The Adenosine triphosphate ATP It was found that chemical energy yielded in the degradation of fuel molecules is recovered by coupled phosphorylation of ADP to yield ATP. The energy rich ATP so formed then transfers it’s energy by donation of its terminal’ high energy phosphate group, to energy requiring functions of the cell: - Biosynthesis - muscle contraction - Active transport against gradients.

Structure and properties of ATP ATP is a nucleotide. It consists of adenine (6 amino derivative of purine, the 5-carbon sugar,ribose, attached to adenine through glucosyl linkage and 3 phosphate group joined to 5‘-position of ribose. ATP is high energy compound because it has a large negative free energy of hydrolysis.

ATP, ADP and AMP occur in all cells. The ATP molecule as it exists in the intact cell is highly charged at pH 7, the three phosphate groups are completely ionized ( 4 negative charges) near the linear phosphate structure.

ATP also forms stable complexes with certain divalent cations as Mg2+ ATP also forms stable complexes with certain divalent cations as Mg2+. Most of ATP in the cell present as Mg2+-complex Mg2+.

Compounds with Δ G more negative than 7 Kcal/mole or 30 KJ/ mole are regarded as high energy compounds. ATP is Carrying four negative charges at pH 7.0 and is neutralized by complexing with Mg2+. The four –ve charges that are very close to each other, repel each other very strongly.

Mechanism of ATP hydrolysis When the terminal phosphate bond is hydrolyzed, some electrostatic stress is relieved, the similar charges are separated as ADP3- + HPO4 2- and they will have very little tendency to combine again because their similar charges repel each other. ATP 4- + H2O ADP3- + HPO4 2- ADP and phosphate as soon as they are formed undergo stabilization by resonance.

The first phosphate of ATP is an ester bonded phosphate and this is not a high energy bond. The outer two phosphates are linked by high energy phosphoric anhydride linkages.

When ATP is incubated under suitable conditions with muscle fibers, it undergoes enzymatic hydrolysis to give ADP + Pi. When this hydrolysis proceeds it gives large liberation of heat.

Why G°' is so high for ATP hydrolysis? 1- Hydrolysis reduces electrostatic repulsion between negative charged oxygen atoms on the phosphorus atoms. 2- ADP and Pi are stabilised by resonance. 3- Entropy (disorder) is higher after the reaction. These 3 factors make ATP hydrolysis favourable and are responsible for the large amount of energy released during hydrolysis.

Factors affecting free energy of ATP hydrolysis This value (-7.3 Kcal) does not represent the real free energy of hydrolysis of ATP in the intact cells (i.e (cellular ATP). This is due to: 1- The conc of ATP, ADP and Pi in the cell are much lower than 1.0 M ( the st thermodynamic concentrations). 2-These substances are not present in equimolar concentrations.

3-Mg2+ forms complex with different affinities with ATP and also ADP. The presence of Mg2+ shifting the equilibrium of ATP hydrolysis.

If appropriate corrections are applied of all these factors, the free energy of hydrolysis of ATP to ADP under intracellular conditions will be about -12 Kcal/ mole. So free energy of ATP hydrolysis inside cells is not necessarily constant.

Free energy of ATP vary from one cell to another. It may vary from time to time depending on the concentration of Mg2+, ATP, ADP and Pi.

Examples of other high energy compounds Enol Phosphates: e.g: Phosphoenol pyruvate (PEP) is high energy phosphate, but for a completely different reason than ATP. - PEP is formed in the glycolytic pathway and is used to synthesize ATP from ADP. - The reaction with ADP to form ATP is essentially irreversible.

Reasons: Enol form of pyruvic acid is less stable than the keto form by about 10-12 kcal/mole. In addition an enol phosphate is less stable than any ordinary phosphate ester by about 3kcal/mole. The phosphate can only exist as the high energy enol form. Thus when phosphate group is removed, the pyruvate can go back to the stable low energy ket form and the surplus energy is released.

Thiol esters e. g: thioesters of coenzyme A as acetyl~Co A: - In this compound, there is a diminshed resonance interaction between electrons of the sulfur and the carbonyl group relative to the resonance in an oxygen ester. -The sulfur will not form a double bond as readily as oxygen.

Phosphocreatine

Biosynthesis work as energy requiring process Biosynthesis is a programmed process that leads from very simple molecule to the living cell itself.

The flow sheet of biosynthesis For the biosynthesis of cell components, two kinds of ingrediants are required. 1- precursors e.g C,H,N 2- ATP Degradative and synthetic pthways between two points given (e.g glucose and pyruvate are not identical (i.e they are not the simple reverse of each other).

Adenylate Energy Charge Some enzymes respond to absolute concentration, but most respond to ratios. Dan Atkinson introduced the concept of ENERGY CHARGE in 1968 to summarize the energy status of a cell. It is a measure of the relative concentration of high-energy phospho-anhydride bonds available in the adenylate pool.

The energy charge, or E. C. , has the range 0 to 1 The energy charge, or E.C., has the range 0 to 1.0. If all the adenylate is in the form of ATP, E.C. = 1.0, and the potential for phosphoryl transfer is maximal. At the other extreme, if AMP is the only adenylate form present, E.C. = 0. Then the relative amounts of the three adenine nucleotides are fixed by the energy charge. The following figure shows the relative changes in the concentrations of the adenylates as energy charge varies from 0 to 1.0.

Regulatory enzymes in energy-producing catabolic pathways show greater activity at low energy charge, but the activity falls off sharbly as E.C. approaches 1.0. In contrast, regulatory enzymes of anabolic sequences are not very active at low energy charge, but their activities increase as E.C. 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 E.C. 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 “R” enzyme to energy charge.

Regulatory enzymes in anabolic pathways, such as acetyl-CoA carboxylase, which initiates fatty acid biosynthesis, respond as “U” enzymes.