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INTERMEDIARY METABOLISM
Introduction to Metabolism and Bioenergetics
Najma Z. Baquer
Emeritus Professor
School of Life Sciences
Jawaharlal Nehru University
New Delhi – 110 067
(02 April 2007)
CONTENTS
General features of metabolism
Catabolic (degradative) and anabolic (biosynthetic) pathways
Experimental approaches to study metabolism
Laws of thermodynamics
Free energy change
Standard free energy change
Key words
Heterotrophs, Autotrophs, Enzymes, Metabolism, Catabolic pathways, Anabolic pathways, Macromolecules,
adenosine triphosphate (ATP), nicotinamide adenine dinucleotide phosphate (NADPH), Mutants, Isotopes,
Endergonic, Exergonic, Intermediate, Oxidized, Reduced.
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General features of metabolism
Myriad, of enzyme catalyzed reactions take place in living cells. Although we collectively refer
to these reactions as “metabolism”, we must not think of cell metabolism in terms of a
membrane-surrounded bag of randomly acting enzymes. Metabolism is a highly coordinated and
purposeful cell activity, in which many multienzyme systems cooperate. Metabolism has four
specific functions:
1. To obtain chemical energy from the degradation of energy rich nutrients from the
environment or from captured solar energy.
2. To convert nutrient molecules into the building block precursors of cell macromolecules.
3. To assemble these building blocks into proteins, nucleic acids, lipids, polysaccharides and
other cell components.
4. To form and degrade biomolecules required in specialized functions of cells.
Although metabolism involves hundreds of different enzyme catalyzed reactions, the central
metabolic pathways, are few in number and they are identical in most forms of life.
Living organisms can be divided into two large groups according to the chemical form of carbon
they require from the environment.
(1)
Autotrophic (self feeding) cells can use carbon dioxide from the atmosphere as the
sole source of carbon and construct all their carbon containing biomolecules from it,
e.g. photosynthetic bacteria and green leafy cells of plants. Some can also use
nitrogen like cyanobacteria to generate nitrogenous components.
(2)
Heterotrophic cells (feeding on others) cannot use atmospheric CO2 and obtain
carbon from their environment in the form of relatively complex organic molecules
such as glucose. In higher animals and microorganisms, cells are mostly
heterotrophic.
Autotrophic and heterotrophic organisms can be divided into subclasses. There are two classes
of heterotrophic organisms - Aerobic and Anaerobic:
(1)
(2)
Aerobes live in air and use molecular oxygen to oxidize their nutrient molecules.
Anaerobes live in the absence of oxygen. Many cells such as yeast, can live either
aerobically or anaerobically, such organisms are called facultative. Anaerobes that
cannot use oxygen at all and indeed may be poisoned by it e.g. microorganisms,
present in deep soils or the ocean floor, are called strict anaerobes.
“Enzymes” are the simplest units of metabolic activity, each catalyzing a specific chemical
reaction. `Metabolism’, however consists of multienzymes sequences each promoting the
sequential catalytic steps involved in a given metabolic pathway. Such enzyme systems may
have anywhere from 2 to 20 enzymes, acting in a consecutive, linked fashion, so that the product
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of the first enzyme becomes the substrate of the second and so on. The “successive
transformation products” in such a pathway [B, C, D, etc.] are known as metabolic intermediates
or “metabolites”. Each of the consecutive steps in these pathways brings about a small specific
chemical change, usually the removal, transfer or addition of a specific atom, a molecule or
functional group. Through such orderly, step by step changes, the incoming biomolecule is
transformed into its metabolic end product. Most metabolic pathways are “linear”, but some are
“circular” or “cyclic”, usually metabolic pathways have branches leading in or out. The term
“Intermediary metabolism” is often used to denote the specific sequences of intermediates
involved in the pathways of cell metabolism.
Regulated step
A
E1
A
E15
B
M
B
E14
E2
C
E2
C
L
E3
E3
E13
D
D
K
E4
E12
E4
E
E
J
E11
E5
P
E5
Regulated step
P
Catabolic (degradative) and anabolic (biosynthetic) pathways
“Intermediary metabolism” has two phases : catabolism and anabolism. Catabolism is the
degradative phase of metabolism in which organic nutrient molecules, e.g. carbohydrates, lipids
and proteins coming either from the “environment” or the “cells own” nutrient stores, are
degraded by stepwise reactions into smaller, simpler end products, e.g. lactic acid, CO2 and
ammonia. Catabolism is accompainied by release of free energy inherent in the complex
structure of large organic molecules. At certain steps in a catabolic pathway, much of the free
energy is conserved, by means of coupled enzymatic reactions in the form of energy carrying
molecule, adenosine triphosphate (ATP). Some may be conserved as energy rich hydrogen
atoms carried by the coenzyme nicotinamide adenine dinucleotide phosphate [NADPH] in the
reduced form.
