Pyruvate kinase regulatory enzymes

The PDHC catalyzes the irreversible conversion of pyruvate to acetyl-CoA (Fig. 42-3) and is dependent on thiamine and lipoic acid as cofactors (see Ch. 35). The complex has five enzymes three subserving a catalytic function and two subserving a regulatory role. The catalytic components include PDH, El dihydrolipoyl trans-acetylase, E2 and dihydrolipoyl dehydrogenase, E3. The two regulatory enzymes include PDH-specific kinase and phospho-PDH-specific phosphatase. The multienzyme complex contains nine protein subunits, including... 

The key regulatory enzymes in glycolysis are phosphory-lase, hexokinase, phosphofructokinase and pyruvate kinase, the activities of which are stimulated by the increase in the concentrations of AMP and phosphate and the decrease in that of phosphocreatine. These mechanisms are discussed in Chapters 6 and 9 Figures 6.16 and 9.27. 

A regulatory effect observed in many biochemical pathways an enzyme, which catalyzes a step (typically a late step) in a pathway, is activated by elevated levels of a precursor of a substrate for that enzyme. A possible example of feed-forward activation may be the action of elevated levels of fructose 1,6-bisphosphate on pyruvate kinase. 

In view of these problems with Ni2+, Mna+ has been used as a probe for Mg2+ with some success. However, it should be noted that there is a difference in radius which may be manifested in different biochemical behaviour (Mn2+, 0.80 A Mg2+, 0.65 A). Thus Mn2+ has been used to probe the Mg2+ site in pyruvate kinase.95 While the Mg2+-activated enzyme is inhibited by Ca2+ and Li+, the Mn2+-activated enzyme is inhibited by Ca2+ and not by Li+. There are also differences in the catalytic and regulatory properties of the NAD+-specific malic enzyme of E. coli,104 depending upon whether the divalent activator is Mn2+ or Mg2+. It is necessary, therefore, to express a cautionary note. These two cations may act in slightly different ways to bring about a similar final result. In the second example it appears that the metal cofactors stabilize two different conformational states of the enzyme. 

When the apparent equilibrium constant exceeds the mass action ratio by more than 20 the reaction is considered to be catalysed far from equilibrium and hence is likely to play a regulatory role in glycolysis. Thus, in the species presented, hexokinase, phosphofructokinase and pyruvate kinase are regulatory enzymes. Data from McManus Smyth (1982) b Behm Bryant (1975a) c Rahman Mettrick (1982) d Beis Barrett (1979) e McManus Sterry (1982). 

The regulatory role of calcium ions in intermediary metabolism is well documented. Calcium has been shown to be involved in activation or inhibition of specific enzyme systems [105], For example, it activates cyclic nucleotide phosphodiesterase, phosphofructokinase, fructose 1 6 biphosphatase, glycerol phosphate dehydrogenase, pyruvate dehydrogenase phosphatase and pyruvate dehydrogenase kinase. Calcium ions inhibit pyruvate kinase, pyruvate carboxylase, Na+/K+-AT-Pase and adenylate cyclase. 

Two other allosteric enzyme regulatory reactions also help to regulate glycolysis the conversion of fructose 6-phosphate to fructose 1,6-diphosphate by phos-phofructokinase and the conversion of phosphoenolpyru-vate to pyruvate by pyruvate kinase. 

C. The activity of regulatory enzymes such as fructose-1,6-bisphos-phatase, hexokinase, phosphofructokinase 1, and pyruvate kinase are frequently controlled by binding allosteric effectors. These allosteric enzymes usually exhibit sigmoidal kinetics. Lactate dehydrogenase is not controlled by allosteric effectors and therefore would be expected to exhibit Michaelis-Menten kinetics. 

The enzyme pyruvate kinase (PK), one of the sites of ADP phosphorylation in glycolysis, provides a good example of adaptation via alteration of enzyme properties. This is because it is modified by the binding of allosteric effectors, it is phosphorylated in the liver and it binds regulatory proteins in the muscle. The reaction catalyzed by PK is as follows ... 

All of the regulatory enzymes of glycolysis exist as tissue-specific isoenzymes, which alter the regulation of the pathway to match variations in conditions and needs in different tissues. For example, in the liver, an isoenzyme of pyruvate kinase introduces an additional regulatory site in glycolysis that contributes to the inhibition of glycolysis when the reverse pathway, gluconeogenesis, is activated. 

Fig. 22.12. Major sites of regulation in the glycolytic pathway. Hexokinase and phos-phofructokinase-1 are the major regulatory enzymes in skeletal muscle. The activity of pyruvate dehydrogenase in the mitochondrion determines whether pyruvate is converted to lactate or to acetyl Co A. The regulation shown for pyruvate kinase only occurs for the liver (L) isoenzyme.

Meixner-Monori, B. Kubicek, C.P. Rohr, H. Pyruvate kinase from Aspergillus niger a regulatory enzyme in glycolysis . Can. J. Microbiol., 30, 16-... 

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