10.1-10.2 Step Zero of the Citric Acid Cycle

Question 1

What are two other names for the citric acid cycle?

Answer 1

Kreb’s or TCA cycle

Question 2

Where does it take place?

Answer 2

Mitochondria

Question 3

How much ATP is NADH worth?

Answer 3

2.5 ATP

Question 4

How much ATP is FADH2 worth?

Answer 4

1.5 ATP

Question 5

How much ATP is GTP worth?

Answer 5

1 ATP

Question 6

When EC is low?

Answer 6

Pyruvate is metabolized by pyruvate dehydrogenase which forms acetyl-CoA and CO2

Question 7

How is pyruvate transported in the mitochondria?

Answer 7

Transmembrane transporter protein

Question 8

What does pyruvate dehydrogenase do?

Answer 8

Converts pyruvate to Acetyl CoA

Question 9

What 5 coenzymes are needed?

Answer 9

NAD+/NADH
FAD/FADH2
CoA/CoAsh
TPP
a-lipoleic acid

Question 10

NAD full name

Answer 10

nicotinamide adenine dinucleotide

Question 11

FAD full name

Answer 11

Flavin adenine dinucleotide

Question 12

CoA name

Answer 12

Coenzyme A

Question 13

TPP name

Answer 13

Thiamine pyrophosphate

Question 14

a-lipoleic acid name

Answer 14

Lipoamide

Question 15

NAD characteristics

Answer 15

Derived from niacin (B3), involved in 200+ reactions, Transfers 2 electrons at once as hydride ion

Question 16

Pellagra

Answer 16

niacin deficiency, rash/diarrhea/neurologic, fish veggies and poultry

Question 17

FAD characteristics

Answer 17

derived from riboflavin (B2), flavoproteins, a precursor to FAD and FMN.
Reduced to FADH2 by transfer of two electrons; one electron transported at a time; semiquinone intermediate

Question 18

Cheilosis

Answer 18

Riboflavin deficiency, swelled or cracked lips, almonds/meat/dairy, destroyed by light

Question 19

CoA characteristics

Answer 19

derived from pantothenic acid (B5), required for energy conversion in citrate cycle, cofactor in production of fatty aces, acetylcholine, heme, cholesterol.
Function - carry acetate in form of CoA

Question 20

Pantothenic acid deficiency

Answer 20

rare, found in most foods (chicken, yogurt, avocados)

Question 21

How do you get CoA across the membrane?

Answer 21

Nucleotide can’t cross; CoA degraded by enzymes into pantothenic acid, travels through circulation, crosses cell membrane, converted back to CoA with ATP and cysteine additon

Question 22

TPP characteristics

Answer 22

derived from thiamin (B1), absorbed in gut and phosphorylated, pulls acetyl group off pyruvate

Question 23

Beriberi

Answer 23

thiamin deficiency; anorexia/cardio problems/neurologic; brown rice and fortified grains - milling white rice removes bran and thiamin; raw fish and silkworms contain thiaminase which degrades thiamine

Question 24

ɑ-lipoic acid characteristics

Answer 24

Synthesized in plants & animals
– Also in broccoli, liver, spinach & tomato
* Provides reactive disulphide for redox rxns
* Receives acetyl from TPP and transfers it to CoA
The ball in the mechanism

Question 25

Citrate Cycle Reaction 0

Answer 25

Substrates: pyruvate, NAD+, CoA
Product: acetyl-CoA, NADH, COs, H+
Enzyme: Pyruvate dehydrogenase complex (E1, E2, E3)
Irreversible

Question 26

Reaction Zero for citric acid cycle

Answer 26

Pyruvate +CoA + NAD+ –> Acetyl CoA + CO2 +NADH +H+

Question 27

Steps 1-3 of pyruvate dehydrogenase

Answer 27

1. E1 binds pyruvate, catalyzes decarboxylation → TPP intermediate & release CO2
2. TPP intermediate reacts with the disulfide of lipoamide group on E2 → lipoamide intermediate
3. E2 lipoamide intermediate carries acetyl group from E1 catalytic site to E2 catalytic site, reacts with CoA → acetyl-CoA & fully reduced dihydrolipoamide

Question 28

Steps 4-5 of pyruvate dehydrogenase

Answer 28

4. Dihydrolipoamide group swings to E3, it’s reoxidized to a disulphide. A coupled rxn forms a dithiol on E3 then transiently forms E3-FADH2
5. The E3-FADH2 coenzyme intermediate is reoxidized in a coupled redox reaction producing NADH + H+

Question 29

Ball and chain mechanism

Answer 29

E2 lipoamide picks up an acetyl group from E1 TPP

Question 30

E2 can swing to bridge the gap

Answer 30

E2 ball and chain delivers acyl group to CoA in E2 catalytic domain

Question 31

Oxidation

Answer 31

loss of electrons

Question 32

Reduction

Answer 32

gain of electrons

Question 33

Oxidant

Answer 33

compounds that oxidize other molecules
– Accept electrons
– Are reduced

Question 34

Reductant

Answer 34

compounds that reduce other molecules
– Donate electrons
– Are oxidized

Question 35

Reduction Potential (E)

Answer 35

Tendency for a molecule to gain electrons, driven by free energy (inverse relationship); favorable +E and -G

Question 36

Using half-reactions to get ΔE

Answer 36

• Half-reactions can be used to determine change in reduction potential