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Biochemistry I - Oxidative Phosphorylation

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    Study Guide Biochemistry I – Oxidative Phosphorylation 1. ATP Synthesis and Chemiosmosis ATP synthesis is the final and most important step of cellular respiration. It uses the energy stored in a proton gradient across the inner mitochondrial membrane to produce ATP. 1.1 Proton Movement and Energy Storage During electron transport, protons (H ⁺ ) are pumped: • From the mitochondrial matrix • Into the intermembrane space This movement creates a proton gradient across the inner membrane. Two important differences are established: • A pH difference (more H ⁺ outside than inside) • An electrical difference (outside becomes positively charged) As a result: • The intermembrane space is acidic and positive • The matrix is basic and negative A helpful shorthand to remember this is: • Positive out, negative in • Acidic out, basic in 1.2 Components of the Chemiosmotic Potential Peter Mitchell proposed that the energy stored in this gradient — called the chemiosmotic potential — has two components :

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    Study Guide 1. Δ pH o Represents the difference in hydrogen ion concentration o Calculated as (pH outside − pH inside) 2. ΔΨ ( Delta Psi) o Represents the electrical potential difference o Arises because protons carry a positive charge Together, these two components determine the total energy stored across the membrane. This equation shows that the membrane potential ( ΔΨ) and the pH gradient ( Δ pH) together define the usable energy. 1.3 ATP Synthase: Converting Gradient Energy into ATP The stored energy of the proton gradient is used by ATP synthase , also called Complex V of the mitochondrial electron transport chain. Figure 1

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    Study Guide ATP synthase has two main parts: • F 0 subunit o Embedded in the inner membrane o Forms a channel for proton movement • F ₁ subunit o Extends into the matrix o Contains the catalytic sites that make ATP 1.4 How ATP Synthase Actually Works The mechanism of ATP synthase is not intuitive : • The F ₁ subunit can form ATP from ADP and phosphate without proton flow • However, ATP cannot be released unless protons flow through the F 0 subunit This means proton movement is essential for ATP release , not just ATP formation. 1.5 Electron Transport and ATP Synthesis Are Not Directly Linked ATP synthase demonstrates that: • Electron transport and ATP synthesis are not rigidly coupled This conclusion is supported by two key observations: 1. Artificial proton gradients can drive ATP synthesis even without electron transport 2. Certain molecules called uncouplers allow proton flow without ATP production 1.6 Uncouplers and Heat Production Uncouplers are molecules that: • Carry protons across the inner membrane • Bypass ATP synthase • Release metabolic energy as heat instead of ATP

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    Study Guide Figure 2 1.7 Dinitrophenol (DNP) • Dinitrophenol is a weak acid • It is hydrophobic , so it dissolves in the inner membrane • It picks up protons in the intermembrane space • Releases them in the matrix Because ATP is not produced: • Energy from food is not stored • Instead, it is released as heat Dinitrophenol was once used as a diet drug , but it caused severe side effects, including liver toxicity , and was withdrawn.

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    Study Guide 1.8 Fatty Acids as Natural Uncouplers Fatty acids can also act as uncouplers because they are: • Weak acids • Able to cross the inner mitochondrial membrane 1.9 Brown Fat and Heat Generation In human infants , heat production occurs through brown fat tissue , found mainly at the base of the neck. Brown fat: • Contains many mitochondria • Appears brown due to iron - rich cytochromes • Is naturally uncoupled In brown fat: • Fat is oxidized • Very little ATP is produced • Most energy is released as heat This heat is essential for keeping the infant’s brain warm and functional . Unfortunately: • Brown fat is largely lost with age • Adult humans cannot burn excess calories as easily through this mechanism Key Takeaway • ATP synthesis is driven by a proton gradient , not directly by electron flow • The gradient has two parts: Δ pH and ΔΨ • ATP synthase uses proton flow to release ATP • Uncouplers convert metabolic energy into heat instead of ATP • Brown fat uses natural uncoupling to generate heat in infants

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    Study Guide 2. Mitochondrial Transport Systems The inner mitochondrial membrane is highly selective and does not allow most molecules to pass freely. However, ATP is made on the matrix side of this membrane and must reach the rest of the cell. To solve this problem, mitochondria use specialized transport proteins . These transport systems use the energy stored in the proton gradient , specifically: • The electrical potential ( ΔΨ) • The pH gradient ( Δ pH) Together, these forces drive the movement of key molecules in and out of the mitochondrial matrix. 2.1 Overall Organization of Transport Key features to remember: • Intermembrane space : acidic and positively charged • Matrix : basic and negatively charged Transporters are arranged to take advantage of this difference.

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    Study Guide 2.2 Transport of ATP and ADP Adenine Nucleotide Translocase ATP made in the matrix must be exported, and ADP must be imported for continued ATP synthesis. This exchange is handled by the adenine nucleotide translocase , which works as an antiporter . Reaction: This transport is driven by the electrical component ( ΔΨ) of the proton gradient: • The intermembrane space is more positive • A negatively charged ATP⁴ ⁻ moving out is energetically favorable As a result, ATP leaves the matrix efficiently while ADP enters. 2.3 Transport of Inorganic Phosphate (Pi) ATP synthase also requires inorganic phosphate (Pi) . Phosphate enters the matrix using the phosphate translocase , which can operate in two different modes. 1. Antiport Mode (OH ⁻ Exchange) Reaction: In this mode: • Hydroxide ions move out of the matrix • Phosphate moves into the matrix • There is no net charge transfer This transport is favored because: • The matrix is more basic • The intermembrane space is more acidic

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    Study Guide 2. Symport Mode (H ⁺ Cotransport) Reaction: In this mode: • Phosphate enters the matrix together with a proton • Transport is driven by the pH gradient • Protons bypass ATP synthase during this process 2.4 Transport of Carboxylic Acids Several key metabolic intermediates must also cross the inner membrane. Examples of Transported Molecules: • Pyruvate → via the pyruvate translocase • Succinate → via the dicarboxylic acid transporter • Citrate → via the tricarboxylic acid transporter Pyruvate Transport • Pyruvate enters the matrix using an antiport mechanism • It is exchanged with hydroxide ions (OH ⁻ ) • This transport uses the pH gradient Transport Driven by Concentration Gradients Other carboxylic acid transporters rely mainly on concentration differences . Example: • When citrate builds up in the matrix • It is transported out into the cytoplasm • Cytoplasmic citrate can inhibit phosphofructokinase • This links mitochondrial activity with regulation of glycolysis

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