Lecture 17 - Electron Transport and Oxidative Phosphorylation Flashcards
What coloured meat has the highest mitochondrial content
Dark meat - contains a lot of cytochrome proteins
What is the mitochondrial membrane dense in?
Protein
What does the chemiosmotic theory describe
Energy conversion in essentially all organisms
What is chemiosmosis
Movement of H+ down the concentration gradient (in mitochondrial IM and ATP is produced)
What does a proton gradient drive?
Electron potential
heat production
NADPH synthesis
ATP ~P
Flagellar rotation
Active transport
plant and animal chemisosmosi
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How does the chemiosmotic proton circuit act like an electrical circuit
Electric current – H+ flow
Battery – electron transport system
Capacitor – H+ gradient
Resistor – ATP synthase
What do uncouplers do
energy converted to heat rather than ATP synthesis
Causes proton leakage
What do blockers do
Shut down H+ flowso DpH and Dy increase, causing cell death
What is an example of an uncoupler
uncoupler proteins
2,4 Dinitrophenol
What is an example of a blocker
Oligomycin
When is uncoupling very beneficial
Hibernating animals create heat to prevent freezing
Bodybuilders take 2,4DNP which causes weight loss, but can be very toxic
What was the first experimental evidence for chemiosmotic theory
evidence that an electrochemical H+ gradient can link directly with an electron transport system and provide energy needed for oxidative phosphorylation (ATP synthesis) using a reconstituted vesicle and ATP synthase from cow hearts and bacterial rhodopsin
Efraim Racker + Walter Stoeckenius, 1973:
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Redox reactions and energy generation
- Measure of phosphoryl transfer potential: DGo’ for hydrolysis of an activated phosphate compound.
- Need expression for electron transfer potential, since oxidative phosphorylation involves converting electron transfer potential of NADH and FADH2 into phosphoryl transfer potential of ATP.
- Biological electron transport: series of linked reduction and oxidation reactions (redox reactions).
- Electron donor (the reductant) is oxidised while transferring electrons to an acceptor (the oxidant). Reduction involves gain of electrons (OILRIG mnemonic).
- Oxidation and reduction must occur together, but it is convenient to consider the two halves
of a redox reaction separately, e.g.Fe2+ + Cu2+ Fe3+ + Cu+
can be described in terms of two half-reactions:Fe2+ Fe3+ + e– Cu2+ + e– Cu+
- For A(red) + B(ox) A(ox) + B(red)
The oxidised plus reduced forms of A and B, i.e. A(ox)/A(red) and B(ox)/B(red) are known as
redox couples
redox pairs
or half cells - Electrons can be transferred from donor to acceptor in four ways:
(a) Directly as electrons, e.g.:
Fe2+ + Cu2+ Fe3+ + Cu+
(b) As hydrogen atoms (a proton and a single electron):
AH2 A + 2e– + 2H+in which AH2 is the hydrogen/electron donor AH2 and A together constitute a redox couple (A/AH2), which can reduce another compound B (or redox couple B/BH2) by transfer of hydrogen atoms: AH2 + B A + BH2
(c) As a hydride ion (:H–), which comprises a proton and two electrons
(d) Direct combination with oxygen
Electron transfers of types (a), (b) and (c) occur in oxidative phosphorylation
Redox potentials
- Tendency of a redox couple to accept or donate electrons depends on the redox potential (or reduction potential or oxidation-reduction potential) of the couple
- The standard redox potential of a couple, Eo’, is measured in an electrochemical cell relative to the standard hydrogen electrode (SHE)
- SHE = hydrogen gas bubbled over a platinum electrode in 1 M acid solution. The reaction 2H+ + 2e– H2 is given an Eo value of 0 volts (V) by convention
- A strong reducing agent (e.g. NADH) is poised to donate electrons and has a negative redox potential
- A strong oxidising agent (e.g. O2 or Fe3+) is ready to accept electrons and has a positive redox potential
- Standard redox potentials for biologically important reactions are measured at pH 7 ([H+] = 10–7 M) instead of pH 0 ([H+] = 1 M).Eo’ = potential of a redox couple in which reduced and oxidised species
are present at 1 M concentration, 25ºC, pH 7.At pH 7, hydrogen electrode E’ = –0.42 V. - In a spontaneous reaction, electrons flow
from redox couple of lower potential
to redox couple of higher potential.NAD+/NADH (Eo’ = –0.32 V) will lose electrons to SHE in 1 M acid (Eo = 0 V)
but
NAD+/NADH will gain electrons from the H electrode at pH 7 (E’ = –0.42 V).
Free energy changes from oxidation/reduction reactions I
- When an electron is moved in an electric field,
work done = (electron charge x potential)
- For electron(s) transferred over potential difference DEº’
DGº’ = –nFDEº’ n = number of electrons transferred F = Faraday constant (96.5 kJ mol–1 V–1) DEº’ = difference in standard reduction potentials between the two redox couples (volts, V) DGº’ is in kJ mol–1
- For a spontaneous reaction (DGº’ negative), DEº’ must be positive
What is the basic pathway for the generation of ATP through electron transport
emf (electron motive force)—> pmf (proton motive force)—-> ATP
What is the trend between strength of negative current and donation of electrons
The stronger the negative charge, the more likely they are to donate electrons
What is the charge of a hydrogen electrode at pH 7
-0.42
How do electrons flow in a spontaneous reaction
Lower potential to higher potential
What does SHE stand for
Standard hydrogen electrode
How do you calculate the electrons transferred over potential difference
DGº’ = –nFDEº’
n = number of electrons transferred F = Faraday constant (96.5 kJ mol–1 V–1) DEº’ = difference in standard reduction potentials between the two redox couples (volts, V) DGº’ is in kJ mol–1
What is the equation for work done
work done = (electron charge x potential)
How do you calculate nFDEo’
DGo’ = –nFDEo’
= –nF[Eo’(acceptor) – Eo’(donor)]
What is the Nernst equation used for
To calculate reduction potentials under non-standard conditions
What is the Nernst Equation
E’ = Eo’ + (2.303 RT / nF) log10 [e– acceptor] / [e– donor]
where R = gas constant (8.314 J K–1 mol–1)
T = absolute temperature in kelvin 2.303 = conversion factor from natural (base e) to common (base 10) logs At 25ºC, (2.303 RT / nF) = 0.059 for 1-electron transfer At 25ºC, (2.303 RT / nF) = 0.0295 for 2-electron transfer
for a spontaneous equation, what must DG be
Positive (>0)
