Electron transport chain/ Biological oxidation /Respiratory chain /Oxidative

phosphorylation
Life is based on the principle concept that everything inside the body must be burnt, which is called
combustion and happens in presence of oxygen. Oxygen will the main similar molecule involved in all
processes of combustion. Without oxygen the efficiency of respiratory chain decreases. In human
body, precisely inside the cell the respiratory chain goes on in mitochondria. Combustion also
explains our breathing process, as we breathe in O2 as input for burning and release CO2 as output
of burning and along with energy. Now this energy is a result of aerobic respiration in mitochondria.
In anaerobic metabolism the absence of oxygen, alters the ATP synthesis and produced ATP is not
optimum.

If we talk about mitochondria, it is present in eukaryotic cells but is not owned by

cell. Because of theory called endosymbiotic theory which says that mitochondria was
actually not an organelle, it was a trapped aerobic bacteria inside the cell. This
relationship was suitable for both, as bacteria was having profits in cell so does the
eukaryotic cell. So evolutionary mitochondria was not present in the cell. Mitochondria
has its beauty in its complexity.

Mitochondria is double membranous structure , outer mitochondrial membrane (OMM) and inner
mitochondrial membrane(IMM) which bounds inter membranous space (IMS).IMM is actually
extended deep into matrix forming cristae.

General glycolytic reactions occur in cytoplasm, gluconeogenesis is partially in cytoplasm and

mitochondria, but processes like electron transport chain and kreb cycle (citric acid cycle or TCA) in
mitochondria. Basically when something has to be burnt at higher anergy level by expecting absolute
amount of ATP to be released. If any process is happening in mitochondria it has to be burning,
oxidizing process with release of ATP.

If we consider OMM:

 It has acyl coA synthetase responsible for initiation of beta oxidation. Acyl is used to define fatty
acid which is generally present in inactive form and therefore we combine it with coA which requires
energy, that’s why we call it synthetase, which belongs to class of ligases.
 Glycerol phosphate acyl transferase

IMM is having :

 Electron carriers : like complex 1 , complex 2, complex 3 and complex 4 which are involved in electron
transport chain.
 ATP synthetase complex ( complex 5)

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 Translocase enzyme which are membrane transporters.

In mitochondrial matrix:

 TCA cycle enzymes

 Fatty acid oxidation (beta-oxidation) enzymes
 PDH complex

Electron transport chain starts from lower redox potential area

and ends up in high redox potential area.

To understand the redox

potential, we must understand
basics like voltage. Voltage is nothing but potential difference,
higher the potential difference, higher the voltage. This
difference decides the movement of entities.

If we consider the following reaction :

Pyruvate lactate

In this reaction we realise that we need to see pairs whether their difference is appropriate so that
they can accept and release H+ and also electrons for that matter, comfortably or less comfortably.
Because if H+ is lost that means that an electron is also lost. Which explains oxidation and reduction
really well as:

 Oxidation is addition of oxygen and removal of H+ or loss of

electron
 Reduction is removal of oxygen and gain of H+ or gain of electron.

We can clearly see in diagram oxidizing reactions where if H2 is lost

to half molecule of O2 i.e O water is formed and if H2 is lost to one
complete molecule of O2 then H2O2 is formed.

Reducing equivalence :

Pyruvate lactate

In above reaction NADH was used up and NAD+ being generated from it. Now if we add 2 molecules
of hydrogen which is equivalent of 2 electrons to NAD+ to make it NADH. Every NADH molecule is

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 2.5 ATP, which means if NADH is generated ,it is equivalent to 2.5 molecules of
ATP. This will be called reducing equivalence.

Some important reducing equivalence we use are:

 NAD+/NADH + (H+)
 FAD+ ( oxidized flavin) /FADH2 ( reduced Flavin)
 FMN/FMH2
 NADPH + (H+) , NOT meant for ATP synthesis in
general.

Remember: If NAD (P) H is mentioned we can take it as

NADH or NADPH.

(Cytochrome P450 hydroxylase cycle)

(Components of mitochondria)

(Extra mitochondrial sources of reducing equivalents)

Now if we consider flow of energy it will be something

like:

Food

Broken down into small units like fatty acids + glycerol, glucose and amino acids.

Finally everything is metabolized into

Acetyl CoA (Mitochondria), which may join citric acid cycle

Giving H molecules which on entering respiratory chain releases H2O and energy.

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In process mentioned above a very basic phenomenon is applied that is:

E = mc2

Energy can neither be created nor destroyed, but it is converted from one form to another just like
petrol id used up while driving and with sunlight’s help our food is prepared. Similar way in human
body food is converted into energy.

