Why is ADP an inhibitor?

Question

Is it even an inhibitor? I know inhibitors decrease enzyme activity, but I'm not quite sure why---is it because ADP has less energy than ATP, because it has 2 phosphate groups instead of three?

Answer 1

Can actually bind to some enzymes and inhibit them (nothing to do with ATP and energy levels).

Eg RNAse is inhibited by ADP. see link

Answer 2

Different cells use different substrates as ligands for receptors.  It is believed that these receptors and receptor functions evolved.  Having said that, ADP has different functions depending on the cell and the other enzymes and receptors associated with these cells.  For example, platelets used ADP to initiate and amplify coagulation by releasing stored ADP which attaches to ADP receptors on their cell surfaces.  [Here it functions as an activator].

Other cells and membrane receptors use ATP to create an intracellular 2nd messenger (cAMP) once a particular ligand attaches to a membrane-bound receptor.  The attachment of the ligand to the receptor activates adenylate cyclase, an enzyme that converts ATP to cAMP. Since most cells have reciprocal concentrations of ATP and ADP, increased levels of ADP means decreased levels of ATP.  In this setting, ADP seems to have an inhibitory role (indirectly).  ADP is not the substrate for the enzyme in question, ATP is. But since the levels are reciprocal, it seems to function in that capacity.  Adding new ADP to a system with a constant amount of ATP will decrease the apparent concentration of ATP without decreasing the actual number of molecules.  Whether this will trigger an inhibitory role is not clear and depends on the circumstance.   Remember that ligand receptor interactions can rely on concentration rather than actual number.