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Phosphoryl Group Transfers and ATP:- ATP Energizes Active Transport and Muscle Contraction

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p504-505

2026-05-30

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Phosphoryl Group Transfers and ATP:- ATP Energizes Active Transport and Muscle Contraction

ATP can supply the energy for transporting an ion or a molecule across a membrane into another aqueous compartment where its concentration is higher. Transport processes are major consumers of en ergy; in human kidney and brain, for example, as much as two-thirds of the energy consumed at rest is used to pump Na⁺ and K⁺ across plasma membranes via the Na K ATPase. The transport of Na and K is driven by cyclic phosphorylation and dephosphorylation of the transporter protein, with ATP as the phosphoryl group donor. Na-dependent phosphorylation of the Na⁺ K⁺ ATPase forces a change in the protein’s conformation, and K⁺-dependent dephosphorylation favors return to the original conformation. Each cycle in the transport process results in the conversion of ATP to ADP and Pi, and it is the free-energy change of ATP hydrolysis that drives the cyclic changes in protein con formation that result in the electrogenic pumping of Na⁺ and K⁺. Note that in this case ATP interacts covalently by phosphoryl group transfer to the enzyme, not the substrate.

In the contractile system of skeletal muscle cells, myosin and actin are specialized to transduce the chemical energy of ATP into motion. ATP binds tightly but noncovalently to one conformation of myosin, holding the protein in that conformation. When myosin catalyzes the hydrolysis of its bound ATP, the ADP and Pi dissociate from the protein, allowing it to relax into a second conformation until another molecule of ATP binds. The binding and subsequent hydrolysis of ATP (by myosin ATPase) provide the energy that forces cyclic changes in the conformation of the myosin head. The change in conformation of many individual myosin molecules results in the sliding of myosin fibrils along actin filaments, which translates into macroscopic contraction of the muscle fiber. As we noted earlier, this production of mechanical motion at the expense of ATP is one of the few cases in which ATP hydrolysis per se, rather than group trans fer from ATP, is the source of the chemical energy in a coupled process.

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