ATP CYCLE

M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

The term high-E compounds or energy rich compounds is usually applied to substances which possess sufficient free E to liberate at least 7 Cal/mol at pH 7.0

The compounds which liberate less than 7.0 Cal/mol are referred to as low-E compounds.

Most of the high E compounds contain PO4

3- (exception acetyl CoA) hence they are called high E phosphate compounds.

Compounds ΔG0 (Cal/mol)

Phosphoenol pyruvate -14.8

Carbamoyl phosphate -12.3

Cyclic AMP -12.0

1,3bisphosphoglycerate -11.8

phosphocreatine -10.3

Acetyl phosphate -10.3

SAM -10.0

Pyrophosphate -8.0

Acetyl CoA -7.7

ADP→AMP+Pi -6.6

Glucose-1-phosphate

-5.0

Fructose-6-Phosphate

-3.8

Glucose-6-phosphate

-3.3

Glycerol-3-phosphate

-2.2

1. Pyrophosphates e.g. ATP 2. Acyl phosphates e.g. 1,3-bisphosphogly 3. Enol phosphates e.g. PEP 4. Thioesters e.g. acetyl CoA 5. Phosphagenes e.g. phosphocreatine

The living objects require a continuous supply of FE mainly for

1. to synthesize macromolecules from simpler & smaller precursors

2. to transport molecules and ions across membranes against gradients and

3. to perform mechanical work, as in the muscle contraction etc.,

The FE in these processes is derived from the environment.

The phototrophs obtain this E by trapping light E from the SUN.

On the other hand, chemotrophs obtain it by the oxidation of food stuffs.

This FE is partly transformed into special form b4 it is used for biosynthesis, tpt, motion and fidelity.

This special carrier of FE is ATP. ATP plays a central role in the transference of FE

from exergonic to the endergonic processes in the cells.

ATP donates much of its chemical E to energy requiring processes (biosynthesis, transport) by undergoing a breakdown to ADP and Pi.

ATP was discovered in extracts of skeletal muscles by Karl Lohmann in GE and by Cyrus Fiske and Y.Subbarow (US) simultaneously in 1929.

Later it was found to be present in all types of cells- animal, plant and microbial.

Fritz.A.Lipmann a GE born US biochemist postulated that ATP is the 10 and universal carrier of chemical E in cells.(Nobel prize)

He also first proposed the ATP Cycle , and is popularly known as Father of ATP cycle.

He introduced squiggle notation to designate E rich bonds of biomolecules such as ATP and ADP.

ATP, ADP & AMP occur not only in cell cytosol but also in MC and the nucleus.

ATP serves as the principal immediate donor of FE in biological systems rather than as a storage form of E.

In a typical cell, an ATP molecule is consumed with in a minute of its formation.

The turnover of ATP is very high. For instance , a resting human consumes

about 40kg ATP in a day.

The endergonic processess such as bio- synthesis, active tpt etc can occur only if ATP is continuously regenerated from ATP

Phototrophs harvest the FE in light to regenerate ATP where as chemotrophs form ATP by the oxidation of food stuffs.