Two receptors bind to a same chemical messenger, but the concentration of this chemical messenger required to reach a certain number of bound messengers for one receptor is twice as great as the concentration for the other receptor. Thus, these two receptors have same , but different , to this chemical messenger

binding specificity; affinity

Explanation:

Receptor binding specificity can be defined as the selectivity of a receptor to bind with a particular chemical messenger. Binding specificity is determined by the interaction between the chemical messenger and its specific receptor, as well as the binding site three-dimensional (3D) structure of both. This concept is contrary to receptor promiscuity, where a receptor can bind to many different chemical messengers. On the other hand, binding affinity can be defined as the overall affinity or avidity of binding. The binding affinity is determined by the accumulated strength of individual attractive forces between a chemical messenger (ligand) and its receptor.

a covalent bond, specfically triple bond