Chapter 7
Biological
Oxidation
Biological oxidation is the
cellular process in which the
organic substances release energy
(ATP), produce CO2 and H2O
through oxidative-reductive
reactions.
organic substances:
carbohydrate, fat and protein
7.1 Principal of Redox Reaction
The electron-donating molecule in a
oxidation-reduction reaction is called
the reducing agent or reductant;
the electron-accepting molecule is the
oxidizing agent or oxidant:
for example:
Fe2+ (ferrous) lose -e
Fe3+ (ferric) gain +e
Redox reaction = reduction-oxidation
reaction
Several forms of Biological Reduction
1. Gain of electrons
2. Hydrogenation
3. Deoxygenation
Several forms of Biological Oxidation
1. Loss of electrons
2. Dehydrogenation
3. Oxygenation
Áoxidation-reduction potential
( or redox potential), E : it is a
measure of the affinity of a
substance for electrons. It decide the
loss (or the gain) of electrons.
ÁA positive E: the substance has a
higher affinity for electrons , accept
electrons easily.
A negative E: the substance has a
lower affinity for electrons , donate
electrons easily.
E0`, the standard redox
potential for a substance :is
measured under stander
condition(25℃, 1mmol/L reaction
substance),at pH7, and is expressed
in volts.
Section 7.2
Respiration Chain and
Oxidative Phosphorylation
7.2.1
Respiratory Chain
• Term:
A chain in the mitochondria
consists of a number of redox
carriers for transferring electrons
from the substrate to molecular
oxygen to form oxygen ion, which
combines with protons to form water.
Redox carriers including 4 protein
complexes
1.Complex I:
NADH:ubiquinone oxidoreductase
NADH:CoQ oxidoreductase
2.Complex II:
Succinate dehydrogenase
3.Complex III:
cytochrome bc1 (ubiquinone Cyt c
oxidoreductase)
4.Complex IV:
cytochrome oxidase
Complex I (NADH:ubiquinone
oxidoreductase)
• Function: transfer electrons from NADH
to CoQ
• Components:
NADH dehydrogenase (FMN)
Iron-sulfur proteins (Fe-S)
complex Ⅰ
NADH→ FMN; Fe-SN-1a,b; Fe-SN-4; Fe-SN-3; Fe-SN-2
→CoQ
R=H: NAD+;
R=H2PO3:NADP+
1. NAD(P)+: Nicotinamide Adenine Dinucleotide Phosphate)
Oxidation of NADH is a 2electron(2e), 2-proton(2H)
reaction
NAD+ or NADP+
NADH or NADPH
2. FMN can transfer 1 or 2 hydride
ions each time
FMN: flavin mononucleotide
Accepts 1 H+ and 1 eto form semiquinone
= stable free radical
Accepts 2 H+ and 2 eto give fully reduced form
3. Iron-sulfur clusters (Fe-S)
transfers 1-electron at a time,
without proton involved
Fe3++e-
Fe2+
4.Ubiquinone (CoQ) is lipid-soluble, not
a component of complex Ⅰ, can transfer 1
or 2 hydride ions each time.
Function: transfer electrons and protons
from complex Ⅰ,Ⅱto complex Ⅲ.
NADH+H+
NAD+
FMN
Reduced Fe-S
Q
FMNH2
Oxidized Fe-S
QH2
Matrix
Intermembrane space
Complex II: Succinate
dehydrogenase (Succinate: CoQ
oxidoreductase)
• Function: transfer electrons from
succinate to CoQ
• Components:
Succinate dehydrogenase (FAD, Fe-S)
Cytochrome b560
Complex Ⅱ
Succinate→ Fe-S1; b560; FAD; Fe-S2 ; Fe-S3
→CoQ
Cytochromes a, b, c are heme proteins,
their heme irons participate redox
reactions of e- transport.
Fe3++e-
Fe2+
Intermembrane space
Matrix
Succinate
Complex III:
cytochrome bc1 (ubiquinone Cyt c
oxidoreductase)
• Function: transfer electrons from CoQ
to cytochrome c
• Components: iron-sulfur protein
cytochrome b(b562, b566)
cytochrome c1
complex Ⅲ
QH2→ b562; b566; Fe-S; c1
→Cyt c
Cytochrome c is soluble, which will transfer
electrons to complex Ⅳ
Intermembrane
space
Matrix
Complex IV: cytochrome
oxidase
• Function: transfer electrons from Cyt c
to molecule oxygen, the final electron
acceptor.
