High Energy Compounds
 ATP often serves as an energy source.
Hydrolytic cleavage of one or both of the "high energy"
bonds of ATP is coupled to an energy-requiring
(non-spontaneous) reaction. (Examples presented earlier.)
 AMP functions as an energy sensor & regulator of
metabolism.
When ATP production does not keep up with needs, a
higher portion of a cell's adenine nucleotide pool is AMP.
AMP stimulates metabolic pathways that produce ATP.
• Some examples of this role involve direct allosteric
activation of pathway enzymes by AMP.
• Some regulatory effects of AMP are mediated by the
enzyme AMP-Activated Protein Kinase.
“High energy” bonds
NH 2
ATP
adenosine triphosphate
O
-O
P
O-
O
O
P
O-
N
N
O
O
P
O
phosphoanhydride
bonds (~)
N
adenine
CH2
O-
N
O
H
H
OH
H
OH
H
ribose
Phosphoanhydride bonds (formed by splitting out H2O
between 2 phosphoric acids or between carboxylic &
phosphoric acids) have a large negative ΔG of hydrolysis.
Why do phosphoanhydride linkages have a high DG
of hydrolysis? Contributing factors for ATP & PPi
include:
 Resonance stabilization of products of hydrolysis
exceeds resonance stabilization of the compound
itself.
 Electrostatic repulsion between negatively
charged phosphate oxygen atoms favors
separation of the phosphates.
Compounds with Δ G more negative than 7
Kcal/mole or 30 KJ/ mole are regarded as high
energy compounds.
ATP has special roles in energy coupling & Pi transfer.
DG of phosphate hydrolysis from ATP is intermediate
among examples below.
ATP can thus act as a Pi donor, & ATP can be synthesized
by Pi transfer, e.g., from PEP.
Roles of "high energy" bonds
 Energy transfer or storage
ATP, PPi, polyphosphate, phosphocreatine
 Group transfer
ATP, Coenzyme A
 Transient signal
cyclic AMP
Examples of other high energy compounds
1- Phosphocreatine :
another compound with a "high
energy" phosphate linkage, is used
in nerve & muscle for storage of
~P bonds.
 Phosphocreatine is produced
when ATP levels are high.
 When ATP is depleted during
exercise in muscle, phosphate is
transferred from phosphocreatine
to ADP, to replenish ATP.
O

O
CH3
H
N
P

O
C
N
O
CH2
NH2+
phosphocreatine
C
O
O
O
C
C
CH2
PEP
O
O
ADP ATP
OPO32
H+
C
C
C
O
O
OH
CH2
enolpyruvate
C
O
CH3
pyruvate
2- Phosphoenolpyruvate (PEP), involved in ATP
synthesis in Glycolysis, has a very high ΔG of Pi
hydrolysis.
Removal of Pi from ester linkage in PEP is spontaneous
because the enol spontaneously converts to a ketone.
The ester linkage in PEP is an exception.
O
Coenzyme A-SH + HO
C
R
O
Coenzyme A-S
C
R
+ H2O
3- A thioester forms between a carboxylic acid & a thiol
(SH), e.g., the thiol of coenzyme A.
Thioesters are ~ linkages. In contrast to phosphate esters,
thioesters have a large negative DG of hydrolysis.
Kinetics vs Thermodynamics
 A high activation energy barrier usually causes
hydrolysis of a “high energy” bond to be very slow in the
absence of an enzyme catalyst.
 This kinetic stability is essential to the role of ATP and
other compounds with ~ bonds.
 If ATP would rapidly hydrolyze in the absence of a
catalyst, it could not serve its important roles in energy
metabolism and phosphate transfer.
 Phosphate is removed from ATP only when the reaction is
coupled via enzyme catalysis to some other reaction useful
to the cell, such as transport of an ion, phosphorylation of
glucose, or regulation of an enzyme by phosphorylation of a
serine residue.
Adenylate Energy Charge
 Many reactions in metabolism are controlled by the
energy status of the cell.
