Mitochondrial Structure, Function, and Cellular Respiration

The Mitochondrion: Structure and Function

Overview of the Mitochondrion

The mitochondrion is a double-membraned organelle found in almost all eukaryotic cells, including plants. Its primary function is to generate ATP through aerobic respiration. The number and location of mitochondria within a cell depend on the cell's energy requirements.

• Presence: Found in nearly all eukaryotic cells.

• Abundance: Varies from 1 to thousands per cell, depending on energy demand (e.g., muscle cells have many mitochondria).

• Localization: Often located near sites of high energy use, such as flagella and cilia.

Mitochondrial Structure

• Outer Membrane (OM): Contains porin proteins, making it permeable to molecules with molecular weight < 5000 Da.

• Inner Membrane (IM): Highly selective, forms cristae to increase surface area (up to 5x greater than OM), and is composed of ~75% protein.

• Intermembrane Space: The region between OM and IM, nearly continuous with the cytosol.

• Mitochondrial Matrix: Contains enzymes, ribosomes, and circular mitochondrial DNA (15,000–20,000 bp), encoding rRNAs, tRNAs, and some inner membrane proteins.

Mutations in mitochondrial DNA are associated with diseases, aging, and neurodegeneration.

Functional Specialization

• Matrix: Site of pyruvate oxidation and the citric acid cycle (TCA cycle).

• Inner Membrane: Houses the electron transport chain (ETC) and ATP synthase for oxidative phosphorylation.

• Intermembrane Space: Involved in proton gradient formation during respiration.

Protein Import into the Mitochondrion

Matrix Protein Import

Most mitochondrial proteins are encoded by nuclear DNA, synthesized in the cytosol, and imported into the mitochondrion in an unfolded state with the help of chaperone proteins (e.g., Hsp70).

• Transit Sequence: Directs proteins to mitochondria.

• TOM Complex: Translocase of the Outer Membrane, recognizes and transports proteins across the OM.

• TIM Complex: Translocase of the Inner Membrane, imports proteins into the matrix.

• ATP Hydrolysis: Required for protein import and release from chaperones.

• Processing: Transit sequence is cleaved by transit peptidase; proteins are folded by mitochondrial chaperonins (e.g., Hsp60).

Stages of Aerobic Respiration

Overview

Aerobic respiration is a highly efficient process for extracting energy from glucose, using oxygen as the final electron acceptor. It consists of five main stages:

1. Glycolysis (cytosol): Glucose is converted to pyruvate, generating ATP and NADH.

2. Pyruvate Oxidation (mitochondrial matrix): Pyruvate is converted to acetyl-CoA, producing NADH and CO2.

3. Citric Acid Cycle (TCA Cycle) (matrix): Acetyl-CoA is oxidized, generating NADH, FADH2, GTP/ATP, and CO2.

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

5. Oxidative Phosphorylation (inner membrane): Proton gradient drives ATP synthesis via ATP synthase.

Key Coenzymes: NADH, FADH2, Coenzyme Q

Comparison: Aerobic Respiration vs. Fermentation

• Aerobic Respiration: Complete oxidation of glucose, external electron acceptor is oxygen, produces much more ATP.

• Fermentation: Incomplete oxidation, internal organic molecules as electron acceptors, less ATP produced.

Pyruvate Oxidation

Transport and Conversion

• Pyruvate enters mitochondria via porins (OM) and a symporter (IM).

• Converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDH) in the matrix.

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

Mechanism: Exergonic coupling attaches CoA, forming a high-energy thioester bond (activated molecule).

The Citric Acid Cycle (TCA Cycle)

Main Steps and Enzymes

The TCA cycle oxidizes acetyl-CoA to CO2, generating high-energy electron carriers and GTP/ATP.

• Step 1: Acetyl-CoA combines with oxaloacetate to form citrate (catalyzed by citrate synthase).

• Step 2: Isomerization of citrate to isocitrate.

• Step 3: Oxidative decarboxylation (isocitrate dehydrogenase): isocitrate → α-ketoglutarate + CO2 + NADH.

• Step 4: Oxidative decarboxylation (α-ketoglutarate dehydrogenase): α-ketoglutarate → succinyl-CoA + CO2 + NADH.

• Step 5: Substrate-level phosphorylation: succinyl-CoA → succinate + GTP/ATP (organism-dependent).