• Components: cytochrome aa3
copper ion (Cu2+)
Cu2+ + eCu+
Complex IV
Cyt c → CuA→a→a3→CuB
→ O2
Cytochrome c
Coenzyme Q
ubiquinone/ol
Sequence of respiratory
chain
Principles:
• e- tend to flow from a redox pair with a
lower E°to one with a higher E°
• In the e--transport chain, e--carriers are
arranged in order of increasing redox
potential, making possible the gradual
release of energy stored in NADH, FADH2
Redox potential
redox pair
E0
There are two respiratory chains
• NADH respiratory chain
NADH
Complex Ⅰ
CoQ
Complex Ⅲ
cytochrome c
Complex Ⅳ
O2
• Succinate (FADH2) respiratory chain
Succinate ComplexⅡ CoQ
ComplexⅢ cytochrome c ComplexⅣ
O2
NADH
respiration
chain
FADH2
respiration
chain
7.2.2 Oxidative Phosphorylation
• The oxidation of organic nutritions produces the
energy-rich molecules, NADH and FADH2.
• The oxidation of NADH or FADH2 in
mitochondrial is the electron transferring
through respiration chain.
• The free energy produced in electron
transferring supports the phosphorylation of
ADP to form ATP.
• The oxidation of NADH or FADH2 and the
formation of ATP are coupled process, called
Oxidation Phosphorylation.
The Chemiosmotic Theory
• The free energy of electron transport
is conserved by pumping protons from
the mitochondrial matrix to the
intermembrane space so as to create
an electrochemical H+ gradient across
the inner mitochondrial membrane.
The electrochemical potential of this
gradient is harnessed to synthesize
ATP.
Peter Mitchell
Electrochemical H+ gradient
(Proton-motive force)
2 components
involved
1. Chemical potential energy
due to difference in [H+]
in two regions separated
by a membrane
2. Electrical potential energy
that results from the
separation of charge when
a proton moves across
the membrane without a
electron.
Complex I:
4 H+ expelled
per e--pair
transferred to
Q
Complex III:
4 H+ expelled per
e--pair transferred
to Cyt c
Complex IV:
2e- + 2 H+ from
matrix convert ½ O2
to H2O; 2 further H+
expelled from
matrix
Proton pumping: Reductiondependent conformational switch of an
e--transport complex
Conformation 1
(high affinity for H+)
Conformation 2
(low affinity for H+).
ATP Synthase
Intermembrane space
Inner
(ab2c9-12)
Membrane
Matrix
C ring
(α3β3γδε)
β-subunit take up ADP and Pi to form ATP
ADP + Pi
ATP
Each of 3 b-subunits
contains an active
site
F1: multisubunit
complex that catalyzes
ATP synthesis
F0 = proton-conducting
transmembrane unit
When protons flow back through F0 channel, γ-subunit
is rotated by the rotation of c ring, then the conformations
of β-subunits are changed, this lead to the synthesis and
release of ATP. To form a ATP need 3 protons flow into
matrix.
H+ flow
β-subunit has three conformations:T (tight), L (loose), O (open)
Translocation of ATP , ADP and Pi.
H+
ADP3- ATP4-
H2PO4- H+
Intermembrane
胞液侧
space
F
0
基质侧
Matrix
F1
H2PO4- H+
ATP4ADP3H+
P/O ratios
• P/O ratio is the rate of phosphate
incorporated into ATP to atoms of O2
utilized. It measure the number of
ATP molecules formed per two
electrons transfer through the
respiratory chain.
• NADH respiratory chain : 2.5,
• FADH2 respiratory chain: 1.5
• During two electrons transfer through
NADH respiratory chain, ten protons
are pumped out of the matrix.
• To synthesis and translocation an ATP,
four protons are needed.
• So, two electrons transport can result
in 2.5 ATP.
• To succinate respiratory chain , two
electrons transport can result in 1.5
ATP.
Regulation of Oxidative
Phosphorylation
• 1.PMF (proton motive force) regulate
the electron transport.
higher PMF
lower rate of
transport
• 2.ADP concentration
resting condition: energy
demanded is low, ADP concentration is
low, the speed of Oxidative
Phosphorytion is low.
active condition: the speed is high.
Inhibitor of Oxidative
Phosphorylation
• 1.Inhibitor of electron transport
Succinate
Antimycin A
Cyanide, Azide
Carbon Monoxide
×
×
×
Retonone
Amytal
• 2.Uncoupling agents
uncoupling protein (in brown adipose tissue),
2,4-dinitrophenol, Pentachlorophenol
heat
H+
Intermenbran space
Ⅰ
Ⅱ
H
Cyt c
uncoupling
protein
F
Q
0
Ⅲ
Ⅳ
F1
Matrix
+ H+
H+
ADP+Pi ATP
2,4-dinitrophnol
3.Oligomycin bonds at the connection of F0
and F1, inhibit the function of ATP synthase.
