L17- TCA Cycle Flashcards
Which enzyme catalyzes the oxidation of pyruvate to acetyl-CoA?
Pyruvate dehydrogenase.
Which three enzymes constitute the pyruvate dehydrogenase complex?
Pyruvate decarboxylase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).
List the coenzymes of pyruvate dehydrogenase.
Coenzyme A/acetyl Coenzyme A (CoASH), thiamine pyrophosphate (TPP), lipoate, FAD and NAD+.
Which pyruvate dehydrogenase coenzyme accepts an acyl group from lipoate, forming acetyl-CoA?
CoASH (coenzyme A).
Coenzyme A serves as a carrier of acyl groups in many metabolic reactions. What high energy bond is formed between CoASH and acyl groups?
Thiol ester bond.
From which vitamin is the coenzyme thiamine pyrophosphate derived?
Vitamin B1 (thiamine).
Name two medical conditions associated with thiamine deficiency.
Beri-beri and Wernicke-Korsakoff syndrome.
What is the function of the pyruvate dehydrogenase coenzyme thiamine pyrophosphate (TPP)?
TPP binds to pyruvate, leading to its decarboxylation. The remaining 2-carbon hydroxyethyl fragment is then transfered to lipoate, another coenzyme of pyruvate dehydrogenase.
Thiamine pyrophosphate (TPP) is bound to which enzyme in the pyruvate dehydrogenase complex?
Pyruvate decarboxylase (E1).
Which structural group on coenzyme A forms a thiol ester linkage with acyl groups?
The sulfhydryl group.
The coenzyme lipoate is bound to which enzyme in the pyruvate dehydrogenase complex?
Dihydrolipoyl transacetylase (E2).
Which enzyme in the pyruvate dehydrogenase complex catalyzes the rapid decarboxylation of pyruvate?
Pyruvate decarboxylase (E1).
Which component of the pyruvate dehydrogenase complex is a target for trivalent arsenic poisoning?
The lipoate coenzyme.
What kind of bond covalently links lipoate to the enzyme dihydrolipoyl transacetylase (E2)?
An amide bond.
The coenzyme flavin adenine dinucleotide (FAD) is derived from which vitamin?
Vitamin B2 (riboflavin).
What is the function of the FAD coenzyme bound to the dihydrolipoyl dehydrogenase enzyme (E3) of the pyruvate dehydrogenase complex?
FAD accepts electrons from the reduced form of lipoate, regenerating oxidized lipoate and forming FADH2. Note that FAD/FADH2 is covalently bonded to E3.
What is the function of the coenzyme NAD+ with regards to the series of reactions catalyzed by the pyruvate dehydrogenase complex?
NAD+ accepts electrons from the FADH2 that is bound to E3, regenerating FAD and producing NADH.
What inhibits each enzyme (E1, E2, and E3) of the pyruvate dehydrogenase complex (PDH)?
Acetyl-CoA inhibits PDH at the E2 component, and NADH inhibits PDH at the E3 component. The E1 component may also be inhibited by phosphorylation on a specific serine residue.
How do elevated concentrations of pyruvate regulate the activity of the pyruvate dehydrogenase complex (PDH)?
Pyruvate inhibits the kinase that inactivates the E1 component by phosphorylation. Therefore, pyruvate helps to keep PDH in its active form.
What regulatory effect does insulin exert on the pyruvate dehydrogenase complex (PDH)?
Insulin activates a phosphatase that can remove the phosphate group on E1. Therefore, insulin helps to activate PDH.
X-linked Leigh’s disease is caused by a deficiency of which enzyme?
Pyruvate dehydrogenase.
What clinical sign would you expect to observe in a patient with partial deficiency of pyruvate dehydrogenase?
Metabolic acidosis due to the buildup of lactate and alanine from the interconversion of pyruvate.
Which TCA cycle enzyme catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate?
Citrate synthase.
Which TCA cycle enzyme catalyzes a substrate-level phosphorylation?
Succinyl-CoA synthetase.
In which cellular compartment does the TCA cycle occur?
Mitochondrial matrix.
Fluoroacetate is able to inhibit which TCA cycle enzyme?
Aconitase.
Which coenzymes are required for proper activity of the enzyme alpha-ketoglutarate dehydrogenase?
Thiamine pyrophosphate (TPP), lipoate, coenzyme A (CoASH), FAD and NAD+.
Which TCA cycle enzymes catalyze irreversible reactions?
Citrate synthase and alpha-ketoglutarate dehydrogenase.
What is the chemical equation for the reaction catalyzed by succinyl CoA synthetase?
Succinyl-CoA + GDP + Pi -> succinate + GTP.
The enzyme succinate dehydrogenase requires which coenzyme(s)?
FAD.
What is the function of the enzyme malate dehydrogenase with respect to the TCA cycle?
Malate dehydrogenase catalyzes the reversible oxidation of malate to oxaloacetate. This regenerates oxaloacetate so that the TCA cycle may go through another round of oxidative metabolism.
Which TCA cycle intermediate is also a precursor for heme synthesis?
Succinyl-CoA.
Which TCA cycle intermediate may be exported to the cytoplasm for synthesis of glucose?
Malate.
List three metabolites that can be directly converted to oxaloacetate.
Pyruvate (by action of the enzyme pyruvate carboxylase), aspartate and malate.
Define the term “anaplerotic” reaction.
An anaplerotic reaction is a reaction that is not part of the TCA cycle but whose product is a TCA cycle intermediate. It is meant to replenish the TCA cycle intermediates.
True or False. Reactions of the TCA cycle are slowed by an increase in the NADH/NAD+ ratio.
True. They are also slowed by an increase in ATP.
Which TCA cycle enzyme(s) is/are subject to allosteric regulation?
Citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.
Which TCA cycle enzyme(s) produce NADH?
Isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase and malate dehydrogenase.
Which TCA cycle enzyme(s) produce FADH2?
Succinate dehydrogenase.
Which TCA cycle enzyme(s) produce GTP via substrate-level phosphorylation?
Succinyl-CoA synthetase.
List the allosteric activators and inhibitors of citrate synthase.
Activators: ADP. Inhibitors: NADH, ATP, succinyl-CoA, citrate
List the allosteric activators and inhibitors of isocitrate dehydrogenase.
Activators: isocitrate, ADP, AMP, Ca++. Inhibitors: NADH, ATP
List the allosteric activators and inhibitors of alpha-ketoglutarate dehydrogenase.
Activators: Ca++. Inhibitors: NADH, succinyl-CoA
Which TCA cycle substrates and coenzymes cannot cross the mitochondrial membrane?
Acyl-CoA compounds (such as acetyl-CoA), oxaloacetate, NAD+ and NADH.
