Lecture 11- Reducing Power

Lecture 11 - Metabolism: Generation and Use of Reducing Power (including Photosynthesis)

Learning Objectives

• Understand the function of the electron transport chain in releasing redox energy.

• Differentiate between two types of electron carriers:

  • Electron carriers that carry electrons only.

  • Electron carriers that carry both H+ and electrons.

• Identify common carriers in each class.

• Sketch the mitochondrial electron transport chain, illustrating how the Proton Motive Force (PMF) is generated and how NADH acts as an electron donor.

• Explain how alternating between different types of electron carriers can lead to pumping H+ against the gradient.

• Recognize how PMF drives ATP production through chemiosmosis and ATP synthase.

• Detail four differences between bacterial and mitochondrial electron transport chains.

• Describe three alternate terminal electron acceptors (other than O2) that bacteria can utilize.

• Explain the oxidase test in diagnostic microbiology.

• Make predictions about whether specific electron donor and acceptor pairs can be used by bacteria based on provided data tables (like slide 16).

• Sketch the "Z-scheme" of oxygenic photosynthesis and indicate where ATP and NADPH are formed.

• Explain the role of water as the electron donor in these processes.

• Compare and contrast the electron transport chains of photosynthesis and respiration.

• Differentiate between photosynthesis in purple bacteria and cyanobacteria/plants.

• Define reverse electron transport and its significance for purple sulfur bacteria, contrasting it with green sulfur bacteria.

• Distinguish between cyclic and non-cyclic photosynthesis.

Vocabulary

• Electron Transport Chain (ETC): A series of protein complexes embedded in the membrane that transfer electrons to generate a proton gradient.

• Terminal Electron Acceptor (TEA): The final electron acceptor in the electron transport chain, often O2 but can be others.

• Electron Carrier: Molecules like NAD, FAD, Quinols, and Cytochromes that shuttle electrons through metabolic pathways.

• NAD (Nicotinamide adenine dinucleotide): A carrier that accepts electrons and protons, becoming reduced to NADH.

• FAD (Flavin adenine dinucleotide): A carrier similar to NAD, becoming reduced to FADH2.

• Quinone: A type of electron carrier that can exist in oxidized (Q) and reduced (QH2) forms.

• FeS Proteins: Iron-sulfur proteins that facilitate electron transfer in respiratory chains.

• Chemiosmosis: The movement of protons across a membrane drive ATP synthesis.

• Oxidative Phosphorylation: The process of generating ATP using the PMF from electron transport.

• Mitchell Hypothesis: A theory explaining ATP synthesis through PMF.

• ATP Synthase: An enzyme that synthesizes ATP using the energy from the proton gradient.

• Substrate-Level Phosphorylation: Direct synthesis of ATP during a metabolic reaction.

• Cytochrome c oxidase: A critical enzyme in the electron transport chain responsible for the reduction of oxygen.

• Photosynthetic Antenna Pigments: Molecules that harvest light energy and funnel it to reaction centers in photosynthetic organisms.

• Photosynthetic Reaction Center: The part of a photosystem where light energy is converted to chemical energy.

• Oxygenic Photosynthesis: Photosynthesis that produces oxygen as a by-product by using H2O as an electron source.

• Non-oxygenic Photosynthesis: Photosynthesis where no oxygen is produced, often using alternative electron donors such as H2S.

• Chlorophyll and Bacteriochlorophyll: Pigments responsible for capturing light energy, with chlorophyll being predominant in plants and bacteriochlorophyll in some bacteria.

• PSI and PSII: Photosystems I and II, which play key roles in the light-dependent reactions of photosynthesis.

• Purple Sulfur Bacteria: A type of photosynthetic bacteria that use H2S for electron donation.

• Green Sulfur Bacteria: Another type of pollution-light photosynthetic bacteria that absorb light differently from purple sulfur bacteria.

Electron Transport Chain Overview

• Electrons travel through the electron transport chain in small energy-releasing steps.

• The final destination for electrons is the terminal electron acceptor; typically O2 (but can be other entities).

• The energy released during this process is harnessed to create the Proton Motive Force (PMF).

Common Electron Carriers in Metabolism

• NADH and FADH2: Both function as electron donors, crucial for driving metabolic reactions.

  • NADH: Reduced form of NAD, operates in biological oxidations.

  • FADH2: Reduced form of FAD, acts similarly in energy-generating pathways. ONLY used in the Krebs Cycle

• Quinone/Quinol: Acts as a mobile electron carrier in membrane-bound complexes; participates in the redox cycle within the ETC.

  • Involved in the Q cycle.

• Heme (cytochrome): Class of proteins that contain heme groups and facilitate electron transport.

• Iron-sulfur proteins: Comprise iron and sulfur, mediating electron transfer in various enzymes and complexes.

Mitochondrial Electron Transport Chain

• Located within the mitochondria of eukaryotic cells.

• Composed of multiple complexes (I-IV) and mobile carriers (like Coenzyme Q and cytochrome c).

