High Energy CompoundsHigh 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” bondsHigh energy” bonds

Phosphoanhydride bonds (formed by splitting out H2O between 2 phosphoric acids or between carboxylic & phosphoric acids) have a large negative ΔG of hydrolysis.

N

NN

N

NH 2

O

OHOH

HH

H

CH 2

H

OPOPOP-O

O

O- O-

O O

O-

adenine

ribose

ATP adenosine triphosphate

phosphoanhydride bonds (~)

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 Compounds with ΔΔ G more negative than 7 G more negative than 7 Kcal/mole or 30 KJ/ mole are regarded as high Kcal/mole or 30 KJ/ mole are regarded as high energy compounds.energy compounds.

ATP has special roles in energy coupling & Pi transfer.

G 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.

Energy transfer or storageEnergy transfer or storage ATP, PPATP, PPii, polyphosphate, phosphocreatine, polyphosphate, phosphocreatine

Group transferGroup transfer ATP, Coenzyme AATP, Coenzyme A

Transient signalTransient signal cyclic AMPcyclic AMP

Roles of "high energy" bondsRoles of "high energy" bonds

O PHN C

O

O

N

N H 2+

C H 2

C H 3

C

O

O

p h o s p h o c r e a t i n e

1- Phosphocreatine :another compound with a "high energy" phosphate linkage, is used in nerve & muscle for storage of ~P bonds.

Examples of other high energy compoundsExamples of other high energy compounds

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.

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.

C

C

O O

O PO 32

C H 2

C

C

O O

O

C H 3

C

C

O O

O H

C H 2

A D P A T P

H +

P E P e n o lp y ru v a te p y ru v a te

Coenzyme A-SH + HO C

O

R

Coenzyme A-S C

O

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 G of hydrolysis.

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.

Kinetics vs ThermodynamicsKinetics vs Thermodynamics

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).

Adenylate Energy ChargeAdenylate Energy Charge

ContCont……

Hence the energy charge is defined as:

Energy charge= [ATP]+1/2[ADP]

Adenylate Kinase catalyze the following reactions:

1- ATP ADP + Pi

2- ATP AMP + PPi

3- ATP+AMP 2ADP

[ATP]+ [ADP]+ [AMP]

Cont…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 metabolismEnergy 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 ATP-regenerating, and U, for ATP-utilizing.

Regulatory enzymes such as PFK and pyrvuate kinase in Regulatory enzymes such as PFK and pyrvuate kinase in glycolysis follow the glycolysis follow the RR response curve as AEC is varied. response curve as AEC is varied.

Note that PFK itself is an ATP-utilizing enzyme, using ATP Note that PFK itself is an ATP-utilizing enzyme, using ATP to phosphorylate fructosto phosphorylate fructosee-6-phosphate to yield fructos-6-phosphate to yield fructosee-1,6--1,6-

bisphosphate. Nevertheless, because PFK acts bisphosphate. Nevertheless, because PFK acts physiologically as the valve controlling the flux of physiologically as the valve controlling the flux of

carbohydrate down the catabolic pathways of cellular carbohydrate down the catabolic pathways of cellular respiration that lead to ATP regeneration, it responds as an respiration that lead to ATP regeneration, it responds as an

““R” enzyme to energy charge.” enzyme to energy charge.

Regulatory enzymes in anabolic Regulatory enzymes in anabolic pathways, such as acetyl-CoA pathways, such as acetyl-CoA

carboxylase, which initiates fatty acid carboxylase, which initiates fatty acid biosynthesis, respond as “biosynthesis, respond as “UU” enzymes.” enzymes.

Cellular energy homoeostasis: maintenance of energy state by creatine kinase (CK) and adenylate kinase (AK)

isoenzymes

A fundamental principle in multicellular A fundamental principle in multicellular organisms is the strict maintenance of stable organisms is the strict maintenance of stable

concentrations of intracellular concentrations of intracellular oxygenoxygen and and ATPATP asas the universal energy currency of the universal energy currency of

biological systems, as well as the tight biological systems, as well as the tight regulation of energy utilization with energy regulation of energy utilization with energy

supply.supply.

Upon activation of excitable cells, such as skeletal Upon activation of excitable cells, such as skeletal and cardiac muscle, or brain and nerve cells, ATP and cardiac muscle, or brain and nerve cells, ATP

turnover rates may increase by several orders of turnover rates may increase by several orders of magnitude within seconds, but [ATP] remains magnitude within seconds, but [ATP] remains

remarkably stable and ATP/ADP ratios, as well as remarkably stable and ATP/ADP ratios, as well as ATP/AMP ratios, are maintained as high as possible ATP/AMP ratios, are maintained as high as possible to guarantee optimal efficiency for cellular ATPases to guarantee optimal efficiency for cellular ATPases

that are at work to perform a multitude of energy-that are at work to perform a multitude of energy-dependent cellular activities, such as muscle dependent cellular activities, such as muscle

contraction, cell motility and ion pumping.contraction, cell motility and ion pumping.

ATP homoeostasis and maintenance of high ATP homoeostasis and maintenance of high ATP/ADP and ATP/AMP ratios are facilitated by the ATP/ADP and ATP/AMP ratios are facilitated by the action of two well-known enzyme systems, working action of two well-known enzyme systems, working

as very fast and efficient energy safeguards. First, as very fast and efficient energy safeguards. First, CKsCKs, efficiently regenerating ATP at the expense of , efficiently regenerating ATP at the expense of

phosphocreatine (PCr) by the following reaction:phosphocreatine (PCr) by the following reaction:

PCr + ADPPCr + ADP ATP + Cr ATP + CrCKs

Second, Adenylate kinase (AK), reconverting Second, Adenylate kinase (AK), reconverting two ADP molecules into one ATP and one two ADP molecules into one ATP and one

AMP.AMP.

These two enzymes, working together in an These two enzymes, working together in an subcellular energy distribution network or subcellular energy distribution network or

circuitcircuit temporally and, due to their subcellular temporally and, due to their subcellular microcompartmentation, to buffer subcellular microcompartmentation, to buffer subcellular

ATPATP level.level.

A common cause of many diseases, like cardiac A common cause of many diseases, like cardiac insufficiency, cardiac hypertrophy as well as most of insufficiency, cardiac hypertrophy as well as most of

the neurodegenerative pathologies, is a generally the neurodegenerative pathologies, is a generally lowered cellular PCr/ATP ratio, indicating a lowered lowered cellular PCr/ATP ratio, indicating a lowered

energy state of cells and tissues.energy state of cells and tissues.

This is often accompanied by elevated calcium This is often accompanied by elevated calcium levels, leading to chronic calcium overload with its levels, leading to chronic calcium overload with its host of negative consequences on cell function and host of negative consequences on cell function and

viability.viability.