Download Electron Transport and oxidative phosphorylation (ATP Synthesis)

Electron Transport and
oxidative phosphorylation
(ATP Synthesis)
Dr. Abir Alghanouchi
Biochemistry department
Sciences college
All of the reactions involved in cellular respiration can be grouped into three main stages
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Glycolysis – occurs in cytoplasm
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The Krebs cycle – occurs in matrix of mitochondria
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Electron transport – occurs across the mitochondrial membrane
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Process in which ATP is formed as a result of transfer of electrons from NADH or FADH2 by a series of electron carriers
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The electron transport chain generates no ATP directly. Rather, its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy
During respiration, most energy flows from glucose ‐>
NADH ‐> electron transport chain ‐> proton‐motive force
‐> ATP.
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Phosphorylation
process
Oxidative
process
O2 einner
membrane
H 2O
H+
ATP
Synthase
ADP+
Pi
H+
outer
membrane
ATP
intermembrane
space
matrix
Figure: Essential features of oxidative phosphorylation
¾
Redox reactions of respiratory chain use electrons to reduce oxygen to water
¾ Energy generated moves protons from matrix to intermembrane space
¾
Inward movement of protons recovers this energy to promote formation of ATP in the matrix.
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Protein
complex
Electron
carrier
Inner
mitochondrial
membrane
Electron
flow
Electron transport chain
ATP synthase
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ATP yield
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Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate‐level phosphorylation (2 from glycolysis and 2 from TCA)
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The vast majority of the ATP (90%) comes from the energy in the electrons carried by NADH and FADH2
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High-energy
electrons carried
mainly byNADH
High-energy
electronscarried
by NADH
Cytosol
Mitochondrion
Glycolysis
Glucose
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Pyruvic
acid
2
Acetyl‐
CoA
Krebs
Cycle
Electron
Transport
Maximum
per
glucose:
by direct
synthesis
by
direct
synthesis
by
ATP
synthase
A Road Map for Cellular Respiration
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Chemical reactions that transfer electrons from one substance to another are called oxidation‐reduction reactions
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REDOX short for oxidation‐reduction reactions
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REDOX FACTS
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A:H A
Reductant ' Oxidant + e‐
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B
B:H
Oxidant + e‐ ' Reductant
(acceptor)
(donor)
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Both oxidation and reduction must occur simultaneously
The reductant of one pair donates electrons and the oxidant of the other pair accepts the electrons
Red1 (AH) + Ox2 (B) Î Ox1(A) + Red2(BH)
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Electrons can move through a chain of donors and acceptors
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In the electron transport chain, electrons flow down a gradient
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Electrons move from a carrier with low reduction potential (high tendency to donate electrons) toward carriers with higher reduction potential (high tendency to accept electrons)
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Potential (EO): measure of the tendency of oxidant to gain electrons, to become reduced, a potential energy.
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∆EO: Standard reduction potential difference between two half reactions
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Succinate Eo' = 0.03V
∆Eo' = 0.07V
I
NADH
Eo' = -0.32V
II
Coenzyme Q
Eo' = 0.10V
∆Eo' = 0.42V
III
∆Eo' = 0.19V
Cytochrome C
Eo' = 0.29V
IV
½ O2
Eo' = 0.82V
∆Eo' = 0.53V
electron flow
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The components of the RC are arranged in order of increasing redox potential
¾
The ∆Eo′ values are the potential differences across the four complexes ( that span the mitochondrial inner membrane)
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Succinate Eo' = 0.03V
∆Eo' = 0.07V
I
NADH
Eo' = -0.32V
II
Coenzyme Q
Eo' = 0.10V
∆Eo' = 0.42V
III
∆Eo' = 0.19V
Cytochrome C
Eo' = 0.29V
IV
½ O2
Eo' = 0.82V
∆Eo' = 0.53V
electron flow
¾
The overall voltage drop
from NADH E0′ = ‐(‐0.32 V)
to O
E0′ = +0.82 V is ∆Eº′ = 1.14 V
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RC exists as four large, multi‐
subunit protein complexes
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`
`
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The respiratory electron transport chain
complex I is a NADH‐
ubiquinone reductase
complex II is succinate dehydrogenase complex III is the ubiquinone ‐
cytochrome c reductase
complex IV is cytochrome oxidase
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Figure: Complex I of the respiratory chain that links NADH and coenzyme Q. `
NADH Dehydrogenase (NADH‐ubiquinone reductase)
accepts 2e‐ from NADH and transfers them to ubiquinone (coenzyme Q), an electron carrier
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Uses two bound cofactors to accomplish this: FMN
(Flavin mononucleotide) and 6 iron‐sulfur (Fe‐S) protein
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Complex II: Succinate-CoQ reductase
Prosthetic groups: FAD; Fe-S
Succinate
FAD
SDH
Fumarate
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`
`
FADH2
CoQ
SDH is succinate dehydrogenase an enzyme of the citric acid cycle (associated with membrane)
2 e‐ transferred from succinate to CoQ 1 mole FADH2 produced
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Electrons from
complex I or II
CoQ
cyt b/cyt c1
Complex III: cytochrome reductase
Prosthetic groups: heme b; heme c1; Fe-S
cyt c
Figure: Complex III of the respiratory chain linking CoQ and cytochrome C. `
Is composed of cytochome b, cytochrome C1 and iron sulphur proteins
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Accepts e‐ from coenzyme Q and transfers e‐ to cytochrome c coupled with the transfer of protons from the matrix to the intermembrane space
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Figure: Complex IV ‐cytochrome oxidase‐ reducing oxygen to water `
Contains cytochromes a/a3 and 2 Cu ions involved in e‐
transfers
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Cytochrome oxidase passes electrons from cytochrome c through a series of heme groups and Cu ions to O2, reducing it to H2O (end product)
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Coenzymes and cytochromes in the complexes act as e‐ donors & acceptors
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Flavin MonoNucleotide (FMN), in Complex I, functions like FAD (which is an electron acceptor that helps electron transfer during Krebs Cycle and Electron Transport Chain in cellular respiration).
