Mitochondrial Bioenergetics Flashcards

Mitochondrial Bioenergetics

Overview

• Mitochondria play a unique role in generating energy under aerobic conditions.
• Key processes include glycolysis, the citric acid cycle (TCA/CAC/Kreb’s Cycle), and oxidative phosphorylation.

Pyruvate Dehydrogenase Complex (PDC)

• Links pyruvate production in the cytoplasm with the TCA cycle in the mitochondrial matrix.
• Large complex with multiple copies of 3 different subunits (E1, E2, E3).
• Requires 5 cofactors:
  • Thiamine pyrophosphate (TPP), lipoamide, and FAD (prosthetic groups).
  • Coenzyme A and NAD+ (co-substrates).

PDC Reaction Mechanism

• E1: Pyruvate dehydrogenase
• E2: Dihydrolipoyl transacetylase
• E3: Dihydrolipoamide dehydrogenase
• Overall reaction: Pyruvate + CoA + NAD+ → Acetyl CoA + CO2 + NADH
• The net reaction has a large, negative \Delta Gº´ (-33 kJ/mol), making it highly favorable/spontaneous.

Summary of PDC

• A soluble complex of 3 enzymatic subunits and 5 cofactors in the mitochondrial matrix.
• Pyruvate is oxidized and activated to form Acetyl CoA.
• Redox decarboxylation reaction, producing CO2 and NADH.
• Large, negative standard free energy change makes it favorable and irreversible.
• Regulation is crucial for control.

Regulation of PDC

• Inhibited by Acetyl CoA and NADH.
• Activated by Pyruvate, ADP, and Calcium.

Covalent Modification

• PD Kinase phosphorylates PDC, inactivating it.
• PD Phosphatase dephosphorylates PDC, activating it.
• PD kinase is stimulated by Acetyl CoA and NADH and inhibited by pyruvate, ADP and Calcium.
• PD phosphatase is stimulated by Ca2+.

Allosteric Control

• Feedback inhibition by Acetyl CoA and NADH on E2 & E3.
• Allosteric control of PD kinase by acetyl CoA, NADH, pyruvate and ADP.
• Ca2+ stimulates PD phosphatase (& PDC activity).

Citric Acid Cycle (TCA/CAC/Krebs Cycle)

• Links the breakdown of fuel molecules to ATP production in oxidative phosphorylation.

Overview of TCA Cycle

• Incoming acetyl group joins with oxaloacetate for 2 oxidative decarboxylation reactions that release CO2 and 2 NADH.
• Regeneration of oxaloacetate producing 1 FADH2, 1 NADH and 1 GTP.
• The GTP is made by substrate-level phosphorylation and can be used as energy.
• Two water molecules are required by reactions in the cycle.
• 4 pairs of electrons are used to reduce and form 3 NADH and 1 FADH2 for oxidative phosphorylation.
• Succinate dehydrogenase is a membrane-bound protein linking the citric acid cycle to oxidative phosphorylation.
• Succinate Dehydrogenase (Complex II): SDH-FADH2 (Complex II) + Q SDH-FAD + QH2. It's present in both TCA cycle and Electron Transport Chain.

Important Points within the cycle

1. What is the net reaction for 1 round of the cycle?
2. Where do the CO2 carbons originate?
3. What intermediates can be made from an amino acid?
4. How can each enzyme be classified?

Thermodynamics

• Pathways may couple unfavorable and favorable reactions to drive product formation.
• Favorable reactions are the steps that are regulated.

Regulation

• The cycle is the hub of mitochondrial oxidation and uses acetyl CoA supplied by the breakdown of glucose, fatty acids, and amino acids.
• The overall pathway is favorable despite the positive standard free energy of Malate Dehydrogenase (MDH).
• Regulation occurs at favorable reactions (negative \Delta G) to control the cycle.
• The TCA is considered an amphibolic pathway linking anabolic and catabolic pathways via Acetyl CoA and CAC intermediates.
• Low energy,Calcium and upstream substrates stimulate the cycle.
• High energy, reduced coenzymes or products inhibit the cycle.

Key Messages

• Pyruvate from carbohydrate breakdown is oxidized to Acetyl CoA by PDC.
• The CAC uses acetyl CoA from glucose, sugars, fatty acids, and amino acids to produce CO2, reduced coenzymes, and GTP.
• NADH and FADH2 made in the mitochondria serve as electron donors for electron transport.
• The energy requirement and availability of molecules used to make energy in the mitochondria can regulate the PDC and CAC.
• Allosteric and/or covalent modification of the PDC and CAC regulate mitochondrial bioenergetics.