Citric acid/Kreb's Cycle | Quizlet

Overall cell resp rxns (aerobic):

Stage1:
Glycolysis (gives pyruvate) ->

choose pyruvate fate (PDC- pyruvate dehydrogenase complex) to make acetyl-CoA ->

Stage 2:
Kreb's cycle ->

Stage 3:
ETC and then chemiosmosis

Which carbons of original glucose get fully oxidized after stage 1 of cellular respiration (glycolysis + making acetyl CoA)?

c 3 and 4

How many NADH are made from first stage of cell resp?

4 (2 from glycolysis, 2 from 2xPDC, one per pyruvate)

How many turns of citric acid cycle to oxidize all 4 carbons (2 per each of the 2 molecules of acetyl-CoA)?

2 turns

Net rxn of Kreb's cycle

Acetyl-CoA fa
+
3NAD+
+
FAD
+
GDP

--->

3NADH
+
FADH2
+
GTP
+
CO2 + CoASH

How many ATP per NADH and FADH2, and GTP?

2.5 ATP per NADH
1.5 ATP per FADH2
1 ATP per GTP

in KREBS CYCLE, 1 acetylCoA = ? ATP
1 glucose = 2 acetylCoA = ? ATP

1 AcetylCoa = 3(2.5) + 1(1.5) + 1 = 10 ATP

1 glucose = 20 ATP

Why cant Kreb's cycle happen without oxygen

Need NAD+ and FADH which cant be regenerated without the ETC functioning via O2

Steps of Kreb's cycle

CIKSSFMO
citrate is krebs specific substrate for making oxaloacetate

Steps 3 and 4, and 8 -> make NADH (oxidative decarboxylation, or dehydrogenation)
Step 5 -> make GTP
Step 6 -> make FAHD2 (forming double bond)

1) acetylCoa + oxaloacetate -> citrate
2) citrate -> isocitrate
3) isocitrate -> alpha-ketoglutarate
4) alpha-ketoglutarate -> succinyl-CoA
5) succinyl-CoA -> succinate
6) succinate -> fumarate
7) fumarate -> malate
8) malate -> oxaloacetate

where does CAC happen

mitochondria-

stay once pdc happens to change pyruvate -> acetylCoa in mitchondria

Step 1 of CAC: Acetyl-CoA + oxaloacetate --> citrate

Acetyl-CoA + oxaloacetate ---> 6 C intermediate, citrate

via citrate synthase

- C-C bond formation
- ACID BASE CATALYSIS w enol intermediate
- Methyl of acetyl-CoA is not a good nucleophile but it is once deprot'd by citrate synthase. AND, carbonyl of oxaloacetate is a good electrophile (so good attacking target)
- rate-limiting step of CAC

synthase vs synthetase

synthase = Not using energy
synthetase = use energy

Citrate synthase open and closed confs: "induced fit"

Open conf: can only bind oxaloacetate, then moves to closed

Closed conf: can then bind acetyl-Coa

Why induced fit in citrate synthase

- Avoids unnecessary hydrolysis of high energy thioester bond in acetyl-CoA until you know the other substrate (oxaloacetate) is ready

Step 2 of CAC: citrate --> cis-Aconitate --> isocitrate

- Need to move hydroxyl one carbon down (so you can go from tertiary to secondary alcohol) make so go through an intermediate, cis-aconitate

- first give (h2o) then take (h2o)



citrate --aconitase RELEASE of water to make double bond -> cis-aconitate (double bond) --aconitase INTAKE of water to make new hydroxyl --> isocitrate

why change citrate to isocitrate

teritary alcohol (citrate) -> secondary (isocitrate) to make it more reacitve

Step 3 of CAC: Isocitrate --> alpha-ketoglutarate

isocitrate --isocitratedehydrogenase--> a-ketoglutarate

- oxidative decarboxylation (the middle COO- leaves as CO2)
- makes NAD -> NADH


1) isocitrate -> oxalosuccinate intermediate with double bond =O in place of -OH
(NAD -> NADH)

2) COO leaves, replaced by H (CO2 released)

Step 4 of CAC: a-ketoglutarate -> succinyl-CoA

COO of C attached to =O leaves as CO2, replaced by -S-CoA

- oxidative decarboxylation (last COO- leaves as CO2)
- makes NAD -> NADH


a-ketoglutarate --a-ketoglutarate dehydrogenase complex--> succinyl-CoA

same as PDC involving TPP, Lipoate, FAD + CoaSH, NAD+

Step 5 of CAC: succinyl-CoA -> succinate

- makes symmetric molecule

succinyl-CoA --succinyl-Coa synthetase --> succinate

- makes GTP (replacing breaking one high energy bond, the thioester, with making another high energy bond, GDP+P-> GTP)

- ex. of substrate level phosphorylation

Step 6 of CAC: succinate -> fumarate

- does alkane->alkene by making double bond across middle carbons
- makes FADH2

succinate --succinate dehydrogenase--> fumarate

FAD gets reduced, succinate gets oxidized
(but why? normally, fumarate reduced and FADH2 oxidized...)

Why is FAD reduced and succinate oxidized in step 6?

FAD is a prosthetic group, covalently bound to succinate dehydrogenase enzyme--> this gives it diff reduction potential properties than if it were free floating FAD

What is the only enzyme in CAC that is located in inner mitochondrial membrane (all others are in matrix)?

succinate dehydrogenase

Step 7 of CAC: fumarate -> L-malate

hydration steps- stay hydrated 7 days of the week

fumarate --fumarase--> L-malate


fumarate -> add OH to get to carbanion -> add H to get to L-malate

One of the H's on CH2 changes to an OH, and it is on the left side of the C so "L" Malate

Step 8 of CAC: L-malate -> oxaloacetate

- just removes an H and changes -OH to =O

L-malate --L-malate dehydrogenase --> oxaloacetate

- makes NAD -> NADH


- unfavorable but kept forward rxn since oxaloacetate gets immediately consumed by citrate synthase

why is succinyl-Coa an important inhibitor?

it will inhibit its formation of alpha-ketoglutarate -> succinylCoA

and a-ketoglutarate will instead go to form amino acids via glutamate

how does citrate inhibit glycolysis AND CAC?

inhibits its own formation (1st step of CAC) + last step of glycolysis, PFK

how does ATP levels regulate PDC?

High atp = active PDC kinase = PDC phosphprylated/inactivated = less acetyl-CoA made

Anaplerotic reactions

produce oxaloacetate and malate to maintain constant levels of citric acid cycle intermediates

Malic enzyme rxn

Malate --malic enzyme, NADP to NADPH--> pyruvate

Beta oxidation of fatty acids to make acetyl-CoA

1) activate fatty acids, turn -COO to -C=O(-S-CoA)
uses 2 ATP

2) oxidize with three reactants, FAD, H2O, and NAD
- First FAD->FADH2 to make double bond bw alpha and beta C
- Then H2O input to insert OH on B side
- Then Turn the OH to =O with NAD -> NADH

3) break C-C bond of alpha and beta C (+ add CoASH). this should remove a 2C acetylCoA and leave behind the rest of the molecule with another -S-CoA attachment for continuing with step 2 and 3 again)

How many cycles of beta-oxidation "choppings" for a 16C fatty acid?

7

Need 8x 2C molecules acetyl-CoA

7 choppings, with the last one leading to 2 2C molecules