Biochemistry - Citric Acid Cycle

Name the rate controlling enzymes of the citric acid cycle.

1. citrate synthase 2. isocitrate dehydogrenase 3. alpha-ketoglutarate dehydrogenase

Name the enzymes of the citric acid cycle that catalyze irreversible reactions.

1. citrate synthase (oxaloacetate + acetyl-CoA -----> citrate) 2. alpha-ketoglutarate dehydrogenase (alpha-ketoglutarate + NAD ------> succinyl-CoA)

What two enzymes in the citric acid cycle have isoforms?

1. isocitrate dehydorgenase 2. succinyl-CoA synthetase

Approximately how many molecules of ATP are produced per molecule of FADH2?

1.5

1 acetyl-CoA molecule generates approximately how many molecules of ATP?

10

How many isoforms does succinyl-CoA synthetase have?

2

Approximately how many molecules of ATP are produced per molecule of NADH?

2.5

One molecule of glucose produces approximately how many molecules of ATP?

20 ATP molecules

How many isoforms does isocitrate dehydrogenase have?

3

In addition to the regulators of enzyme activity within the citric acid cycle, two enzymes outside cycle profoundly affect its activity. Which ones? A. Isocitrate dehydrogenase and isocitrate oxidase B. Lactic acid dehydrogenase and pyruvate kinase C. Acetalaldehyde dehydrogenase and citrate synthase D. Glucokinase and phosphofructokinase E. Pyruvate carboxylase and pyruvate dehydrogenase

E. Pyruvate carboxylase and pyruvate dehydrogenase

The electrons extracted from the citric acid cycle by the reduced of NAD+ and FAD are used in the ____________.

ETC

The citric acid cycle has both anabolic and catabolic elements. Therefore, it is said to be _________________.

amphibolic

Where in the cell does glycolysis occur?

cytosol

Where in the cell does ATP phosphorylation occur?

cytosol and mitochondria

Where in the cell does the citric acid cycle occur?

mitochondrial matrix

High cellular concentrations of which molecule would inhibit the entry of pyruvate into the citric acid cycle? A. AMP B. Acetyl-CoA C. NADH D. Pyruvate

B. Acetyl-CoA

Isocitrate dehydrogenase (IDH) IDH is allosterically activated by ADP at high concentrations. How does this interaction helps regulate the Krebs cycle? A. Low levels of ADP stimulate the Krebs cycle, leading to the production of excess ATP that is stored for later use. B. High levels of ADP stimulate the Krebs cycle, leading to the increased conversion of ADP to ATP during oxidative phosphorylation. C. High levels of ADP inhibit the Krebs cycle because ADP prevents IDH from catalyzing the conversion of isocitrate to α-ketoglutarate. D. Low levels of ADP inhibit the Krebs cycle because IDH requires ADP as a substrate for the reaction it catalyzes.

B. High levels of ADP stimulate the Krebs cycle, leading to the increased conversion of ADP to ATP during oxidative phosphorylation.

Which substance is a product of glycolysis, a precursor of gluconeogenesis, and a precursor of the TCA cycle? A. Glucose B. Pyruvate C. ATP D. CoA

B. Pyruvate

Gifblaar, a small shrub native to Southern Africa, is a common cause of lethal poisoning in cattle and other livestock living in the region. The plant's extreme toxicity is due to fluoroacetate, a potent metabolic poison that is present in all parts of the plant. In the body, fluoroacetate is converted into fluorocitrate, which irreversibly binds the enzyme that converts citrate to isocitrate in step 2 of the Krebs cycle. As a result, the cycle is unable to proceed past step 1. Which of the following best describes an effect of gifblaar poisoning? A. Elevated levels of α-ketoglutarate would be found in the blood and urine. B. The production of NADH and FADH2 would decrease, but ATP levels would remain constant. C. ATP would be depleted, inhibiting many biological processes. D. Isocitrate would accumulate in tissues throughout the body.

C. ATP would be depleted, inhibiting many biological processes.

An investigator is measuring the activity of various enzymes involved in reactions of intermediary metabolism. One of the enzymes has greatly decreased activity compared to reference values. The buffer of the assay contains citrate. Which of the following enzymes will most likely be directly affected by the use of citrate? A. Fructose-2,6-bisphosphatase B. Isocitrate dehydrogenase C. Phosphofructokinase D. Pyruvate carboxylase

C. Phosphofructokinase

Conversion of pyruvate to acetyl-Coenzyme A is catalyzed by pyruvate dehydrogenase (oxidoreductase) which requires thiamine pyrophosphate as the coenzymes. Which other enzyme system in the citric acid cycle also has a requirement for thiamine pyrophosphate? A. Citrate synthase B. Citrate isomerase C. Isocitrate oxidoreductase D. α-Ketoglutarate oxidoreductase (dehydrogeanase) C. Malate oxidoreductase

D. α-Ketoglutarate oxidoreductase (dehydrogeanase)

Where in the cell is the electron transport chain?

inner mitochondrial membrane

High concentrations of acetyl-CoA activate which enzyme?

pyruvate carboxylase (pyruvate ---> oxaloacetate)

High concentrations of acetyl-CoA inhibit which enzyme?

pyruvate dehydrogenase (pyruvate ----> acetyl-CoA)

Alpha-ketoglutarate is similar to what other enzyme, in terms of its cofactor requirement?

pyruvate dehydrogenase complex ( cofactor: TPP)