ATP concentrations. During citric acid production ATP concentrations are likely to arise (ATP produced in glycolysis) and either of these enzymes could, if inhibited, slow down the process. In fact all of the evidence suggests that both enzymes are modified or controlled in some way such that they are insensitive to other cellular metabolites during citric add production. 

It is relevant also to compare the results in Fig.5 with previously published data for PAC production under similar environmental conditions, where with higher concentrations of initial benzaldehyde (600 mM), pyruvate (400 mM) and PDC activity (8.4 U ml ) a similar maximum concentration of PAC of 330 mM was produced [6]. PDC stability was similar in both processes with half life values of approximately 27h. However, PAC production was much faster in the benzaldehyde emulsion system, presumably due to higher initial enzyme concentration. 

There are also voices critical of the rTCA cycle Davis S. Ross has studied kinetic and thermodynamic data and concludes that the reductive, enzyme-free Krebs cycle (in this case the sequence acetate-pyruvate-oxalacetate-malate) was not suitable as an important, basic reaction in the life evolution process. Data on the Pt-catalysed reduction of carbonyl groups by phosphinate show that the rate of the reaction from pyruvate to malate is much too low to be of importance for the rTCA cycle. In addition, the energy barrier for the formation of pyruvate from acetate is much too high (Ross, 2007). 

A key enzyme in the process of recycling lactate from muscle to liver is lactate dehydrogenase (LD) which catalyses the reversible interconversion of lactate and pyruvate. This important reaction is shown in Figure 7.10. 

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