biological oxidation and electron transport chain · 2020. 4. 13. · oxidative phosphorylation 24...
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BIOLOGICAL OXIDATION &
ELECTRON TRANSPORT CHAIN
Lord Shiva College of Pharmacy, Sirsa
Presented By: Ramesh Kumar
(Assist. Prof., L.S.C.P. Sirsa)
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RESPIRATION
2
Organisms can be classified based on the mechanism to obtain energy-Autotrophs: are able to produce their own organicmolecules through photosynthesis
Heterotrophs: live on organic compounds produced by other organisms ( e.g. Starch/ Glucose)All organisms use cellular respiration to extract
energy from organic molecules.
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RESPIRATION
3
❑Respiration –a process by which cells derive
energy with a controlled reaction between H+
and O2; the end product being water.
❑Aerobic organisms are able to capture a far
greater proportion of the available free energy of
respiratory substrates than anaerobic organisms.
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RESPIRATION
4
The objective of respiration is toproduce ATP.❑Energy is released from oxidation reactions in the form of electrons❑Electrons are shuttled by electron carriers (e.g. NAD+) to an electron transport chain❑Electron energy is converted to ATP in the electron transport chain
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METABOLISM
5
▪ Metabolism is the sum of the chemical
reactions in an organism.
▪ Catabolism is the energy-releasing processes.
▪ Anabolism is the energy-using processes.
▪ Catabolism provides the building blocks and
energy for anabolism.
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METABOLISM
6
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ATP COUPLES ENERGYBETWEEN
CATABOLISM AND ANABOLISM
7
catabolism
anabolism
ADPATP + Pi
Energy from food (fuel molecules) or from
photosynthesis
Energy available for work & chemical synthesis (e.g.
movement, signal amplification, etc.
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BIOLOGICAL OXIDATION
8
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CELLULAR METABOLISM
9
Part 1:Breakdown of large
macromolecules to simple subunits
Part 2: Breakdown of simple subunits to acetyl CoAaccompanied by production of limited amounts of ATP andNADH
Part 3:Complete oxidation of
acetyl CoA to H2O and CO2 accompanied by production of large amounts of NADH and ATP in mitochondrion
fats
fatty acids andglycerol
polysaccharides
simple sugars
proteins
amino acids
glucose
Citricacidcycle
Acetyl CoA
CoA
2 CO2
8 e- (Reducing power as NADH)
oxidative phosphorylation
O2
H2O
ATP
glyc
oly
sis
pyruvate
ATP
NADH
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ATP ISTHE PRINCIPAL CARRIER OF
CHEMICAL ENERGY IN THECELL!
10
High Energy compound.
Go’ = -7.3 kcal/mol
▪ Major activities promoted by ATP:
▪ -locomotion
▪ -membrane transport
▪ -signal transduction
▪ -keeping materials in the cell
▪ -nucleotide synthesis
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BIOLOGICAL OXIDATION
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❑Involves the transfer of electrons
❑oxidation being termed for the removal of
electrons & reduction for gain of electrons
❑Oxidation is always accompanied by reduction of
an e- acceptor
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ENZYMES INVOLVED IN OXIDATION
AND REDUCTION REACTIONS
12
❑Are called as Oxidoreductases which include :
oxidases, dehydrogenases, hydroperoxidases and
oxygenases.
❑Oxidases use oxygen as an electron acceptor
❑Dehydrogenases can’t use O2 as an electron acceptor
❑Hydroperoxidases use H2O2 as a substrate
❑Oxygenases catalyze the direct transfer of O2 into the
substrate
❑Oxidases & dehydrogenases are involved in
respiration; hydroperoxidases neutralize free radicals &
oxygenases are involved in biotransformation reactions.
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OXIDASES
13
❑Catalyze the removal of hydrogen from a substrate with the involvement of oxygen as a H –acceptor, forming water or hydrogen peroxide.
❑Exist in two different forms :❖Some of them are copper containing suchas, Cytochrome oxidase , the terminal component of ETC which transfer the e - finally to O2.
