Krebs Cycle, Oxidative Phosphorylation, and ATP Synthesis: Study Guide

Krebs Cycle (Citrate Cycle)

Overview and Metabolic Significance

The citrate cycle (also known as the Krebs cycle or tricarboxylic acid cycle) is a central metabolic pathway that connects carbohydrate, fat, and protein metabolism. It is considered the hub of metabolism for several reasons:

• Integration Point: It links catabolic and anabolic pathways, processing acetyl-CoA from carbohydrates, fats, and proteins.

• Energy Production: Generates high-energy electron carriers (NADH, FADH2) and GTP/ATP.

• Precursor Supply: Provides intermediates for biosynthetic pathways (e.g., amino acids, nucleotides).

Cycle Accomplishments per Turn

• Oxidizes acetyl-CoA to CO2

• Produces 3 NADH, 1 FADH2, 1 GTP (or ATP), and 2 CO2

• Regenerates oxaloacetate for continued cycling

Metabolic Engine Analogy

• Fuel: Acetyl-CoA

• Exhaust: CO2

• Output: NADH, FADH2, GTP/ATP

Redox Reactions and Free Energy Calculations

Redox reactions are fundamental to energy conversion in biochemistry. The free energy change ($\Delta G^\circ'$) can be calculated using reduction potentials:

• Equation:

$\Delta G^\circ' = -nF\Delta E^\circ'$

• Where n is the number of electrons transferred, F is Faraday's constant, and $\Delta E^\circ'$ is the difference in reduction potentials.

Vitamin Cofactors in Pyruvate Dehydrogenase (PDH) Reaction

• Niacin (Vitamin B3): Required for NAD+ formation (step: electron transfer).

• Riboflavin (Vitamin B2): Required for FAD formation (step: electron transfer).

• Pantothenic Acid (Vitamin B5): Component of Coenzyme A (step: acetyl group transfer).

• Thiamine (Vitamin B1): Required for thiamine pyrophosphate (step: decarboxylation).

Disorders and Inhibition

• Pellagra: Caused by niacin deficiency, impairs NAD+ production, affecting PDH and citrate cycle.

• Arsenic Inhibition: Arsenic binds to lipoic acid, inhibiting PDH and citrate cycle enzymes.

Eight Reactions of the Citrate Cycle

• Each reaction is classified as exergonic or endergonic.

• Key steps produce NADH, FADH2, CO2, and GTP.

Net Reaction:

$\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{GDP} + \text{Pi} + 2\text{H}_2\text{O} \rightarrow 2\text{CO}_2 + 3\text{NADH} + 3\text{H}^+ + \text{FADH}_2 + \text{GTP} + \text{CoA}$

Regulation and Anaplerotic Reactions

Metabolic Flux Regulation

• AMP, ADP: Activate the cycle (signal low energy).

• ATP, NADH: Inhibit the cycle (signal high energy).

• Metabolic Logic: The cycle is upregulated when energy is needed and downregulated when energy is abundant.

Anaplerotic Reactions

• Pyruvate Carboxylase: Converts pyruvate to oxaloacetate, replenishing cycle intermediates.

• Other Anaplerotic Reactions: Include amino acid degradation pathways that supply cycle intermediates.

Oxidative Phosphorylation and Chemiosmotic Theory

Proton Motive Force and Chemiosmotic Theory

The chemiosmotic theory explains how ATP is synthesized using the energy from a proton gradient across the mitochondrial membrane.

• Proton Motive Force: Generated by electron transport chain (ETC) activity.

• Difference in Mitochondria vs. Chloroplasts: Both use proton gradients, but the direction and location differ.

Racker and Stoeckenius Experiment

• Demonstrated ATP synthesis driven by artificial proton gradients, validating chemiosmotic theory.

Electron Transport System (ETS) Enzymes

• Complex I: NADH dehydrogenase

• Complex II: Succinate dehydrogenase

• Complex III: Cytochrome bc1 complex

• Complex IV: Cytochrome c oxidase

Electron donors and acceptors are arranged vectorially along the pathway, facilitating efficient electron transfer.

Energy Calculations for Proton Transport

Energy required for proton transport is related to reduction potentials:

$\Delta G^\circ' = -nF\Delta E^\circ'$

NADH and oxygen have large differences in reduction potential, driving ATP synthesis.

ATP Synthase and ATP Synthesis

Structure and Organization of ATP Synthase

• F0 Subunit: Membrane-embedded, forms proton channel.

• F1 Subunit: Catalytic, synthesizes ATP.

Mechanism of ATP Synthesis

• Proton flow through F0 causes rotation, inducing conformational changes in F1 that drive ATP synthesis.

Binding Change Mechanism

• Three basic principles: substrate binding, conformational change, product release.

• Experimental evidence: rotational forces observed in single-molecule studies.

Transport of ADP, ATP, and Pi

• Specific transporters move ADP, ATP, and Pi across the inner mitochondrial membrane.

Electron Transfer Mechanisms and ATP Yield

• Liver: Malate-aspartate shuttle transfers electrons from NADH.

• Muscle: Glycerol-3-phosphate shuttle transfers electrons.

• ATP Yield from Glucose: Varies by tissue due to shuttle differences.

Regulation and Inhibitors of Electron Transport and ATP Synthesis

Activators and Inhibitors

• Activators: ADP, Pi, NADH

• Inhibitors: ATP, NAD+, certain drugs and toxins

2,4-Dinitrophenol and Thermogenin

• 2,4-Dinitrophenol: Uncouples electron transport from ATP synthesis, dissipating proton gradient as heat.

• Thermogenin: Protein in brown fat, allows adaptive heat production in animals.

Mitochondrial Disorders and Maternal Inheritance

• Mitochondrial DNA is inherited exclusively from the mother, explaining maternal transmission of mitochondrial disorders.