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Energy yielding nutrients
Cell
Macromolecules
Carbohydrates
Fats
Catabolism
Release
(Degradative)
Energy poor
End products
CO2
H2O
NH3
Proteins
Polysaccharides
Lipids
Nucleic Acids
release
of
energy
Chemical
Energy
Anabolism
(Biosynthesis)
ATP
NADPH
Precursor molecules
Amino acids
Sugars
Fatty Acids
Nitrogenous bases
In anabolism also called biosynthesis, the building up or synthetic phase of metabolism, small
precursor or building block molecules are built up into large macromolecular components of
cells, such as proteins and nucleic acids. Since biosynthesis results in increased size and
complexity of structure, it requires input of free energy which is furnished by the breakdown of
ATP to ADP and phosphate. Biosynthesis of some cell components also requires high-energy
hydrogen atoms, which are donated by NADPH. Catabolism and anabolism take place
simultaneously in cells, and their rates are regulated “independently”.
Catabolic pathways converge to a few end products
The enzymatic degradation of each of the major energy-yielding nutrients of cells
(carbohydrates, lipids and proteins) proceeds in a stepwise manner through a number of
consecutive enzymatic reactions.
There are three major stages in aerobic catabolism.
In Stage I, cell macromolecules are degraded to their major building blocks. Thus
polysaccharides are degraded to hexoses or pentoses; lipids are degraded to fatty acids, glycerol
and other components, and proteins are hydrolyzed to their 20 component amino acids.
In Stage II of catabolism, the various products formed in stage I are collected and converted into
smaller number of yet simpler molecules. Thus the hexoses, pentoses and glycerol from stage I
are degraded to a single 3-C intermediate Pyruvate, which is then converted into a single 2-C
unit, the acetyl group of Acetyl-CoA. Similarly the fatty acids and the carbon skeletons of most
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of the amino acids are also broken down to form acetyl groups in the form of acetyl-CoA.
Acetyl-CoA is thus the common end product of stage II of catabolism.
Proteins
Stage 1
Polysaccharides
Pentoses
Amino acids
Stage II
Lipids
Hexoses
Glucose
Glycerol, fatty acids
Pyruvate
Acetyl CoA
Stage III
Citric acid cycle
NH3
H2O
CO2
Fig. 1: The three stages of catabolism of major energy-yielding nutrients
In Stage III the acetyl group of acetyl-CoA is fed into the citric acid cycle - the “final” common
pathway by which most energy yielding nutrients are ultimately oxidized to CO2, water and NH3
(or other nitrogenous products) which are the other end products of catabolism.
It is important to note that the pathways of catabolism converge towards the citric acid cycle in
stage III. The final pathway of catabolism thus resembles a widening river, fed by many
tributaries and streams.
Biosynthetic (anabolic) pathways diverge to yield many products
Anabolism, or biosynthesis, also takes place in three stages, beginning with smaller precursor
molecules. For e.g. protein synthesis begins with the formation of α-keto acids and other
precursors. In the next stage the α-ketoacids are aminated by amino group donors to form α-
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aminoacids. In the final stage of anabolism the amino acids are assembled into polypeptide chain
to form many different proteins.
Similarly acetyl groups are built up into fatty acids and they in turn are assembled to form
various lipids. Just as catabolism is a converging process, anabolism is a diverging one, since it
begins with a few simple precursor molecules from which a large variety of different
macromolecules are made.
Each major stage in the anabolism or catabolism of a given biomolecule is catalyzed by a
multienzyme system. The sequential chemical changes taking place in the central routes of
metabolism are virtually identical in all forms of life e.g. the catabolism of D-glucose to give
pyruvic acid is accomplished through the same chemical intermediates and through the same
number of reactions in most living organisms.
ATP carries energy from catabolic to anabolic reactions
Complex nutrient molecules such as glucose contain potential energy because of their high
degree of structural order. When it is degraded to CO2 and H2O much free energy becomes
available. Free energy is that form of energy capable of doing work under constant temperature
and pressure, this energy must therefore be captured and conserved or it will appear as heat.
