Assignment On Bioenergetics

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North Maharashtra University
School Of Life Sciences
Assignment On
Bioenergetics
Submitted To
Mr. Bharat M. Bhalerao
Submitted By
Miss. Mugdha P. Padhye

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Index-
Sr.
No.
Contents
Page
No.
1 Introduction to Bioenergetics 3
2 History and Need for Bioenergetics 4
3 Electron Transport Chain 5
4 Organization of Carrier 7
5 Functions of Carrier Complexes 9
6 Proton Gradient 10
7 Iron Sulphur Proteins & Cytochromes 11
8 Reversed Electron Transfer 12
9 References 13

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Introduction-
Growth, development and metabolism are some of the central phenomena in the study of
biological organisms. The role of energy is fundamental to such biological processes. Energy is
available for work or for other processes when weak bonds are broken and stronger bonds are
made. The production of stronger bonds allows release of usable energy.
This Bioenergetics is the subject or a field of biochemistry that concerns energy flow through
living systems during in making and breaking of chemical bonds in the molecules found in
biological organisms.
This includes the study of thousands of different cellular processes such as cellular respiration
and the many other metabolic processes that can lead to production and utilization of energy in
forms such as ATP molecules.

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History and Need for Bioenergetics-
The term ‘Bioenergetics’ was apparently first used by Albert Szent-Gyorgyi as the title of a
small book in 1957. However, the name did not come into general use until the publication of the
first edition of ‘Bioenergetics’ by Albert L. Leninger in 1963. In the discussion at the conference,
Leninger described how he had been casting about for a title while writing the book; when he
suggested ‘Bioenergetics’ several friends disapproved saying, “that’s too flashy”. Nevertheless,
he chose that word, and the name is now generally accepted.
Bioenergetics is needed for studying-
 Energy transpositions during varying metabolic conditions like rest, differing intensities
of exercise and work, and pathological states such as obesity, diabetes, cardiovascular
disease and other disease states.
 To determine how energy produced and released by the metabolic process is harnessed to
perform the cellular work and activities necessary to sustain the life process.
 The link between cellular or molecular events.
 The field of bioenergetics is therefore inherently interdisciplinary and incorporates many
of the basic science areas.
Sigmund Freud
In his efforts to uncover the underlying causes of mental and nervous disorders, he
developed his theory of psychoanalysis, examination of the mind. He acknowledged the
body, attributing vital, energetic processes of the organism as being libido-driven (pleasure
seeking, avoiding pain).
Wilhelm Reich
Reich, a student of Freud, had as his theory that the body structure is the physiological
manifestation of emotional disturbance. Reich's experience showed him that neurotic
patterns were reflected in both psychology and physiology personality traits as well as the
body's muscular holding patterns.
As a psychoanalyst and former student of Freud, Reich became Lowen's therapist and
mentor.
Alexander Lowen
Lowen is the founder of Bioenergetic Analysis. He acknowledges the profound genius and
influence of Dr. Wilhelm Reich's work. Lowen extended Reich's direct emphasis on the
physical body during therapy. He applied appropriate hands-on physical manipulations
directly to a patient as an essential component to treatment.