 One index of the energy status is the energy charge,
which is proportional to the mole fraction of ATP plus
half the mole fraction of ADP, given that ATP contains
two anhydrid bonds whereas ADP contains one.
 It is a measure of the relative concentration of high-energy
phospho - anhydride bonds available in the adenylate pool.
 The energy charge can have a value ranging from 0 (all AMP)
to 1 (all ATP).
Cont…
 Hence the energy charge is defined as:
Energy charge=
[ATP]+1/2[ADP]
[ATP]+ [ADP]+ [AMP]
 Adenylate Kinase catalyze the following reactions:
1- ATP
2- ATP
ADP + Pi
AMP + PPi
3- ATP+AMP
2ADP
Cont…
 Danil Atkinson showed that ATP-generating
pathways (catabolic) are inhibited by a high energy
charge.
 It is evident that control of these pathways has
evolved to maintain the energy charge within rather
narrow limits. In other words the energy charge like
the pH of a cell is buffered. The energy charge of
most cells range from 0.8 to 0.95.“
 A high Energy Charge signals the slow down of
metabolism. A low Energy Charge signals up
regulation of metabolism.
Energy charge regulate metabolism
High concentrations of ATP inhibit the relative rates of a
typical ATP-generating (catabolic) pathway and stimulate
the typical ATP-utilizing (anabolic) pathway.
Regulatory enzymes in energy-producing catabolic
pathways show greater activity at low energy charge, but
the activity falls off sharply as AEC approaches 1.0.
In contrast, regulatory enzymes of anabolic sequences are
not very active at low energy charge, but their activities
increase as AEC nears 1.0 .
These contrasting responses are termed R, for ATPregenerating,
and
U,
for
ATP-utilizing.
Regulatory enzymes such as PFK and pyrvuate kinase in
glycolysis follow the R response curve as AEC is varied.
Note that PFK itself is an ATP-utilizing enzyme, using ATP
to phosphorylate fructose-6-phosphate to yield fructose-1,6bisphosphate.
Nevertheless,
because
PFK
acts
physiologically as the valve controlling the flux of
carbohydrate down the catabolic pathways of cellular
respiration that lead to ATP regeneration, it responds as an
“R” enzyme to energy charge.
Regulatory enzymes
in anabolic
pathways,
such
as
acetyl-CoA
carboxylase, which initiates fatty acid
biosynthesis, respond as “U” enzymes.
Cellular energy homoeostasis:
maintenance of energy state by creatine kinase
(CK) and adenylate kinase (AK) isoenzymes
A fundamental principle in multicellular
organisms is the strict maintenance of stable
concentrations of intracellular oxygen and
ATP as the universal energy currency of
biological systems, as well as the tight
regulation of energy utilization with energy
supply.
Upon activation of excitable cells, such as skeletal
and cardiac muscle, or brain and nerve cells, ATP
turnover rates may increase by several orders of
magnitude within seconds, but [ATP] remains
remarkably stable and ATP/ADP ratios, as well as
ATP/AMP ratios, are maintained as high as possible
to guarantee optimal efficiency for cellular ATPases
that are at work to perform a multitude of energydependent cellular activities, such as muscle
contraction, cell motility and ion pumping.
ATP homoeostasis and maintenance of high
ATP/ADP and ATP/AMP ratios are facilitated by
the action of two well-known enzyme systems,
working as very fast and efficient energy safeguards.
First, CKs, efficiently regenerating ATP at the
expense of phosphocreatine (PCr) by the following
reaction:
PCr + ADP
CKs
ATP + Cr
Second, Adenylate kinase (AK), reconverting
two ADP molecules into one ATP and one
AMP.
These two enzymes, working together in an
subcellular energy distribution network or
circuit temporally and, due to their subcellular
microcompartmentation, to buffer subcellular
ATP level.
A common cause of many diseases, like cardiac
insufficiency, cardiac hypertrophy as well as most of
the neurodegenerative pathologies, is a generally
lowered cellular PCr/ATP ratio, indicating a lowered
energy state of cells and tissues.
This is often accompanied by elevated calcium
levels, leading to chronic calcium overload with its
host of negative consequences on cell function and
viability.