Introduction to metabolism.
Specific and general pathways
of carbohydrates, lipids and
The series of changes that a substance
undergoes after absorption from the
gastrointestinal tract where by it is used
for synthesis of some of the tissue
components or is broken down or
otherwise altered and eliminated from the
body through urine, feces, sweat or
CATABOLISM AND ANABOLISM
The process by which it is used in the
synthesis of tissue components are
referred to as “anabolism”
And the process by which it is broken
down into simpler products are referred
to as “catabolism”
This is process of substance convertion in
some part of metabolism.
They can be :
(glycolysis, β-oxidation of fatty acids)
(Kreb’s cycle, urea synthesis cycle)
(Pentose pathway of glucose oxidation).
The modern views on the biological oxidation
All the enzymes involved in this process of biological
to the major class of oxidoreductases
AH2 + O2 —————— ——> A + H2O
Cytochrome oxidase, which is the terminal component
of ETC, belongs to this category. It contains heme and
is described under the components of ETC.
2. Aerobic Dehydrogenases
AH2 + O2 ----—> A + H2O2.
These enzymes are flavoproteins and the product is
usually hydrogen peroxide.
3. Anaerobic Dehydrogenases
AH2 (reduced) + B (oxidised) → A (oxidised) + BH2 (reduced)
- NAD+ linked dehydrogenases AH2 + NAD+ → A + NADH + H+
- FAD-linked Dehydrogenases
4. Hydroperoxidases (All these enzymes use H2O2 as a
a)Peroxidase: H2O2 + AH2 —(peroxidase)——> 2H2O + A
b) Catalase 2H2O2 -—----(catalase)— —> 2H2O + O2
5. Oxygenases These are enzymes which catalyse
oxygen is transferred and incorporated into a substrate
a) Mono-oxygenases A-H + O2 + BH2 —(hydroxylase)—»
A-OH + H2O + B
b) Di-oxygenases A + O2 → AO2
FADH2 to the
Electrons of NADH or FADH2 are used to reduce molecular oxygen to
A large amount of free energy is liberated.
The electrons from NADH and FADH2 are not transported directly to O2 but
are transferred through series of electron carriers that undergo reversible
reduction and oxidation.
The flow of electrons through carriers leads to the pumping of protons out
of the mitochondrial matrix.
protons generates a
pH gradient and a
that creates a
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.
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
succinate FAD Fe-S
Fe-S cyt c1
High-Energy Electrons: Redox Potentials and Free-Energy
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 phos-phate compound).
∆G°' for ATP = -7.3 kcal mol-1
Electron transfer potential is expressed as E'o, (also
called redox potential, reduction potential, or
oxidation-reduction potential) require 0.34 EV for 1
E'o (reduction potential) is a measure of how easily a
compound can be reduced (how easily it can accept
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 Evolts (strong reducing agent)
E'o of O2 = +0.82 Evolts (strong oxidizing agent)
Important characteristic of ETC is the amount of energy
released upon electron transfer from one carrier to
This energy can be calculated using the formula:
n – number of electrons transferred from one carrier to
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
And 43.4 kcal/mol (FADH2).
• Mobile coenzymes: ubiquinone
(Q) and cytochrome c serve as
links between ETC complexes
• Complex IV reduces O2 to water
THE RESPIRATORY CHAIN CONSISTS OF FOUR
Components of electrontransport chain are
arranged in the inner
membrane of mitochondria
in packages called
cyt c 1
Complex I (NADH-ubiquinone oxidoreductase)
Transfers electrons from NADH to Co Q (ubiquinone)
- enzyme NADH dehydrogenase (FMN - prosthetic
group) - iron-sulfur clusters.
NADH reduces FMN to FMNH2.
Electrons from FMNH2 pass to a Fe-S clusters.
Fe-S proteins convey electrons to ubiquinone.
QH2 is formed.
The flow of two electrons from NADH to coenzym Q leads
to the pumping of four hydrogen ions out of the matrix.
Complex II (succinate-ubiquinon oxidoreductase)
Transfers electrons from succinate to Co Q.
Form 1 consist of:
- enzyme succinate dehydrogenase (FAD –
- iron-sulfur clusters.
Succinate reduces FAD to FADH2.
Then electrons pass to Fe-S proteins
which reduce Q to QH2
Form 2 and 3 contains enzymes acyl-CoA dehydrogenase
(oxidation of fatty acids) and glycerol phosphate dehydrogenase
(oxidation of glycerol) which direct the transfer of electrons
from acyl CoA to Fe-S proteins.
Complex II does not contribute to proton gradient.
Complex III (ubiquinol-cytochrome c oxidoreductase)
Transfers electrons from ubiquinol to cytochrome c.
Consist of: cytochrome b, Fe-S clusters and cytochrome c1.
electron transferring proteins containing a heme prosthetic
group (Fe2+ ⇔ Fe3+).
Oxidation of one QH2 is accompanied by the translocation
of 4 H+ across the inner mitochondrial membrane. Two H+
are from the matrix, two from QH2
Complex IV (cytochrome c oxidase)
Transfers electrons from cytochrome c to O2.
Composed of: cytochromes a and a3.
Catalyzes a four-electron reduction of molecular oxygen (O2) to
O2 + 4e- + 4H+ → 2H2O
Translocates 2H+ into the intermembrane space
The Chemiosmotic Theory
• Proposed by Peter Mitchell in the
1960’s (Nobel Prize, 1978)
• Chemiosmotic theory: electron
transport and ATP synthesis
are coupled by a proton
gradient across the inner
Mitchell’s postulates for chemiosmotic theory
1. Intact inner mitochondrial membrane is required
2. Electron transport through the ETC generates a proton
3. ATP synthase catalyzes the phosphorylation of ADP in a
reaction driven by movement of H+ across the inner
membrane into the matrix
Active Transport of ATP, ADP and Pi Across the
Inner Mitochondrial Membrane
• ATP must be transported to the cytosol, and ADP and Pi must
enter the matrix
• ADP/ATP carrier, adenine nucleotide translocase, exchanges
mitochondrial ATP4- for cytosolic ADP3• The exchange causes a net loss of -1 in the matrix (draws some
energy from the H+ gradient)
• Phosphate (H2PO4-) is transported into matrix in symport with
H+. Phosphate carrier draws on ∆pH.
• Both transporters consume proton-motive force
The most important factor in determining the rate of
oxidative phosphorylation is the level of ADP.
The regulation of the rate of oxidative
phosphorylation by the ADP level is called respiratory
REGULATION OF OXIDATIVE
Coupling of Electron Transport with ATP Synthesis
Electron transport is tightly coupled to phosphorylation.
ATP can not be synthesized by oxidative phosphorylation
unless there is energy from electron transport.
Electrons do not flow through the electron-transport chain
to O2 unless ADP is phosphorylated to ATP.
Important substrates: NADH, O2, ADP
Intramitochondrial ratio ATP/ADP is a control mechanism
High ratio inhibits oxidative phosphorylation as ATP
allosterically binds to a subunit of Complex IV