BIO C HE MIA I I – ĆW ICZ E NI A L I S T A 2

do wykładu dr. hab. inŜ. P. Dobryszyckiego

C y k l k w a s u c y t r y n o w e g o

1. If a eukaryotic cell were broken open and the subcellular organelles were separated by zonal ultracentrifugation on a sucrose gradient, in which of the following would the citric acid cycle enzymes be found?

(a) nucleus

(d) mitochondria

(b) lysosomes

(e) endoplasmic reticulum

(c) Golgi complex

2. Match the cofactors of the pyruvate dehydrogenase complex in the left column with their corresponding enzyme components and with their roles in the enzymatic steps that are listed in the right column.

(a) coenzyme A

(1) pyruvate dehydrogenase component

(b) NAD+

(2) dihydrolipoyl dehydrogenase

(c) thiamine pyrophosphate

(3) dihydrolipoyl transacetylase

(d) FAD

(4) oxidizes the hydroxyethyl group

(e) lipoamide

(5) decarboxylates pyruvate

(6) oxidizes dihydrolipoamide

(7) accepts the acetyl group from acetyllipoamide

(8) provides a long, flexible arm that conveys intermediates to different enzyme components

(9) oxidizes FADH2

3. Which of the following statements concerning the enzymatic mechanism of citrate synthase is correct?

(a) Citrate synthase uses an NAD+ cofactor.

(b) Acetyl CoA binds to citrate synthase before oxaloacetate.

(c) The histidine residues at the active site of citrate synthase participate in the hydrolysis of acetyl CoA.

(d) After citryl CoA is formed, additional structural changes occur in the enzyme.

(e) Each of the citrate synthase subunits binds one of the substrates and brings the substrates into close proximity to each other.

4. Which of the following answers complete the sentence correctly? Succinate dehydrogenase (a) is an iron-sulfur protein like aconitase.

(b) contains FAD and NAD+ cofactors like pyruvate dehydrogenase.

(c) is an integral membrane protein unlike the other enzymes of the citric acid cycle.

(d) carries out an oxidative decarboxylation like isocitrate dehydrogenase.

5. The conversion of malate to oxaloacetate has a ∆G°' = +7.1 kcal/mol, yet in the citric acid cycle the reaction proceeds from malate to oxaloacetate. Explain how this is possible.

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6. Given the biochemical intermediates of the pyruvate dehydrogenase reaction and the citric acid cycle (Figure 17.1), answer the following questions: (a) Name the intermediates:

A

B

(b) Draw the structure of isocitrate and show those atoms that come from acetyl CoA in bold letters.

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(c) Which reaction is catalyzed by α-ketoglutarate dehydrogenase?

(d) Which enzyme catalyzes step 2?

(e) Which reactions are oxidations? Name the enzyme catalyzing each of them.

(f) At

which

reaction

does

a

substrate-level

phosphorylation

occur?

Name

the

enzyme

and the products of this reaction.

(g) Which of the reactions require an FAD cofactor? Name the enzymes.

(h) Indicate the decarboxylation reactions and name the enzymes.

7. If the methyl carbon atom of pyruvate is labeled with 14C, which of the carbon atoms of oxaloacetate would be labeled after one turn of the citric acid cycle? Note that the "new" acetate carbons are the two shown at the bottom of the first few structures in the cycle, because aconitase reacts stereospecifically.

(a) None. The label will be lost in CO2.

(b) α

(c) β

(d) γ

(e) δ

8. Although O2 does not participate directly in the reactions of the citric acid cycle, the cycle operates only under aerobic conditions. Explain this fact.

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9. Which of the following answers complete the sentence correctly? The pyruvate dehydrogenase complex is activated by

(a) phosphorylation of the pyruvate dehydrogenase component (E1).

(b) stimulation of a specific phosphatase by Ca2+.

(c) inhibition of a specific kinase by pyruvate.

(d) decrease of the NADH/NAD+ ratio.

(e) decreased levels of insulin.

10. First select the enzymes in the left column that regulate the citric acid cycle. Then match those enzymes with the appropriate control mechanisms in the right column.

(a) citrate synthase

(1) feedback inhibited by succinyl CoA

(b) aconitase

(2) allosterically activated by ADP

(c) isocitrate dehydrogenase

(3) inhibited by NADH

(d) α-ketoglutarate dehydrogenase

(4) regulated by the availability of acetyl CoA

(e) succinyl CoA synthetase

and oxaloacetate

(f) succinate dehydrogenase

(5) inhibited by ATP

(g) fumarase

(f) malate dehydrogenase

11. Match the intermediates of the citric acid cycle in the left column with their biosynthetic products in mammals, listed in the right column.

(a) isocitrate

(1) aspartic acid

(b) α-ketoglutarate

(2) glutamic acid

(c) succinyl CoA

(3) cholesterol

(d) cis-aconitate

(4) porphyrins

(e) oxaloacetate

(5) none

12. Which of the following answers complete the sentence correctly? Anaplerotic reactions (a) are necessary because the biosynthesis of certain amino acids requires citric acid cycle intermediates as precursors.

(b) can convert acetyl CoA to oxaloacetate in mammals.

(c) can convert pyruvate into oxaloacetale in mammals.

(d) are not required in mammals, because mammals have an active glyoxylate cycle.

(e) include the pyruvate dehydrogenase reaction operating in reverse.

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