Citric Acid Cycle (Ben) Flashcards Preview

Y Biochemistry 2 > Citric Acid Cycle (Ben) > Flashcards

Flashcards in Citric Acid Cycle (Ben) Deck (24)
Loading flashcards...
1

What is the first reaction of the citrate cycle?

Reactants?

Enzyme?

Product?

Notes?

Reactants: Oxaloacetate  + Acetyl-CoA

Enzymes: Citrate Synthase

Products: Citrate + CoA-SH + H+

Notes: IRREVERSIBLE + oxaloacetate can be considered catalytic because it is regenerated by the cycle

2

What is the first part of the second reaction of the citrate cycle?

Reactants?

Enzyme?

Product(s)?

Dehydration of citrate to an intermediate...

Reactants: Citrate

Enzyme: Aconitase

Product: Cis-Aconitate Intermediate

 

 

 

3

What two things can inhibit aconitase and what results from this?

  • buildup of its product isocitrate
    • results in free citrate which can be removed to the cytosol for fatty acid synthesis

 

  • fluoracetate - in plants or metabolized from chemo meds
    • inhibits aconitase via fluoracetyl-CoA > fluorocitrate formation + causes citrate accumulation

4

What is the 2nd part of the second reaction of the citrate cycle?

Reactants?

Enzyme?

Product(s)?

Rehydration of an intermediate to isocitrate...

Reactants: Cis-aconitate intermediate

Enzyme: Aconitase

Product: Isocitrate

5

What is the 1st part of the 3rd reaction of the citric acid cycle (acting on the isomer formed in reaction 2)?

Reactants?

Enzyme?

Product(s)?

Notes?

Dehydrogenation...

Reactant: Isocitrate + NAD+

Enzyme: Isocitrate Dehydrogenase

Product: Oxalosuccinate + NADH + H+

Notes: IRREVERSIBLE + product remains enzyme-bound for 2nd part of rxn 

6

What is the 2nd part of the third reaction of the citrate cycle?

Reactants?

Enzyme?

Product(s)?

Notes/requirements?

Oxidative decarboxylation...

Reactants: Oxalosuccinate 

Enzyme: Isocitrate Dehydrogenase

Product: α-Ketoglutarate

Notes: IRREVERSIBLE + requires Mg2+ or Mn2+

 

7

What is the fourth reaction of the citrate cycle, occurring after decarboxylation via isocitrate dehydrogenase?

Reactants?

Enzyme?

Product(s)?

Notes (especially about the enzyme)?

Another oxidative decarboxylation...

Reactants: α​-Ketoglutarate dehydrogenase + NAD+ + HS-CoA 

Enzyme: α​-Ketoglutarate dehydrogenase complex

Product: Succinyl-CoA + NADH

Notes: IRREVERSIBLE + α​-KGDH Complex is very similar to PDH complex (same E3)

 

8

Considering that alpha-ketoglutarate dehydrogenase complex is so similar to PDH complex...

what are its prosthetic groups + cofactors?

Prosthetic Groups:

  • Thiamin Pyrophosphate
  • Lipoic Acid
  • FAD

Cofactors: NAD + CoA (via CoA-SH)

9

What is the 5th reaction of the citrate cycle, coming after the reaction catalyzed by an enzyme complex?

Reactants?

Enzyme?

Product(s)?

Notes?

Substrate level phosphorylation...

Reactants: Succinyl-CoA + Pi + GDP

Enzyme: Succinyl-CoA Synthetase

Products: Succinate + CoA-SH + GTP

Notes: GTP can pass the phosphoryl group to ADP via NDP Kinase

10

What is special about Succinyl-CoA Synthetase in different tissues with regard to its effect on nucleoside diphosphates?

Two isoenzymes exist:

Gluconeogenic tissues (liver/kidney): GDP/ADP-specific versions

  • GDP version makes GTP for decarboxylation of oxaloacetate to phosphoenolpyruvate
  • links citric acid cycle + gluconeogenesis

Non-gluconeogenic tissues: only ADP-specific version

11

What is the sixth reaction of the citrate cycle, following a substrate level phosphorylation?

Reactants?

Enzyme?

Product(s)?

Dehydrogenation of succinate...

Reactants: Succinate + FAD

Enzyme: Succinate Dehydrogenase

Products: Fumarate + FADH2

 

12

What is special about the location and function of succinate dehydrogenase?

- it is a membrane-bound flavoprotein bound to the inner surface of the inner mitochondrial membranse

- it is part of complex II of the respiratory chain

- it reduces CoQ via electrons transported by its Iron-Sulfur Proteins

 

13

What is the 7th reaction of the citrate cycle, right after the step catalyzed by a membrane bound enzyme?

Reactants?

Enzyme?

Products?

Notes?

Hydration of fumarate...

Reactants: Fumarate

Enzyme: Fumarase

Product: L-Malate

Note: the hydration is stereospecific, always forming the L-enantiomer

14

What inhibits succinate dehydrogenase?

Via what kind of inhibition?

Malonate

- via competitive inhibition

15

What is the 8th and final step of the citrate cycle, reforming the intial compound that contributes to the first step?

Reactants?

Enzymes?

Products?

 

Oxidization of malate...

Reactants: Malate + NAD+

Enzyme: Malate Dehydrogenase

Products: Oxaloacetate + NADH

 

 

 

16

What 3 citric acid cycle steps produce NADH?

​Remember: all dehydrogenase steps except succinate dehydrogenase.

  1. Isocitrate Dehydrogenase (oxidation of isocitrate to oxalosuccinate)
  2. Alpha-ketoglutarate Dehydrogenase (oxidative decarboxylation of AKG to succinyl-CoA)
  3. Malate Dehydrogenase (oxidation of L-malate to oxaloacetate)

 

17

Which step of the citrate cycle produces FADH2?

Succinate Dehydrogenase

- also technically E3 of AKGDH Complex I guess, but that remains bound to E3 as its prosthetic group

18

Which step of the citrate cycle is a substrate-level phosphorylation?

Succinyl-CoA Synthetase

19

What is special about the free energy change of the final step of the citrate cycle?

And because of this, what moves it forward?

non-spontaneous under standard conditions

- driven by high [malate]/[oxaloacetate] and [NAD+]/[NADH] ratios

20

Which two steps of the citrate cycle produce CO2?

​The two oxidative decarboxylations leading up to ADP/GDP phosphorylation

  1. Isocitrate Dehydrogenase (oxalosuccinate to AKG)
  2. AKG-Dehydrogenase Complex (AKG to Succinyl-CoA)

21

What is the net reaction of the citrate cycle?

22

What inhibits alpha-ketoglutarate dehydrogenase complex?

Arsenite

- also high [ammonia]

23

What are the 3 irreversible steps of the citrate cycle?

  1. Citrate Synthase
  2. Isocitrate Dehydrogenase
  3. AKG-Dehydrogenase Complex

24

What activates the citrate cycle?

Think ratios of products and reactants.

  • low [NADH]/[NAD+]
  • low [ATP]/[ADP]
  • low [succinyl-CoA]/[CoA]
  • Ca2+ (as in working muscle)