<P> As protons cross the membrane through the channel in the base of ATP synthase, the F proton - driven motor rotates . Rotation might be caused by changes in the ionization of amino acids in the ring of c subunits causing electrostatic interactions that propel the ring of c subunits past the proton channel . This rotating ring in turn drives the rotation of the central axle (the γ subunit stalk) within the α and β subunits . The α and β subunits are prevented from rotating themselves by the side - arm, which acts as a stator . This movement of the tip of the γ subunit within the ball of α and β subunits provides the energy for the active sites in the β subunits to undergo a cycle of movements that produces and then releases ATP . </P> <P> This ATP synthesis reaction is called the binding change mechanism and involves the active site of a β subunit cycling between three states . In the "open" state, ADP and phosphate enter the active site (shown in brown in the diagram). The protein then closes up around the molecules and binds them loosely--the "loose" state (shown in red). The enzyme then changes shape again and forces these molecules together, with the active site in the resulting "tight" state (shown in pink) binding the newly produced ATP molecule with very high affinity . Finally, the active site cycles back to the open state, releasing ATP and binding more ADP and phosphate, ready for the next cycle . </P> <P> In some bacteria and archaea, ATP synthesis is driven by the movement of sodium ions through the cell membrane, rather than the movement of protons . Archaea such as Methanococcus also contain the A A synthase, a form of the enzyme that contains additional proteins with little similarity in sequence to other bacterial and eukaryotic ATP synthase subunits . It is possible that, in some species, the A A form of the enzyme is a specialized sodium - driven ATP synthase, but this might not be true in all cases . </P> <P> Molecular oxygen is an ideal terminal electron acceptor because it is a strong oxidizing agent . The reduction of oxygen does involve potentially harmful intermediates . Although the transfer of four electrons and four protons reduces oxygen to water, which is harmless, transfer of one or two electrons produces superoxide or peroxide anions, which are dangerously reactive . </P>

Vitamin ____ is present in the body as coenzyme fad and fmn