❖Other are flavoproteins such as , L – amino acidoxidase (FMN linked) and Xanthine oxidase (FADlinked)
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DEHYDROGENASES
14
Perform 2 main functions:❑Transfer hydrogen from one substrate to another in a coupled Oxidation /Reduction reaction❑As components of Electron transport chain such as cytochromesDehydrogenases use coenzymes – nicotinamides & riboflavin - as hydrogen carriers❑ Nicotinamides can be in the form of NAD + or
NADP+
❑ Riboflavin can be – FMN or FAD same as oxidases
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HYDROPEROXIDASES
15
❑Includes 2 sets of enzymes : catalases and
peroxidases
❑Peroxidases reduce H2O2 at the expense of
several other substances
H2O2 + AH2 2H2O + A
❑Catalases uses H2O2 as electron acceptor &
electron donor
2H2O2 2H2O
Peroxisomes are rich in oxidases and catalases
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OXYGENASES
16
Catalyze the incorporation of O2 into substrates in 2 steps
❑ Oxygen is bound to the active site of the
enzyme
❑ Bound O2 is reduced or transferred to the
substrate
Consist of two sets of enzymes
1. Dioxygenases : incorporate both atoms of
oxygen into the substrate ; A + O2 AO2
2. Monooxygenases : incorporates one atom of
oxygen into the substrate & the other is
reduced to water
A – H + O2 + ZH2 A – OH + H2O + Z
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MITOCHONDRION
17
❑Mitochondria, have
been termed the
"powerhouses" of the
cell since the final
energy release takes
place in the
mitochondria only.
❑Mitochondria have
an outer membrane
that is permeable to
most metabolites, an
inner membrane that
is selectively
permeable, enclosing
a matrix within .
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MITOCHONDRION
❑The outer membrane is characterized by the
presence of various enzymes, including acyl-
CoA synthetase and glycerol phosphate
dehydrogenase.
❑Adenylyl kinase and creatine kinase are found
in the intermembrane space.
❑The phospholipid cardiolipin is concentrated in
the inner membrane together with the enzymes of
the respiratory chain, ATP synthase and various
membrane transporters.
❑The matrix encloses the enzymes of TCA
cycle, beta oxidation and pyruvate dehydrogenase
complex. 19
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MITOCHONDRION
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THE GENERATION OF ATP
ATP is generated by the phosphorylation of ADP
20
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THE GENERATION OF ATP
21
ATP can be generated either by-
❑Substrate level phosphorylation or
❑By Oxidative phosphorylation
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SUBSTRATE-LEVEL PHOSPHORYLATION
22
❑Substrate-level phosphorylation is the transfer of a high-energy PO4
- to ADP at the expense of the energy of the substrate without involving the electron transport chain.❑The substrate has higher energy level than the product, the surplus energy is used up for ATP formation.
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SUBSTRATE LEVEL PHOSPHORYLATION
23
Substrate-level phosphorylation – transferring a
phosphate directly to ADP from another molecule
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OXIDATIVE PHOSPHORYLATION
24
Oxidative phosphorylation is the process by which
the energy stored in NADH and FADH2 is used to
produce ATP.
A. Oxidation step: electron transport chain
NAD+ + H2O
FAD + H2O
NADH + H+ + O2
FADH2 + O2
B. Phosphorylation step
ADP + Pi ATP
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REDOX REACTIONS
25
During redox reactions, electrons carry energy from
one molecule to another.
1) NAD+ as an electron carrier.
❑NAD accepts 2 electrons and 1 proton to become
NADH
❑ The reaction is reversible.
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NICOTINAMIDE COENZYME: NAD+
26
Always a 2-electron reaction transferring 2 e- and 2H+
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THE FLAVIN COENZYMES /FLAVOPROTEINS
27
H3C
H3C NN
O
OH OH OH O O
CH2 CH CH CH CH2 O P O P O CH2
N N O OH OH
O
N
N
NH2
N
N
flavin adenine dinucleotide (FAD)
OH OH
Always a 2-electron
reaction transferring
2 e-
and 2 H+
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OXIDATION AND REDUCTION OF FLAVIN
COENZYMES
28
It can accept/donate 1 or 2 e-. FMN has an
important role in mediating e- transfer
between carriers that transfer 2 e- (e.g.,
NADH) and those that transfer 1 e- (e.g.,
Fe+++).