(Schema of electron transport chain)

ETC happens on inner mitochondrial membrane

(complex 1,2,3,4)

Iron is required for oxidative metabolism. If we

consider presence if iron in ETC, it can be obtained
from green leafy vegetables or meat, as in leaf
chlorophyll will be present in turn chloroplast which
is made up of cytochrome with central iron. In meat, myoglobin is present which is tertiary
structured heme proteins and have Fe.

Complex 1 takes electron or H+ from NADH and now offers to the only non proteinaceous component
of ETC that is quinone (Q) which is structurally related to vitamin k

Now complex one passes electron to Q and Q is now in reduced form

Q also accepts the electron from complex 2 called succinate Q reductase

Now complex 3 accepts electron from reduced quinone and now complex 3 will pass it to cytochrome
C and therefore it is called Q-cyt c oxidoreductase

Now cytochrome C will offer electrons to complex 4(cytochrome oxidase) and complex 4 will give
electron to oxygen

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(Iron sulphur proteins in ETC )

( Final picture of ETC )

(Reduced form of quinol)

In ETC there are some fixed molecules and some moving ones :

Fixed complexes Complex 1,2,3,4

Moving complex Cytochrome C and Q

(floating) (have to move to pass the
electrons)
(Q cycle)

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 Biological oxidation respiratory chain oxidative phosphorylation
 The quinone can actually exist in three forms:
 Quinone the oxidized version
 Semi quinone which is partially oxidized and partially reduced
 Quinol the completely reduced version.
 Complex Q is shuttling between 1 and 2 on one side and cytochrome being on the other side.
 Q can be regenerated whenever required by Q-cycle.
 If we consider an example of NADH + (H+) becoming NAD.
 is the movable unit and once it takes the electron and H enters complex 3. Here is becomes
immobile.
 Quinone is movable unit in the matrix but inside complex 3 it is immobile.
 Inside the complex Q may have two types:
o Initially 2 out of the 4 electrons given by mobile Q is given to Qo which then take the
H and put it in the space of complex 3. Now the electrons will be given to FeS which
further passes them to cytochrome C.
o The other two electrons are passed to b(L) unit. From b(L ) they move to b(H) and then
they reach Qi. Qi will accept two H+ from the space. Now electron from Qi will be
given to Q so that it becomes QH2.
 So the point is that for transfer of electron the electrons will be transferred by Q as it will
change its forms from Q to QH2.
(chemiosmotic theory of oxidative phosphorylation)
 When electrons are
moving in a direction of
complexes then protons
will be jetting out into
intermembrane space.
During this jetting out
4H+ are given by
complex1 and 3 whereas
only 2H+ are given by
complex 4. There is no
pumping from complex 2.
Basically as the
electrons are moving
from one complex to
other protons are getting pumped into intermembranous space. For 1 NADH+ + (H+) moving
through the complexes 10H+ are given in intermembranous space along with 2.5 ATP.

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 But if movement is through complex 2 then no. of H+ pumped will be 6 and
1.5 ATP will be produced.
 The movement of protons in this case is against the gradient and perpendicular to direction in
which electrons are moving. As the inter membranous space already have high concentration of
H+. so pumping as against gradient. So, the electron movement creates electromotive energy
which helps the protons to move against concentration gradient.
 Now as concentration of H+ increases in intermembranous space the H+ will be waiting to move
from high to low conc. the created difference in concentrations is called proton gradient.
 Now we create channels for movement of H+ ions, and this movement will create ripple energy
which is capable of producing ATP.
 So, in the process oxidation happens when H+ are combine with O and phosphorylation happens as
ATP are produced. This is a coupling between two processes and that’s why is called as oxidative
phosphorylation.
 The channel has two portions one in intermembranous space (Fo) and one inside matrix (F1). These
both are collectively forming ATP synthetase complex. This is referred as complex 5.
(complex 5)
 The Fo has lots of compartments inside. F1 is having 3
alpha-beta pairs and a delta unit which helps in
connection with Fo.
 Both the units are combined at two places one by gamma
stalk and b2 branch. That’s why the gamma stalk rotates
in middle.
 Between each alpha-beta there is space for ADP and Pi
and as the rotate the ADP and Pi will form ATP. And with
the next rotation ATP is knocked off.
 This way we can explain Peter Mitchell’s chemiosmotic
theory, as it states that alpha beta rotations result in
formation of ATP. Beta units have catalytic sub unit and
alpha has an accepting nature for ADP and Pi.
 Fo is called so because it can be blocked by oligomycin
and shows oligomycin sensitivity.

(Conditions limiting the rate of respiration)

 Availability of oxygen affect the rate of

phosphorylation as if oxygen is not present no
combustion, no glycolysis, no NADH, no electron
transfer between complexes, no concentration
gradient and thus no rotation and no ATP.

