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Biochemistry > Chapter 13: Krebs Cycle > Flashcards

Chapter 13: Krebs Cycle Flashcards

1
Q

what is the energy produced in the krebs cycle conserved as?

A

conserved as reducing power when NAD+ and ubiquinone are reduced to NADH and QH2

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2
Q

why is the krebs cycle not exclusively a catabolic pathway for the oxidation of acetyl CoA?

A
  • oxaloacetate, citrate, alpha ketoglutarate and succinyl CoA all lead to biosynthetic pathways
  • the pool of intermediates can be replenished by the catabolism of amino acids or fatty acids
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3
Q

why is the krebs cycle considered a multi-step catalyst reaction?

A
  • only small amounts of each intermediate are needed to convert large quantities of acetyl CoA to products
  • the rate at which the krebs cycle metabolizes acetyl CoA is very sensitive in changes in the concentration of intermediates
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4
Q

where is the pyruvate dehydrogenase complex located?

A

mitochondria

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5
Q

how is pyruvate transported from the cytoplasm into the mitochondria so it can be converted to acetyl CoA?

A

pyruvate translocase transports pyruvate in symport with H+ across the inner membrane

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6
Q

why can molecules cross the outer membrane of the mitochondria?

A

presence of porins

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7
Q

How is oxaloacetate, which can be a substrate for gluconeogenesis, transported to the cytoplasm from the mitochondria?

A

oxaloacetate is converted to phosphoenolpyruvate (catalyzed by PEPCK) and a PEP transporter relocates it across the membrane

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8
Q

how is acetyl CoA transported from the mitochondria to the cytoplasm for fatty acid synthesis?

A
  • tricarboxylic acid transporter that exports citrate

- in the cytoplasm, citrate is converted to oxaloacetate and acetyl CoA

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9
Q

how do species that lack mitochondrial PEPCK export oxaloacetate?

A
  • malate-aspartate shuttle

- oxaloaceate is converted to malate by malate dehydrogenase

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10
Q

a large complex of enzymes and cofactors that catalyzes the conversion of pyruvate to acetyl CoA, so it can enter the krebs cycle

A

pyruvate dehydrogenase complex

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11
Q

is the conversion of pyruvate to acetyl CoA an oxidation or reduction?

A
  • oxidation

- NAD+ is concomitantly reduced

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12
Q

what are the 3 enzymes complexes of pyruvate dehydrogenase?

A
  • pyruvate dehydrogenase (E1)
  • Dihydrolipoyl transacetylase (E2)
  • Dihydrolipoyl dehydrogenase (E3)
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13
Q

what is the prosthetic group of E1?

A

TPP

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14
Q

what is the prosthetic group of E2?

A

lipoamide

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15
Q

what is the prosthetic group of E3?

A

FAD

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16
Q

what are the cosubstrates of the pyruvate dehydrogenase complex?

A
  • HS-CoA

- NAD+

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17
Q

What is the role of TPP?

A

binds pyruvate in E1, catalyzing decarboxylation to form hydroxyethyl-TPP

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18
Q

what is the role of lipoamide?

A
  • E2 swinging arm
  • hydroxyethyl is transferred to lipoamide
  • transfer of acetyl group to HS-CoA leaves lipoamide in reduced dithiol form
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19
Q

what is the role of FAD?

A
  • E3

- becomes reduced in regeneration of oxidized lipoamide

20
Q

describe the swinging arm mechanism

A
  1. Lipoamide accepts acyl group from E1, disulfide reduced to thiol and thioester
  2. Acyl group transferred to CoA; lipoamide fully reduced to dithiol
  3. E3 oxidizes dithiol back to disulfide
21
Q

where is the pyruvate dehydrogenase complex located in eukaryotes?

A

mitochondrial maxtrix

22
Q

where is the pyruvate dehydrogenase complex located in bacteria?

A

cytosol

23
Q

How is E1 regulated?

A
  • pyruvate dehydrogenase kinase
    • activated by: NADH and Acetyl CoA
    • inhibited by: NAD+, HS-CoA, ADP, Pyruvate
  • pyruvate dehydrogenase is active when dephosphorylated
24
Q

How is E2 regulated?

A
  • activated by: HS-CoA

- inhibited by: acetyl CoA

25
Q

How is E3 regulated?

