Protein kinase and Allosteric effector

allosteric effectors have their own specific sites for binding to enzyme. they can bring positiveor negative effect. that depends on the natre of effector.

Ahh... A fine question. The answer: When a substrate acts as an effector there are homotropic effects. The substrate binds to the active site and induces allosteric-like effects.

BPG in hemoglobin means allosteric effector, that binds to the site that is completely remote from that active site for oxygen. The amount of BPG in red cells determines the oxygen affinity of hemoglobin.

It is not the fourth one specifically that binds easier, O2 is a positive allosteric effector (activator) of Haemoglobin and the binding of O2 facilitates further binding of O2. I'm not sure why this is though.