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 (sites of inhibition)

 There can be multiple site and multiple

agents that block the oxidative
phosphorylation:
o Piericidin A, amobarbital, rotenone and
alkyl guanides can block the communication
between complex 1 and Q.
o Malonate blocks the conversion of
succinate into fumarate so no FAD or FeS. The action is according to competitive inhibition.
o Carboxin TTFA blocks flow of electrons from complex 2 to complex Q.
o Dimer caprol (BAL) and antimycin-A blocks the functioning of complex 3. Along with
naphthoquinone.
o H2S, CO, CN- can block the complex 4. This is the reason for people who work as cleaners in
underground sanitation area die because the inhale the toxic gases in savage. As there are
present H2S, CO and CN-.
o Uncouplers: allow oxidation bur blocks phosphorylation. Oligomycin prevents stimulation of
oxygen uptake and that stops formation of ATP.
Atractyloside block translocase enzyme blocking the entry of ADP, so no ATP.
 Uncoupler can be artificial or physiological. Uncoupling means both the reactions will be
separated. This blocks ATP synthesis.
 Artificial uncoupler: 2,4 DNP,cresol, chloro carbomyl cyanide phenyl extract (CCCP) and
valinomycin or inophore. They work by creating holes in the membrane that causes disturbance in
gradient formation and this way at last there is not enough ATP to be formed because not much
H+ are to be passed through the channel.
 Ca2+ also behaves as artificial uncoupler.
 Physiological uncouplers: thermogenin is physiologic uncoupler which uses the energy created in
form of heat and not ATP.
 Thyroxine T3/T4 also make sure that instead of ATP the energy is released as heat. Therefore,
in hyperthyroidism the person may have heat intolerance. In hypothyroidism more ATP is
produced so there is lot of unused energy without heat that gets stored and makes the person
fat and cold intolerant.
 Excessive amount of free fatty acids
 Unconjugated bilirubin
 High dose aspirin will act as pharmacological uncoupler of oxidative phosphorylation which blocks
phosphorylation so the energy is in form of heat and sometimes my cause high temperature.
 Infection and inflammation can release interleukin and they stimulate synthesis of uncouplers
which will increase oxidation with decreased phosphorylation causing energy to be converted
into heat and this will increase our body temperature.
 Fatal infantile mitochondrial myopathy and renal dysfunction (FIMM-RD) have decreased amount
of oxidoreductases in ETC so it stops oxidative phosphorylation. These patients can have lactic

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 acidemia. The linker reaction involves pyruvate becoming acetyl co-A in
presence of PDH enzyme but in this case the mitochondrial enzyme that is PDH won’t work and
this causes pyruvate conversion to lactic acid, causing lactic acidemia. As acetyl-coA is not
formed, no further oxidation and no energy, this causes lethargy and weakness.
 MELAS: mitochondrial encephalopathy + lactic acidosis + seizures happen because of NADH-
coA-oxidoreductase defect or complex 1 defect. 30% chances are that complex 4 defects can
also cause MELAS.
 Leigh’s disease
 MERRF: Mitochondrial disorder along with ragged red fiber syndrome

Extra edge concepts in ETC and energy kinetics

(Transports across mitochondrial membrane)
 ETC is happening on inner mitochondrial membrane (IMM) with
matric and intermembranous space (IMS) being on surrounding.
 There are transports happening through IMM:
o Every time a H2PO4 goes in and hydroxyl radical comes out
o Every time pyruvate enters it takes H+ along with it. This is
symport activity.
o For every HPO4-2 coming out malate will be going in.
o For every malate coming out citrate along with H+ will be
going in. Sometimes alpha-ketoglutarate may go in for each
malate coming out. This is antiporter action.
o When ADP goes in ATP comes out. This is done by
translocase enzyme. As ADP enters the F tunnel, in presence
ATP synthetase the rotations combine the ADP with Pi
forming ATP.
o There are different blockers for these transporters.
 Glycerophosphate shuttle: NADH cannot cross the membrane so
it gives the hydrogen to dihydroxyacetone phosphate making it glycerol-3-phosphate which can
enter through the membrane. In this way the electrons and hydrogen from NADH reaches IMS
where the glycerol-3-phosphate will again become DHAP releasing the electron and H+ for FAD
to become FADH2 respiratory chain at inner membrane. In this case NADH will only be able to
provide 1.5 ATP because the final product id FADH2. We can say that NADH using glycerol-
phosphate shuttle only gives 1.5 ATP.
 Malate aspartate shuttle: malate and oxaloacetate are 4C compounds. In this shuttle NADH will
give hydrogen and electrons to OA to become malate in presence of malate dehydrogenase, malate
crosses the inner membrane and reaches the matrix and again becomes OA while releasing the H
and electrons which are accepted by NAD+ to become NADH + H+ and therefore 2.5ATPs will be
produced. But now the OA cannot cross the membrane back, so it chooses to be converted into
alpha-ketoglutarate to avoid a futile cycle. This happens by support of glutamate as it provides

