Electron Transport

Metabolism, like combustion, essentially involves transfer of electrons from fuel (carbohydrates, fats, etc.) to oxygen

metabolic processes produce reducing equivalents (NADH, FADH₂) when fuels are oxidized
electrons can be carried only by NAD⁺, FAD, FMN, heme, Fe/S cluster proteins, certain lipids (ubiquinone), and cysteine

Oxidative Phosphorylation – ATP made by electron transfer from reducing equivalents to O₂

This process accounts for 90% of ATP that is made in all body tissues (except RBC’s) and more than 99% of O₂ use

occurs only in mitochondria inner membrane – most energy metabolism in matrix

Two separate steps:

(1) oxidation – electron transfer from NADH/FADH₂ to O₂ producing H₂O and NAD⁺/FAD (can be reused)

e^- transfer produces change in free energy

Faraday’s Equation: D G = -nFD E (n = number of e^-, F = Faraday’s constant, D E = voltage change)

D E for NADH (FADH₂) is 1.14 V (0.82 V), so D G for one NADH (FADH₂) is -52 kcal/mol (-38 kcal/mol)

electron transfer causes buildup of proton and electrochemical gradient across inner mitochondrial membrane

(2) phosphorylation – energy storage in ATP, coupled to entry of protons into mitochondrial matrix, down their gradient

Oxidation – Electron Transport Chain – Consists of 12 Steps in Four Protein Complexes:

Complex I – receives 2 e^- from NADH, transfers to ubiquinone (coenzyme Q, a lipid that can carry electrons)

Complex II – includes TCA enzyme succinate dehydrogenase – receives 2 e^- from FADH₂, transfers to ubiquinone

Complex III – transfers 2 e^- from ubiquinone to cytochrome C (a peripheral membrane protein containing heme)

Each complex (except complex II) pumps 3 protons out of mitochondria through membrane – this produces two gradients:

total is about 5.2 kcal/mol – two protons is more than enough to make one ATP (7.4 kcal/mol) – need 2-4 experimentally
NADH uses three proton pumping complexes (I, III, IV) for 3 ATP/O₂– FADH₂ only uses two (III and IV) for 2 ATP/O₂
stoichiometry of e^- transport: cytochromes and Fe/S centers can only carry one e^-; NADH can accept/donate pairs of e^-; FMN and coenzyme Q can accept 1-2 e^- and donate them one at a time to carriers that can only accept 1 e^-

Phosphorylation – Consists of F₀F₁ ATPase Running in Reverse

The ATPase allows protons to migrate down their concentration/voltage gradient into the matrix to produce ATP
very many ATPase proteins – inner mitochondrial membrane is most protein-rich in body
ATP transported into cytoplasm by ATP/ADP Antiporter – 1:1 ratio, so conc. of adenine inside matrix is constant
inorganic phosphate enters matrix by PO₄^-/H⁺ Symporter – coupled to proton gradient

Respiratory Control – Regulation of Oxidative Phosphorylation is by Product Inhibition

High energy (low ADP): ATPase cannot keep up, electrochemical gradient increases, e^- transport slows (D G too high)

Low energy (high ADP): ATP synthesis allows electrochemical gradient to decrease, e^- begins again

Inhibitors of Oxidative Phosphorylation – Four Types:

(1) Electron Transport Inhibitors – example: cyanide (CN^-) binds complex IV

binds complex IV Þ blocks e- transport Þ respiration stops Þ H+ gets consumed, electron gradient ß Þ ATP ß

(2) ATP Synthesis Inhibitors – example: oligomycin binds F₀F₁ ATPase

(3) ATP/ADP Exchange Inhibitors – example: atractyloside binds exchange molecule

(4) Uncouplers – weak acids that are permeable to inner mitochondrial membrane – shuttle protons

example: 2,4-dinitrophenol (2,4-DNP) – causes temperature elevation

brown adipose tissue – used by bears for heat production – leaky membrane Þ O₂ use without ATP production