Chapter 18 - Citric Acid Cycle Preparation Flashcards
What reaction serves to link glycolysis and the citric acid cycle and what is the enzyme that catalyses it?
The pyruvate dehydrogenase complex catalyzes the following reaction linking glycolysis and citric acid cycle
Pyruvate + CoA + NAD+ —> acetyl CoA + NADH + H+
What are the 5 enzymes (incl. regulatory) that constitute the pyruvate dehydrogenase complex?
Which reactions do they catalyze?
Pyruvate dehydrogenase - catalyzes the decarboxylation of pyruvate and the formation of acetyl lipoamide
Dihydrolipoyl transacetylase - catalyzes the formation of acetyl CoA
Dihydrolipol dehydrogenase - catalyzes the reduction of the oxidized lipoic acid
PDH kinase - associated with the complex phoshorylates and inactivates the complex
PDH phosphatase - dephoshorylates and activates the comples
The conversion of pyruvate into acetyl CoA consists of three steps. What are these steps?
Decarboxylation
Oxidization
Transfer of the resultant acetyl group to CoA
The conversion of pyruvate into acetyl CoA commits the carbon atoms to either of two principal fates. What are the fates?
Oxidization to CO2 by the citric acid cycle or incorporation into lipids.
What coenzymes are required by the pyruvate dehydrogenase complex, and what are their roles?
Thiamine pyrophosphate plays a role in the decarboxylation of pyruvate
Lipoamide transfers the acetyl group
Coenzyme A accepts the acetyl group from lipoic acid to form acetyl CoA.
FAD accepts the electron and hydrogen ions when oxidized lipoic acid is reduced
NAD+ accepts electrons and a proton from FADH2
Distinguish between catalytic coenzymes and stoichiometric coenzymes in the pyruvate dehydrogenase complex.
Catalytic enzymes (TTP, Lipoic acid, and FAD) are modified but regenerated in each reaction cycle
Stoichiometric coenzymes (Coenzyme A and NAD+) are used in only one reaction because they are the components of the products and reactants
When lipoamide is reoxidized, what is the immediate electron receptor of the reaction? Why is the observed electron transfer unusual?
The electrons are transferred from reduced lipoamide to FAD initially and then to NAD+. This transfer is unusual because the electrons are passed to NAD+ from FADH2. The transfer is usually in the other direction.
Compare the regulation of the pyruvate dehydrogenase complex in muscle and in liver.
In muscle, the acetyl CoA generated by the complex is used for free energy generation. Signals that indicate an energy-rich state inhibit the complex. whereas the reverse conditions stimulate the enzyme. Calcium as the signal for muscle contraction also stimulates the enzyme.
In liver, acetyl CoA derived from pyruvate is used for biosynthetic purposes such as fatty acid synthesis. Insulin, the hormone denoting the fed state, stimulates the complex.
What are the thioesters in the reaction catalyzed by pyruvate dehydrogenase complex?
acetyl lipoamide and acetyl CoA
Thioesters are important not only in the reaction catalyzed by the pyruvate dehydrogenase complex but also for the generation of pyruvate itself. What is the thioester in glycolysis that helps to generate pyruvate?
A thioester is a key intermediate in the formation of 1,3-biphosphoglycerate from glyceraldehyde 3-phosphate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
1,3-biphosphoglycerate is subsequently metabolized to pyruvate.
