citric acid cycle

Question 1

where does the citric acid cycle occur?

Answer 1

mitochondrial matrix

Question 2

what does the citric acid cycle do?

Answer 2

Produces large amounts of ATP, unlike Glycolysis which only produces 2 ATPs

uses a combination of catabolic/anabolic processes

it does not use O2 as a reactant

it oxidizes molecules to form NADH and FADH2

90% of aerobic cell energy production in collab with Oxidative phophorylation

Cyclical and efficient; small number of molecules = a lot of NADH & FADH2

This cycle has evolved to harvest the full potential energy of nutrients to produce ATP.

Question 2

what is the transition stage of the citric acid cycle under aerobic conditions?

Answer 2

Fatty acids → Acetyl CoA (Beta-oxidation)

2 Pyruvate → 2 Acetyl CoA
(entrance to the main energy pathway of the citric acid cycle)

Catalyst - pyruvate dehydrogenase

This process involves pyruvate dehydrogenase complex (E1 + E2 + E3)

Question 3

what are the end products of the citric acid cycle?

Answer 3

2 NADH & 2 Acetyl CoA

Question 4

what is the E1 complex? what does it do?

Answer 4

E1 Pyruvate dehydrogenase - Decarboxylation of pyruvate

(TPP reacts with pyruvate pops off carbon in form of CO2) → TPP-Acetyl group

Question 5

what is the E2 complex? what does it do?

Answer 5

E2 Dihydrolipoyl transacetylase - Transfer of CoA complex to Acetyl group

(TPP-Acetyl reacts with the Lipoate in disulphide form) → Oxidized lipoyllysine

(Lipoate has an Acetyl group now)

(Acetyl group transfered to CoA complex)

(Lipoate has two Thiol (-SH) groups now - two hydrides) → Reduced lipoyllysine

(Formation of Acetyl CoA) → used in Citric Acid Cycle

Question 5

what is the E3 complex? what does it do?

Answer 5

E3 Dihydrolipoyl dehydrogenase - Recycle of Lipoyllysine (FAD+ reduction then NAD+ reduction)

(Lipoate with two Thiol (-SH) groups has two hydrides) → Reduced lipoyllysine

it releases 2e- in the form of 2H-

(Lipoate returns to disulphide form (-S) groups) → Oxidized lipoyllysine

Which are taken by:

Initially FAD+ → FADH2

THEN NAD+ pickpockets them:-

2 NAD+ → 2 NADH - which then pass electrons to O2 in the electron transport chain)

Question 6

what is the pyruvate dehydrogenase complex?

Answer 6

a massive enzyme complex that contains tens of copies of each enzyme subunit, which are responsible for the conversion of Pyruvate to Acetyl CoA, to continue on in the Citric Acid Cycle

Question 7

what are the end products of glycolysis (using one glucose)?

Answer 7

2 NADH & 2 ATP
Glucose → 2 Pyruvate

Question 8

describe the kreb cycle (CIKSSFMO):

Answer 8

Oxaloacetate (OAA) + Acetyl CoA → Citrate
Catalyst - citrate synthase

[2] Citrate → Isocitrate
Catalyst - aconitase

[3] Isocitrate → α-ketoglutarate (2NADH GAIN) (2CO2 LOSS)
Catalyst - isocitrate dehydrogenase
Irreversible exergonic reaction

[4] α-ketoglutarate → Succinyl CoA (2NADH GAIN) (2CO2 LOSS)
Catalyst - α-ketoglutarate dehydrogenase
Irreversible exergonic reaction

[5] Succinyl CoA → Succinate (2ATP GAIN)
Catalyst - Succinyl-CoA synthetase

(GDP + Pi = GTP) then ADP steals Pi and turns into ATP (Substrate-level Phosphorylation)

[6] Succinate → Fumarate (2FADH2 GAIN)
Catalyst - succinate dehydrogenase

[7] Fumarate → Malate
Catalyst - fumarase

[8] Malate → Oxaloacetate (2NADH GAIN)
Catalyst - malate dehydrogenase

Question 9

what are the end products of the kreb cycle (using 2 Acetyl CoA)?

Answer 9

6 NADH & 2 FADH2 & 4 CO2 & 2 ATP

Question 10

Explain the regulation and control of Pyruvate dehydrogenase (PDH) in the Transition Stage.

Answer 10

PDH is the enzyme (E1) involved in the complex that converts 2 Pyruvate into 2 Acetyl CoA which is an irreversible reaction.

PDH Kinase is an enzyme that inhibits PDH E1 by putting a phosphate on it.

Regulation depending on the the needs of the cell:

(A) If the cell has enough energy

Inhibition of PDH through (Stimulating PDH Kinase inhibits PDH):

• ↑ NADH
• ↑ Acetyl CoA
• ↑ ATP

(B) If the cell needs energy

Stimulation of PDH through (inhibiting PDH Kinase stimulates PDH):

• ↑ Pyruvate
• ↑ ADP

Question 11

how do the control and regulation points during Krebs Cycle allow re-direction of cellular resources?

Answer 11

Blocking isocitrate dehydrogenase causes citrate build up (isocitrate and citrate are interconvertable), which moves citrate into the cytoplasm causing phosphofructokinase to stop glycolysis

When α-ketoglutarate dehydrogenase is inactive, α-ketoglutarate builds up, which switches its use to production of amino acids.

Question 12

Explain the point of control and regulation of α-ketoglutarate dehydrogenase during Krebs Cycle

Answer 12

(A) If the cell has enough energy

Inhibition or negative regulation of α-ketoglutarate D.H. through:

• ↑ NADH
• ↑ ATP
• ↑ Succinyl CoA

(B) If the cell needs energy

Stimulation or positive regulation of α-ketoglutarate D.H. through:

• ↑ Ca2+

Question 12

Explain the point of control and regulation of Isocitrate dehydrogenase in Krebs Cycle

Answer 12

Isocitrate dehydrogenase is an enzyme involved in the conversion of Isocitrate → α-ketoglutarate during Krebs Cycle.

(A) If the cell has enough energy

Inhibition or negative regulation of Isocitrate D.H. through:

• ↑ NADH
• ↑ ATP

(B) If the cell needs energy

Stimulation or positive regulation of Isocitrate D.H. through:

• ↑ ADP

Question 13

Citric acid cycle is an amphibolic pathway.

What does that mean?

Answer 13

it means that this pathway can serve both catabolic and anabolic processes.

Question 14

The Citric Acid Cycle can provide biosynthetic precursors through its own intermediates, but this can lead to the depletion of the intermediates.

Describe how can we can replenish oxaloacetate when the muscle cells require ATP:
e.g. of an anaplerotic reaction

Answer 14

Oxaloacetate (OAA) can be replenished through the gluconeogenesis pathway:

When Pyruvate → Oxaloacetate (OAA) in the mitochondria using pyruvate carboxylase.

Then it can be used in the Citric Acid Cycle:

Oxaloacetate (OAA) + Acetyl CoA → Citrate

Pyruvate carboxylase is only active when Acetyl CoA is present, so ↑ Acetyl CoA activates it, leading to the conversion of pyruvate into oxaloacetate to be used in the Citric Acid Cycle.

Question 15

what are anaplerotic reactions?

Answer 15

they are chemical reactions that form intermediates of a metabolic pathway.