Complex V - ATP Synthase
ATP synthase is a membrane protein complex that synthesizes ATP. Although ATP synthase is often referred to as Complex V, it does not play a direct role in the electron transport chain (ETC). Rather, it provides the mechanism by which protons travel down their charge and concentration gradient back to the matrix.
Explore the 3D structure of ATP Synthase:
ATP synthase is composed of several subunits that work together to produce ATP. ADP molecules can be seen bound to both the alpha and beta subunits, although only the beta subunits are catalytic.
This model shows the moment just before a molecule of ADP is phosphorylated with its associated beta subunit adopting a half-closed conformation.
Click the annotations to explore the subunits of ATP synthase and the embedded molecules of ADP:
A closer look at ATP Synthase
The beta subunits contain active sites for ATP synthesis.
Hydrogen bonds from amino acid residues in the beta subunits hold molecules of ADP in place by stabilizing ADP’s base and sugar. A magnesium ion can also be found covalently bonded to the ADP molecule, where it stabilizes and neutralizes the charge of the phosphate groups.
The F₁ domain is made up of the protein subunits that sit in the mitochondrial matrix and are involved in catalysis of ATP.
ATP synthesis and hydrolysis occur in the alpha (α) and beta (β) subunits. They are held in place by the immobile OSCP, F6, b and d subunits.The gamma (γ), epsilon (ε) and delta (δ) subunits make up the central stalk of ATP synthase.
The a and c subunits make up the F₀ domain of ATP synthase and are involved in proton pumping.
The c subunit is composed of eight to fifteen alpha helices (in this model, eight) and is anchored to the lipid bilayer by the a subunit.
A Molecular Machine
ATP synthase is composed of several mechanical parts.
Chemical Motor
The chemical motor is made up of the alpha, α, and beta, β, subunits. The beta subunits catalyze the synthesis of ATP through conformational changes which force a phosphodiester bond to form between ADP and Pᵢ.
Stator
The stator is immobile and is composed of the OSCP, F6, a, b, and d subunits. The a subunit of the stator holds ATP synthase in place in the inner membrane while the b subunit of the stator stabilizes the α₃β₃ hexamer ring and prevents it from rotating.
Rotor/Axle
The rotor/axle forms a central stalk that runs through the hexamer ring. The axle rotates in a counterclockwise direction which causes conformational changes in the beta subunits, stimulating ATP synthesis and release from the active site.
Electrical Motor
The electrical motor is composed of the c-ring. The c-ring contains proton binding sites and rotates in a counterclockwise direction. The number of protons translocated for every 360° rotation of the c-ring is dependent on c-ring stoichiometry.
ATP Synthesis
Chemiosmotic Theory
The chemiosmotic theory was postulated by Peter Mitchell in 1961. It suggests that most ATP in metabolic cells is synthesized by using energy stored in the electrochemical gradient generated by the breakdown of glucose across the inner mitochondrial membrane. The theory makes three assumptions:
The ETC generates energy for the transport of protons (H⁺) across the inner membrane
ATP synthase synthesizes or hydrolyzes ATP through the pumping of H⁺
The inner membrane is impermeable to ionic species such as H⁺
Chemiosmosis is the movement of ions across a semipermeable membrane through a membrane-bound structure and is the process by which ATP synthesis occurs. Each protein complex, with the exception of CII, in the ETC pumps H⁺ across the mitochondrial inner membrane to the intermembrane space (IMS). As a result, the mitochondrial matrix has a lower concentration of H⁺ than the intermembrane space. In addition, because H⁺ are ions, the IMS will be more positively charged, or p-doped while the matrix will be more negatively charged, or n-doped. Due to the selective permeability of the phospholipid bilayer, H⁺ is prevented from flowing down its concentration and charge gradient into the matrix. This produces an electrochemical gradient which is then utilized by ATP synthase to form ATP.
Powered by Protons
Protons flow through ATP synthase through an access channel in the a subunit that is only open to the IMS. The protons temporarily bind to helices of the c subunit; as protons flow down their electrochemical gradient, energy is released, causing the the c-ring to rotate.
As the c-ring rotates in a counterclockwise direction, protons exit through an egress channel to the matrix. To prevent the backflow of protons into the IMS, amino acid chains in the c-ring repel H⁺.
The torque generated in F₀ is then transferred to F₁ through the axle, which rotates counterclockwise within the α₃β₃ hexamer ring. As the gamma subunit rotates, it causes conformational changes in the catalytic beta subunits. This conformational change in the beta subunits forces the catalysis of a phosphodiester bond between ADP and Pᵢ, resulting in another conformational change in the beta subunit.
At any given time, the beta subunits exist in one of three different conformational states (sort of).
Like the chemiosmotic theory, the structure and conformational states of ATP synthase is still under scientific scrutiny. The Binding Change Mechanism, developed by Paul Boyer (1979), posits that ATP synthesis is dependent on the conformational changes generated by the rotation of the gamma subunit coupled with proton translocation. As protons are pumped, gamma rotates, causing the beta subunits to change shape. Specifically, the active site of a beta subunit cycles between three conformational states for every ~120° rotation: open, loose, and tight.
Each beta subunit is in one of these conformations at any given time and interconverts between these states as catalysis proceeds.
In the open (O) state, molecules of ADP and Pᵢ diffuse into the active site.
In the loose (L) state, the beta subunit loosely closes up around the molecules of ADP and Pᵢ.
In the tight (T) state, the beta subunit closes around the ADP and Pᵢ, forcing them together to form ATP.
As the beta subunits cycle from the tight state to the open state, the newly formed ATP is released into the matrix.
Intermediate states have been observed between the open, loose, and tight states, and are still in the process of being described.
The exact mechanism by which ATP is synthesized and the structure of ATP synthase are still under scientific investigation.
The mostly widely accepted mechanism for ATP synthesis is the chemiosmotic theory, as proposed by Peter Mitchell in 1961. With contemporary bioanalytical techniques, new research on the mechanism of ATP synthesis suggest that the chemiosmotic theory may need to be updated to reflect more recent findings.