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BIOL 201 - Lectures 13 to 15

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Biology (Sci)
BIOL 201
Greg Brown

Lecture 13 ATP synthase structure F1Fointeractions ∗ δ tightly linked to an α, and to 2 b subunits ∗ γ loosely associated with α/β ring, tightly associated with c ring ∗ β subunits⇨ catalytic ∗ α subunit has two half-channels ∗ ε tightly associated with γ and some c subunits ∗ c, γ, ε subunits rotate in unison ∗ Rotation driven by discharge of proton gradient ∗ Proton enters a half channel, associates with c ∗ After nearly one rotation, leaves via another a half channel Binding change mechanism ∗ Rotation of γ driven by rotation of c subunit ringof F caused by H translocation o ∗ Rotating γ induces cycle of conformational changesin β subunits ∗ Nucleotide binding site of 1 β subunit cycles through 3 conformational states: ∗ O: very poor ATP binding, poor binding for ADP + Pi ∗ L: binds ADP + Pi more strongly ∗ T: very tight binding of ADP + Pi ↔ATP ∗ O → L → T → O cycle: ADP + Pi → ATP Proton translocation and c ring rotation Once a proton enters the cytoplasmic half-channel, the c subunit moves away from the a subunit into the membrane, driving the rotation of the c subunits until the proton comes into contact with the matrix half-channel and is released into the matrix. ATP/ADP translocase (antiporter) ∗ As the γ subunit rotates, β subunit changes conformation. ∗ Exchanges cytosolic ATP for mitochondrial ATP ∗ Involves conformational changes driven by membrane potential ∗ Phosphate transported into mitochondria via - antiport with OH, combines with translocated proton ∗ One translocated protein allocated to ATP/ADP and Pi transport processes + Regenerating cytosolic NAD Anti-porting aspartate for Glutamate. Malate dehydrogenase helps regenerate cytosolic NAD+ that was used in glycolysis. Oxaloacetate itself does not move across the mitochondrial inner membrane. Involves two transporters: - Aspartate/Glutamate - Malate/α-Ketoglutarate How many ATP for each NADH oxidized? - + ∗ Transfer of electrons from 1 NADH (2 e) to O →2transfer of 10 H from matrix to IMS ∗ 10 c subunits in ATP synthase + ∗ 1 full rotation of c ring: translocation of 10 H from IMS to matrix ∗ 3 ATP synthesized for each full rotation of c ring ∗ 1 ATP per 3.3 (3) protons translocated ∗ 1 proton translocated from IMS to matrix for each ADP/ATP pair translocated + ∗ Net synthesis/transport of 1 ATP requires translocation of 4 H ∗ 2.5 ATP synthesized and transported per NADH oxidized ∗ Note the difference between synthesized and transported! Respiratory control Oxygen consumption over time in a suspension of mitochondria provided with fuel to oxidize. Note that the rate of O2 consumption shoots up (steeper slope) when ADP is added. The discharge or the proton gradient is coupled to the synthesis of ATP. Inhibitors Uncouplers Acidic aromatic compounds such as 2, 4 dinitrophenol can uncouple oxidation and phosphorylationthey carry protons across inner membrane (discharge proton gradient) Release of energy: C 6 12+ 6 O6→ 6 H O + 62CO 2 2 Lecture 14 The ultimate source of biological energy is sunlight Photosynthetic organisms capture light energy and use it to: ∗ transfer the H atoms from water to acceptor molecules ∗ form molecular oxygen ∗ synthesize ATP from ADP and phosphate ∗ transfer H atoms from acceptor to carbons derived from CO to fo2m glucose (carbon fixation) Net: 6 H O + 6 CO → C H O + 6 O 2 2 6 12 6 2 Photosynthesis: overview ∗ CO 2 H 02→ (CH O) 2 O . E2dergonic, driven by absorption of light + + ∗ Light reactions: 2 H 2O + 2 NADP + 3 ADP + 3 Pi → O + 22H + 2 NADPH + 3 ATP + + ∗ Dark reactions: CO +22 H + 2 NADPH + 3 ATP → CH O + H O2+ 2 N2DP + 3ADP + 3 Pi (Calvin cycle) Light absorption and energy transformation in photosynthesis Light absorbed by pigments in the thylakoid membrane. Photosynthesis “action spectrum” corresponds to absorbance spectrum of three pigments Chlorophyll has a prophirin ring, another ring, and a long hydrophobic side chain. It is overall a very hydrophobic molecule. Highlighted electrons get excited when light is absorbed. How does the absorption of light drive an (otherwise) energetically unfavourable redox reaction ? Because of the configuration of donor/acceptor molecules in a photosystem, excited electrons can get transferred to an acceptor molecule instead of returning to the ground state. Photoinduced charge separations occur in photosystem reaction centers When the chlorophyll has a strong positive charge it becomes a strong oxidizing agent and can grab electrons. Light harvesting complexes Light energy can be transferred from one compound to the next through resonance energy transfer (No net gain or loss, excitation energy is simply transferred from molecule to molecule). Energy gets funneled to a special pair of chlorophylls in the photosynthetic reaction center. Resonance energy transfer Two cooperating photosystems, photosynthetic electron transport and photophosphorylation Two photosystems cooperate to generate NADPH, proton gradient ∗ Photosystem II charge separation: H O → ½ O , Q → QH 2 2 2 ∗ Transfer of electrons from QH t2 plastocyanin (Pc) catalyzed by cytochrome b/fcomplex: QH + 2 2Pcox Q + Pc red + ∗ Photosystem I charge separation: 2 PC red→ 2 PC ,oxADP → NADPH ∗ = Linear electron flow → acidification of lumen ∗ Proton movement from lumen back to stroma mediatedby chloroplast ATP synt
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