Metabolism: Reducing Power & Photosynthesis

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

• Electron transport chains release redox energy in small steps as electrons transit electron transport molecules.
• The electrons end up at a terminal electron acceptor like O₂, and the released energy generates a proton motive force (PMF).

Common Electron Carriers

• NAD and FAD are common electron carriers transporting both electrons (e-) and protons (H+).
• Heme and FeS proteins are common electron carriers for electrons only.

Eukaryotic Electron Transport

• This occurs in mitochondria where transport "complexes" pump H+ to create the PMF.
• Complex I uses a proton pump. The pump works like the slots on a coin-operated washer.
• Complex III has a proton pump due to quinone/quinol (Q) loop.
• The Q loop includes protons from cytoplasm and electrons from an electron donor to reduce quinone.
• Electrons from Quinol then reduce FeS. Protons are deposited on the outside of membrane.
• The PMF generates ATP via chemiosmosis and ATP synthase. This is also known as oxidative phosphorylation or Mitchell Hypothesis.

Bacterial Electron Transport

• Bacterial electron transport is more diverse than mitochondrial transport.
• Bacteria can use different transport complexes and terminal electron acceptors.
• They can pump variable numbers of H+, incorporate multiple transport chains at once (branched chains), and may or may not have cytochrome c oxidase (complex IV).
• The presence/absence of cytochrome c oxidase is an important diagnostic test (oxidase test). For example, E. coli utilizes different electron transport pathways.

Chemolithotrophs and Chemoorganotrophs

• Electron transport in these organisms requires the electron donor to have higher energy than the electron acceptor.
• Enzymes are needed to recognize both donor and acceptor substrates.

Photosynthesis

• Photosynthesis consists of light and dark reactions.
• Light reactions involve harvesting light energy, electron transport, NADPH production, and ATP synthesis by photophosphorylation.
• Dark reactions involve carbon fixation and carbohydrate synthesis via the Calvin-Benson Cycle.

Light Reactions - Harvesting Light

• Light must be absorbed by a pigment to drive a biological reaction. Each pigment has unique properties tied to its chemical structure.
• Chlorophyll, the primary pigment in chloroplasts, appears green because it absorbs many wavelengths of light but transmits green wavelengths.
• When a photon is absorbed, the electron configuration of the pigment changes.
• Electrons are usually stable at the lowest energy or ground state.
• When a photon moves the electrons to the excited state, it can lose its energy in one of four ways: motion (heat), fluorescence, passing energy to a neighboring molecule, or driving a chemical reaction.
• Pigment molecules in the antenna complex form a compact on the thylakoid.
• Energy from light striking the antenna complex is funneled to a reaction center chlorophyll that then directly participates in photosynthesis.
• Light energy oxidizes chlorophyll, releasing electrons in a reaction center.
• Antenna pigments channel light to the reaction center with electrons going to the electron transport chain.

Oxygenic Photosynthesis

• The stages involve two photosystems.
• Electrons move from PSII to PSI in order to make ATP.
• Electrons move from PSI in order to make reducing power (NADPH).
• Electron "holes" in PSII are filled by electrons from H₂O or H₂S, which forms O₂ or S.

Zigzag (Z) Scheme of Electron Transport

• The photosystems are arranged so electrons can be passed among them to generate ATP and NADPH

Purple and Green Sulfur Bacteria

• Purple Sulfur bacteria use one chlorophyll (bacteriochlorophyll).
• Low energy light excites electrons, but not enough to go "downhill" to NAD+.
• Electrons pass through bc₁ complex to produce PMF, which generates ATP.
• Some of the PMF energy is used to push some e- "uphill" to create NADH.
• An electron donor (H₂S, malate, etc.) must be used.
• Green Sulfur bacteria also use one chlorophyll (bacteriochlorophyll).
• Higher energy light excites electrons to a higher (neg.) redox potential than Fd.
• The Electron path splits at FeS protein, which then goes through bc₁ complex to make PMF.
• Some e- go to Fd, and from there "downhill" to NAD+ to create NADH.
• PMF is used to make ATP and donors like H₂S, malate, etc., must be used because not all e- return to P840.