Adenosine Triphosphate (Atp)
Adenosine 5'-triphosphate (ATP) was discovered in 1929 by the German chemist Karl Lohmann. Its chemical structure consists of an adenosine nucleotide structure to which three phosphoryl groups are sequentially attached via cleavable bonds. It is the most common energy "currency" of living cells, functioning as an intermediate which drives "energy requiring" (endergonic) reactions through the cleavage of its phosphate groups. The processes that maintain life require an input of energy in order to proceed and are driven by the exergonic (energy yielding) reactions of nutrient oxidation. This coupling is most often mediated through the syntheses of high-energy intermediates, such as ATP, whose consumption provides energy for the endergonic processes. In biosynthesis, for example, ATP is the immediate product of all processes leading to the chemical storage of energy.
The capacity of ATP to act as the biochemical energy "currency" comes from its high potential to transfer its phosphate groups. The molecule can be cleaved in several different ways. For example transfer of the orthophosphate to alcoholic hydroxyl groups, acid or amide groups and the release of ADP has a free energy of hydrolysis of 29.4 kJ/mol. The enzymes, which catalyse these reactions are kinases, which in some cases (e.g., creatine kinase) can also catalyse the hydrolysis of ATP from ADP.
Another way to cleave ATP is to remove two pyrophosphate groups (two phosphates) and release adenosine monophosphate (AMP). The free energy of hydrolysis for this reaction is 36.12 kJ/mol and it occurs, for example, in the course of purine synthesis.
The energy stored in ATP is used in the vast majority of central biochemical reactions, including the synthesis of macromolecules such as proteins, fats and carbohydrates from their corresponding monomeric subunits. Such endergonic reactions are often driven forward by enzymatic coupling to the hydrolysis of ATP. Many catabolic pathways, including glycolysis, require an "investment" of ATP, which is later re-synthesized. Essentially all anabolic pathways require ATP, either directly or indirectly. In the human body ATP provides energy for the contraction of muscles. The cleavage of ATP to ADP and inorganic phosphate is generally coupled to another process in the cell so that the free energy released by ATP is used to drive another endergonic process. In the actomyosin complex in muscle, the cleavage of ATP provides energy for contraction. Also, the active transport of many substances across membranes depends on a source of ATP. For example in the Na+/K+ pump in cell membranes, the energy required for the transport of ions against their concentration gradients is provided by ATP. ATP is also released together with acetylcholine at synapses in the peripheral nervous system and has been proposed as a modulator of nervous transmission, as it inhibits the release of acetylcholine.
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