• Use to make the PMF to make ATP

• Complex I: Accepts electrons from NADH, pumps protons out, contributing to PMF.

• Complex II: Accepts electrons from FADH2 but does not pump protons. Which then goes to coenzyme Q

• Complex III: Engages in the Q-loop, further pumping protons into the intermembrane space, supporting PMF.

• Complex IV: Reduces oxygen and contributes the last step to forming water from electrons and protons, completing the electron transport.

Complex I Details

• Proton Pump Mechanism: Movement of protons across the membrane creates a charge difference, analogous to mechanical work in a physical system, enabling ATP production. FMM is the electron acceptor from NADH. As electrons flow through the proteins of complex I, they are attracted to positively charged amino acids, causing a conformational change.

• As electrons move through, they’re alternating the structures of proteins, causing them to bend, creating the PMF, allowing H ions to make their way through

Complex III Mechanics

• Q-loop: A mechanism where electrons from reduced quinol donate electrons while protons are translocated, contributing to the proton gradient. Protons from cytoplasm plus electrons from donor (cytochrome B) to pass electrons from quinone to quinol (reduction). Quinol binds, offloads H ions to pass electrons to Sulfur to go to cytochrome C. As they're offloading they’re passing electrons outside the membrane. Allowing the loop to keep going.

ATP Production: Chemiosmosis and Oxidative Phosphorylation

• Proton Gradient: Utilized by ATP synthase to convert ADP + Pi into ATP.

• Chemiosmosis: The process where the flow of protons back across the membrane through ATP synthase drives ATP generation.

• ATP synthesis linked to electron transport relies on the Mitchell Hypothesis, which posits that a PMF drives ATP synthesis. To synthesize ATP from phosphate. Ion gradients can help synthesize ATP.

Differences Between Bacterial and Mitochondrial Electron Transport Chains

• Bacterial electron transport pathways are incredibly diverse:

  • Can utilize varied transport complexes.

  • Can use one or more bacterial electron transport chains, which causes branching.

  • Utilize multiple types of terminal electron acceptors.

  • Variable numbers of protons can be pumped.

  • May operate multiple branched transport chains simultaneously.

  • Cytochrome c oxidase presence can vary, crucial for the oxidase test in diagnostics.

Example of Bacterial Electron Transport Chain

• E. coli: Utilizes various pathways for electron transport, showcasing the flexibility of bacterial electron transport.

  • NADH Dehydrogenase (NDH): Transfers electrons from NADH to Q.

  • Fumarate reductase and nitrate reductase: Common enzymes that utilize alternate electron acceptors (like nitrate).

Electron Transport in Chemolithotrophs and Chemoorganotrophs

• Chemolithotrophs: Utilize inorganic substrates (like H2 gas, H2S, etc.) for energy via oxidation.

• Chemoorganotrophs: Utilizes organic substrates to drive their metabolic activities.

• Important role in biogeochemical cycles such as nitrogen fixation by nitrifying bacteria.

Photosynthesis Overview

• Divided into two main sets of reactions:

  1. Light Reactions: Harvest light energy, leading to NADPH production and ATP generation through photophosphorylation.

  2. Dark Reactions: FIX carbon and synthesize carbohydrates via the Calvin-Benson Cycle.

Light Reactions Explained

• Pigments must absorb light for photosynthesis:

  • Chlorophyll is the principal pigment found in chloroplasts, which appears green.

  • Absorption of light excites electrons within the chlorophyll, leading them into higher energy states.

Excitation of Electrons in Chlorophyll

• Upon absorbing light, electrons transition from a ground state to an excited state and return to ground state, releasing energy in various forms:

  1. Heat (motion).

  2. Fluorescence.

  3. Energy transfer to neighboring pigment molecules.

  4. Driving chemical reactions.

Structure of Light-Harvesting Complex

• Antenna Complex: Aggregates of pigment molecules that channel light energy to the reaction center.

  • Key role in converting light energy into chemical energy.

Photosynthetic Mechanism

• Light energy induces oxidation of chlorophyll at the reaction center.

  • Antenna pigments transfer absorbed light energy to chlorophyll, facilitating electron transport.

• Key components include:

  • Light reactions occurring in the thylakoid membranes.

Z-Scheme of Oxygenic Photosynthesis

• Photosystem II (PSII): Triggers photolysis of water, releasing O2 and building the proton gradient.

• Photosystem I (PSI): Utilizes the electrons from PSII and produces NADPH.

• Proton Gradient: Generated during light reactions, essential for ATP synthesis.

Photosynthetic Electron Transport Summary

• Oxygenic photosynthesis features dual photosystems; PSII generates ATP and transfers electrons downhill to PSI, where reducing agents (like NADPH) are produced.

• Electron donors affect the pathways and the overall metabolic output of photosynthetic organisms, including purple and green sulfur bacteria:

  • Purple Sulfur Bacteria: Use low-energy light, operate in a unique manner compared to higher organisms, using direct electron donations like H2S.

  • Green Sulfur Bacteria: Utilize a higher energy input to excite electrons, achieving reductions via electron pathways leading to NADH formation.