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 Phosphoanhydride bonds (formed by splitting out H 2 O between 2 phosphoric acids or between carboxylic & phosphoric acids) have a large negative Δ G of hydrolysis.

Why do phosphoanhydride linkages have a high DG of hydrolysis? Contributing factors for ATP & PP i 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 & P i transfer.  G of phosphate hydrolysis from ATP is intermediate among examples below. ATP can thus act as a P i donor, & ATP can be synthesized by P i transfer, e.g., from PEP.

 Energy transfer or storage ATP, PP i, polyphosphate, phosphocreatine ATP, PP i, polyphosphate, phosphocreatine  Group transfer ATP, Coenzyme A ATP, Coenzyme A  Transient signal cyclic AMP cyclic AMP Roles of "high energy" bonds

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 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 Δ G of P i hydrolysis. Removal of P i from ester linkage in PEP is spontaneous because the enol spontaneously converts to a ketone. The ester linkage in PEP is an exception.

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 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 Charge

Cont… Hence the energy charge is defined as: Energy charge= [ATP]+1/2[ADP] Adenylate Kinase catalyze the following reactions: 1- ATP ADP + P i 2- ATP AMP + PP i 3- ATP+AMP 2ADP [ATP]+ [ADP]+ [AMP]

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 ATP- regenerating, 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-1,6- bisphosphate. 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 “” enzyme to energy charge. Note that PFK itself is an ATP-utilizing enzyme, using ATP to phosphorylate fructose-6-phosphate to yield fructose-1,6- bisphosphate. 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. 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 energy- dependent 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 ATP + Cr PCr + ADP ATP + Cr CKs

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