How ATP is Synthesized at “Energy-Coupling” Membranes:Chemiosmotic Coupling
Module 0210101:
Molecular Biology and Biochemistry of the Cell
Lecture 13
Dale Sanders
23 February 2009
Objectives
1. the notion that NADH oxidation can be coupled to ATPsynthesis via a transmembrane electrochemical potentialfor protons (or protonmotive force, PMF);
2. how the energy stored in transmembrane solute potentialdifferences and ion potential differences can be quantifiedin terms of kJ/mol or mV;
3. how uncouplers increase the rate of electron transport;
4. that the PMF is used in a wide array of biological systemsto transduce free energy from one form to another.
By the end of the lecture you should understand…
Reading
Voet & Voet (2004) Biochemistry (3rd Ed.) Chapter22 and especially pp. 827-835
As in previous lectures, all the topics today are wellcovered by the standard big biochemistry textbooks. Anexample is
Also useful for a more in-depth treatment is
Nicholls, DG & Ferguson, SJ (2002) Bioenergetics 3pp. 46 & 47 and Chapter 4
How is ATP Synthesis Coupled to theRedox Reactions?
NADH
NAD+
½O2 + 2H+
H2OADP+ Pi
ATP
Phosphorylation is energized indirectly,
via generation of a H+ potential across
the inner mitochondrial membrane:
Redox
Phosphorylation
The chemiosmotic hypothesis (Peter Mitchell, 1960s):
1. The coupling complexes in the redox chain are H+ pumps.
2. Low membrane permeability to H+ allows build up of transmembrane H+
potential.
3. Passive return of H+ across membrane energises ATP synthesis.
Cytosol Mito matrix
Note: Redox enzymes+ ATP synthase arediscrete elements inmembrane
UQ
Cyt c
Innermembrane
A hydraulic analogy of chemiosmotic coupling
EnergyinputMainselectricitye- transport
PumpH2O pumpH+ pump
Energy outputLightATP
Couplingenergy:Potentialenergydifferencefor H2O H+
High potential H2O H+
Low potentialH2OH+
Energy transducerWater wheel/dynamoATP synthase
http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_4/hydroelectric_dam.jpg
Economic Exploitation of Hydraulic EnergyCoupling
biologicinstitute.org/2008/04/03/perspectives/
The F-Type ATP Synthase
Twosectors
ADPbindingsite
Rotation
Quantifying the Free Energy inTransmembrane Solute Potentials
(i) Uncharged solute:
[S]o [S]i
membrane
Chemical potential of S = μs = μos + RT ln [S] (1)
Chemical potential difference, inside with respect tooutside is
ΔμS = (μS)i – (μS)o = RT ln [S]i (2)
[S]o
ΔμS = (μS)i – (μS)o = RT ln [S]i (2)
[S]o
= RT x 2.303 log10 [S]i[S]o
e.g. if [S]i = 10 mM, [S]o = 1 mM
ΔμS = 8.314 x 298 x 2.303 x1 = +5.7 kJ/mol
Note: 1. Units are those of Gibbs free energy.
2. Polarity: + ve value says that reaction
So Si
is “uphill”
conversely, Si So is “downhill”
Will release 5.7 kJ/mol
Quantifying the Free Energy inTransmembrane Solute Potentials
(ii) Charged solute (ion):
[Sz]o [Sz]i
membrane
Electrochemical potential of Sz = μs = μos +RT ln[Sz] + zF
(3)
Electrochemical potential difference, inside with respect tooutside is
ΔμS = (μS)i – (μS)o = RT ln [Sz]i + zF (4)
[Sz]o
z = valence
= electrical potential
i -o = io
ΔμS = (μS)i – (μS)o = RT ln [Sz]i + zF (4)
[Sz]oUnits of Eq. 4: J/mol.
