15 - oxidative phosphorylation

explain the Coupling of the Electron Transport and Oxidative Phosphorylation

• is uncoupling a thing?

• ATP production linked to exergonic steps of electron transport

• Phosphorylation tightly coupled to respiration (due to intact membrane)

• Intact membrane

• Uncoupling → can be done by some molecules that dissipate the proton gradient therefore ATP cannot be made)

• Inhibition

what 2 things drive ATP synthesis

• chemical potential (decrease in pH/increase in H)

• electrical potential

what is the chemiosmotic hypothesis?

the proton motive force is enough to drive ATP synthesis

ATP synthase and bacteriorhodopsin

• bacteriorhodopsin is a protein on the membrane of a vesicle

• when light shines on the bacteriorhodopsin, it moves protons across membrane to build up the H+ concentration in the membrane

• the vesicle will then associate with ATPase and cause ATP synthesis as H+ leave the vesicle

describe the structure of ATP synthase

• Fo

  • stationary

  • a helices form a wheel in membrane

• F1

  • rotates which is driven by H+ movement

  • spindle is a gamma subunit that does rotation

  • at head, 3 active sites btwn each pair of alpha-beta subunits where ATP is synthesized

Boyer’s “Binding Change” names of active site

F1 being observed as if it is fixed in place

• O = open

  • inactive active site - doesn't bind substrates (Pi and ADP) very well

• L = loose

  • low affinity of S also catalytically inactive

• T = taut

  • binds substrate strongly, is active

Boyer’s “Binding Change” Mechanism of ATP Synthesis

• substrates (ADP and Pi) bind to loose active site. Taut active site still has ATP product from last reaction

• energy is added and spindle rotates so the substrates are now in the Taut active site and reaction occurs.

• old ATP leaves moves to the open active site and leaves soon, a dehydration reaction occurs

• new ATP is in the Taut active site until next reaction occurs and then it will move to the loose active site

Uncoupling Oxidative Phosphorylation

• what happens

• what are the characteristics of the molecules that do this?

• ETC will still run, but not making ATP so it becomes futile, just using up resources

• molecules:

  • all are hydrophobic (dissolved in membrane)

  • need weakly acidic or basic FGs:

    • easily lose H⁺ at physiological pH

    • pKa near physiological pH

    • dissipate gradient via acid-base chemistry

Biological Uncoupling

• what is the protein called?

• what does it do?

• in what organisms is it found?

thermogenin

• uncoupling protein

  • creates heat, found in organisms that need to generate heat

  • has channel that protons can travel through back to matrix to dissipate H+ gradient which generates heat

how is thermogenin (UPC1) regulated?

stimulated: hormonal regulation by noradrenaline that stimulates cascade of events that liberates fatty acids to bind UCP1 open channel → allows H+ out and back into matrix, dissipates gradient and generates heat

inhibited: ATP, ADP, GTP, GDP binds to UPC1 and black the channel, no gradient is dissipated

Uncoupling in Plants

• oxidase protein transfers H+ and electrons to O2

• this takes H+ and electrons out of the ETC so no ATP is made

• also generates heat

Transport & Energetics

• ATP-ADP Translocase

  • antiporter

  • translocates ATP from matrix into IM space and ADP from IM into matrix

  • energetic cost of 1 H⁺ to this translocation due to charge diff btwn ATP and ADP

  • drives reaction by brining in substrate and removing product

• Phosphate Translocase

  • makes up the charge difference between ATP and ADP translocation

  • symporter of H2PO4^- and H⁺

how many total ATP are made per glucose?

• 30 or 32

• Depends on shuttle system transferring reducing equivalents into the mitochondrion

what is the P/O ratio?

• P/O ratio

• phosphorylation/oxidation

  • NADH → 5ATP/2NADH = 2.5 ratio

  • FADH2 → 3ATP/2FADH2 = 1.5 ratio

respiratory control

• primary regulatory ratio is ATP/ADP

  • increase the ratio will inhibit ATP making pathways, decreasing will have the opposite effect

• secondary regulatory ratio is NADH/NAD⁺

  • increase the ratio will inhibit NADH making pathways, decreasing will have the opposite effect

ATP Synthase Inhibitors

• 2,4-dinitrophenol, dicumarol, FCCP

  • uncouplers → dissipate the proton gradient to reduce chemiosmotic potential

• DCCD, oligomycin

  • ATP synthase inhibitors → bind to ATP synthase and block it