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iron‐sulfur (Fe‐S proteins): Fe‐S centers transfer e‐ in Complexes I, II and III
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Coenzyme Q (ubiquinone), lipid soluble, floats in the membrane and doesn’t require protein
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Cytochromes (b, c1, c, a, a3; contain heme): transfer e‐ in Complexes III and IV, Cytc is the only soluble cytochrome
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NAD+, FMN, CQ are carriers of e‐ and hydrogen while cytochromes are carriers of electrons only.
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ATP‐synthase (complex V), present in the inner mitochondrial membrane, actually makes ATP from ADP
and Pi.
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ATP used the energy of an existing proton gradient to power ATP synthesis.
◦ This proton gradient develops between the intermembrane space and the matrix.
◦ This concentration of H+ is the proton‐motive force.
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¾
The ATP synthase molecules are the only place that will allow H+ to diffuse back to the matrix
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This flow of H+ is used by the enzyme to generate ATP a process called chemiosmosis.
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Chemiosmosis: (osmos = push) is the oxidative phosphorylation that results in ATP production in the inner membrane of mitochondria.
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Properties of ATP Synthase
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Multisubunit transmembrane protein
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Molecular mass = ~450 kDa
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Functional units
◦ F0: water‐insoluble transmembrane protein
(up to 8 different subunits)
◦ F1: water‐soluble peripheral membrane protein (5 subunits) ,contains the catalytic site for ATP synthesis
Flow of 3 protons through ATP synthase leads to phosphorylation of 1 ADP
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Cytosolic NADH (glycolysis) must enter the mitochondria to fuel oxidative phosphorylation but NADH and NAD+ cannot diffuse across the inner mitochondrial membrane
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Two shuttle systems for reducing equivalents:
1.
Glycerol phosphate shuttle: insect flight muscles
2.
Malate Malate‐aspartate shuttle: predominant in liver and other mammalian tissues
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In muscle and brain
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Each NADH converted to FADH2 inside mitochondrion
◦ FADH2 enters later in the electron transport chain
◦ Produces 2 ATP
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In liver and heart
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NADH oxidized while reducing oxaloacetate to malate
◦ Malate dehydrogenase
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Malate crosses membrane
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Malate reoxidized to oxaloacetate
◦ Malate dehydrogenase
◦ NAD+ reduced to NADH
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NADH via electron transport yields 3 ATP
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Respiratory inhibitors
¾ These compounds prevent the passage of e‐ by binding a component of the ETC blocking the oxidation/reduction reaction
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Complex
designation
Functional
groups
Function
I–
----------NADH-Q
reductase
FMN (flavin ----------mono‐
nucleotide); Fe‐S
oxidizes NADH to NAD+; Rotenone
----------------------transfers electrons to coenzyme Q
II – SuccinateQ reductase
-----------
FAD; Fe‐S
oxidizes succinate to fumarate with reduction of FAD to --------------FADH2; electron transfer to CoQ
III Cytochrome
----------reductase
heme b; heme c
1; ---------Fe‐S
transfers electrons between coenzyme Q -------------and cytochrome C (C becomes reduced)
Antimycin A
----------
IV ----------C
Cytochrome
oxidase
heme a‐a3; ---------Cu
oxidizes cytochrome C; ------------reduces ½O
2 to H2O
Carbon monoxide
---------Cyanide
----------
Inhibitors
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Lippincot's Illustrated Reviews Biochemistry
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Lechinger's Principles of Biochemistry 4th edition. D. L. Nelson and M.M. Cox, Worth Publishers.
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Harpers illustrated biochemistry 25th edition. Robert K. Murray; Darly K. Granner: Victor W Rodwell
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www.rpi.edu/dept/bcbp/molbiochem
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www.med.ufl.edu/biochem/rcohen/rcohen.html
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http://courses.cm.utexas.edu/jrobertus/ch339k
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