To convert to volts (recall ΔG Em): ÷ F. Thus
ΔμS = RT (2.303) log10 [Sz]i + z (5)
F [Sz]oFor the special case of the proton (H+), z = +1 ;
log10 [H+]i = -pHi ; log10 1/[H+]o = pHo ;
RT (2.303) = (8.314)(298)(2.303) = 0.059 V = 59 mV
Thus can rewrite Eq 5 as
ΔμH = 59 (pH0 – pHi) + (6)
F
96500F
F
Electrochemical Potential Difference of the Proton
ΔμH/F is Known as The Protonmotive Force(PMF)
From Eq 6 we can write
PMF = 59 (pHo – pHi) + (7)
Units: mV
A measure of the free energy in the electrochemicalpotential difference for protons across a membrane;
An important biological parameter because so much ofenergy transduction in biology is through the PMF
pHcytosol = pH0 = 7.5 ; pHmatrix = pHi = 8.0
= -170 mV (inside negative)
From Eq 7,
PMF = 59 (7.5 – 8.0) + (-170)
= -30 – 170 = -200 mV
THE PMF IN COUPLED MITOCHONDRIA
Proton flows, stoichiometries and energetics
Note:1. Total of 10 H+ pumped out per
2e– flowing through redoxchain
2. 4 mol H+ used directly foreach mol ATPsynthesised
Requirement for > 1 mol H+/mol ATP
Energy available for ATP synthesis is
ΔμH = (PMF)(F) = (-0.2)(96500) = -19.3 kJ/mol
But ΔG’ATP = -48.8 kJ/mol (Lecture 6)
i.e. for ATP synthesis, +48.8 kJ/mol is required
Reaction is
nH+o + ADP + Pi ↔ nH+
i + ATP
Energetically, this reduces to
nF(PMF) + ΔG’ATP < 0 for ATP generation.
By coupling ATP synthesis to translocation of 4 H+/ATP,
(-19.3)(4) = -77.2 kJ is available for each mol of ATP
This is more than enough energy!!
-
H+ Flows and Impact on Redox-Phosphorylation Coupling
1. Why do redox reactions speed up when ADP isadded? [See slide 20, Lecture 12]
Answer:
ADP facilitates flow of H+ through ATP synthase.
This decreases the PMF slightly.
Decrease in opposing driving force enables fasteroperation of redox coupled H+ pumps.
2. The action of uncouplers
[A brilliant prediction of the chemiosmotic hypothesis]
Some compounds (eg 2, 4 dinitrophenol)
(i) abolish ATP synthesis “uncouple”
(ii) speed up respiration
Chemiosmotic explanation:
Uncouplers catalyse passive H+ flow:
They are “protonophores”: abolish PMF
Thus (i) no driving force for ATP synthesis
(ii) no force opposing redox reactions
H+ Flows and Impact on Redox-Phosphorylation Coupling
How Uncouplers Work
UncouplerUncoupler
ATP synthase
Respiratorycomplexes
Membrane-bound ATP Synthase is Ubiquitous atEnergy-Coupling Membranes
No resp. pumps:
ATPase inhydrolytic mode
Evolved as H+ pump??
cc
compared with mitosC.
ATP Synthase and the PMF:
Recurring Themes in Bioenergetics
From previous slide…
Although energy sources and mechanisms of H+
pumps are variable,
presence and mechanism of ATP synthase isconserved through evolution.
Furthermore (next slide)…
Bacteria have evolved alternative physiological usesfor the PMF…
Solute uptake
PMF
ChampionSprinter
Summary
1. NADH oxidation is coupled to ATP synthesis viageneration of a PMF.
2. The energy stored in ion potential differencescan be expressed in terms of kJ/mol or mV andcomprises electrical and chemical components.
3. For H+, PMF = 59(pH0 - pHi) + Δψ (in mV).
4. Uncouplers (and ADP to some extent) tend toincrease the rate of e- transport by dissipating thePMF.
5. The PMF is also used to generate ATP atbacterial cell membranes and thylakoids….and…..
6. …powers bacterial solute uptake andswimming.