Download Oxidative phosphorylation (1)

Respiratory Chain
The Oxidative Phosphorylation
Objectives
• ATP as energy currency
• Mitochondria and the electron transport chain organization
• Inhibitors of the electron transport chain
• Oxidative phosphorylation and the uncoupling proteins
• Inherited defects in oxidative phosphorylation.
• The role of mitochondria in apoptosis
Adenosine Triphosphate (ATP)
Biochemists call ATP the ‘common
currency of metabolism’ because it
allows energy from fuel metabolism
to be used for work, transport and
biosynthesis.
1 gm fat gives 9 kcal while 1 gm
carbohydrate and protein give 4
kcal only,
about 40% of food energy is
conserved as ATP, and the
remaining 60% is liberated as heat.
Energy-rich molecules as glucose, fatty acids & amino acids
are metabolized by a series of metabolic reactions yielding CO2 and H2O
&
Energy is produced as ATP or heat
Electron Transport Chain (ETC) =
Respiratory chain
Electron transport chain (ETC) is the
final common pathway in aerobic
cells by which electrons and
hydrogen (NADH & FADH2) derived
from foodstuffs are transferred to
oxygen to form water and finally
produce ATP (energy)
ETC is located in the inner
mitochondrial membrane
& is the final common pathway of
metabolism
(oxidative phosphorylation)
Electron Transport Chain (ETC)
• Energy-rich molecules as glucose or fatty acids are metabolized by a
series of metabolic reactions yielding CO2 and H2O .
• The metabolic intermediates of these reactions give electrons to
specialized co-enzymes NAD and FAD to form NADH and FADH2 which
donate a pair of electrons to specialized set of electrons carriers,
named the electron transport chain, ECT (Respiratory chain).
• As electrons are passed down the electron transport chain, they lose
much of their energy. Part of this energy can be taken and stored by
production of ATP from ADP and inorganic phosphate Pi (Oxidative
phosphorylation).
The remainder of the free energy not trapped as ATP is released as heat
Electron Transport Chain (ETC)
Electron transport chain
Organization of the respiratory chain:
The inner mitochondrial membrane contains 5 separate enzyme
complexes(Complexes I, II, III ,IV and V )
a)Each complex accepts or donates electrons to relatively mobile carriers
such as coenzyme Q and cytochrome C
B) Each carrier of the electron transport chain can receive electrons from a
donor and can subsequently donate electrons to the next carrier in the
chain. The electrons finally combine with O2 and proton (H+) to form H2O.
This requirement for O2 makes the electron transport process named
respiratory chain
Electron transport chain
Components of respiratory chain:
All components of the electron transport chains are protein in nature,
except Coenzyme Q.
1)Complex I: contain enzyme called NADH dehydrogenase
2)Complex II : contain enzyme called flavoprotein dehydrogenase
3)Complex III: contains cytochrome b enzyme and cytochrome c1
4) Complex IV: contains cytochrome a+a3 (cytochromes are conjugated
proteins with heme ring)
Electron transport chain
5)Complex V : ATP synthase it is formed of 2 subunits
i)F1 subunit which protrudes into the matrix
ii)F0 subunit which is present in the inner mitochondrial membrane
The protons outside the inner mitochondrial membrane can re-enter the
mitochondrial matrix by passing through F0-F1 complex .this results in
synthesis of ATP from ADP+Pi
Electron transport chain
Reactions of respiratory chain :
1)Entry via NADH+H :
NADH+H can join the chain giving electrons to complex I to coenzyme Q to
complex III to complex IV to final acceptor O2
2)Entry via FADH2 :
FADH2 obtained from complex II can join the chain directly giving electrons
to coenzyme Q then complex III to complex IV to final acceptor O2
The transport of a pair of electrons from NADH to oxygen via the
ETC produces energy which is sufficient to produce 3 ATPs. The
transport of a pair of electrons from FADH2 to oxygen via the ETC
produces sufficient energy to produce 2 ATPs .
Energy released from NADH & FADH2 entering ETC
P : O ratios
It is the ratio between numbers of ADP changed into ATP to the number of
oxygen atom utilized by respiratory chain . It is 3:1 if electrons enter through
NADH+H and it is 2:1 if electrons enter through FADH2.