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Electron Transport Chain-
An electron transport chain (ETC) couples electron transfer between an electron donor (such
as NADH) and an electron acceptor (such as O2) with the transfer of H+
ions (protons) across a
membrane. The resulting electrochemical proton gradient is used to generate chemical energy in
the form of adenosine triphosphate (ATP). Electron transport chains are the cellular mechanisms
used for extracting energy from sunlight in photosynthesis and also from redox reactions, such as
the oxidation of sugars (respiration).
The electron transport chain consists of a spatially separated series of redox reactions in which
electrons are transferred from a donor molecule to an acceptor molecule. The underlying force
driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free
energy is the free energy available to do work. Any reaction that decreases the overall Gibbs free
energy of a system will proceed spontaneously.
The function of the electron transport chain is to produce a transmembrane proton
electrochemical gradient as a result of the redox reactions. If protons flow back through the
membrane, they enable mechanical work, such as rotating bacterial flagella. ATP synthase, an
enzyme highly conserved among all domains of life, converts this mechanical into chemical
energy by producing ATP, which powers most cellular reactions.
At the mitochondrial inner membrane, electrons from NADH and succinate pass through the
electron transport chain to oxygen, which is reduced to water. The electron transport chain
comprises an enzymatic series of electron donors and acceptors. Each electron donor passes
electrons to a more electronegative acceptor, which in turn donates these electrons to another
acceptor, a process that continues down the series until electrons are passed to oxygen, the most
electronegative and terminal electron acceptor in the chain. Passage of electrons between donor
and acceptor releases energy, which is used to generate a proton gradient across the
mitochondrial membrane by actively “pumping” protons into the intermembrane space,
producing a thermodynamic state that has the potential to do work. The entire process is called
oxidative phosphorylation, since ADP is phosphorylated to ATP using the energy of hydrogen
oxidation in many steps.
A small percentage of electrons do not complete the whole series and instead directly leak to
oxygen, resulting in the formation of the free-radical superoxide, a highly reactive molecule that
contributes to oxidative stress and has been implicated in a number of diseases and aging.

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Schematic Representation of ETC
Energy obtained through the transfer of electrons down the ETC is used to pump protons from
the mitochondrial matrix into the intermembrane space, creating an electrochemical proton
gradient across the mitochondrial inner membrane (IMM) called ΔΨ. This electrochemical
proton gradient allows ATP synthase (ATP-ase) to use the flow of H+
through the enzyme back
into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate.
Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle
electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q
(ubiquinone; labeled UQ), which also receives electrons from complex II (succinate
dehydrogenase; labeled II). UQ passes electrons to complex III (cytochrome bc1 complex;
labeled III), which passes them to cytochrome c (cyt c). Cyt c passes electrons to Complex IV
(cytochrome c oxidase; labeled IV), which uses the electrons and hydrogen ions to reduce
molecular oxygen to water.
Four membrane-bound complexes have been identified in mitochondria. Each is an extremely
complex transmembrane structure that is embedded in the inner membrane. Three of them are
proton pumps. The structures are electrically connected by lipid-soluble electron carriers and
water-soluble electron carriers. The overall electron transport chain-
NADH → Complex I → Q → Complex III → cytochrome c → Complex IV → O2
↑
Complex II
↑
FADH

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Organization of Carrier-
Mitochondrial carriers are proteins from the solute carrier family which transfers molecules
across the membranes of the mitochondria.
A variety of substrate carrier proteins, which are involved in energy transfer, have been found in
the inner membranes of mitochondria and other eukaryotic organelles such as the peroxisome
which are encoded by nuclear genes. Such proteins include: ADP, ATP carrier protein
(ADP/ATP translocase); 2-oxoglutarate/malate carrier protein; phosphate carrier protein;
tricarboxylate transport protein (or citrate transport protein); Graves disease carrier protein; yeast
mitochondrial proteins MRS3 and MRS4; yeast mitochondrial FAD carrier protein; and many
others.
Most of them contain a primary structure exhibiting regions of 100 homologous amino acid
repeats, the N and C termini face the inter-membrane space and there are six definable trans-
membrane segments in each carrier. All carriers also contain a common sequence, referred to as
the MCF motif, in each repeated region, with some variation in one or two signature sequences.
Amongst the members of the mitochondrial carrier family that have been identified, it is the
ADP/ATP carrier (AAC) that is responsible for importing ADP into the mitochondria and
exporting ATP out of the mitochondria and into the cytosol following synthesis. The AAC is an
integral membrane protein that is synthesized lacking a cleavable pre-sequence, but instead
contains internal targeting information. It forms a dimmer of two identical subunits and consists
of a basket shaped structure with six trans-membrane helices that are tilted with respect to the
membrane, 3 of them kinked at the level of proline residues.
Mitochondrial ADP/ATP carrier