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 the amino group for transamination and gets converted into alpha-KG and OA
becomes aspartate. Now alpha-KG and aspartate will be back in IMS and gets converted into OA
and glutamate. As aspartate becomes OA and alpha-KG will become glutamate.
 Basically, shuttles are the changes that are made to any compounds to get its work done without
crossing the membrane.
 High energy compound is a compound which on burning releases 7 kcal per mole.
 Free energy change explains the energy difference between substrate and product. If the
difference is positive that means energy is accepted by the substrate to form product and the
reaction is endergonic, but in case of negative free energy change the substrate releases energy
and this is called exergonic reactions. And these reactions will be spontaneous. Exothermic means
specifically related to heat as it is a part of exergonic reactions.
 All exothermic reactions are exergonic reactions but all exergonic reactions are not exothermic
reactions.
 If delta G is negative the reaction is spontaneous but if delta G is positive the reaction is non-
spontaneous and require energy.
 So, for high energy compounds delta G is -7 kcal per mole. These compounds will react
spontaneously. Like when one mole ATP is broken to ADP and Pi the energy released is 7 kcal.
 In the first step of glycolysis ATP is broken and the energy liberated is used for phosphorylation.
 The energy released in converting ATP to AMP is 10.7 kcal per mole whereas for ADP the energy
released is 7.3 kcal per mole.
 Glucose-1-phosphate breaking down to glucose and phosphate will release 5 kcal per mole. So
glucoe-1-phosphate is low energy compound.
 Glucose-6-phosphate breaking down to glucose and phosphate will release 3.3kcal per mole. So
glucoe-6-phosphate is low energy compound and has lesser energy than glucose and glucose-1-
phosphate.
 Glycerol-3-phosphate releases 2.2 kcal per mole.
 Phospho-enol pyruvate breakdown releases the maximum energy in glycolysis by giving 14.8 kcal
per mole.
 Calorific value is the energy liberated when something is burnt.
 Carbohydrates have 4.1 kcal, proteins have 5.1-5.5 kcal and lipids have 9.4-9.6 kcal energy release
if their one mole is burnt.
 The finest fuel for human body is glucose.
 Respiratory coefficient: it is CO2 liberated in reaction / oxygen used in reaction. RQ of
carbohydrates is 1 that’s why it is best fuel. In lipids RQ is 0.71 and in proteins RQ is 0.81.
 Alcohol is non-nutritious but has high calorific value. It make the person chubbier but not
energetic.
 RQ of alcohol is 0.66.
 Carbohydrates in food should be 60-70% which also means that carbs should fulfill the energy
requirement till 70%. The fibers of carbs can be soluble and insoluble. The insoluble ones are

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 roughage and are hygroscopic and unabsorbed. It doesn’t provide energy and
doesn’t make the person obese. But it may cause water retention.
 The normal amount of protein a person should have is 1 gram per kg of body weight. Excess protein
is bad for a person because its breakdown will release amino acids and this will increase the load
on kidneys. It may lead to kidney failure.
 The amount of insoluble fiber should be 14 gram per 1000 calories.
 The food should be a balanced diet and RQ should be between 0.7-1 and ideal will be 0.8
 If fat metabolism increases and carbs are spared, RQ will fall.
 RQ is least in ketolysis that is when ketone bodies are getting utilized.
 Basal metabolic rate: even if we are at rest our body is continuously working and it needs energy
and energy required by our body at compete physical, mental and digestive rest is BMR.
 Resting metabolic rate: energy require at physical and mental rest.
 BMR is higher when person is active and it decrease when we sleep.
 BMR and RMR can be measured by Atawater Benedict Roth test.
 Every one degree rise in body temperature will increase BMR by 10-15%. So person should eat
during fever to supply energy required.
 Most common organs must be considered to check BMR are CNS and kidney and then heart and
other organs.
 BMR can be decreased by:
o Starvation/malnutrition: it is an adaptive method when our body adjust accordingly and our
energy demand and functioning capacity both decrease.
o Sleep
o Muscle mass can also affect BMR proportionally
o Decreased thyroxine causes decrease in BMR
 Increase in BMR is caused by:
o Hyperthyroidism as it will cause uncoupling of oxidative phosphorylation which will cause
decreased ATP production with high heat energy.
o Fever
o Males have higher BMR then females.
o Polar bear will have high BMR.
o In hill station you feel more hunger than usual because of increased BMR.
 Specific dynamic action: it is the activation energy. Carbohydrates will have 5% SDA, proteins
have 30%SDA and lipids have 15%SDA. In a balanced diet we have 60% carbs, 20% protein and
20% lipid, this combination decreases the SDA by 10%.
 BMR will be different according to work performed by the person whether he is highly active
moderately or lightly active.
 Higher amount of protein in food needs high energy to break it causing increased temperature
of body.

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