Pyruvate reduction

FIGURE 19.30 (a) Pyruvate reduction to ethanol in yeast provides a means for regenerating NAD consumed in the glyceraldehyde-3-P dehydrogenase reaction, (b) In oxygen-depleted muscle, NAD is regenerated in the lactate dehydrogenase reaction. 

Problem 19.19 What is the stereochemistry of the pyruvate reduction shown in Figure 19.15 Does NADH lose its pro-R or pro-S hydrogen Does addition occur to the Si face or Re face 1 of pyruvate ... 

We can now look at the partial reactions involved in LDH-catalyzed pyruvate reduction. 

Scheme 6.16 TA-catalyzed transamination coupied with pyruvate reduction.

LDH. Rather surprisingly, the properties of LDH have been investigated fully only in Hymenolepis spp. Burke et al. (108) purified 128-fold a single species of LDH from H. diminuta which resembled the H form of mammalian LDH. Other workers (448, 567, 630, 926) were able to distinguish electrophoietically at least two LDH isoenzymes in H. diminuta and H. microstoma. The kinetic parameters for LDH, similar in both species (Table 5.7), indicate that pyruvate reduction is favoured over lactate oxidation and that pyruvate, once formed, would be rapidly reduced to lactate, with the subsequent oxidation of NADH. 

Problem 19.19 What is the stereochemistry of the pyruvate reduction shown in Figure 19.15 Does... 

Pig heart Lactate DH -17.5 -11.6 0.9 Glyoxalate reduction Pyruvate reduction/lactate oxidation Glyoxalate oxidation 430 192/85 115 Rel 71... 

Similar studies of the enzyme from pig skeletal muscle have been reported 175,183). In the earlier work, a fast burst of NADH formation in the dead-time of the apparatus was observed, equal in amplitude to the active center concentration at pH 8.0, but smaller at lower pH values. The suggestion that slow isomerization of the ternary product complex before pyruvate release may be the step responsible for the low steady-state maximum rate of lactate oxidation seems to be inconsistent with the full burst observed at pH 8.0, since it might be expected to result in partial equilibration of the reactant and product ternary complexes. Direct studies of the oxidation of E-NADH by pyruvate at pH 9.0 did indicate that reverse hydride transfer from NADH to pyruvate is indeed fast, but the absence of a deuterium isotope effect suggested that the observed rate constant of 246 sec, equal to the maximum steady-state rate of pyruvate reduction, may reflect an isomerization of the ternary complex preceding even faster hydride transfer. More recent studies 183) with improved techniques, however, appear to indicate no burst of enzyme-bound NADH formation preceding the steady-state phase of lactate oxidation at pH 8.0. On the basis of stopped-flow studies of lactate oxidation in the presence of oxamate, which forms a dead-end complex with E-NADH and can serve as an indicator of the rate of formation... 

Work with bacterial enzymes has been partially paralleled by studies with animal and plant preparations. Particles with molecular weights over 10 have been shown to contain lipoic acid and to carry out oxidation of pyruvate, reduction of DPN, and formation of acetyl CoA. The number of protein molecules involved and their relation to each other remain to be established. 

In an MER, pyruvate reduction proceeds nine times faster than conventionally. However, since enzymatic regeneration of cofactor FDH through acetone reduction is not trivial (Figure 4.4), it may appear that this approach is not easily reproducible with other questions. 

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