Oxidative Phosphorylation Free Energy of Electron Transfer
Introduction • Living cells save up metabolic energy predominantly in the form of fats and carbohydrates. • They “spend” this energy for biosynthesis, membrane transport, and movement. • In both directions, energy is exchanged and transferred in the form of ATP
Introduction • Glycolysis and the TCA cycle convert some of the energy available from stored and dietary sugars directly to ATP. • However, most of the metabolic energy that is obtainable from substrates entering glycolysis and the TCA cycle is funneled via oxidation-reduction reactions into NADH and FADH2. • The cells oxidize NADH and FADH2 and convert their reducing potential into the chemical energy of ATP, which is named as Oxidative Phosphorylation
An Overview • Electron Transport: Electrons carried by reduced coenzymes (NADH and FADH2) are passed through a chain of proteins and coenzymes to drive the generation of a proton gradient across the inner mitochondrial membrane. • Phosphorylation: The proton gradient runs downhill to drive the synthesis of ATP. • It all happens in or at the inner mitochondrial membrane
Defining Ox-Phos Oxidative Process that involves electron extraction. Electrons passed through ETS and ultimately to terminal electron acceptor. Generates energy and pumps protons. Phosphorylation Synthesis of ATP from ADP + Pi Converts energy stored as proton gradient into energy stored in chemical bond This is in contrast to substrate-level phosphorylation (as in glycolysis)
Recovering Energy Oxidative phosphorylation Converting electron energy into ATP Indirectly coupled Very efficient recovery of energy Last phase of extracting energy out of biomolecules Follow the electrons
Redox Thermodynamics Oxidation is loss; reduction is gain If you extract electrons from one donor, you must deposit them on acceptor FADH2 + Q ←→ FAD + QH2 Reduced oxidized oxidized reduced Reaction can be expressed as half-reactions Only look at one substance at a time Q + 2H+ + 2 e- ←→ QH2
Redox Thermodynamics Standard reduction potential, E0’ Tendency of the oxidized form to be reduced (accept electrons) The more + the value, the greater the tendency Actual reduction potential depends on concentrations of species R is gas constant (8.3145 J/K/mol) T is temp (Kelvin) n = number of electrons transferred F is Faraday’s constant (96,485 J/V/mol)
Nernst equation At 25℃ (298K): Reduction potential predicts flow of electrons Flow from substance with lower to higher potential Lower or negative reduction potential means it is more likely to donate than receive electrons
Free Energy Changes Reduction potential predicts flow of electrons Example: e- flow from NADH (-0.315) to Q (+0.0045)