Ch 9 (Part 3): 9.4 - E.T.C./ Oxidative Phosphorylation

● So far, in glycolysis & the Krebs cycle, 1 glucose molecule has resulted in:  4 ATPs (2 from glycolysis, 2 from Krebs)  10 NADH (2 from gly., 2 from acetyl-CoA step, 6 from Krebs Cycle)  2 FADH2 (from Krebs Cycle) x2

● Following glycolysis and the Krebs cycle, NADH and FADH2 account for most of the energy extracted from food ● These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

ELECTRON TRANSPORT CHAIN (E.T.C.) ● E.T.C. = a collection of molecules (mostly protein complexes) embedded in the inner membrane of mitochondrion (foldings of inner membrane form CRISTAE)

The Pathway of Electron Transport ● the groups along the chain alternate between reduced & oxidized states as they accept and donate electrons ● each successive group is more electronegative than the group before it, so the electrons are “pulled downhill” towards OXYGEN (the final electron carrier!)

Free energy (G) relative to O2 (kcal/mol) NADH 50 FADH2 Multiprotein complexes 40 I FMN FAD Fe•S Fe•S II Q III Cyt b Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Fe•S 30 Cyt c1 Free energy (G) relative to O2 (kcal/mol) IV Cyt c Cyt a ATP ATP ATP Cyt a3 20 10 2 H+ + 1/2 O2 H2O

● as molecular oxygen (O2) is reduced, it also picks up H+ from the environment to form water (H2O)

ATP Production of the E.T.C. Typically, the ATP produced is as follows: 1 NADH  3 ATP 1 FADH2  2 ATP (FADH2 is “dropped off” at a lower point in the E.T.C., so it generates fewer ATPs) “exchange rate”

Chemiosmosis: The Energy-Coupling Mechanism ● Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space (active transport) ● H+ (protons) then move back across the membrane, passing through channels in ATP synthase

Chemiosmosis: The Energy-Coupling Mechanism ● ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP ● This is an example of CHEMIOSMOSIS, the use of energy in a H+ gradient to drive cellular work

● The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis ● The H+ gradient is referred to as a PROTON-MOTIVE FORCE, emphasizing its capacity to do work

the ATP synthase complex which causes the phosphorylation of ADP to (inner matrix) ● protons then diffuse back across the membrane through the ATP synthase complex which causes the phosphorylation of ADP to form ATP! (intermembrane space)

A rotor within the membrane spins as shown when H+ flows past INTERMEMBRANE SPACE H+ A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. ADP + ATP P i MITOCHONDRAL MATRIX

Electron transport chain Inner mitochondrial membrane Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis ATP ATP ATP H+ H+ H+ H+ Protein complex of electron carriers Cyt c Intermembrane space Q IV I III ATP synthase Inner mitochondrial membrane II 2H+ + 1/2 O2 H2O FADH2 FAD NADH + H+ NAD+ ADP + P ATP i (carrying electrons from food) H+ Mitochondrial matrix Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane Oxidative phosphorylation

SUMMARY: ● most energy flows in this sequence: Glucose  NADH  E.T.C.  proton  ATP motive force

TOTAL ATPs PROCESS ATP produced by subs. phos. Reduced coenz. ATP produced by oxid. phos. (in the E.T.C.) TOTAL ATPs Glycolysis 2 ATP 2 NADH (go to ETC) 4-6 ATP 6-8 oxid. of pyruvate to acetyl CoA   2 NADH (go to ETC) 6 ATP 6 Krebs cycle 6 NADH 2 FADH2 18 ATP 4 ATP 24 36-38!

● approximately 40% of energy in glucose is converted to ATP ● the remaining energy is lost as heat

CYTOSOL Electron shuttles span membrane MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis 2 Pyruvate 2 Acetyl CoA Citric acid cycle Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level phosphorylation by substrate-level phosphorylation by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 36 or 38 ATP Maximum per glucose: **actual ATP total’s are slightly less – when we factor in “real” exchange rates and the energetic cost of moving the ATP formed in the mitochondrion out into the cytosol, where it will be used**