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ELECTRON TRANSPORT CHAIN
29
❑The electron transport chain (ETC) is a series of membrane-bound electron carriers.❑ Embedded in the mitochondrial inner membrane
❑Electrons from NADH and FADH2 are transferred to complexes of the ETC❑Each complex transfers the electrons to the next complex in the chain
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ELECTRON TRANSPORT CHAIN -
SERIES OF REDOX REACTIONS
30
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COMPLEXES OF ELECTRON
TRANSPORT CHAIN
31
▪ Electrons flow through the respiratory chain through a redox span of 1.1 V from NAD+/NADH to O2/2H2O passing through three large proteincomplexes;
▪ NADH-Q oxidoreductase (Complex I), where electrons are transferred from NADH to coenzyme Q(Q) (also called ubiquinone);
▪ Q-cytochrome c oxidoreductase (Complex III), which passes the electrons on to cytochrome c; and
▪ Cytochrome c oxidase (Complex IV), which completes the chain, passing the electrons to O2 and causing it to be reduced to H2O .
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COMPLEXES OF ELECTRON TRANSPORT
CHAIN
32
▪ Some substrates with more positive redox potentialsthan NAD+/NADH (eg, succinate) pass electrons toQ via, succinate Q reductase (Complex II), ratherthan Complex I.
▪ The four complexes are embedded in the inner mitochondrial membrane, but Q and cytochrome c are mobile.
▪ Q diffuses rapidly within the membrane, while Cytochrome c is a soluble protein.
▪ The flow of electrons through Complexes I, III, andIV results in the pumping of protons from the matrix across the inner mitochondrial membrane into the intermembrane space
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ELECTRON CARRIERS IN ETC
33
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FLAVOPROTEINS
34
Flavoproteins are important components of Complexes
I and II. The oxidized flavin nucleotide (FMN or FAD)
can be reduced in reactions involving the transfer of two
electrons (to form FMNH2 or FADH2), but they can also
accept one electron to form the semiquinone
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IRON-SULFUR CENTERS (CLUSTERS)
35
Iron-sulfur centers (Fe-S) are prosthetic groups containing 1-4 iron atoms
Iron-sulfur centers transfer only one electron, even if they contain two or
more iron atoms.
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UBIQUINONE
36
Coenzyme Q (CoQ, Q or ubiquinone) is lipid-soluble. It dissolves in the hydrocarbon core of a membrane.
•the only electron carrier not bound to a protein.it can accept/donate 1 or 2
e-. Q can mediate e- transfer between 2 e- that transfer and 1 e- carriers
Free CoQ can undergo a 2 e- oxidation/reduction:Q + 2 e- + 2 H+ QH2.
When bound to special sites in respiratory complexes, CoQcan accept 1 e- to form a semiquinone radical (Q•-).
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CYTOCHROMES
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CYTOCHROMES
38
❑Cytochromes are electron carriers containing heme .
❑Heme in the 3 classes of cytochromes (a, b,c) differ in substituents on the porphyrin ring.
❑Some cytochromes(b,c1,a,a3) are part of large integral membrane protein complexes.
❑Cytochrome c is a small, water-soluble protein.
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STRUCTURE OF CYTOCHROME
❑Heme is a prosthetic group of cytochromes.❑Heme contains an iron atom in a porphyrin ring system.
❑The heme iron can undergo 1 e- transition between ferric and ferrous states: Fe3+ + e-
→
Fe2+
❑Copper ions besides two heme A groups (a and a3) act as electron carriers in Cyta,a3Cu2++e-→Cu+
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ELECTRON CARRIERS
❑NAD+, flavins andCo Q carry electrons and H+
❑Cytochromes and non-haem iron proteins carry only electrons.
❑NAD+ FAD undergoes only a 2 e- reaction;
❑Cytochromes undergo only 1e- reactions
❑FMN Q undergoes 1e- and 2 e- reaction
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ORDER AND REDOX POTENTIALS-0.32
-0.3
+0.045
+0.03+0.077
+0. 22 +0. 25
+0. 55
+0. 29
+0.82
The electron carriers are arranged in terms of rising
redox potential in the electron transport chain.
