Lecture #10: citric acid cycle and ATP synthesis

CAC Step 1

Acetyl CoA condenses with oxaloacetate to form citrate
Enzyme: citrate synthase

CAC step 6

Succinate is oxidized to fumarate, producing FADH2
Enzyme: Succinate dehydrogenase

The citric acid cycle produces

1 ATP, 3 NADH, 1 FADH2 per turn

Citric Acid Cycle

consists of nine sequential reactions that oxidize acetyl CoA to CO2, capturing high energy electrons in NADH and FADH2

There are __ oxidation-reduction reactions in the citric acid cycle

4

CAC step 5

Succinyl CoA is converted to succinate, generates GTP

Net Equation from Pyruvate through the Citric Acid Cycle

Pyruvate + 2H2O + 4 NAD+ + FAD + GDP + Pi --> 3CO2 + 4NADH + 1 FADH2 + 4H+ + GTP + HS-CoA

CAC Net

4 NADH, 1 FADH2, 1 GTP/ATP for each molecule of pyruvate

x2 for each molecule of glucose

CAC yields

purpose is not to yield large quantities of ATP
produces high energy carrier molecules

Malate-asparate shuttle (MAS)

requires no additional energy cost
cytosolic NADH from glycolysis enters respiratory chain as NADH in complex I

Glycerol-3-phosphate shuttle (GPS)

requires an energy cost
NADH from glycolysis enters the respiratory chain as FADH2 in complex II

3 ATPs per pair

from NADH generated in the mitochondria

2 ATPs per pair

FADH2 generated in the mitochondria

2 ATPs per pair

shuttled from cytoplasmic NADH to mitochondrial using the GPS

3 ATPs per pair

shuttled from cytoplasmic NADH to mitochondrial using the MAS

Fatty acids

converted to acetyl CoA by a series of four enzyme-catalyzed reactions in mitochondria

Stages of Aerobic Oxidation

1. Glycolysis in cytosol, creating pyruvate
2. Pyruvate converted to Acetyl CoA which then enters the Citric Acid cycle, generating NADH and FADH2
3. The ETC in the inner mitochondrial membrane transfers electrons from NADH and FADH2, creating a proton gradient
4. Oxidative phosphorylation utilizes the proton-motive force to synthesize ATP via ATP synthase

Fatty acyl CoA undergoes beta oxidation in mitochondria

shorted by two carbon atoms, forming acetyl CoA with each turn of cycle

Strong reducing agent pairs with

weak oxidizing agent

weak reducing agent pairs with

strong oxidizing agent

oxidizing agents

stronger electron affinities than reducing agents

Weak reducing agent

stronger electron affinity than strong reducing agent

ETC complex I

NADH dehydrogenase complex, transfers electrons to ubiquinone

ETC complex II

succinate dehydrogenase, generates FADH2
transfers electrons to ubiquinone

ETC complex III

Cytochrome Bc1, transfers electrons to cytochrome c

ETC complex IV

cytochrome c oxidase, reduces O2 to H2O

Ubiquinone (coenzyme Q)

lipid-soluble molecule
can accept and donate two electrons and protons

ubiquinone

fully oxidized

ubisemiquinone

partially reduced free radical of ubiquinone
intermediate carrier in ETC

ubiquinol

fully reduced form of ubiquinone

ETC carrier order

increasing redox potential from strongly reducing --> more oxidizing agent
NADH strong reducing agent
Last electron acceptor is O2

cytochrome c

A peripheral protein used for electron transport between complex III and IV

substrate-level phosphorylation

direct energy input into ATP synthesis by a transfer of a high energy phosphate bond to ADP to make ATP

Oxidative phosphorylation

indirect energy input into ATP synthesis. Direct energy input into rotational catalysis but no transfer of a high energy phosphate bond

chemiosmotic hypothesis

the proton-motive force across the inner mitochondrial membrane is the immediate source of energy for ATP synthesis

movement of protons through ATP synthase

alters the binding affinity of the active site

Spinning of gamma subunit

causes change in conformation of active sits on subunits of F1

The Role of the F0 Portion of ATP Synthase

c subunits of the F0 base form a ring
Protons moving through the membrane rotate the ring
rotation of the ring provides twisting force that drives ATP synthesis