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BMB 402 > Chapter 10 The Citrate Cycle > Flashcards

Chapter 10 The Citrate Cycle Flashcards

1
Q

What is the citrate cycle?

A

a central pathway in oxidative respiration that converts acetyl units to CO2 with the concomitant production of FADH2, NADH, and ATP

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2
Q

Why is the citrate cycle considered the “hub” of cellular metabolism?

A
  1. it is central to aerobic metabolism and ATP production by generating the bulk of NADH and FADH2, which are oxidized by the ETC to generate ATP by oxidative phosphorylation
  2. it links the oxidation of various metabolic fuels (carbs, fatty acids, and proteins) to ATP synthesis through shared intermediates
  3. it provides metabolites for numerous biosynthetic pathways
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3
Q

Where does the citrate cycle occur?

A

in the mitochondrial matrix

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4
Q

What is the primary function of the citrate cycle?

A

oxidize acetyl-CoA

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5
Q

What does each turn of the citrate cycle generate in terms of reducing power?

A

each turn transfers 8 electrons from intermediates to NAD+ and FADH, generating 3 NADH, 1 FADH2, and 1 GTP

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6
Q

Who pieced together/discovered the citrate cycle?

A

Hans Krebs

fled Germany, described cycle in 1937, won Nobel Prize in 1953 with Fritz Lipmann, who discovered the role of acetyl-CoA in metabolism

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7
Q

Why is oxaloacetate so important to the cycle?

A

It is the substrate for the first reaction (catalyzed by citrate synthase) and the product of the last reaction (catalyzed by malate dehydrogenase)

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8
Q

What controls the flow of acetyl-CoA into the cycle?

A

pyruvate dehydrogenase

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9
Q

What controls the flow of oxaloacetate into the cycle?

A

pyruvate carboxylase

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10
Q

What inhibits pyruvate dehydrogenase?

A

ATP and NADH (high levels signal acetyl-CoA to be diverted to fatty acid synthesis)

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11
Q

How much CO2 is generated per turn of the citrate cycle? In what steps?

A

2 CO2 given off as waste; produced in steps 3 and 4

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12
Q

What is the overall net reaction of the citrate cycle?

A

Acetyl-CoA + 3NAD+ + FAD + GDP + P + 2H2O —-> CoA + 2CO2 + 3NADH + 3H+ + FADH2 + GTP

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13
Q

What are the key enzymes of the citrate cycle?

A
  • pyruvate dehydrogenase (technically outside of the cycle, but important because it regulates flux of acetyl-CoA)
  • citrate synthase (step 1)
  • isocitrate dehydrogenase (step 3)
  • alpha-ketogluterate dehydrogenase (step 4)
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14
Q

Which enzyme in the citrate cycle is activated by CoA?

A

pyruvate dehyrogenase

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15
Q

oxidation

A

molecule or functional group loses electrons

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16
Q

reduction

A

molecule or functional group gains electrons

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17
Q

Oxidant/oxidizing agent

A

accept electrons and are reduced

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18
Q

Reductant/reducing agent

A

lose electrons and are oxidized

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19
Q

In linked metabolic pathways, the oxidants in subsequent reactions must ….

A

have progressively higher standard reduction potentials

20
Q

Define vitamin

A

any of a group of organic compounds which are essential for normal growth and nutrition and are required in small quantities because the body does not synthesize them

21
Q

What five coenzymes are required for pyruvate dehydrogenase to work?

A
  1. NAD+
  2. FAD
  3. CoA
  4. Thiamine pyrophosphate (TPP)
  5. alpha-lipoic acid (lipoamide)
22
Q

Where is NAD+ derived from?

A

niacin/vitamin B3

23
Q

Where is FAD derived from?

A

riboflavin/vitamin B2

24
Q

Where is CoA derived from?

A

pantothenic acid/vitamin B5

25
Q

Where it TPP derived from?

A

thiamine/vitamin B1

26
Q

Where do we get alpha lipoic acid from?

A

It is synthesized in the body (not a vitamin)

27
Q

What three subunits are part of the pyruvate dehydrogenase complex?

A

E1: pyruvate dehydrogenase
E2: dihydrolipoyl acetyltransferase
E3: dihydrolipoyl dehydrogenase

28
Q

What are the five catalytic steps performed by the pyruvate dehydrogenase enzyme complex?