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Q CYCLE :THE MECHANISM OF H+
TRANSPORT IN COMPLEX III
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SITE SPECIFIC INHIBITORS OF ETC
43
Amobarbital, Rotenone, Piericidin AGuanethidine
Malonate, Carboxin, TTFA
AntimycinA, BAL, Hypoglycemi c agents
CN, CO, H2S,Azide
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INHIBITORS OF ETC
44
❑Barbiturates such as Amobarbital inhibit electron transport via Complex I by blocking the transfer from Fe-S to Q. At sufficient dosage, they are fatal in vivo.
❑Antimycin A and dimercaprol inhibit the respiratory chain at Complex III.
❑ The classic poisons H2S, carbon monoxide, and cyanide inhibit Complex IV and can therefore totally arrest respiration.
❑Malonate is a competitive inhibitor of Complex
II.
❑Atractyloside inhibits oxidative phosphorylation by inhibiting the transporter of ADP into and ATP out of the mitochondrion .
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H+ /PROTON TRANSPORT
Complexes I, III, and IV act as proton pumps. Since the inner mitochondrial membrane is impermeable to ions in general and particularly to protons, these accumulate in the intermembrane space, creating the proton motive force predicted by thechemiosmotic theory.
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PROTON TRANSPORT
4H+ are pumped per 2e- passing through complex III.
The H+/e- ratio is less certain for the other complexes: probably 4H+/2e-
for complex I; 2H+/2e- for complex IV.
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ATP SYNTHASE COMPLEX
❑ATP synthase is embedded in the inner membrane,
together with the respiratory chain complexes .
❑Several subunits of the protein form a ball-like
shape arranged around an axis known as F1, which
projects into the matrix and contains the
phosphorylation mechanism .
❑F1 is attached to a membrane protein complex
known as F0, which also consists of several protein
subunits.
❑F0 spans the membrane and forms a proton
channel.
❑The flow of protons through F0 causes it to rotate,
driving the production of ATP in the F1 complex.
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ATP SYNTHASE COMPLEX
48
❑The enzyme complex consists
of an F0 subcomplex which is a
disk of "C" protein subunits.
❑Attached is a Υ subunit in the
form of a "bent axle." Protons
passing through the disk of "C"
units cause it and the attached Υ
subunit to rotate.
❑The Υ subunit fits inside the F1
sub complex of three α and
three βsubunits, which are fixed
to the membrane and do not
rotate.
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PROTON MOTION AND ROTATIONOF C RING
49
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OXIDATIVE PHOSPHORYLATION- CHEMIOSMOSIS
50
❑As the electrons are transferred, some electron
energy is lost with each transfer.
❑This energy is used to pump protons (H+) across
the membrane from the matrix to the inner
membrane space.
❑ A proton gradient is established.
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OXIDATIVE PHOSPHORYLATION-
CHEMIOSMOSIS
51
❑The higher negative charge in the matrix attracts the protons (H+) back from the intermembrane space to the matrix.
❑The accumulation of protons in the intermembrane space drives protons into the matrix via diffusion.
❑Most protons move back to the matrix through ATP synthase.
❑ATP synthase uses the energy of the proton gradient to synthesize ATP from ADP + Pi.
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CHEMIOSMOTIC HYPOTHESIS
52
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OXIDATIVE PHOSPHORYLATION
The chemiosmotic
theory, proposed by
Peter Mitchell in 1961,
postulates that the two
processes are coupled
by a proton gradient
across the inner
mitochondrial
membrane so that the
proton motive force
caused by the
electrochemical
potential difference
(negative on the matrix
side) drives the
mechanism of ATP
synthesis
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OXIDATIVE PHOSPHORYLATION
54
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ATP/ ADP EXCHANGE
55
▪ ATP/ADPexchange transporter is inhibited by
Atractyloside and Bongregate
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UNCOUPLERS OF OXIDATIVE PHOSPHORYLATION
❑Uncouplers dissociate oxidation in the respiratory chain
from phosphorylation.
