Adenosine triphosphate – ATP
Adenosine 5′-triphosphate (ATP) is a multifunctional nucleotide that is most important as a “molecular currency” of intracellular energy transfer. In this role ATP transports chemical energy within cells for metabolism. It is produced as an energy source during the processes of photosynthesis and cellular respiration and consumed by many enzymes and a multitude of cellular processes including biosynthetic reactions, motility and cell division. ATP is also incorporated into nucleic acids by polymerases in the processes of DNA replication and transcription. In signal transduction pathways, ATP is used as a substrate by kinases that phosphorylate proteins and lipids, as well as by adenylate cyclase, which uses ATP to produce the second messenger molecule cyclic AMP.
The structure of this molecule consists of a purine base (adenine) attached to the 1′ carbon atom of a pentose (ribose). Three phosphate groups are attached at the 5′ carbon atom of the pentose sugar. When ATP is used in DNA synthesis, the ribose sugar is first converted to deoxyribose by ribonucleotide reductase. ATP was discovered in 1929 by Karl Lohmann, and was proposed to be the main energy-transfer molecule in the cell by Fritz Albert Lipmann in 1941.
Metabolic processes that use ATP convert it back into its original form, thus it is constantly being recycled. The human body contains only about 250 grams (8.8 oz) of ATP, yet turns over its own body weight equivalent in ATP every day.
ATP can be produced by redox reactions using simple and complex sugars (carbohydrates) or lipids as an energy source. For ATP to be synthesized from complex fuels, they first need to be broken down into their basic components. Carbohydrates are hydrolysed into simple sugars, such as glucose and fructose. Fats (triglycerides) are metabolised to give fatty acids and glycerol.
The overall process of oxidizing glucose to carbon dioxide is known as cellular respiration and can produce up to 30 molecules of ATP from a single molecule of glucose. ATP can be produced by a number of distinct cellular processes; the three main pathways used to generate energy in eukaryotic organisms are glycolysis, the citric acid cycle/oxidative phosphorylation, and beta-oxidation. The majority of this ATP production by a non-photosynthetic aerobic eukaryote takes place in the mitochondria, which can make up nearly 25% of the total volume of a typical cell.