What is the enzyme complex that depends on hydrogen ions in order to make ATP molecules?
ATP synthase
The hydrogen ions are allowed to pass through the thylakoid membrane through an embedded protein complex called ATP synthase.
The enzyme complex that depends on hydrogen ions (protons) to make ATP molecules is called ATP synthase. It is located in the inner membrane of the mitochondria in eukaryotic cells and in the plasma membrane of prokaryotic cells.
ATP synthase uses the energy stored in a proton gradient across the membrane to drive the synthesis of ATP from ADP and inorganic phosphate (Pi) in a process called oxidative phosphorylation. As protons flow down their concentration gradient through the enzyme complex, the energy is used to power the rotation of a molecular rotor that drives the synthesis of ATP from ADP and Pi. This process is called chemiosmosis and is a key part of the electron transport chain, which generates the proton gradient by transferring electrons from electron donors to electron acceptors.
ATP synthase is composed of two main parts: a transmembrane proton channel called F0 and a catalytic knob-like structure called F1. The F0 unit is responsible for proton translocation across the membrane, while the F1 unit is responsible for ATP synthesis.
The F0 unit is made up of a ring of c-subunits that forms a transmembrane channel, and a subunit called a and b, which anchor the c-ring to the membrane. As protons are pumped from the mitochondrial matrix or cytoplasm into the intermembrane space or extracellular space, they flow through the channel created by the c-ring. This flow of protons causes the c-ring to rotate, and this rotational energy is then used to drive ATP synthesis by the F1 unit.
The F1 unit is composed of three catalytic subunits (α, β, and γ) that form a hexameric ring structure. The γ-subunit sits at the center of the ring and is connected to the c-ring by a central stalk. As the c-ring rotates, it causes the central stalk and γ-subunit to rotate as well, inducing conformational changes in the α- and β-subunits that drive ATP synthesis.
Overall, ATP synthase is a remarkable molecular machine that converts the energy stored in a proton gradient into the universal energy currency of the cell, ATP.
The synthesis of ATP by ATP synthase is a highly efficient process, with each ATP synthase molecule able to generate hundreds of ATP molecules per second. The exact number of ATP molecules produced per proton translocated depends on the organism and the specific conditions but is typically around three ATP molecules per ten protons translocated.
The proton gradient that powers ATP synthesis is established through the electron transport chain, which transfers electrons from electron donors (such as NADH and FADH2) to electron acceptors (such as oxygen) through a series of redox reactions. As electrons are transferred down the electron transport chain, they are used to pump protons across the mitochondrial or bacterial membrane, creating a proton gradient that drives ATP synthesis by ATP synthase.
In addition to its role in oxidative phosphorylation, ATP synthase is also involved in other cellular processes that require ATP hydrolyses, such as muscle contraction and the transport of ions across the mitochondrial membrane. In these cases, ATP synthase operates in reverse, hydrolyzing ATP to pump protons across the membrane and establish a proton gradient.