❑These compounds are toxic in vivo, causing respiration to
become uncontrolled, since the rate is no longer limited by
the concentration of ADP or Pi.
❑2,4-dinitrophenol
❑2, 4- dinitrocresol
❑CCCP
❑TCCP
❑Valinomycin
❑High dose of Aspirin
❑The antibiotic oligomycin completely blocks oxidation and
phosphorylation by blocking the flow of protons through ATP
synthase
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UNCOUPLERS OF OXIDATIVE PHOSPHORYLATION
57
Physiological Uncouplers❑Long chain fatty acids❑Thyroxin❑Brown Adipose tissue-Thermogenin (or the uncoupling protein) is a physiological uncoupler found in brown adipose tissue that functions to generate bodyheat, particularly for the newborn and during hibernation in animals❑Calcium ions
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MECHNISM OF ACTIONOF UNCOUPLING PROTEIN-THERMOGENIN
58
▪ Proton gradient is dissipated, no ATP formation
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P: O RATIO
59
❑Defined as the number of inorganic phosphate molecules incorporated in to ATP for every atom of oxygen consumed.❑Oxidation of NADH yields 3 ATP molecules(P: O ratio 3, Latest concept 2.5)❑Oxidation of FADH2 yields 2 ATP molecules (P: O ratio 2, Latest concept 1.5)
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REGULATION OF ATP SYNTHESIS
60
❑The rate of respiration of mitochondria can be
controlled by the availability of ADP.
❑This is because oxidation and phosphorylation are
tightly coupled; ie, oxidation cannot proceed via the
respiratory chain without concomitant
phosphorylation of ADP.
❑This is called respiratory control or acceptor
control.
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SUBSTRATE SHUTTLES
61
❑Oxidation of Extra mitochondrial NADH Is Mediated by
Substrate Shuttles.
❑NADH cannot penetrate the mitochondrial membrane, but it is
produced continuously in the cytosol by 3-
phosphoglyceraldehyde dehydrogenase, an enzyme in the
glycolysis sequence.
❑However, under aerobic conditions, extra mitochondrial
NADH does not accumulate and is presumed to be oxidized
by the respiratory chain in mitochondria.
❑The transfer of reducing equivalents through the mitochondrial
membrane requires substrate pairs, linked by suitable
dehydrogenases on each side of the mitochondrial membrane
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SUBSTRATE SHUTTLES
62
Two main shuttle systems are of importance
❑Glycerophosphate shuttle – present in skeletal
muscle and brain
❑Malate shuttle – present in liver, kidney and of
more universal utility
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GLYCEROL-3-PHOSPHATE SHUTTLE
63
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MALATE ASPARTATE SHUTTLE
64
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MITOCHONDRIAL INNERMEMBRANE
TRANSPORTERS
65
▪ Mitochondrial inner membrane is selectively permeable. The
movement of biomolecules is mediated through specific transporters
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CLINICAL ASPECT
66
❑The condition known as fatal infantile mitochondrial myopathy and renal dysfunction involves severe diminution or absence of most oxidoreductases of the respiratory chain.❑MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke) is an inherited condition due to NADH:Q oxidoreductase (Complex I) or cytochrome oxidase (ComplexIV) deficiency.
❑It is caused by a mutation in mitochondrial DNA and may be involved in Alzheimer's disease and diabetes mellitus.❑A number of drugs and poisons act by inhibition of oxidative phosphorylation.
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SUMMARY
67
▪ Virtually all energy released from the oxidation of carbohydrate, fat, and protein is made available in mitochondria as reducing equivalents (—H or e–). These are funneled into the respiratory chain, where they are passed down a redox gradient of carriers to their final reaction with oxygen to form water.
▪ The redox carriers are grouped into four respiratory chain complexes in the inner mitochondrial membrane. Three of the four complexes are able to use the energy released in the redox gradient to pump protons to the outside of the membrane, creating an electrochemical potential between the matrix and the inner membrane space.
▪ ATP synthase spans the membrane and acts like a rotary motor using the potential energy of the proton gradient or proton motive force to synthesize ATP from ADP and Pi. In this way, oxidation is closely coupled to phosphorylation to meet the energy needs of the cell.