Mitochondrial Structure and Cellular Respiration: Pyruvate Oxidation, Citric Acid Cycle, and Fatty Acid Catabolism

Mitochondrial Structure and Function

Overview of the Mitochondrion

The mitochondrion is a double-membraned organelle found in nearly all eukaryotic cells, including plants. Its structure is closely linked to its role in energy production through aerobic respiration.

• Number of mitochondria: Varies by cell type and energy demand (from 1 to thousands per cell; muscle cells have many).

• Location: Often found near regions of high energy demand, such as flagella and cilia.

Mitochondrial Membranes

• Outer Membrane (OM): Contains porins—transmembrane proteins that allow passage of molecules with molecular weight < 5000 Da. It is relatively non-selective.

• Inner Membrane (IM): Highly selective, contains cristae (folds) that increase surface area (up to 5x greater than OM). Rich in proteins (~75% by weight).

Mitochondrial Compartments

• Intermembrane Space: Located between OM and IM; nearly continuous with cytosol.

• Mitochondrial Matrix: Enclosed by IM; contains enzymes, ribosomes, and circular mitochondrial DNA (15,000–20,000 bp). Encodes rRNAs, tRNAs, and some IM protein polypeptides.

• Intracristal Spaces: Discrete regions within cristae; important for proton accumulation during electron transport.

Additional info: Mutations in mitochondrial DNA are linked to diseases, aging, and neurodegeneration.

Import of Matrix Proteins

Protein Targeting and Translocation

Most mitochondrial proteins are encoded by nuclear DNA, synthesized in the cytosol, and imported into the mitochondrion.

• Chaperone Proteins: Cytosolic proteins (e.g., Hsp70) keep precursor proteins unfolded for import.

• Transit Sequence: N-terminal signal sequence directs proteins to mitochondria.

• Translocases: TOM (Translocase of Outer Membrane) and TIM (Translocase of Inner Membrane) complexes mediate import.

• ATP Hydrolysis: Required for translocation and release of chaperones.

• Transit Peptidase: Cleaves the transit sequence once inside the matrix.

Additional info: Proper folding of imported proteins is assisted by mitochondrial chaperones (e.g., Hsp60).

Stages of Cellular Respiration

Overview

Aerobic respiration is the process by which cells completely oxidize glucose to generate ATP, using oxygen as the final electron acceptor. It is more efficient than fermentation.

• Coenzymes involved: NADH, FADH2, Coenzyme Q

Five Stages of Respiration

1. Glycolysis: Occurs in the cytosol; glucose is converted to pyruvate.

2. Pyruvate Oxidation: Pyruvate is transported into the mitochondrion and converted to acetyl CoA.

3. Citric Acid Cycle (TCA/Krebs Cycle): Acetyl CoA is oxidized, generating NADH, FADH2, and ATP/GTP.

4. Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through protein complexes, pumping protons into the intermembrane space.

5. Oxidative Phosphorylation: ATP is synthesized as protons flow back into the matrix via ATP synthase.

Additional info: In bacteria, the plasma membrane serves the role of the mitochondrial inner membrane.

Pyruvate Oxidation

Conversion of Pyruvate to Acetyl CoA

Pyruvate produced by glycolysis is transported into the mitochondrial matrix, where it undergoes oxidative decarboxylation.

• Enzyme: Pyruvate Dehydrogenase Complex (PDH)

• Reaction: Pyruvate + CoA + NAD+ → Acetyl CoA + CO2 + NADH

• Thioester Bond: Acetyl CoA contains a high-energy thioester bond, making it an activated molecule.

Equation:

$\text{Pyruvate} + \text{CoA} + \text{NAD}^+ \rightarrow \text{Acetyl CoA} + \text{CO}_2 + \text{NADH}$

Citric Acid Cycle (TCA/Krebs Cycle)

Main Steps and Products

The citric acid cycle oxidizes acetyl CoA, generating high-energy electron carriers and ATP/GTP.

• Step 1: Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

• Step 2: Isomerization, oxidation, and decarboxylation steps produce NADH, FADH2, CO2, and ATP/GTP.

• Key Enzymes: Dehydrogenases (e.g., isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinate dehydrogenase).

• Products per Acetyl CoA: 2 CO2, 3 NADH, 1 FADH2, 1 ATP (plants/bacteria) or GTP (animals).

Equation:

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

Regulation of Pyruvate Oxidation and Citric Acid Cycle

Regulation ensures efficient energy production and prevents wasteful activity when energy is abundant.

• Allosteric Regulation: NADH acts as a feedback inhibitor for dehydrogenases in the cycle.

• Phosphorylation: Pyruvate dehydrogenase is inactivated by phosphorylation (when ATP is high) and activated by dephosphorylation.

• Enzymes: PDH kinase (adds phosphate, inactivates PDH), PDH phosphatase (removes phosphate, activates PDH).

Beta-Oxidation of Fats

Fatty Acid Catabolism

Fats are highly reduced molecules that store more energy than carbohydrates. Beta-oxidation breaks down fatty acids to generate acetyl CoA, NADH, and FADH2.

• Triacylglycerols: Hydrolyzed to fatty acids and glycerol; glycerol enters glycolysis.

• Fatty Acid Activation: Fatty acids are linked to CoA, coupled to ATP hydrolysis.

• Beta-Oxidation Steps: Oxidation (FADH2 produced), hydration, oxidation (NADH produced), thiolysis (acetyl CoA released).

• Location: Mitochondria and peroxisomes in eukaryotes; cytosol in bacteria.

• Energy Yield: Fats generate more NADH and FADH2 than carbohydrates.

Equation (for one cycle):

$\text{Fatty Acyl CoA} + \text{FAD} + \text{NAD}^+ + \text{H}_2\text{O} + \text{CoA} \rightarrow \text{Fatty Acyl CoA (shortened by 2C)} + \text{FADH}_2 + \text{NADH} + \text{Acetyl CoA}$

Protein Catabolism

Proteolysis and Entry into TCA Cycle

Proteins are hydrolyzed by proteases into small peptides and amino acids, which can be further degraded and enter the citric acid cycle as intermediates.

• Proteases: Enzymes that break down proteins.

• Peptidases: Endopeptidases (cleave internal peptide bonds) and exopeptidases (cleave terminal amino acids).

• Amino Acids: Can be converted into TCA cycle intermediates.

Summary Table: Key Steps and Products of Cellular Respiration

Stage	Main Location	Key Products	Key Enzymes
Glycolysis	Cytosol	Pyruvate, ATP, NADH	Glycolytic enzymes
Pyruvate Oxidation	Mitochondrial Matrix	Acetyl CoA, CO2, NADH	Pyruvate Dehydrogenase Complex
Citric Acid Cycle	Mitochondrial Matrix	CO2, NADH, FADH2, ATP/GTP	Dehydrogenases, Synthases
Electron Transport Chain	Inner Membrane	H+ gradient, H2O	Complexes I-IV, ATP Synthase
Beta-Oxidation	Mitochondrial Matrix/Peroxisome	Acetyl CoA, NADH, FADH2	Dehydrogenases, Thiolase

Key Points for Exam Preparation

• Understand how mitochondrial structure supports its function in energy metabolism.

• Know the mechanisms of protein import into the mitochondrial matrix.

• Be able to describe the steps and regulation of pyruvate oxidation and the citric acid cycle.

• List and explain the steps of beta-oxidation of fatty acids.

• Recognize how protein catabolism integrates with central metabolism.

Additional info: Study regularly and review these concepts for upcoming quizzes and exams.