• Step 6: Oxidation: succinate → fumarate + FADH2.

• Step 7: Hydration: fumarate → malate.

• Step 8: Oxidation: malate → oxaloacetate + NADH.

Products per Acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP/ATP, 2 CO2

Regulation of Pyruvate Oxidation and TCA Cycle

• Allosteric Regulation: NADH inhibits dehydrogenases in the cycle.

• PDH Regulation: Controlled by phosphorylation (inactive when phosphorylated, active when dephosphorylated).

• High ATP: Promotes PDH phosphorylation and inactivation.

• Enzymes: PDH kinase (phosphorylates/inactivates PDH), PDH phosphatase (dephosphorylates/activates PDH).

Beta-Oxidation of Fats

Overview

Fatty acids are highly reduced molecules that store more energy than carbohydrates. Beta-oxidation is the process by which fatty acids are broken down to generate acetyl-CoA, NADH, and FADH2.

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

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

• Steps: Oxidation, hydration, oxidation, thiolysis (cleavage).

• Products per cycle: 1 acetyl-CoA, 1 NADH, 1 FADH2 (for each round).

• Even-numbered fatty acids: Completely converted to acetyl-CoA.

• Odd-numbered fatty acids: Yield propionyl-CoA in the final round (requires further metabolism).

Protein Catabolism

Proteolysis

• Proteases: Enzymes that hydrolyze proteins into small peptides and amino acids.

• Peptidases: Further degrade peptides into amino acids.

• Endopeptidases: Cleave internal peptide bonds.

• Exopeptidases: Remove terminal amino acids.

• Amino acids: Can enter the TCA cycle as intermediates after deamination.

Summary Table: Key Features of Mitochondrial Respiration

Stage	Location	Main Inputs	Main Outputs	Key Enzymes/Complexes
Glycolysis	Cytosol	Glucose, NAD+, ADP	Pyruvate, NADH, ATP	Various glycolytic enzymes
Pyruvate Oxidation	Mitochondrial Matrix	Pyruvate, NAD+, CoA	Acetyl-CoA, NADH, CO2	Pyruvate Dehydrogenase Complex
Citric Acid Cycle	Mitochondrial Matrix	Acetyl-CoA, NAD+, FAD, GDP/ADP	NADH, FADH2, GTP/ATP, CO2	Citrate Synthase, Dehydrogenases
Electron Transport Chain	Inner Membrane	NADH, FADH2, O2	ATP, H2O	Complexes I-IV, ATP Synthase
Beta-Oxidation	Mitochondrial Matrix	Fatty Acyl-CoA, NAD+, FAD	Acetyl-CoA, NADH, FADH2	Acyl-CoA Dehydrogenase, others

Key Equations

• Pyruvate Oxidation: $\text{Pyruvate} + \text{CoA} + \text{NAD}^+ \rightarrow \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH}$

• Citric Acid Cycle (per Acetyl-CoA): $\text{Acetyl-CoA} + 3\ \text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}_i + 2\ \text{H}_2\text{O} \rightarrow 2\ \text{CO}_2 + 3\ \text{NADH} + 3\ \text{H}^+ + \text{FADH}_2 + \text{GTP} + \text{CoA}$

• Beta-Oxidation (per cycle): $\text{Fatty Acyl-CoA} + \text{FAD} + \text{NAD}^+ + \text{H}_2\text{O} + \text{CoA} \rightarrow \text{Fatty Acyl-CoA (shortened by 2C)} + \text{FADH}_2 + \text{NADH} + \text{Acetyl-CoA} + 2\ \text{H}^+$

Summary

• The structure of the mitochondrion is closely linked to its function in energy metabolism.

• Matrix proteins are imported from the cytoplasm via TOM and TIM complexes, requiring ATP hydrolysis.

• Pyruvate from glycolysis is transported into the mitochondrion and oxidized, generating NADH and CO2.

• The citric acid cycle produces NADH, FADH2, and GTP/ATP, and is regulated by allosteric mechanisms.

• Fatty acids are catabolized by beta-oxidation to produce acetyl-CoA, NADH, and FADH2.

Example: Muscle cells, which require large amounts of ATP, contain thousands of mitochondria located near contractile fibers to efficiently supply energy for contraction.