Much of the free energy released from glucose and other cellular fuels during their catabolism is
conserved by the coupled synthesis of adenosine triphosphate (ATP) from adenosine diphosphate
(ADP) and inorganic P.
ATP
CO2
1.
2.
3.
4.
H2O
Catabolism
Biosynthesis
Contraction and Motility
Active Transport
Transfer of genetic
information
O2
Fuels
ADP +Pi
Energy dependent activities of cells depend upon transmission of energy by ATP.
The energy conserved in the form of ATP can do four different kinds of work.
1.
2.
3.
Biosynthesis, the terminal P group transferred to precursor molecules are “energized”.
Cell motility or contraction.
Active transport of nutrients and ions across membranes.
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4.
Genetic information transfer during the biosynthesis of DNA, RNA and proteins.
Thus we have an energy cycle in cells, in which ATP serves as the energy carrying link
between energy-yielding and energy requiring cellular processes.
NADPH carries energy in the form of reducing power
A second way of carrying chemical energy from reactions of catabolism to the energy requiring
reactions of biosynthesis is in the form of hydrogen atoms or electrons. When glucose is formed
from CO2 during photosynthesis, or when fatty acids are made from acetate in the liver of an
animal, reducing power in the form of hydrogen atoms is required for the reduction of double
bond to single bonds, hydrogen atoms must have considerable free energy. Such high energy
hydrogen atoms are obtained from cell fuels by dehydrogenases which catalyze removal of
hydrogen atoms from fuel molecules and their transfer to specific coenzymes, particularly to the
oxidized form of nicotinamide adenine dinucleotide phosphate (NADP+). The reduced or
hydrogen carrying form of this coenzyme, designated NADPH is a carrier of energy-rich
electrons from catabolic reactions to electron requiring biosynthetic reactions, just as ATP is a
carrier of energy-rich phosphate groups.
Reduced fuel
Catabolism
NADP
Oxidized product
NADPH
Carrier of energy electrons
Reduced
Biosynthetic
Products
Reductive
Biosynthetic
reactions
Oxidized
Precursor
Cell metabolism is an economical tightly regulated process
Cell metabolism operates at maximum economy. The overall rate of energy yielding catabolism
is controlled by the needs of the cell for energy in the form of ATP and NADPH. Thus cells
conserve just enough nutrients to meet the energy utilization at any given time. Similarly the
rate of biosynthesis of building block molecules and of cell macromolecules is also adjusted to
immediate needs.
Many animals and plants can store energy-supplying and carbon supplying nutrients, such as fat
and carbohydrates, but they generally cannot store protein, nucleic acids, or simple building
block molecules, which are made only when needed and in amounts required. Catabolic
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pathways are very sensitive and responsive to changes in energy needs, the regulatory
mechanisms of central metabolic pathways, particularly those providing energy as ATP, are
capable of responding to metabolic needs quickly and with great sensitivity.
Experimental approaches to study metabolism
Three major approaches are used singly or in combination, to work out the chemical details of a
metabolic pathway.
In vitro studies
The first and the most direct is to study the pathway in vitro (in glass i.e. in the test tube) in a cell
free extract of a tissue capable of catalyzing the overall metabolic process e.g. conversion of
glucose into ethanol and CO2 by yeast, was discovered by Buchner, 1898. Subsequently it was
found that the breakdown of glucose in such extracts required the addition of inorganic
phosphate, which disappeared from the extract as glucose was consumed. It was then found that
phosphorylated intermediates (derivatives) of a hexose accumulated in the medium. Once the
intermediates were identified, an enzyme was found in the yeast extract that acted upon it to
form another product. Addition of enzyme inhibitors to yeast extract caused other intermediates
to accumulate. By a combination of such approaches the 11 metabolites that are the
intermediates in the conversion of glucose into ethanol in yeast were ultimately identified and
isolated. Each of the 11 enzymes involved in this sequence have been isolated and identified.
Many other metabolic pathways have been worked out by this direct approach in which the
successive enzymes and intermediates degrading and forming them have been identified one by
one. When the entire sequence is known it can be reconstituted in the test tube from purified
components.