A
  1. E1 subunit (pyruvate dehydrogenase) binds pyruvate, catalyzing decarboxylation and releasing CO2
  2. hydroxyethyl-TPP moiety of E1 reacts with lipoamide on the N-terminal domain of the E2 subunit (dihydrolipoyl acetyltransferase); generates acetyl-dihydrolipoamide
  3. E2 acetyl-dihydrolipoamide carries acetyl group from E1 site to E2 catalytic site , yielding acetyl-CoA
  4. dihydrolipoamide is reoxidized on the E3 subunit (dihydrolipoyl dehydrogenase), eventually forming E3-FADH2
  5. E3-FADH2 coenzyme intermediate is reoxidized to produce NADH
29
Q

Which enzyme requires CoASH to produce acetyl-CoA?

A

pyruvate dehyrogenase

30
Q

Why does arsenic poisoning affect you more quickly than a Vitamin B deficiency?

A

Arsenic prevents the reformation of cyclic lipoamide, which is an essential component of the pyruvate dehydrogenase complex. Without it, the Citrate Cycle shuts down. With a deficiency in any of the B Vitamin precursors to pyruvate dehydrogenase coenzymes, the Citrate Cycle would slowly decrease in its ability to produce energy over time.

31
Q

What are two diseases caused by deficiencies in B Vitamins?

A

Pellagra, a deficiency in niacin/B3, causes inflamed skin, diarrhea, dementia, and sores in the mouth. Beriberi, a deficiency in thiamine/B1, is observed in chronic alcoholics and has neurological and cardiovascular symptoms.

32
Q

What is the catalytic “workhorse” of pyruvate dehydrogenase?

A

lipoamide

33
Q

What are the three metabolic fates of acetyl-CoA?

A
  1. can be metabolized in the citrate cycle, ultimately converting redox energy to ATP by oxidative phosphorylation
  2. can be used to store metabolic energy in the form of fatty acids, which are transported to adipose tissues as triglycerides
  3. is a building block for the synthesis of membrane lipids, cholesterol, and ketones
34
Q

How is pyruvate dehydrogenase regulated?

A

Phosphorylation, NADH, ATP, and Acetyl-CoA all play a role in regulation. High levels of these molecules would provide negative feedback and downregulate the Citrate Cycle.

35
Q

Which steps of the Citrate Cycle produce reducing agents?

A

Steps 3, 4, and 8 produce NADH, and Step 6 produces FADH2.

36
Q

What is the protonation state of citrate at physiological pH?

A

Citrate is fully deprotonated at physiological pH, so it is not acidic.

37
Q

Why do NADH and ATP inhibit many of the key enzymes involved in the Citrate Cycle?

A

These molecules downregulate pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. High levels indicate that the cycle is running faster than the amount of input provided, so the cycle must slow down.

38
Q

How is pyruvate carboxylase activated?

A

activated allosterically by acetyl-CoA, leading to increased production of oxaloacetate

39
Q

What inhibits the regulatory enzymes?

A

High NADH to NAD+ ratio, NADH, ATP

citrate synthase and alpha-ketogluterate dehydrogenase are also inhibited by succinyl-CoA

citrate synthase inhibited by citrate

40
Q

The poison compound 1080 converts fluoroacetate to fluorocitrate. Which enzyme in the citrate cycle is inhibited by this poison?

A

aconitase

41
Q

amphibolic pathway

A

a metabolic pathway that functions in both catabolism and anabolism; ex. citrate cycle

42
Q

anaplerotic reaction

A

replenishes Citrate Cycle intermediates that have been shunted to biosynthetic pathways to produce other molecules

43
Q

What citrate cycle intermediates serve as precursors for other pathways?

A

citrate
alpha ketogluterate
succinyl-CoA
malate
oxaloacetate

44
Q

What enzyme catalyzes the primary anaplerotic reaction?

A

pyruvate carboxylase

it balances the input of oxaloacetate with acetyl-CoA by providing a way to convert pyruvate into OAA using an ATP dependent carboxylation reaction

45
Q

How is the citrate cycle used to produce ATP?

A

substrate-level phosphorylation reactions utilizing GTP

BMB 402 (14 decks)

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