How the free energy generated by the transport of electrons
by ECT is used to produce energy (ATP)
Coupling of ECT to phosphorylation of ADP to ATP
Transfer of electrons across electron transport chain
FREE ENERGY RELEASED
Transport of protons (H+) across the inner mitochondrial membrane from the
matrix to the intermembrane space.
This creates an electrical gradient with more +ve charge on the outside of the
membrane than on the inside and a pH gradient with lower pH on outside.
Proton Pump
Electron transport is coupled to the phosphorylation of ADP by the transport of
protons (H+) across the inner mitochondrial membrane from the matrix to the
intermembrane space. This creates an electrical gradient with more +ve
charge on the outside of the membrane than on the inside and a pH gradient
with lower pH on outside.
Protons reenter (goes back) the mitochondrial matrix by passing through a
channel in the complex V (ATP synthase complex) giving an energy that are
required for the phosphorylation of ADP to ATP.
Oxidative Phosphorylation (in mitochondria):
Oxidation: electron flow in electron transport chain (with production of energy)
Phosphorylation: phosphorylation of ADP to ATP
1- Inhibitors of oxidation via ETC are compounds that prevent the
passage of electrons by binding to a component of the chain and
subsequently blocking the oxidation/reduction reactions.
As ETC and oxidative phosphorylation are tightly coupled, inhibition of
the ECT also inhibits ATP synthesis. E.g: cyanide and CO poisoning
2- inhibitor of phosphorylation (Oligomycin)binds to ATP synthase closing
the H+ channel preventing reentry of protons to the matrix & thus
preventing phosphorylation of ADP to ATP
3- Uncoupling proteins (UCP)
Uncoupling proteins (UCPs) are located in the inner mitochondrial membrane
leading to proton leak as they allow protons to reenter the mitochondrial
matrix with no accompanying synthesis of ATP (no phosphorylation of ADP
to ATP). No energy is utilized for the process of ATP synthesis, although ETC
is functioning….i.e. The process of ETC is not coupled to posphorylation.
However, energy is released in the form of heat.
• The first discovered was uncoupling protein-1 (UCP1), formerly known as
thermogenin, which is found exclusively in brown adipose tissue. Brown
adipose tissue is abundant in the newborn and in some adult mammals, and it
is brown because of its high content of mitochondria. In humans, brown adipose
tissue is abundant in infants, but it gradually diminishes and is barely detectable
in adults. UCP1 provides body heat during cold stress in the young and in some
adult animals. It accomplishes this by uncoupling the proton gradient, thereby
generating heat (thermogenesis) instead of ATP. Uncoupling proteins are
expressed at high levels in hibernating animals, permitting them to maintain
body temperature without movement or exercise.
Synthetic Uncouplers
Synthetic uncouplers are compounds that can uncouple ETC &
phosphorylation by increasing the permeability of the inner
mitochondrial membrane to protons (thus will not reenter through ATP
synthase)
Examples:
* 2,4-dintrophenol
An uncoupler that causes electron transport to proceed at a
rapid rate without phosphorylation & thus energy is released
as heat rather than being used to synthesize ATP
* High dose of aspirin (salicylates)
uncouples oxidative phosphorylation causing fever (observed with toxic
overdose of aspirin)
Inherited defects in oxidative phosphorylation
• Mitochondrial DNA (mtDNA) is small circular DNA maternally inherited as
mitochondria of sperm cell do not enter the fertilized ova.
• Mitochondrial DNA (mtDNA) codes only for 13 polypeptide (of total 120)
required for oxidative phosphorylation.
(while the remaining are synthesized in the cytosol & are transported into the
mitochondria).
• Defects of oxidative phosphorylation usually results from alteration in mtDNA
(mutation rate 10 times more than that of nuclear DNA).
• Tissues with greater ATP requirement (as CNS, sk.ms. & heart muscles, kidney
& liver) are most affected by defects in oxidative phosphorylation.
Mitochondria and apoptosis
initiated by the formation of pores in the outer
Mitochondrial membrane
pores allow cytochrome c to leave and enter
the cytosol
+ proapoptotic factors, activates a family of
proteolytic enzymes ,the caspases
cleavage of key
proteins and
resulting in the
morphologic
and biochemical
changes
characteristic
of apoptotic
cell death