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Oxidation of Cytoplasmic Reduced NAD (NADH+H+)
+
NADH+H is continuously formed in the cytoplasm by glycolysis and it must be oxidized
+
to regenerate cytoplasmic NAD which is important for the process of glycolysis to
proceed normally.
I- In the absence of oxygen
1- To regenerate NAD+ under anaerobic conditions, two electrons are transferred from
+
each NADH+H molecule to pyruvic acid forming lactic acid by the action of the lactate
dehydrogenase enzyme (LDH).
+
Pyruvic acid + NADH+H
lactic acid + NAD+
2- Anaerobic bacteria and yeasts reduce pyruvic acid to ethanol and CO2 with oxidation of
+
NADH+H to NAD+ by the alcohol dehydrogenase enzyme, thereby regenerating NAD+.
Pyruvate Decarboxylase
Pyruvic acid
acetaldehyde
CO2
Alcohol Dehydrogenase
Ethanol
+
NADH+H
NAD+
II- In the Presence of Oxygen
The inner mitochondrial membrane is impermeable to NADH+H+. Therefore, NADH+H+
produced during glycolysis cannot pass directly into the mitochondria to be oxidized. Two
shuttles serve to transport reducing equivalents from NADH+H+ in cytosol to the electron
transport chain in mitochondria; glycerol-3-phosphate shuttle and malate aspartate shuttle.
1- Glycerol-3-Phosphate Shuttle
The glycerol-3-phosphate shuttle is a secondary mechanism for the transport of reducing
+
equivalents from cytosolic NADH+H into the mitochondrion to be oxidized via the
electron transport chain. The shuttle involves two different glycerol-3-phosphate
dehydrogenases (glycerol-3PDH): one is cytosolic, acting to produce glycerol-3-phosphate
from dihydroxyactone phosphate (DHAP), and the other is mitochondria, which is an
integral protein of the inner mitochondrial membrane, oxidizes the glycerol-3-phosphate
produced by the cytosolic enzyme to dihydroxyactone phosphate with reduction of FAD to
FADH2
FADH2 is then oxidized in the electron transport chain giving 2 ATP.
2
NAD+
Glycerol-3P
Cytoplasmic
glycerol-3PDH
+
NADH+H
FAD
Mitochondrial
glycerol-3PDH
DHAP
Cytosol
FADH2
Inner mitochondrial
membrane
Mitochondrial
matrix
Glycerol-3-phosphate shuttle
2- Malate Aspartate Shuttle
The malate-aspartate shuttle is the principal mechanism for the movement of reducing
+
equivalents (in the form of NADH+H ) from the cytoplasm to the mitochondria. It is more
complex than the glycerol-3-phosphate shuttle but more efficient.
Malate aspartate shuttle occurs in two phases: phase A and phase B.
Phase A
•
•
•
•
Cytoplasmic Malate Dehydrogenase (MDH) reduces Oxaloacetate (OAA) to
+
malate while oxidizing NADH+H to NAD+.
Malate is transported to the interior of the mitochondrion.
Inside the mitochondrion, malate is oxidized once again to Oxaloacetate by the
+
mitochondrial MDH, giving rise to NADH+H
+
NADH+H is then oxidized in the electron transport chain giving 3 ATP.
Phase B
•
•
•
•
Oxaloacetate formed inside mitochondrion is converted to aspartate by
transamination, the amino group being donated by glutamate which is converted to
α-ketoglutarate. This transamination occurs by mitochondrial glutamic oxaloacetic
transaminase (GOT).
Aspartate then leaves the mitochondrion and enters the cytosol. Also, αketoglutarate leaves the mitochondrion to the cytosol.
In the cytoplasm, aspartate is converted to oxaloacetate while α-ketoglutarate is
converted to glutamate by transamination by cytoplasmic GOT.
The glutamate enters into the mitochondrion and the oxaloacetate begins a new turn
of the cycle.
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Malate
Malate
+
NAD
+
NAD
MDH
MDH
NADH+H+
NADH+H+
OAA
OAA
Glutamate
Glutamate
GOT
GOT
α-KG
α-KG
Aspartate
Aspartate
Cytosol
Inner mitochondrial
membrane
Mitochondrial
matrix
Malate Aspartate Shuttle
+
Energy from Oxidation of Cytosolic NADH+H
+
1- Cytosolic NADH+H is oxidized by lactate dehydrogenase in absence of oxygen and
+
gives no energy but serves to regenerate NAD .
2- Glycerol-3-phosphate shuttle generates 2 ATP for every cytosolic NADH+H+ molecule
oxidized, as FADH2 bypasses the first phosphorylation site in the electron transport chain.
3- Malate aspartate shuttle generates 3 ATP for every cytosolic NADH+H+ molecule
oxidized. So, it is more efficient than the glycerol-3-phosphate shuttle.
Bioenergetics
Bioenergetics is the study of thermodynamics (energy transformations) in living systems.
Cells convert potential energy (energy that has not yet been used), usually in the from of
covalent bonds between carbon atoms or in the form of ATP molecules, into kinetic energy
(energy in use) to accomplish cell division, growth, biosynthesis, active transport and all
other processes that need energy.
Although complicated, biological systems obey the fundamental laws of thermodynamics.