Mutants of organisms allow identification of intermediate steps
Another important approach to elucidation of a metabolic pathway is the study of genetic
mutations of organisms, in which a given enzyme fails to be synthesized in active form. Such a
defect, if not lethal, may result in the accumulation and excretion of the substrate of the defective
enzyme. From genetic defects in specific enzymes occurring in people it has been possible for
e.g., to deduce the nature of certain steps in the metabolism of amino acids. Such human genetic
disorders are rare and do not lend themselves to systematic investigation. However, genetic
defects in microorganisms can be produced at will by subjecting them to mutagenic agents e.g.
irradiation with X-rays or treatment with certain chemicals, which can alter the structure of
specific genes in their DNA. Such mutant microorganisms in which one enzyme or another is
defective are powerful tools for study of metabolism.
Isotopic tracers provide a powerful method of studying metabolism
Another powerful method for establishing the general out line of a metabolic pathway is to use
an isotopic form of an element to label a given metabolite. For example, the radioactive isotope
14
C (the normal or average atomic weight of carbon is 12.01) is frequently used to label a
specific carbon atom in an organic molecule. Such a 14C-labelled molecule is chemically
indistinguishable from a normal labeled molecule, but can easily be detected and measured
through its radioactivity. e.g. acetic acid can be synthesized in the laboratory in such a way that
its carboxyl carbon atom is enriched in 14C which otherwise occurs in only extremely small and
constant amounts in the carbon compound found in biosphere and geosphere.
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When such a sample of radioactively labeled acetate is fed to an animal, its metabolic fate can be
readily traced. For example, the respiratory CO2 exhaled by the animal will be found to contain
14
C indicating that some of the acetate is metabolized in such a way that its carboxyl carbon atom
is converted into CO2.
Moreover, if palmitic acid is subsequently isolated, from the liver lipids of the animals it will be
also found to contain 14C indicating that the carboxyl carbon atom of acetate is a biosynthetic
precursor of palmitic acid. Furthermore when the labeled palmitic acid molecule is chemically
degraded it will be found to have excess 14C in only the alternate carbon atom beginning with the
carboxyl carbon.
H
O
H C-C*
H
O
O
*
Acetate labeled in carboxylic atom
OFed to rats and palmitic acid isolated from the liver
C
CH2
*
CH2
CH2
*
CH2
CH2
*
CH2
CH2
*
CH2
CH2
*
CH2
CH2
*
CH2
CH2
*
CH2
CH3
Figure 1: Use of an isotope of carbon to trace the metabolic fate of the carboxyl-C atom of
acetate
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Much appears as exhaled 14CO2, significant amount is found in the palmitic acid of liver lipids
isotope found in alternating C atom telling us that palmitic acid is made by 8 molecules of
acetate joined in a head to tail fashion.
However if acetate containing 14C only in the methyl group is fed, again palmitic acid will be
labeled, but in alternate carbon atoms starting from the α, or 2 carbon atoms. These observations
led to the conclusion that all the carbons of palmitic acid ultimately derive from acetate
molecules in a pathway that result in a head to tail linkage of the carbon skeleton of acetate
molecules.
Rate of metabolic processes
The isotope tracer method can also be used to determine the rate of metabolic processes in intact
organisms. One of the most significant advances made with this powerful method is the
discovery that the macromolecular components of cells and tissues undergo constant metabolic
turnover i.e. they exist in a dynamic steady state in the cell, in which constant biosynthesis is
exactly counter balanced by an equal rate of degradation. For e.g. the isotopic measurements
have shown that the protein of rat liver have a half life of about 5 to 6 days. On the other hand,
the protein of skeletal-muscle or the brain turnover much more slowly.
Metabolic pathways are compartmented in cells
1.
Prokaryotic and eukaryotic cells
Prokaryotic cells contain no compartments separated by internal membranes, yet there is
some degree of segregation of certain enzyme systems in bacteria, e.g. the enzymes
participating in the biosynthesis of proteins are located in the ribosomes and some of the
enzymes participating in the biosynthesis of phospholipids are located in the bacterial cell
membrane.
2.
Animal or plant tissues
These are first gently homogenized in a isotonic sucrose medium, a process that ruptures
the plasma membrane but leaves most of the internal organelles intact. Sucrose is used
because it does not pass through membranes readily and thus does not cause internal
organs such as chloroplast and mitochondria to swell.
The subcellular organelles, e.g. nuclei and mitochondria, which differ in size and specific gravity
and thus sediment at different rates in a centrifugal field can then be isolated from the
“homogenate” by differential centrifugation.
