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Questions Based on the Action of Pyruvate

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Hi, I need some assistance with the following questions:

1. The disease beriberi, which results from a dietary deficiency of vitamin B1 (Thiamine) is characterized by neurological and cardiac symptoms, as well as increased levels of pyruvate and alpha- ketoglutarate in the blood. How does a deficiency of thiamine account for the increased levels of pyruvate and alpha- ketoglutarate?

2. Patients in shock experience decreased delivery of oxygen to tissues, decreased activity of the pyruvate dehydrogenase complex, and increased anaerobic metabolism. Excess pyruvate is converted to lactate, which accumulates in tissues and in the blood, causing lactic acidosis.
a. Since oxygen is not a reactant or a product in the citric acid cycle, why do low levels of oxygen decrease the activity of the pyruvate dehydrogenase complex?
b. To alleviate lactic acidosis, shock patients are sometimes given dichloroacetate, which inhibits pyruvate dehydrogenase kinase. How does this treatment affect the activity of the pyruvate dehydrogenase complex?

3. The reaction for which the enzyme pyruvate kinase is named never occurs in the cell. Briefly discuss this.

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Solution Preview

1. Thiamine is needed as an essential co-factor for the pyruvate dehydrogenase complex. The catalytic activity of the complex actually consists of three enzymes: pyruvate dehydrogenase, dihydrolipoamide acyltransferase and dihydrolipoamide dehydrogenase. Five non-protein co-factors are required for the enzymes to be catalytically active. They are: NAD+, CoA, TPP, lipoic acid and FAD.

TPP stands for thiamine pyrophosphate. Without this co-factor, the enzyme is inactive. Thiamine pyrophosphate is made from thiamine (B1). Therefore, with insufficient thiamine, the pyruvate dehydrogenase complex will not function well, and as a result, pyruvate will accumulate.

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Glycolysis and Gluconeogenesis

The following questions refer to the mitochondrial enzyme MALATE DEHYDROGENASE.

What is the chemical reaction, within the citric acid cycle, that is catalyzed by malate dehydrogenase, and which substrates, products, and/or cofactors are involved?

The reaction from the previous question is classified as a dehydrogenation, which means that the reaction represents a(n) _______________ wherein ________________.

a. oxidation reaction; malate loses electron(s)
b. oxidation reaction; malate gains electron(s)
c. reduction reaction; malate loses electron(s)
d. reduction reaction; malate gains electron(s)
e. hydrolysis reaction; malate loses hydrogen(s)

The ΔG'° for the chemical reaction catalyzed by malate dehydrogenase is +29.7 kJ/mol. Under standard biochemical conditions, is this reaction exergonic, endergonic, or close to equilibrium? Under these conditions, in which direction would this chemical reaction be expected to proceed?

Under typical cellular conditions of glycolysis and the citric acid cycle, does the malate dehydrogenase reaction represent a reversible catalytic step or an essentially irreversible step? Based on this answer, is the actual ΔG expected to be greater than, less than, or about equal to the ΔG'°? Why?

Within the context of the citric acid cycle the dehydrogenation of malate proceeds to the right (i.e. formation of oxaloacetate is favored). Why is this is so, and what other reactions or substrates/products of the TCA cycle are important here?

When gluconeogenesis is favored (and glycolysis is downregulated), describe the chemical reaction that is catalyzed by malate dehydrogenase. Under such conditions, is the formation of oxaloacetate or malate favored? Explain why the reaction proceeds in the direction you describe.

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