The Citric Acid Cycle (Krebs Cycle)

Overview and Significance

The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway that completes the oxidation of glucose derivatives, generating high-energy electron carriers and ATP. It occurs in the mitochondrial matrix of eukaryotic cells and is essential for cellular respiration.

• Purpose: Oxidizes acetyl CoA to CO2, producing NADH, FADH2, and ATP/GTP.

• Location: Mitochondrial matrix in eukaryotes; cytoplasm in prokaryotes.

• Key Intermediates: Citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

Entry into the Cycle: Acetyl CoA

Acetyl CoA is the molecule that enters the citric acid cycle, formed from pyruvate after glycolysis.

• Structure: Contains an acetyl group linked to coenzyme A.

• Function: Donates the acetyl group to oxaloacetate to form citrate.

Key Steps and Intermediates

The cycle consists of eight main steps, each catalyzed by a specific enzyme.

• Step 1: Acetyl CoA combines with oxaloacetate to form citrate.

• Step 2-8: Citrate is converted through a series of intermediates back to oxaloacetate, releasing CO2 and capturing energy in NADH, FADH2, and ATP/GTP.

• Carboxylic Acids: All intermediates are carboxylic acids; adding any one to cells increases respiration rate.

Products per Cycle

For each molecule of pyruvate:

• 2 CO2

• 3 NADH

• 1 FADH2

• 1 ATP or GTP

For each molecule of glucose (2 pyruvate):

• 4 CO2

• 6 NADH

• 2 FADH2

• 2 ATP or GTP

Enzyme Types in the Citric Acid Cycle

Most enzymes in the citric acid cycle are dehydrogenases, which catalyze oxidation-reduction reactions, transferring electrons to NAD+ or FAD.

• Dehydrogenases: Remove hydrogen atoms and electrons from substrates.

• Isomerases: Rearrange molecular structures.

• Lyases: Remove or add groups to molecules without hydrolysis.

Regulation of the Citric Acid Cycle

The cycle is tightly regulated to match cellular energy needs.

• Allosteric Regulation: ATP inhibits key steps, preventing excess energy production.

• Competitive Regulation: NADH inhibits certain enzymes, ensuring balance between NAD+ and NADH.

Examples of Intermediates

• Citrate: First product formed from acetyl CoA and oxaloacetate.

• Alpha-ketoglutarate: Intermediate that can be labeled with isotopic acetyl groups.

• Fumarate: Contains a double bond; involved in later steps of the cycle.

Electron Transport Chain (ETC)

Overview and Function

The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane. It receives electrons from NADH and FADH2 and transfers them to oxygen, the terminal electron acceptor, generating a proton gradient.

• Location: Inner mitochondrial membrane (cristae).

• Function: Transfers electrons, pumps protons, and produces ATP.

Mechanism of Electron Transport

• Electrons from NADH and FADH2 are passed through complexes I-IV.

• Each complex has a higher redox potential than the previous, ensuring directional electron flow.

• Energy released is used to pump H+ ions from the matrix to the intermembrane space.

Proton Gradient and ATP Synthesis

The proton gradient across the inner membrane creates a proton-motive force that drives ATP synthesis via the enzyme ATP synthase.

• ATP Synthase: Large protein complex that synthesizes ATP as protons flow back into the matrix.

• Oxidative Phosphorylation: Process of ATP generation using energy from the proton gradient.

• Equation:

Regulation and Inhibition

• ATP synthesis halts if the inner membrane is disrupted, as the proton gradient collapses.

• Mutations in ATP synthase can prevent ATP production.

Summary of Cellular Respiration

ATP Yield and Variability

• Maximum observed ATP yield per glucose: 29 ATP.

• Yield depends on electron donor and terminal electron acceptor.

• Each NADH can generate up to 3 ATP; each FADH2 up to 2 ATP.

Alternative Substrates and Acceptors

• Glucose is not the only substrate; other molecules can enter the cycle.

• Oxygen is not the only terminal electron acceptor; alternatives exist in some organisms.

Table: Citric Acid Cycle Intermediates and Products

Table: Electron Transport Chain Complexes

Additional info: The notes have been expanded to include definitions, context, and tables for clarity. Images included are directly relevant to the explanation of each paragraph and reinforce the educational content.