Biochemistry I - Oxidative Phosphorylation - Quick Guides

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Study GuideBiochemistry I–Oxidative Phosphorylation1.ATP Synthesis and ChemiosmosisATP synthesis is the final and most important step of cellular respiration. It uses the energy stored in aproton gradientacross the inner mitochondrial membrane to produce ATP.1.1Proton Movement and Energy StorageDuring electron transport, protons (H⁺) are pumped:•From the mitochondrial matrix•Into the intermembrane spaceThis movement creates aproton gradientacross the inner membrane.Two important differences are established:•ApH difference(more H⁺outside than inside)•Anelectrical difference(outside becomes positively charged)As a result:•Theintermembrane spaceisacidic and positive•Thematrixisbasic and negativeA helpful shorthand to remember this is:•Positive out, negative in•Acidic out, basic in1.2Components of the Chemiosmotic PotentialPeter Mitchell proposed that the energy stored in this gradient—called thechemiosmotic potential—hastwo components:

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Study Guide1.ΔpHoRepresents the difference in hydrogen ion concentrationoCalculated as(pH outside − pH inside)2.ΔΨ (Delta Psi)oRepresents the electrical potential differenceoArises because protons carry a positive chargeTogether, these two components determine the total energy stored across the membrane.This equation shows that themembrane potential (ΔΨ)and thepH gradient (ΔpH)together definethe usable energy.1.3ATP Synthase: Converting Gradient Energy into ATPThe stored energy of the proton gradient is used byATP synthase, also calledComplex Vof themitochondrial electron transport chain.Figure 1

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Study GuideATP synthase has two main parts:•F0subunitoEmbedded in the inner membraneoForms a channel for proton movement•F₁subunitoExtends into the matrixoContains the catalytic sites that make ATP1.4How ATP Synthase Actually WorksThe mechanism of ATP synthase isnot intuitive:•TheF₁subunit can form ATPfrom ADP and phosphatewithout proton flow•However,ATP cannot be releasedunless protons flow through theF0subunitThis means proton movement is essential forATP release, not just ATP formation.1.5Electron Transport and ATP Synthesis Are Not Directly LinkedATP synthase demonstrates that:•Electron transportandATP synthesisare not rigidly coupledThis conclusion is supported by two key observations:1.Artificial proton gradientscan drive ATP synthesis even without electron transport2.Certain molecules calleduncouplersallow proton flow without ATP production1.6Uncouplers and Heat ProductionUncouplers are molecules that:•Carry protons across the inner membrane•Bypass ATP synthase•Release metabolic energy asheat instead of ATP

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Study GuideFigure 21.7Dinitrophenol (DNP)•Dinitrophenol is aweak acid•It ishydrophobic, so it dissolves in the inner membrane•It picks up protons in the intermembrane space•Releases them in the matrixBecause ATP is not produced:•Energy from food is not stored•Instead, it is released asheatDinitrophenol was once used as adiet drug, but it caused severe side effects, includinglivertoxicity, and was withdrawn.

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Study Guide1.8Fatty Acids as Natural UncouplersFatty acids can also act as uncouplers because they are:•Weak acids•Able to cross the inner mitochondrial membrane1.9Brown Fat and Heat GenerationInhuman infants, heat production occurs throughbrown fat tissue, found mainly at the base of theneck.Brown fat:•Contains many mitochondria•Appears brown due to iron-rich cytochromes•Is naturallyuncoupledIn brown fat:•Fat is oxidized•Very little ATP is produced•Most energy is released asheatThis heat is essential for keeping the infant’sbrain warm and functional.Unfortunately:•Brown fat is largely lost with age•Adult humans cannot burn excess calories as easily through this mechanismKey Takeaway•ATP synthesis is driven by aproton gradient, not directly by electron flow•The gradient has two parts:ΔpHandΔΨ•ATP synthase uses proton flow to release ATP•Uncouplers convert metabolic energy intoheat instead of ATP•Brown fat uses natural uncoupling to generate heat in infants

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Study Guide2.Mitochondrial Transport SystemsTheinner mitochondrial membraneis highly selective and does not allow most molecules to passfreely. However, ATP is made on thematrix sideof this membrane and must reach the rest of thecell. To solve this problem, mitochondria usespecialized transport proteins.These transport systems use the energy stored in theproton gradient, specifically:•Theelectrical potential (ΔΨ)•ThepH gradient (ΔpH)Together, these forces drive the movement of key molecules in and out of the mitochondrial matrix.2.1Overall Organization of TransportKey features to remember:•Intermembrane space: acidic and positively charged•Matrix: basic and negatively chargedTransporters are arranged to take advantage of this difference.

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Study Guide2.2Transport of ATP and ADPAdenine Nucleotide TranslocaseATP made in the matrix must be exported, and ADP must be imported for continued ATP synthesis.This exchange is handled by theadenine nucleotide translocase, which works as anantiporter.Reaction:This transport is driven by theelectrical component (ΔΨ)of the proton gradient:•The intermembrane space ismore positive•A negatively charged ATP⁴⁻moving out is energetically favorableAs a result, ATP leaves the matrix efficiently while ADP enters.2.3Transport of Inorganic Phosphate (Pi)ATP synthase also requiresinorganic phosphate (Pi). Phosphate enters the matrix using thephosphate 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 isno net charge transferThis transport is favored because:•The matrix ismore basic•The intermembrane space ismore acidic

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Study Guide2. Symport Mode (H⁺Cotransport)Reaction:In this mode:•Phosphate enters the matrixtogether with a proton•Transport is driven by thepH gradient•Protons bypass ATP synthase during this process2.4Transport of Carboxylic AcidsSeveral key metabolic intermediates must also cross the inner membrane.Examples of Transported Molecules:•Pyruvate→ via thepyruvate translocase•Succinate→ via thedicarboxylic acid transporter•Citrate→ via thetricarboxylic acid transporterPyruvate Transport•Pyruvate enters the matrix using anantiport mechanism•It is exchanged withhydroxide ions (OH⁻)•This transport uses thepH gradientTransport Driven by Concentration GradientsOther carboxylic acid transporters rely mainly onconcentration differences.Example:•When citrate builds up in the matrix•It is transported out into the cytoplasm•Cytoplasmic citrate caninhibit phosphofructokinase•This links mitochondrial activity with regulation of glycolysis

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