ATP Production Flashcards
Anaerobic Fermentation
pyruvate + NADH → lactate + NAD+
3 Stages of converting glucose to ATP
Stage 1: The fuel molecules are oxidized to two-carbon fragments in the form of the acetyl group of acetyl~CoA (the ~ bond indicates that this is a high-energy bond, meaning that the hydrolysis of the bond releases a great deal of energy can be used to drive a chemical reaction).
Stage 2: The acetyl groups enter the tricarboxylic acid (TCA) cycle which produces CO2 and the reduced energy carriers NADH and FADH2
Stage 3: Stage 3: The NADH and FADH2 are oxidized in the electron transport chain (ETC), producing H+ and electrons, and then the electrons are transferred to O2,
producing water. The protons are used in a process known as oxidative phosphorylation (OxPhos) to produce ATP
Coenzyme A (CoA)
Coenzyme A (CoA) is a coenzyme that is derived from ATP and the B vitamin pantothenic acid (B5). It contains a reactive –SH (thiol) group that is covalently linked to the acetyl group of acetyl~CoA via a thioester bond. Thioester bonds are relatively high-energy bonds, and so acetyl~CoA can readily donate acetyl groups to other acceptors.
Production of Acetyl~CoA from Pyruvate
Pyruvate is formed in the cytoplasm by glycolysis, and then is transported into the mitochondrial matrix, where it is converted to acetyl~CoA by the pyruvate dehydrogenase (PDH) complex with the release of CO2.
3 types of α-ketoglutarates
Pyruvate Dehydrogenase
Branched chain α-ketoacid dehydrogenase
α-ketoglutarate dehydrogenase
The PDH complex contains three enzymes present in multiple copies:
pyruvate dehydrogenase (aka pyruvate decarboxylase) (E1) dihydrolipoyl transacetylase (E2) dihydrolipoyl dehydrogenase (E3)
Which subunits are identical between members of the α-ketoacid dehydrogenase family?
The E3 subunits are identical between members of the α-ketoacid dehydrogenase family.
What coenzymes/cosubstrates/prosthetic groups are associated with the PDH Complex?
In addition to the E1, E2, and E3 subunits, the PDH complex contains two stoichiometric coenzymes/cosubstrates (NAD, CoA) and three catalytic coenzyme prosthetic groups (thiamine pyrophosphate (TPP), FAD, and lipoic acid).
Overall reaction scheme of oxidative decarboxylation of pyruvate by the PDH complex
- Pyruvate is decarboxylated to form a hydroxyethyl derivative bound to the reactive carbon of thiamine pyrophosphate, the coenzyme of pyruvate decarboxylase (E1)
- The hydroxyethyl intermediate is oxidized by transfer to the disulfide form of lipoic acid covalently bound to dihydrolipoyl transacetylase (E2)
- The acetyl group, bound as a thioester to the side chain of lipoic acid, is transferred to CoA
- The sulfhydryl form of lipoic acid is oxidized by FAD-dependent dihydrolipoyl dehydrogenase (E3) leading to the regeneration of oxidized lipoic acid.
- FADH2 on E2 is reoxidized to FAD as NAD+ is reduced to NADH + H+
Function of long lysine side chain of dihydrolipoyl transacetylase
The long lysine side chain of dihydrolipoyl transacetylase (E2) serves as a swinging arm to transfer electrons and the acetyl group from pyruvate dehydrogenase (E1) to dihydrolipoyl dehydrogenase (E3):
The swinging arm of E2 keeps the acetyl group and the electrons from pyruvate formed by E1 in close proximity to E3. This means that none of the intermediates can diffuse away from the complex during the reaction, markedly enhancing the efficiency of the whole process. This containment of the intermediates within the complex is an example of what is known as substrate channeling.
Niacin and thiamine deficiency
Niacin and thiamine deficiency can cause severe central nervous system deficiency. Niacin is one of the precursors to NAD+/NADH, while thiamine is
the precursor to TPP, the coenzyme for E1. A lack of either NAD+/NADH or thiamine will reduce/block the activity of of the PDH complex, resulting in the eventual reduction in ATP synthesis. The CNS is heavily dependent on ATP to support its high level of activity, and so a reduction in ATP synthesis means reduced CNS function.
Arsenic
Arsenic in the form of trivalent arsenite forms a stable complex with the thiol groups of lipoic acid, rendering it unable to serve as a coenzyme for PDH.
Regulation of the PDH Complex: activation of E1
E1 is inactivated by phosphorylation by PDH kinase, and E1 is activated by dephosphorylation by PDH phosphatase.
Positive Allosteric Effectors of PDH Kinase
ATP, Acetyl CoA and NADH
Negative Allosteric Effectors of PDH Kinase
Pyruvate
Positive Allosteric Effectors of PDH Phosphatase
Ca2+
Negative Feedback Inhibition of PDH Complex
NADH and Acetyl CoA
Step 1 of TCA Cycle: Formation of citrate
The condensation of acetyl~CoA and oxaloacetate (OAA) is catalyzed by citrate synthase. The reaction is made irreversible by the hydrolysis of the thioester bond if
acetyl~CoA and facilitated by an enzyme-bound intermediate, citroyl~CoA. The CoA liberated in this reaction can then be used by the PDH complex for oxidative decarboxylation of another pyruvate.
Step 2 of TCA Cycle: Isomerization of citrate to isocitrate
Citrate is isomerized to isocitrate by the enzyme aconitase via a cis-aconitate intermediate. The equilibrium of the reaction is actually towards the left (formation of citrate (note the (+)ve ∆Go), but since isocitrate is rapidly consumed in the net step, the reaction is pulled to the right in vivo.
Step 3: Oxidation of isocitrate to α-ketoglutarate and CO2
Isocitrate dehydrogenase catalyzes the irreversible decarboxylation of isocitrate, along with the generation of NADH. Due to the loss of CO2, this is an irreversible reaction
Step 4: Oxidation of α-ketoglutarate to Succinyl~CoA and CO2
This reaction is catalyzed by the α-ketoglutarate dehydrogenase complex, which is very similar to the PDH complex and uses the same coenzymes/cosubstrates (NAD, CoA) and catalytic coenzyme prosthetic groups (thiamine pyrophosphate (TPP), FAD, and lipoic acid) as PDH.
This reaction is irreversible and is another site of generation of NADH. Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are responsible most of the CO2 produced and exhaled.