Transcript Substrate and oxidative phosphorylation

Substrate and oxidative
phosphorylation
• Substrate-level phosphorylation is a type of
chemical reaction that results in the formation and
creation of adenosine triphosphate (ATP) by the
direct transfer and donation of a phosphoryl (PO3)
group to adenosine diphosphate (ADP) from a
reactive intermediate. While technically the
transfer is PO3, or a phosphoryl group, convention
in biological sciences is to refer to this as the
transfer of a phosphate group. In cells, it occurs
primarily and firstly in the cytoplasm (in
glycolysis) under both aerobic and anaerobic
conditions.
• Unlike oxidative phosphorylation, here the
oxidation and phosphorylation are not coupled or
joined, although both types of phosphorylation
result in ATP.
• It should be noted that there is an oxidation
reaction coupled to phosphorylation, however this
occurs in the generation of 1,3bisphosphoglycerate from 3phosphoglyceraldehyde via a dehydrogenase. ATP
is generated in a separate step (key difference
from oxidative phosphorylation) by transfer of the
high energy phosphate on 1,3-bisphosphoglycerate
to ADP via a kinase.
ATP is synthesized when protons flow back to the
mitochondrial matrix through an enzyme complex
ATP synthase.
The oxidation of fuels and the phosphorylation of
ADP are coupled by a proton gradient across the
inner mitochondrial membrane.
Oxidative
phosphorylation is
the process in which
ATP is formed as a
result of the
transfer of electrons
from NADH or
FADH2 to O2 by a
series of electron
carriers.
OXIDATIVE PHOSPHORYLATION IN
EUKARYOTES TAKES PLACE IN MITOCHONDRIA
Two membranes:
outer membrane
inner membrane (folded into
cristae)
Two compartments:
(1) the intermembrane space
(2) the matrix
The outer membrane
is permeable to small
molecules and ions
because it contains
pore-forming protein
(porin).
The inner membrane
is impermeable to ions
and polar molecules.
Contains transporters
(translocases).
Location of mitochondrial complexes
• Inner mitochondrial membrane:
Electron transport chain
ATP synthase
• Mitochondrial matrix:
Pyruvate dehydrogenase complex
Citric acid cycle
Fatty acid oxidation
THE ELECTRON TRANSPORT CHAIN
Series of enzyme complexes (electron carriers)
embedded in the inner mitochondrial membrane,
which oxidize NADH2 and FADH2 and transport
electrons to oxygen is called respiratory
electron-transport chain (ETC).
The sequence of electron carriers in ETC
NADH
FMN
Fe-S
succinate FAD Fe-S
Co-Q
Fe-S
cyt b
cyt c1
cyt c
cyt a
cyt a3
O2
High-Energy Electrons: Redox Potentials
and Free-Energy Changes
In oxidative phosphorylation, the electron
transfer potential of NADH or FADH2 is
converted into the phosphoryl transfer
potential of ATP.
Phosphoryl transfer potential is G°' (energy
released during the hydrolysis of activated phosphate compound). G°' for ATP = -7.3 kcal mol-1
Electron transfer potential is expressed as E'o,
the (also called redox potential, reduction
potential, or oxidation-reduction potential).
E'o (reduction potential) is a measure of how easily a
compound can be reduced (how easily it can accept
electron).
All compounds are compared to reduction potential of
hydrogen wich is 0.0 V.
The larger the value of E'o of a carrier in ETC the better
it functions as an electron acceptor (oxidizing factor).
Electrons flow through the ETC components spontaneously
in the direction of increasing reduction potentials.
E'o of NADH = -0.32 volts (strong reducing agent)
E'o of O2 = +0.82 volts (strong oxidizing agent)
NADH
FMN
Fe-S
succinate FAD Fe-S
Co-Q
Fe-S
cyt b
cyt c1
cyt c
cyt a
cyt a3
O2
Important characteristic of ETC is the amount of
energy released upon electron transfer from one
carrier to another.
This energy can be calculated using the formula:
Go’=-nFE’o
n – number of electrons transferred from one carrier
to another;
F – the Faraday constant (23.06 kcal/volt mol);
E’o – the difference in reduction potential between
two carriers.
When two electrons pass from NADH to O2 :
Go’=-2*96,5*(+0,82-(-0,32)) = -52.6 kcal/mol