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Carrier Complexes in ETC-
Complex I
In Complex I (NADH dehydrogenase, also called NADH:ubiquinone oxidoreductase; EC
1.6.5.3) two electrons are removed from NADH and transferred to a lipid-soluble carrier,
ubiquinone (Q). The reduced product, ubiquinol (QH2) freely diffuses within the membrane, and
Complex I translocates four protons (H+
) across the membrane, thus producing a proton gradient.
Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus
being one of the main sites of production of harmful superoxide.
The pathway of electrons occurs as follows-
NADH is oxidized to NAD+
, by reducing Flavin mononucleotide to FMNH2 in one two-electron
step. FMNH2 is then oxidized in two one-electron steps, through a semiquinone intermediate.
Each electron thus transfers from the FMNH2 to an Fe-S cluster, from the Fe-S cluster to
ubiquinone (Q). Transfer of the first electron results in the free-radical (semiquinone) form of Q,
and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH2.
During this process, four protons are translocated from the mitochondrial matrix to the
intermembrane space.
Complex II
In Complex II (succinate dehydrogenase; EC 1.3.5.1) additional electrons are delivered into the
quinone pool (Q) originating from succinate and transferred (via FAD) to Q. Complex II consists
of four protein subunits: SDHA, SDHB, SDHC, and SDHD. Other electron donors (e.g., fatty
acids and glycerol 3-phosphate) also direct electrons into Q (via FAD).
Complex III
In Complex III (cytochrome bc1 complex; EC 1.10.2.2), the Q-cycle contributes to the proton
gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH2
at the QO site and sequentially transferred to two molecules of cytochrome c, a water-soluble
electron carrier located within the intermembrane space. The two other electrons sequentially
pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol. A
proton gradient is formed by two quinol (4H+4e-) oxidations at the Qo site to form one quinol
(2H+2e-) at the Qi site. (In total six protons are translocated: two protons reduce quinone to
quinol and four protons are released from two ubiquinol molecules).
When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as
antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide
formation.
Complex IV
In Complex IV (cytochrome c oxidase; EC 1.9.3.1) four electrons are removed from four
molecules of cytochrome c and transferred to molecular oxygen (O2), producing two molecules
of water. At the same time, four protons are translocated across the membrane, contributing to
the proton gradient. The activity of cytochrome c is inhibited by cyanide.

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Functions of Carrier Complexes-
Functions of Complex I-
o Transport of electrons from NADH to ubiquinone
 Electron source: NADH
 Co-factor: Flavin mononucleotide
 Transport: via eight redox groups, iron–sulphur clusters
 Electron acceptor: Ubiquinone
 Ubiquinone function: Transfers of electrons to next complex in the chain
(Complex III)
o Simultaneous shunting of protons
 Out of mitochondrial matrix
 Across inner mitochondrial membrane
 Into intermembrane space
o Stoichiometry: 4H+
/2e-
Functions of Complex II-
o Mitochondrial respiratory chain
 Catalyzes oxidation of succinate to fumarate
 Transfers electrons to ubiquinone pool of respiratory chain
o Krebs cycle
Functions of Complex III-
 Transfers electrons from ubiquinol to cytochrome c
 Coupled with transfer of electrons across inner mitochondrial membrane
 Contains 3 redox centers-
o Cytochrome b
o Cytochrome c
o Rieske FeS protein
Functions of Complex IV and V (ATP synthase)-
o Regulate Proton flow from intermembrane space to matrix
o Conversion of ADP + inorganic phosphate to ATP
o Molecular motor: Proton translocation at interface of subunits c and a
 Drives rotation of subunit c oligomer
 Induces conformational changes in F1
 Facilitates synthesis of ATP

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Proton Gradient-
Proton gradients (differences in the concentrations of hydronium ions) can be calculated as a
thermodynamic measure, termed electrochemical potential.
In biological processes, the direction of an ion moves by diffusion or active transport across a
membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts,
proton gradients are used to generate a chemi-osmotic potential that is also known as a proton
motive force. This potential energy is used for the synthesis of ATP by oxidative
phosphorylation.
The proton gradient can be used as intermediate energy storage for heat production and flagella
rotation. In addition, it is an inter-convertible form of energy in active transport, electron
potential generation, NADPH synthesis, and ATP synthesis/ hydrolysis.
Two protons are expelled at each coupling site, generating the proton motive force (PMF). ATP
is made indirectly using the PMF as a source of energy. Each pair of protons yields one ATP.
The electron transport chain in the mitochondrion is the site of oxidative phosphorylation
in eukaryotes. The NADH and succinate generated in the citric acid cycle are oxidized,
providing energy to power ATP synthase.

