adenosine triphosphate

adenosine triphosphate
Name	Adenosine triphosphate
IUPACName	Adenosine 5'-(tetrahydrogen triphosphate)
OtherNames	ATP

adenosine triphosphate. Adenosine triphosphate (ATP) is a crucial organic compound that serves as the primary energy currency for all living cells. It is a nucleoside triphosphate consisting of an adenine base, a ribose sugar, and three phosphate groups. The molecule's high-energy phosphate bonds enable it to store and transfer chemical energy within cells, powering a vast array of biological processes from muscle contraction to nerve impulse propagation.

Chemical structure and properties

ATP is composed of three distinct molecular components: the nitrogenous base adenine, the five-carbon sugar ribose, and a chain of three phosphate groups. The adenine and ribose together form the nucleoside adenosine, to which the triphosphate tail is attached. The phosphate groups are labeled alpha (α), beta (β), and gamma (γ), proceeding from the ribose outward. The key chemical feature of ATP is the presence of two phosphoanhydride bonds linking the phosphate groups, which are high-energy bonds. In aqueous solution at physiological pH, ATP carries multiple negative charges, making it a highly soluble molecule that typically exists complexed with magnesium or other divalent cations. This chelation is critical for its biological activity, as it influences the molecule's conformation and interaction with enzymes like ATP synthase and myosin.

Biological functions

The principal biological function of ATP is to serve as a universal energy intermediary, coupling energy-releasing (exergonic) processes with energy-requiring (endergonic) cellular activities. It acts as an immediate donor of free energy, rather than a long-term storage molecule like triglycerides or glycogen. Beyond its energetic role, ATP is a substrate for kinase enzymes in phosphorylation reactions, which regulate the activity of proteins such as those in the MAPK/ERK pathway. It is also a building block for nucleic acid synthesis during processes like DNA replication catalyzed by DNA polymerase. Furthermore, ATP functions as an extracellular signaling molecule, acting as a neurotransmitter in systems like the purinergic signalling pathways within the central nervous system.

Biosynthesis

ATP is continuously synthesized from adenosine diphosphate (ADP) and inorganic phosphate through two primary mechanisms: substrate-level phosphorylation and oxidative phosphorylation. Substrate-level phosphorylation occurs in the cytosol during pathways like glycolysis and in the mitochondrial matrix during the citric acid cycle. The more prolific producer of ATP is oxidative phosphorylation, which occurs across the inner mitochondrial membrane in eukaryotes or the plasma membrane in prokaryotes. This process is driven by the electron transport chain, which creates an electrochemical gradient used by the ATP synthase complex. In chloroplasts of plants and algae, photophosphorylation uses light energy to generate ATP during the light-dependent reactions of photosynthesis.

Hydrolysis and free energy

The hydrolysis of ATP to adenosine diphosphate and inorganic phosphate is a highly exergonic reaction, releasing approximately -30.5 kJ/mol under standard conditions, and even more within the cellular environment due to factors like pH and magnesium concentration. This large negative Gibbs free energy change arises from factors including relief of electrostatic repulsion between the negatively charged phosphate groups, greater resonance stabilization of the products, and increased stabilization by hydration after hydrolysis. The reaction is catalyzed by a class of enzymes known as ATPases, such as the sodium-potassium pump and the myosin head in muscle fibers. The energy released is not lost as heat but is harnessed to drive endergonic processes via mechanisms like energy coupling.

Role in cellular processes

ATP is indispensable for nearly all cellular work. It provides the energy for muscle contraction by powering the cross-bridge cycling of actin and myosin filaments. It fuels active transport across membranes, such as the operation of the sodium-potassium pump which maintains the resting potential of neurons. During biosynthesis, ATP drives the formation of complex molecules, including the peptide bond formation in protein synthesis on the ribosome and the polymerization of nucleotides by RNA polymerase. It is also essential for cell motility, enabling the movement of cilia and flagella through the action of dynein motors, and for intracellular transport along microtubules via kinesin.

History and discovery

The discovery of ATP unfolded through the work of several key biochemists in the early 20th century. In 1929, the German chemist Karl Lohmann first isolated the molecule from muscle extracts while working in the laboratory of Otto Meyerhof, who was awarded the Nobel Prize in Physiology or Medicine in 1922 for his research on muscle metabolism. The correct structure of ATP was determined several years later. A pivotal understanding of its biological role came from the research of Fritz Albert Lipmann, who later received the Nobel Prize in Physiology or Medicine in 1953 for his concept of ATP as the universal "energy currency" of the cell. The mechanism of its synthesis was elucidated by the work of Peter D. Mitchell, who won the Nobel Prize in Chemistry in 1978 for his chemiosmotic theory explaining oxidative phosphorylation.

Category:Biomolecules Category:Nucleotides Category:Energy metabolism