The nucleic, mitochondria and other fractions obtained in this way can be tested for their ability
to catalyze a given metabolic sequence. From this approach it can be found that different
metabolic pathways take place in different intracellular locations in eukaryotic cells.
Cytosol
Mitochondria
glycolysis conversion of glucose
TCA, electron transport
lactate
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Principle of thermodynamics
Bioenergetics or Biochemical thermodynamics describes the transfer and utilization of energy in
biological systems. It concerns only the initial and final energy states of reaction components,
not the mechanism or how much time is needed for the chemical change to take place. In short,
bioenergetics predicts if a process is possible. “Biological systems follow the general laws of
thermodynamics.”
Law of thermodynamics
First law of thermodynamics
The total energy of a system, including its surroundings, remains constant. This is also the law of
conservation of energy. It implies that within the total system, energy is neither lost nor gained
during any changes.
Second law of thermodynamics
The total entropy of a system must increase if a process is to occur spontaneously.
Significance
Thus, Bioenergetics makes use of a few basic ideas from the field of thermodynamics,
particularly the concept of free energy. Changes in free energy (∆G) provide a measure of the
energetics feasibility of a chemical reaction and can allow prediction of whether a reaction will
take place. The free energy concept is essential for understanding the unique role adenosine
triphosphate (ATP) plays in transferring energy from energy yielding catabolic processes to
energy – requiring reaction.
∆G: changes in free energy
• Energy available to do works.
• Approaches zero as reaction proceeds to equilibrium.
• Predicts whether a reaction is favourable.
∆H: changes in enthalpy
• Heat released or absorbed during a reaction.
• Does not predict whether a reaction is favourable.
∆S: Changes in Entropy
• Measure of randomness.
• Does not predict whether the reaction is favourable.
Relation between changes in free energy (G), Enthalpy (S) and Entropy (S).
∆G = ∆H – T∆S --------------------------------------------------- (I)
Where T is the absolute temperature in degree Kelvin (oK): ok = bC + 273
Under the conditions of biochemical reactions, because ∆H is approximately equal, to ∆E, the
total changes in internal energy of the reaction, the above relationship may be expressed in the
following way:
∆G = ∆E – T∆S --------------------------------------------------- (II)
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A. Free energy change- Three varieties Go and ∆Go, ∆Go/
∆G- Is more general because it predicts the change in free energy and thus, the
a reaction, at any specified concentration of products.
direction of
∆Go- Is the change in standard free energy, when reactants and products are at 1mol/lt.
∆Go/- The standard free energy change at pH 7 i.e. in the standard state when the
concentration of protons in assumed to be 10-7 mol/lt.
a. Sign of ∆G predicts the direction of a reaction: The change in free energy, ∆Go, can be
used to predict the direction of a reaction at constant temperature and pressure. Consider
the reaction:
A
B
A. Negative ∆G: if ∆G is negative, there is a net loss of energy, and the reaction goes
spontaneously and the reaction is said to be exergoic.
B. Positive ∆G: if ∆G is positive, there is a net gain of energy. And the reaction does
not go spontaneously and the reaction is said to be endergonic. The energy must
be added to the system to make the reaction to go.
C. ∆G is zero: If ∆G = 0, the reactants are in equilibrium.
b. ∆G of the forward and backward reactions: The free energy of the forward reaction
(A B) is equal in magnitude but opposite in sign to that of the back reaction (B
A).
For example, if ∆G of the reaction is –5000 cal/mol, then that of the back reaction is +
5000 cal/mol.
c. ∆G depends on the concentration of reactants and products: ∆G of the reaction A
B depends on the concentration of the reactant and product. At constant temperature and
pressure, the following reaction can be derived:
o
[B]
∆G = ∆G + RT In
[A]
Where, ∆G is the standard free energy change.
R is the gas constant (1.987 cal/mol -degree)
T is the abosulte Temperature (oK)
[A] and [B] are the actual concentration of reactant and product.
In represent the natural logarithm
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Fig.2: Change in free energy (∆G) during a reaction A. The product has a lower free
energy (G) than reactants B. The product has a high free energy than the reactant
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d. Sign of ∆G can be different from that of ∆Go: A reaction with a positive ∆Go can
proceed in the forward direction (have a negative overall ∆G) if the ratio [B]/[A] is
sufficiently small (that is ratio of reactants to products is large). For example, consider
the reaction:
(B) (A) (A) (A) (A) (A)
(A) (A) (A) (A)
∆G = -0.96 Kcal/mol
(B) (A) (A) (A) (B) (A)
(A) (B) (A) (A) (B) (A)
(A)
(B)
Glucose 6-PO4
Fructose 6-PO4
(i) Shows reaction conditions in which the concentration of reactant, glucose 6-phosphate, is
high compared to the concentration of product, fructose 6-phophate. This means
(ii) that the ratio of the product to reactant is small, and RT in
Fructose 6- PO4
Glucose 6- PO4
(iii) is large and negative, causing ∆G to be negative despite ∆Go being positive. Thus, the
reaction can proceed in the forward direction.