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Iron Sulphur Proteins and Cytochromes-
Iron-sulfur proteins are proteins characterized by the presence of iron-sulfur clusters containing
sulfide-linked di-, tri-, and tetra- iron centers in variable oxidation states. Iron-sulfur clusters are
found in a variety of metalloproteins, such as the ferredoxins, as well as NADH dehydrogenase,
hydrogenases, Coenzyme Q - cytochrome c reductase, Succinate - coenzyme Q reductase and
nitrogenase. Iron-sulfur clusters are best known for their role in the oxidation-reduction reactions
of mitochondrial electron transport. Both Complex I and Complex II of oxidative
phosphorylation have multiple Fe-S clusters.
In almost all Fe-S proteins, the Fe centers are tetrahedral and the terminal ligands are thiolato
sulfur centers from cysteinyl residues. The sulfide groups are either two- or three-coordinated.
Three distinct kinds of Fe-S clusters with these features are common-
2Fe-2S Clusters, 4Fe-4S clusters and 3Fe-4S clusters.
2Fe-2S Clusters 4Fe-4S clusters
Cytochromes are pigments that contain iron. They are found in two very different environments.
Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile
macromolecular structures imbedded in the membrane. The mobile cytochrome electron carrier
in mitochondria is cytochrome c. Bacteria use a number of different mobile cytochrome electron
carriers.
Other cytochromes are found within macromolecules such as Complex III and Complex IV. They
also function as electron carriers, but in a very different, intramolecular, solid-state environment.
Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone
carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, etc.) enter
the electron transport chain at the cytochrome level. When electrons enter at a redox level greater
than NADH, the electron transport chain must operate in reverse to produce this necessary,
higher-energy molecule.

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Reversed Electron Transfer-
Oxidative phosphorylation is a partially reversible process and in the presence of an artificially
high ATP/ADP ratio electrons from a weak reducing agent like succinate can be forced
backwards through the respiratory chain carriers to yield a stronger reductant such as NADH:
Succinate + NAD+
+ Energy => Fumarate + NADH + H+
(overall reaction)
Ethanol is relatively poor reducing agent, and will only reduce a tiny proportion of any added
NAD. However, in an artificial system containing sub-mitochondrial particles, added alcohol
dehydrogenase, a trace of NAD and excess NADP, electrons from ethanol can be forced
backwards via a small pool of NADH through the energy-linked transhydrogenase to form large
amounts of the excellent reducing agent, NADPH. The reaction requires a source of energy:
either added ATP, or respiration using another segment of the respiratory chain.
Ethanol + NAD+
=> Acetaldehyde + NADH + H+
NADH + NADP+
+ Energy => NAD+
+ NADPH
The process has little physiological relevance, but gave enormous insight into mitochondrial
function. Reversed electron transport from ethanol to NADP in rotenone-blocked sub-
mitochondrial particles can be driven either by energy from external ATP (reversing the normal
operation of the F1ATPase) or by the energy from succinate oxidation via complex 2, complex 3
and complex 4. Both routes are sensitive to uncouplers, but oligomycin only blocks the process
when it is driven by ATP. This key observation showed that there must be a common high
energy intermediate between the coupling sites on the respiratory chain and the manufacture of
ATP. The intermediate is known as the high energy pool.

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References-
1. Leninger’s Principles of Biochemistry by D. L. Nelson and M. M. Cox (2000)
2. Biochemistry by L. Stryer, W. H. Freeman and Co. (4th
Edition) (1992)
3. Fundamentals of Biochemistry by Ed Voet, J. D. Voet and C.W. Pratt (1999)
4. http://ssrl.slac.stanford.edu/research/highlights_archive/electransfer.html
5. http://www.trinity.edu/lhunsick/ironsulfur_cluster_proteins.htm
6. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellularRespiration.html

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