Function
ATP hydrolysis provides the energy needed for many essential processes in organisms and cells. These include intracellular signaling, DNA and RNA synthesis, Purinergic signaling, synaptic signaling, active transport, and muscle contraction. These topics are not an exhaustive list but include some of the vital roles ATP performs.
ATP in Intracellular Signaling
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Signal transduction heavily relies on ATP. ATP can serve as a substrate for kinases, the most numerous ATP- binding protein. When a kinase phosphorylates a protein, a signaling cascade can be activated, leading to the modulation of diverse intracellular signaling pathways.[4] Kinase activity is vital to the cell and, therefore, must be tightly regulated. The presence of the magnesium ion helps regulate kinase activity.[5] Regulation is through magnesium ions existing in the cell as a complex with ATP, bound at the phosphate oxygen centers. In addition to kinase activity, ATP can function as a ubiquitous trigger of intracellular messenger release.[6] These messengers include hormones, various-enzymes, lipid mediators, neurotransmitters, nitric oxide, growth factors, and reactive oxygen species.[6] An example of ATP utilization in intracellular signaling can be observed in ATP acting as a substrate for adenylate cyclase. This process mostly occurs in G-protein coupled receptor signaling pathways. Upon binding to adenylate cyclase, ATP converts to cyclic AMP, which assists in signaling the release of calcium from intracellular stores.[7] The cAMP has other roles, including secondary messengers in hormone signaling cascades, activation of protein kinases, and regulating the function of ion channels.
DNA/RNA Synthesis
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DNA and RNA synthesis requires ATP. ATP is one of four nucleotide-triphosphate monomers that is necessary during RNA synthesis. DNA synthesis uses a similar mechanism, except in DNA synthesis, the ATP first becomes transformed by removing an oxygen atom from the sugar to yield deoxyribonucleotide, dATP.[8]
Purinergic Signaling
Purinergic signaling is a form of extracellular paracrine signaling that is mediated by purine nucleotides, including ATP. This process commonly entails the activation of purinergic receptors on cells within proximity, thereby transducing signals to regulate intracellular processes. ATP is released from vesicular stores and is regulated by IP3 and other common exocytotic regulatory mechanisms. ATP is co-stored and co-released among neurotransmitters, further supporting the notion that ATP is a necessary mediator of purinergic neurotransmission in both sympathetic and parasympathetic nerves. ATP can induce several purinergic responses, including control of autonomic functions, neural glia interactions, pain, and control of vessel tone.[9][10][11][12]
Neurotransmission
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The brain is the highest consumer of ATP in the body, consuming approximately twenty-five percent of the total energy available.[13] A large amount of energy is spent on maintaining ion concentrations for proper neuronal signaling and synaptic transmission.[14] Synaptic transmission is an energy-demanding process. At the presynaptic terminal, ATP is required for establishing ion gradients that shuttle neurotransmitters into vesicles and for priming the vesicles for release through exocytosis.[14]Neuronal signaling depends on the action potential reaching the presynaptic terminal, signaling the release of the loaded vesicles. This process depends on ATP restoring the ion concentration in the axon after each action potential, allowing another signal to occur. Active transport is responsible for resetting the sodium and potassium ion concentrations to baseline values after an action potential occurs through the Na/K ATPase. During this process, one molecule of ATP is hydrolyzed, three sodium ions are transported out of the cell, and two potassium ions are transported back into the cell, both of which move against their concentration gradients.
Action potentials traveling down the axon initiate vesicular release upon reaching the presynaptic terminal. After establishing the ion gradients, the action potentials then propagate down the axon through the depolarization of the axon, sending a signal towards the terminal. Approximately one billion sodium ions are necessary to propagate a single action potential. Neurons will need to hydrolyze nearly one billion ATP molecules to restore the sodium/potassium ion concentration after each cell depolarization.[13]Excitatory synapses largely dominate the grey matter of the brain. Vesicles containing glutamate will be released into the synaptic cleft to activate postsynaptic excitatory glutaminergic receptors. Loading these molecules requires large amounts of ATP due to nearly four thousand glutamate molecules stored into a single vesicle.[13] Significant stores of energy are necessary to initiate the release of the vesicle, drive the glutamatergic postsynaptic processes, and recycle the vesicle as well as the left-over glutamate.[13] Therefore, due to the large amounts of energy required for glutamate packing, mitochondria are close to glutamatergic vesicles.[15]
ATP in Muscle Contraction
Muscle contraction is a necessary function of everyday life and could not occur without ATP. There are three primary roles that ATP performs in the action of muscle contraction. The first is through the generation of force against adjoining actin filaments through the cycling of myosin cross-bridges. The second is the pumping of calcium ions from the myoplasm across the sarcoplasmic reticulum against their concentration gradients using active transport. The third function performed by ATP is the active transport of sodium and potassium ions across the sarcolemma so that calcium ions may be released when the input is received. The hydrolysis of ATP drives each of these processes.[16]
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