A
  • activated by: NAD+

- inhibited by: NADH

26
Q

what are the starting molecules of the krebs cycle?

A
  • acetyl CoA
  • oxaloacetate
  • NAD+
27
Q

what are the products of the krebs cycle?

A
  • 2 CO2
  • 3 NADH
  • FADH2 (QH2)
  • GTP (ATP)
  • oxaloacetate
28
Q

how many ATP equivalents are produced per acetyl CoA in the krebs cycle?

A

10 ATP equivalents

29
Q

how many ATP equivalents are produced per glucose in the krebs cycle?

A

20 ATP equivalents

30
Q

what is the first reaction of the krebs cycle?

A
  • reaction of acetyl CoA with oxaloacetate to form citrate and HS-CoA
  • catalyzed by citrate synthase (transfers acetyl group)
  • delta G= -32
31
Q

why does the 1st reaction of the krebs cycle have a negative delta G?

A

due to the hydrolysis of the high energy thioester bond

32
Q

instead of being couple to ATP synthesis, the large negative G of the first step in the krebs cycle is used to……?

A

ensure that the reaction happens even when oxaloacetate concentrations are low

33
Q

what is the 2nd reaction of the krebs cycle?

A
  • conversion of citrate to isocitrate
  • catalyzed by aconitase
  • near equilibrium reaction
  • isomerization required since citrate is a tertiary alcohol so it cannot be further oxidized
34
Q

what helps aconitase in the binding of citrate?

A

iron-sulfur cluster

35
Q

what is the 3rd step of the krebs cycle?

A
  • conversion of isocitrate to alpha-ketoglutarate
  • decarboxylation catalyzed by isocitrate dehydrogenase
  • NAD+ reduced to NADH
  • CO2 formed
36
Q

what is the 4th step of the krebs cycle?

A
  • conversion of alpha-ketoglutarate to succinyl CoA
  • catalyzed by alpha-ketoglutarate dehydrogenase complex
  • oxidative decarboxylation
  • reaction analogous to pyruvate dehydrogenase
  • NAD+ reduced to NADH
  • CO2 formed
37
Q

what is the 5th step of the krebs cycle?

A
  • conversion of succinyl CoA
  • catalyzed by succinyl CoA synthetase
  • substrate level phosphorylation –> formation of GTP
38
Q

what is the 6th step of the krebs cycle?

A
  • conversion of succinate to fumarate (pro-chiral)
  • catalyzed by succinate dehydrogenase complex (inhibited by malonate)
  • FAD reduced to FADH2
  • contains iron-sulfur clusters
  • protons and electrons are moved to ubiquinone, forming QH2
  • same enzyme complex as complex II in ETC
39
Q

what is the 7th step of the krebs cycle?

A
  • conversion of fumarate to malate
  • catalyzed by fumarase
  • near equilibrium
  • addition of water
40
Q

what is the 8th step of the krebs cycle?

A
  • conversion of malate to oxaloacetate
  • catalyzed by malate dehydrogenase
  • NAD+ reduced to NADH
  • delta G = + 29
41
Q

how does the conversion of malate to oxaloacetate proceed if the delta G is + 29?

A

the low concentration of oxaloacetate drives the reaction

42
Q

what are the regulation points of the krebs cycle?

A
  • citrate synthase
  • isocitrate dehydrogenase
  • alpha-ketoglutarate dehydrogenase
43
Q

how is citrate synthase regulated?

A

inhibited by: ATP and NADH

44
Q

how is isocitrate dehydrogenase regulated?

A
  • allosterically activated by: Ca2+ and ADP

- allosterically inhibited by: NADH

45
Q

how is the alpha-ketoglutarate dehydrogenase complex regulated?

A

activated by: Ca2+

46
Q

what is the glyoxylate pathway/shunt?

A
  • route that bypasses the two CO2 producing reactions of the krebs cycle
  • one of the ways that fatty acids can produce glucose
47
Q

what happens at the end of the glyoxylate pathway?

A

both malate and oxaloacetate can be converted into phosphoenolpyruvate, whch is the product of phosphoenolpyruvate carboxykinase, the first enzyme in gluconeogenesis

Biochemistry (9 decks)

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