Intermembrane space
Matrix
Oligomycin
C ring
Succinate
Ⅱ
Retonone
Amytal
Ⅰ
Antimycin A
×
Ⅲ
×
×
Oligomycin
Uncoupling
agent
Ⅴ
×
×
Ⅳ
ATP and other Energy-rich
compounts
ATP has two energy-rich phosphoric acid anhydride
bonds, the hydrolysis of each bond release more
energy than simple phosphate esters.
NH 2
N
N
OH
O= P
OH
OH
OH
O ～p O ～p
OH
N
N
OCH2 O
H
OH
H
H
H
OH OH
AMP
ADP
ATP
Some Energy-rich compounds
Structure
Exemple
creatine phosphate
phosphoenolpyruvate
acetyl phosphate
Acetyl CoA
ΔGº’
• The hydrolysis of energy-rich bond:
ΔGº’ = -5～-15kcal/mol
• The compounds with energy-rich bond
are high-energy compounds.
• The hydrolysis of low-energy bond:
ΔGº’ = -1～-3kcal/mol
• The compounds with low energy bond
are low-energy compounds.
Transport of high-energy bond
energies
• 1.Substrate level phosphorylation
Glycerate 1,3-biphosphate + ADP
Glycerate 3-phosphate +ATP
ΔGº’ = -4.5kcal/mol
Phosphoenolpyruvate +ADP
Pyruvate + ATP
ΔGº’ = -7.5kcal/mol
2.ATP is the center of energy producing and
utilizing.
ATP
Oxidative
Phosphorylation
Energy
utilization
Substrate level
phosphorylation
~P
~P
ADP
3.Other nucleoside triphosphates
are involved in energy transport.
• GTP:
gluconeogenesis
protein synthesis
• UTP: glycogen
• CTP: lipid synthesis
4.Transport of the terminal
phosphate bond of ATP to the
other nucleoside
• Function of nucleoside
diphosphate kinase
ATP + UDP
ATP + CDP
ATP + GDP
ADP + UTP
ADP + CTP
ADP + GTP
• Function of adenylate kinase
ADP + ADP
ATP + AMP
7.3 Energy from cytosolic
NADH
• A mitochondrial NADH produce 2.5
ATP
• A cytosolic NADH must be
transported into mitochondrial for
oxidation by two methods.
Glycerol phosphate shuttle
1.5 ATP
Malate aspartate shuttle
2.5 ATP
Glycerol phosphate shuttle
CH2OH
CH2OH
Electron chain
NADH+H+
Glycerol
phosphate
dehydrogenase
NAD+
C=O
C=O
CH2O- Pi
CH2O- Pi
dihydroxyacetone
phosphate
dihydroxyacetone
phosphate
CH2OH
CH2OH
CHOH
CHOH
CH2O- Pi
CH2O- Pi
Glycerol
phosphate
Glycerol
phosphate
Intermembran
space
FADH2
FAD
Glycerol
phosphate
dehydrogenase
Inner menbran
Malate aspartate shuttle
O
Aspartate
-OOC-CH2-C-COO-
H 3N
Malate
α-ketoglutarate
carrier
-
+
OOC-CH2-C-COO
H
oxaloacetate
-
Aspartate
oxaloacetate
Glutamate
Glutamate
Electron chain
NADH
+H+
NADH
+H+
NAD+
α-ketoglutarate
α-ketoglutarate
OH
NAD+
-OOC-CH2-C-COOH
Malate
cytosol
Glutamate-aspartate
carrier
inner mitochondrial
membran
Malate
matrix
7.4 Other Biological Oxidations
• Monoxygenases
dioxygenase --add 2 atoms of O2
oxygenase to organic compounds.
monoxygenase (mixed-function oxidase,
hydroxylase)
--adds 1 oxygen atom to organic
compounds as a hydroxyl group.
RH + NADPH + H+ + O2
ROH + NADP+ + H2O
The chief compounds of monoxygenase:
Cyt b5, Cyt P450, Cyt P450 reductase(FAD,FMN)
Free Radical Scavenging
Enzymes
Free Radical: the groups with an
unpaired electron. (such as O2﹣、
H2O2、•OH)
1.Superoxide dismutases(SODs)
2O2﹣+ 2H+
SOD
H2O2 + O2
peroxidase
H2O + O2
• 2.Glutathione peroxidase
H2O2
(ROOH)
2G –SH
Glutathion
e
Glutathione
reductase
peroxidase
H2O
(ROH+H2O)
NADP+
G –S – S – G
NADPH+H+
• 3.Catalase (in peroxisomes)
2H2O2
catalase
2H2O + O2
summary