Krebs Cycle Flashcards

1
Q

What is the PDH Complex?

A
  • Multi-subunit enzyme
  • Takes pyruvate and converts it into acetyl Co-A (releases CO2)
  • Allosterically regulated
  • Many cofactors
  • Can be considered the 1st control point of the citric acid cycle
  • It catalyses a 3,4,5 reaction in which there are 3 subunits that catalyse a 4 step reaction using 5 cofactors
  • Pyruvate dehydrogenase complex (E1, E2 and E3)
  • Cofactors are TPP, lipoate, FAD, NAD+ and CoA-SH

Allosterically regulated by:
Negative -> ATP, acetyl-CoA and NADH
Positive -> ADP and pyruvate

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2
Q

1st step

A
  1. Conversion of acetyl-CoA to citrate
    - The enzyme is citrate synthase
    - acetyl-CoA reacts with oxaloacetate to produce citrate
    - Highly exergonic process seeing as there is hydrolysis of the thioester
    - H2O is a cofactor
    - CoA-SH is produced
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3
Q

2nd step

A
  1. Aconitase
    - Isomerisation of citrate to isocitrate
    - Produced in two steps:
    - In the first step water is removed and cis-Aconitate is formed
    - In the second step water is introduced and isocitrate is formed
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4
Q

3rd step

A
  1. Conversion of isocitrate into a-ketoglutarate
    - Isocitrate dehydrogenase
    - NAD+ to NADH (reduction)
    - CO2 is released
    - Reaction is oxidation
  • Negative modulator is ATP and NADH
  • Positive is ADP
  • 2nd control point
  • Produces 2.5 ATP
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5
Q

4th step

A
  1. a-ketaglutarate to succinyl-CoA
    - a-ketoglutarate dehydrogenase complex (similar to PDH complex)
    - NAD+ to NADH
    - CoA-SH is an input
    - CO2 is produced as a by-product
  • Negative modulators are ATP, succinyl-CoA and NADH
  • 3rd control point
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6
Q

5th step

A
  1. Succinyl CoA to succinate
    - Succinyl CoA synthetase
  • GDP + Pi is converted to GTP
  • CoA-SH is removed
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7
Q

6th step

A
  1. Succinate to fumarate
    - Succinate dehydrogenase
  • FAD is reduced to FADH2
  • 1.5 ATP
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8
Q

7th step

A
  1. Fumarate to L-malate
    - Fumarase
  • Carbanion intermediate
  • The enzyme is highly stereospecific and so it does not like the cis isomer or D-malate isomer
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9
Q

8th step

A
  1. Fumarate to oxaloacetate
    - Malate dehydrogenase
  • NAD+ is reduced to NADH
  • 2.5 ATP
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10
Q

What are the free energy change patterns?

A

Reactions 2, 6 and 8 are positive in free energy change.

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