Reduction of pyruvate

Biological reduction of pyruvic acid catalyzed by the enzyme... 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

The method is quite effective, but is not widely used now because of the ubiquity of digital computers. Zuman and Patel - 36. show circuit designs for some kinetic schemes. Williams and Bruice made good use of the analog computer in their study of the reduction of pyruvate by 1,5-dihydroflavin. In this simulation eight rate constants were evaluated variations in these parameters of 5% yielded discemibly poorer curve fits. 

Problem 9.25 Lactic acid buildup in tired muscles results from reduction of pyruvate. If the reaction occurs from the Re face, what is the stereochemistry of the product ... 

The successful synthetic application of this electroenzymatic system has first been shown for the in-situ electroenzymatic reduction of pyruvate to D-lactate using the NADH-dependent D-lactate dehydrogenase. Electrolysis at — 0.6 V vs a Ag/AgCl-reference electrode of 50 mL of a 0.1 M tris-HCL buffer of pH 7.5 containing pentamethylcyclopentadienyl-2,2 -bipyridinechloro-rhodium(III) (1 x 10 3 M), NAD+ (2 x 10 3 M), pyruvate (2 x 10 2 M), 1300 units D-lactate dehydrogenase (divided cell, carbon foil electrode) after 3 h resulted in the formation of D-lactate (1.4 x 10 2 M) with an enantiomeric excess of 93.5%. This means that the reaction occurred at a rate of 5 turnovers per hour with respect to the mediator with a 70% turnover of the starting material. The current efficiency was 67% [67],... 

As we have seen a stereoselective reaction is one in which there is a preponderance of one isomer irrespective of the stereochemistry of the reactant. The enzymatic reduction of pyruvic acid is stereoselective when the chiral molecules of the enzyme complexes with achiral pyruvic acid, they given a preponderance of one form of pyruvic acid-enzyme complex which then gives a single form of lactic acid. 

On the other hand a stereospecific reaction is one in which a particular stereoisomeric form of the reactant reacts to give a stereoisomeric form of the product. The enzymatic reduction of pyruvic acid is not stereospecific because the reactant can not existin stereoisomeric form. Therefore, all reactions that are stereospecific are necessarily stereoselective but not all stereoselective reactions are stereospecific. 

Kosicki, G. W., Westheimer, F. H. Oxaloacetate decarboxylase from cod. Mechanism of action and stereoselective reduction of pyruvate by borohydride. Biochemistry 7, 4303—4309 (1968). 

Phillips, T. M., Kosicki, G. W., Schmidt, D. E., Jr. Stereoselective reduction of pyruvate by sodium borohydride catalyzed by pyruvate kinase. Biochim. Biophys. Acta 293, 125-133 (1973). 

In marked contrast, nature s reducing agent, reduced nicotinamide adenine dinucleotide (NADH), delivers hydride in a stereospecific manner because it is a cofactor in an enzyme-catalysed reaction. For example, reduction of pyruvic acid to lactic acid in vertebrate muscle occurs via attack of hydride to produce just one enantiomer, namely (5)-lactic acid. 

It can now be seen that, in the enzymic reduction of pyruvic acid to lactic acid, hydride is delivered to the Re face of the pyruvic acid. 

The active form of lactate dehydrogenase (mass 144 kDa) is a tetramer consisting of four subunits (1). Each monomer is formed by a peptide chain of 334 amino acids (36 kDa). In the tetramer, the subunits occupy equivalent positions (1) each monomer has an active center. Depending on metabolic conditions, LDH catalyzes NADH-de-pendent reduction of pyruvate to lactate, or NAD -dependent oxidation of lactate to pyruvate (see p. 18). 

Figure 6-1. The steps of glycolysis. Feedback inhibition of glucose phosphorylation by hexokinase, inhibition of pyruvate kinase, and the main regulatory, rate-limiting step catalyzed by phosphofructoki-nase (PFK-I) are indicated, pyruvate formation and substrate-level phosphorylation are the main outcomes of these reactions. Regeneration of NAD occurs by reduction of pyruvate to lactate during anaerobic glycolysis.

But the results provided an exciting bonus. The reactions proved to be stereospecific. NAD+ that has been reduced with dideuterioethanol has both a hydrogen atom and a deuterium atom in the 4-position of the pyridine ring, but reductions of pyruvate or of acetaldehyde with NAD(D) prepared enzymically in this way quantitatively transfer deuterium, and leave the hydrogen atom in place. The 1954 diagram showing the stereospecific transfer of hydrogen from the 4-position of NADH is presented in (5). 

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate. As mentioned earlier, some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to C02) produce lactate from glucose even under aerobic conditions. The reduction of pyruvate is catalyzed by lactate dehydrogenase, which forms the l isomer of lactate at pH 7 ... 

Dehydrogenases often act primarily to reduce a carbonyl compound rather than to dehydrogenate an alcohol. These enzymes may still be called dehydrogenases. For example, in the lactic acid fermentation lactate is formed by reduction of pyruvate but we still call the enzyme lactate dehydrogenase. In our bodies this enzyme functions in both directions. However, some enzymes that act mainly in the direction of reduction are called reductases. An example is aldose reductase, a member of a family of aldo-keto reductases71 73 which have (a / P)8-barrel structures.74 76... 

Figure 17-7 Outline of the glycolysis pathway by which hexoses are broken down to pyruvate. The ten enzymes needed to convert D-glucose to pyruvate are numbered. The pathway from glycogen using glycogen phosphorylase is also included, as is the reduction of pyruvate to lactate (step 11). Steps 6a-7, which are involved in ATP synthesis via thioester and acyl phosphate intermediates, are emphasized. See also Figures 10-2 and 10-3, which contain some additional information.

This system has been successfully applied to the in-situ electroenzymatic reduction of pyruvate to D-lactate using the NADH-dependent D-lactate dehydrogenase or the reduction of 4-phenyl-2-butanone to (5)-4-phenyl-2-butanol using the NADH-dependent horse liver alcohol dehydrogenase (HLADH) with high enantioselectivity (Fig. 22.4) [65]. 

The final form of metabolic regulation is effected by the use of iso-enzymes, which are multiple forms of an enzyme. For example, lactate dehydrogenase exists in five forms in a rat. They differ in primary structure and have different isoelectric points, but they all catalyse the reversible reduction of pyruvate to lactate. 

Regeneration of NAD+ by reduction of pyruvate to lactate. Because NAD+ is a necessary participant in the oxidation of glyceraldehyde-3-phosphate to glycerate-l,3-bisphosphate, glycolysis is possible only if there is a way by which NADH can be reoxidized. 

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