Download 1.Oxidative phosphorylation

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)
Δ Gº -7.3 kcal/mol/bond
ATP is a product of catabolic reactions and a driver of biosynthetic reactions
ATP readily forms a complex with magnesium ion, and it is this
complex that is required in all reactions in which ATP participates,
including its synthesis. A magnesium deficiency impairs virtually all
of metabolism, because ATP can neither be made nor utilized in
adequate amounts
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
Electron transport chain is formed from five separate enzyme complexes
called complexes I, II, III, IV and V
Complexes I, II, III and IV contain parts of the electron transport chain
(oxidation), while complex V catalyzes ATP synthesis (phosphrylation).
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
All components of the electron transport chains are protein in nature,
except Coenzyme Q. The coenzyme Q and cytochrome C are the mobile
components of the electron transport chain
Energy released from NADH & FADH2 entering ETC
P : O ratios
The transport of a pair of electrons from NADH (and FMNH2) to oxygen via the
electron transport chain produces energy which is more than sufficient to
produce 3 ATPs from 3 ADP and 3 Pi. The transport of a pair of electrons from
FADH2 to oxygen via the ETC produces sufficient energy to produce 2 ATPs
from 2ADPs.
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
Mitochondrial permeability transition pore (MPTP)
Located in the inner mitochondrial membrane, the MPTP is a nonselective pore
that is a critical factor in cell death. It is normally closed, but will open when
cells are reperfused after a period of ischemia (ischemic reperfusion injury,
IRI), and small molecules will leave the mitochrondrial matrix. Opening of the
MPTP is now considered a key feature of IRI in which cellular damage is much
greater than that produced by ischemia alone. Cascades of reactions occuring
in response to IRI lead to apoptosis and necrosis, and cell death.
Ischemia, such as that found in heart attacks, is usually caused by a clot that
blocks an artery. Clot busters such as streptokinase can be administered to
dissolve clots and reperfuse ischemic cells. But, if the ischemic state has been
prolonged before administration of a clot buster, death may result from
reperfusion injury and opening of the MPTP. This occurs all too often in heart
attack patients. Several drugs, such as cyclosporin A, inhibit the MPTP from
opening and may protect cells from necrosis or apoptosis after the
administration of a clot buster. In fact, a large clinical trial of cyclosporin A is
currently being conducted with heart attack patients in conjunction with the
administration of a clot buster.