B. Standard free energy change, ∆G: ∆Go is called the standard free energy change
because it is equal to the free energy change, ∆G, under standard conditions that is when
reactants and products are kept at 1mol/lit concentration
[B]
=1
[A]
[B]
So,
In
= 0 [In 1 = 0]
[A]
And therefore the equation III becomes
∆G
= ∆Go + 0
∆Go is predictive only under standard conditions
Under standard conditions, ∆G can be used to predict the direction a reaction proceeds,
because under these conditions, ∆Go is equal to ∆G. However, ∆Go cannot predict the
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direction of a reaction under physiological condition, because it is composed solely of
constants (R, T and Keq) and is therefore not altered by changes in product or substrate
concentrations.
Endergonic processes proceed by couping to exergonic processes
The vital processes e.g.1. Synthetic reactions, muscular contraction, nerve impulse. Conduction,
and active transport obtain energy by chemical linkage, or coupling, to oxidative reactions.
A
Heat
D
Chemical
C
B
A+C
B+D+ Heat
A
D
Free energy
[E]
-[E]
B
C
Exergonic reactions
The conversion of metabolite A to metabolite B occurs with release of free energy. It is coupled
to another reaction, in which free energy is required to convert metabolite C to metabolite D. As
some of the energy liberated in the degradative reaction is transferred to the synthetic reaction in
a form other than heat, the chemical terms “exothermic” and “endothermic” cannot be applied to
the these reactions. Rather, the terms exergonic and endergonic are used to indicate that a
process is accompanied by loss or gain, respectively, of free energy, regardless of the form of
energy involved.
1
2
E
3
4
Endergonic
processes
Syntheses
Muscular
contraction
Nervous
Excitation
Active transport
Fig. 3. Transfer of free energy from an exergonic to Fig 4. Transfer of energy through a common high
An endergonic reaction via a high – energy inter
energy compound to energy – requiring (endergonic)
mediate compound.
processes.
In practice, an endergonic process cannot exist independently but must be a component of a
coupled exergonic reactions are termed as catabolism (generally, the breakdown or oxidation of
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the molecules), whereas the synthetic anabolic process constitute metabolism. One possible
mechanism of coupling could be envisaged if a common obligatory intermediate (I) took part in
both reactions, i.e.
A+C
I
B+D
A. Energy carried by ATP
(High energy phosphates play a central role in energy capture and transfer) In order to maintain
living processes, all organisms must obtain supplies of free energy from their environment.
Heterotrophic organisms obtain free energy by coupling their metabolism to the break down of
complex organic molecules in their environment. In all these organisms ATP plays a central role
in the transference of free energy from the exergonic to the endergonic processes.
NH2
N
N
N
N9
Adenosine
Mg
OO-
P
OO
P
OO
P
O CH2
O
C
C
Phosphate
H H
OH
Ribose
H
H
OH
ATP
B. Energy carried by ATP
ATP consists of a molecule of adenosine to which three phosphate groups are attached. If one
phosphate is removed, adenosine diphosphate (ADP) is produced. If two phosphate are removed,
adenosine monophosphate (AMP) results.
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1. ∆Go of ATP : The standard free energy of hydrolysis of ATP, ∆Go, is approximately –7300
cal/mol for each of the two terminal phosphate groups. Because of this large, negative ∆Go, ATP
is called a high –energy phosphate compound.