The First law of thermodynamics states that energy is always conserved, it cannot be
created or destroyed. From this law we can conclude the following points:
•
Energy lost by the system, must be gained by the surroundings and vice versa
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•
The change in energy (∆E) of a system refers to the change that occurs when
energy flows into, or out of, the system
•
The internal energy (E) of a system is the sum of all the kinetic and potential
energy contained within the system.
•
If E1 is the internal energy within the initial system, and E2 is the internal energy at
some later time, then the change in energy (∆E) is simply:
∆E = E2 - E1
•
If ∆E is positive, it means that energy flowed into the system from the
surroundings.
•
If ∆E is negative, it means that energy flowed out of the system into the
surroundings.
The second law of thermodynamics states that the universe (i.e. all systems) tends to the
greatest degree of randomization. This concept is defined by the term entropy (∆S) which
is a measure of disorder. An ordered state is low entropy, while a disordered state is high
entropy.
Gibbs Free Energy
Gibbs free energy (∆G) is that energy which is available for useful work. Gibb's free
energy calculation allows to determine whether a given reaction will be
thermodynamically favorable or not. Also, Gibbs free energy is an indicator of spontaneity
of a reaction.
Gibbs free energy (∆G) = ∆H – T∆S
Where:
o ∆H is the enthalpy. For simplicity, it equals the heat content (H) of the system
at constant pressure and volume that are usually found in biological systems.
o T is the temperature in Kelvin degrees
o ∆S is the entropy which is a measure of disorder
If ∆G less than 0 (negative), the reaction is exergonic (releases free energy) and
spontaneous reaction
If ∆G is greater than 0 (positive), the reaction is endergonic i.e. requires energy to proceed
i.e. the reaction is not spontaneous.
At ∆G = 0, the reaction is at equilibrium
The two factors that determine the change in free energy in a reaction are the intrinsic
properties of the substances and the concentrations of the reactants and products.
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Standard Free Energy (∆G°)
The standard free energy (∆G°) is the free energy change under standard conditions; all
reactants and products have an initial concentration of 1 mol/L (1.0M), temperature= 25
°C and the pressure= 1 atmosphere.
∆G° is used to calculate the equilibrium constant of reactions.
The reactions that occur in living cells may be exergonic reactions releasing free energy or
endergonic reactions which require energy. Usually, catabolic reactions are exergonic and
the anabolic reactions are endergonic.
Living cells utilize the energy liberated from the exergonic reactions to synthesize high
energy intermediate (mainly ATP) which in turn gives the energy to energy requiring
processes.
ATP (Adenosine Triphosphate)
It is a high energy compound which is considered as the energy currency of the cell.
It is adenosine triphosphate, a nucleotide formed of adenine, ribose and 3 inorganic
phosphates.
ATP Production
The energy needed for ATP synthesis is mostly obtained from:
A- Oxidative Phosphorylation in which the electrons produced by oxidation of foodstuffs
are transferred through the electron transport chain to react finally with oxygen and the
produced energy is utilized for ATP synthesis
B- Substrate Level Phosphorylation
ATP can be formed by transferring the high energy from substrates directly to ADP.
ATP is formed at substrate levels from:
1) - 1,3 diphosphoglycerate by phosphoglycerate kinase (in glycolysis)
2) - Phosphoenol pyruvate by pyruvate kinase (in glycolysis)
3) - Succinyl CoA by succinate thiokinase (in citric acid cycle)
4) - Creatine phosphate by creatine kinase enzyme
5) - ADP by myokinase (adenyl kinase) enzyme
Functions of ATP
It is the source of energy for:
1- Biosynthetic reactions
1- Muscle contraction
2- Nerve conduction
3- Active absorption and secretion
4- Active transport across biological membranes
5- Activation of monosaccharides, fatty acids and amino acids
6- Formation of creatine phosphate, which is the energy store in muscles.
7- Biosynthesis of cAMP
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Low Energy Compounds
They are compounds that contain low energy bonds.
Low energy bond is that bond which on hydrolysis gives energy less than 4000 calories.
Examples of low energy bonds include:
i- Ester bonds in lipids
ii- Glycosidic bonds in carbohydrates
iii- Peptide bonds in proteins
iv- Phosphate ester bond in glucose 6 phosphate and fructose 6 phosphate.
High Energy Compounds
They are compounds that contain one or more high-energy bonds.
High-energy bond is that bond which on hydrolysis gives energy more than 7000 calories.
Types of High Energy Bonds
High energy bonds may be classified into:
A- High energy phosphate bonds
B- High energy sulfur bonds
A- High Energy Phosphate Bonds
1- Pyrophosphate bond which occurs in ATP, GTP and CTP, ADP, GDP, CDP and UDP.
2- Carboxyl phosphate bond that occurs in 1,3 diphosphoglycerate
3- Enol phosphate bond that occurs in phosphoenol pyruvic acid
4- Guanidine phosphate that occurs in creatine phosphate and arginine phosphate
B- High Energy Sulfur Bonds
1- Thioester bonds present in acetyl CoA (active acetate) and succinyl CoA (activate
succinate).
2- Sulfur bond in active methionine (S-adenosyl methionine)
3- Sulfur bond in active sulfate (PAPS, phospho-adenosyl-phospho-sulfate).