Pyruvate dehydrogenase reactions

The mechanism of the pyruvate dehydrogenase reaction is a tour de force of mechanistic chemistry, involving as it does a total of three enzymes (a) and five different coenzymes—thiamine pyrophosphate, lipoic acid, coenzyme A, FAD, and NAD (b). 

Based on the action of thiamine pyrophosphate in catalysis of the pyruvate dehydrogenase reaction, suggest a suitable chemical mechanism for the pyruvate decarboxylase reaction in yeast ... 

The pyruvate dehydrogenase reaction. Acetyl-CoA is transported out of the mitochondria, via citrate (Figure 11.3). Acetyl-CoA is converted to malonyl-CoA, catalysed by acetyl-CoA carboxylase. 

The pyruvate dehydrogenase reaction takes place in the mitochondrial matrix (see p. 210). Three different enzymes [E1-E3] form the PDH multienzyme complex (see B). 

The acetyl-CoA that supplies the cycle with acetyl residues is mainly derived from p-oxidation of fatty acids (see p. 164) and from the pyruvate dehydrogenase reaction. Both of these processes take place in the mitochondrial matrix. 

Leucine, isoleucine, lysine, and tryptophan form acetyl CoA or ace toacetyl CoA directly, without pyruvate serving as an intermediate (through the pyruvate dehydrogenase reaction, see p. 107). As men tioned previously, phenylalanine and tyrosine also give rise to acetoacetate during their catabolism (see Figure 20.7). Therefore, there are a total of six ketogenic amino acids. 

Describe the subunit structure of the enzyme pyruvate dehydrogenase. Discuss the functioning of each of the coenzymes that are associated with these subunits and write detailed mechanisms for each step in the pyruvate dehydrogenase reaction. 

Thus far we have discussed the two most important enzymes regulating the supplies of acetyl-CoA and oxaloacetate for the TCA cycle. This leaves us with the three enzymes, all within the cycle, that regulate the activity of the cycle. The first of these, citrate synthase, catalyzes the formation of citrate from acetyl-CoA and oxaloacetate. Regulation, by energy charge and other parameters, of the rate of glycolysis and of the pyruvate dehydrogenase reaction play... 

Net reaction for the pyruvate dehydrogenase reaction plus the citric acid cycle (10 reactions)... 

Following this route under aerobic conditions, pyruvate is converted to acetyl CoA by the enzyme pyruvate dehydrogenase and the acetyl CoA then enters the citric acid cycle. The pyruvate dehydrogenase reaction is an oxidative decarboxylation (see Topic LI for details) ... 

Answer Thiamine is essential for the formation of thiamine pyrophosphate (TPP), one of the cofactors in the pyruvate dehydrogenase reaction. Without TPP, the pyruvate generated by glycolysis accumulates in cells and enters the blood. 

If glucose-l-14C is converted to acetyl-CoA via glycolysis and the pyruvate dehydrogenase reaction and enters the Krebs cycle, then where is radioactivity found in succinate before the first run of the Krebs cycle is completed ... 

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

This irreversible reaction is the link between glycolysis and the citric acid cycle. (Figure 17.4) Note that, in the preparation of the glucose derivative pyruvate for the citric acid cycle, an oxidative decarboxylation takes place and high-transfer-potential electrons in the form of NADH are captured. Thus, the pyruvate dehydrogenase reaction has many of the key features of the reactions of the citric acid cycle itself... 

Recall also that NADH is generated in the formation of acetyl CoA from pyruvate by the pyruvate dehydrogenase reaction. 

Melzer E. and Schmidt H. L. (1987) Carbon isotope effects on the pyruvate dehydrogenase reaction and their importance for relative carbon-13 depletion in lipids. J. Biol. Chem. 262(17), 8159-8164. 

NADH is produced by the a-ketoglutarate dehydrogenase, isocitrate dehydrogenase, and malate dehydrogenase reactions of the TCA cycle, by the pyruvate dehydrogenase reaction that converts pyruvate to acetyl CoA, by (3-oxidation of fatty acids, and by other oxidation reactions. 

Transfer of a two-carbon unit from a 2-keto sugar to the carbonyl carbon (Ci) of an aldose by a transketolase, which requires thiamine pyrophosphate and magnesium as cofactors. A covalent enzyme-substrate intermediate is formed similar to the one that occurs during the pyruvate dehydrogenase reaction (Chapter 13). 

O -glycerophosphate derived in situ from glucose. The glucose can also be converted into fatty acids from acetyl-CoA generated via glycolysis and the pyruvate dehydrogenase reaction and from NADPH obtained via the pentose phosphate pathway. 

The mechanism of the pyruvate dehydrogenase reaction is wonderfully complex, more so than is suggested by its simple stoichiometry. The reaction requires the participation of the three enzymes of the pyruvate dehydrogenase complex and five coenzymes. The coenzymes thiamine pyrophosphate (TPP), lipoic acid, and FAD serve as catalytic cofactors, and CoA and NAD" are stoichiometric cofactors. 

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