2. Very high – energy phosphate compounds: Compounds exist that contain phosphate with
energy higher than that of ATP includes
∆Go/
Compounds
Phosphoenolpyruvate
Carbomyl phosphate
1, 3 bisphosphoglycerate
Creatine phophate
KJ/mol
Kcal/mol
-61.9
-51.4
-49.3
-43.1
-14.8
-12.3
-11.8
-10.3
3. Low energy phosphate compounds: Other phosphate containing compounds having standard
free energy of hydrolysis less than ATP include:
∆Go
Compounds
ADP
AMP +Pi
Pyrophosphate
Glucose 1-phosphate
Fructose 6-phosphate
AMP
Glucose 6-phosphate
Glucose 3-phosphate
KJ/mol
Kcal/mol
-26.7
-27.6
-20.9
-15.9
-14.2
-13.8
-9.2
-6.6
-6.6
-5.0
-3.8
-3.4
-3.3
-2.2
4. ATP as an intermediate in phosphate transfer: ATP occupies an intermediate position on
the bioenergetic scale of phosphate containing compounds. ADP can serve as an acceptor of
phosphate groups containing very high energy to form ATP, which, in turn can donate phosphate
group in the cell to form lower energy phosphates. There are no enzymes in cells that can
transfer phosphate groups directly from very high – energy donors to low – energy acceptors
without their first being transferred to ATP.
The high free energy change on hydrolysis of ATP is due to relief of charge repulsion of adjacent
negatively charged oxygen atoms and to stabilization of the reaction products, especially
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phosphate. As resonance hybrids, other biologically important compounds that are classified as
“high – energy compounds” are thiol, amino acid esters involved in protein synthesis, S-adenosyl
methionine (active methionine) UDP Glc. (uridine diphosphate glucose) and PRPP (S –
phosphoribosyl-1-pyrophosphate).
5. High-energy phosphates are designated by ~(P): To indicate the presence of the highenergy phosphate group, Lipmann introduces the symbol ~(P). The symbol indicates that the
group attached to the bond, on transfer to an appropriate acceptor, results in transfer of the large
quantity of free energy. Thus, ATP contains two high – energy phosphate groups and ADP
contains one, whereas the phosphate in AMP is of the low – energy type, since it is a normal
ester link.
O-
O-
P
O~P
O
O
O-
Adenosine
O
O~
P
OO-
Adenosine
O P O~
OP O-
ATP
ADP
O
O
High – energy phosphate act as the “Energy currency of the cell”
There are three major sources of ~(P) taking part in energy conservation or energy capture:
1. Oxidative phosphorylation
Oxidative phosphorylation is the greatest quantitative sources of ~(P) in aerobic organism. The
free energy to drive this process comes from respiratory chain oxidation using molecular O2
within mitochondria.
2. Glycolysis
A net formation of two ~(P) results from the formation of lactate from one molecule of glucose
generated in two reaction catalyzed by Phosphoglycerate Kinase and Pyruvate Kinase,
respectively.
Phosphoglycerate Kinase
1, 3 Bisphosphoglycerate
3 –Phosphoglycerate
ADP
Mg+2
ATP
Pyruvate Kinase
Phosphoenolpyruvate
3 –Phosphoglycerate
ADP
Mg+2
ATP
19
3. The citric acid cycle
One ~(P) is generated directly in the cycle at the succinyl thiokinase step.
Succinyl thiokinase
Mg+2 ADP + Pi
ATP
Succinyl-CoA
Succinate
CoASH
Another group of compounds, phosphagens, act as storage forms of high – energy phosphate.
These include creatine phosphate, occurring in vertebrate skeletal muscle, heart, spermatozoa
and brain and arginine phosphate, occurring in invertebrate muscle.
ATP allows the coupling of thermodynamically unfavorable reactions to favorable one: The first
reaction of glycolysis, the phosphorylation of glucose to glucose – 6 phosphate is highly
endergonic and cannot proceed as such under physiological condition
(1) Glucose + Pi
glucose 6-PO4 + H2O
(∆Go/ = +13.8 KJ/mol)
For the reaction to proceed, it must be coupled with another reaction that is more exergonic than
the phosphorylation of glucose is endergonic. Such a reaction is the hydrolysis of the terminal
phosphate of ATP.
(2) ATP
ADP + Pi
(∆Go/ = -30.5 KJ/mol)
When (1) and (2) are coupled in a reaction catalyzed by Hexokinase, phosphorylation of glucose
readily proceeds in a highly exergonic reaction which under physiological conditions is far from
equilibrium and thus irreversible for practical purpose
Hexokinase
Glucose + ATP
glucose 6-PO4 + ADP (∆Go/ = -16.7 KJ/mol)
Many “activation” reactions follow this pattern.
Suggested Reading
1.
Harpers Biochemistry, Prentice Hall International.
2. Concepts of Biochemistry, L.M. Srivastava. CBS publishers and distributors (2004).