Which of these is a correct expression for fractional saturation of binding sites in a protein:
(a) Y = [ P L ] / [ P ] [ L ]
(b) Y = [ P L ] / [ P ] + [ L ]
(c) Y = [ P L ] /[ P ] + [ P L ]
(d) Y = 1 / 1 + 1 K A [ L ]
To determine the correct expression for fractional saturation of binding sites in a protein, let's analyze each option:
(a) Y = [ P L ] / [ P ] [ L]
This expression represents the ratio of the complex formed between the protein and ligand ([PL]) to the product of the concentration of the protein ([P]) and the concentration of the ligand ([L]). However, this expression lacks the denominator, making it incorrect.
(b) Y = [ P L ] / [ P ] + [ L]
This expression represents the ratio of the complex formed between the protein and ligand ([PL]) to the sum of the concentration of the protein ([P]) and the concentration of the ligand ([L]). This expression is a valid form of fractional saturation, as it considers both the protein and ligand concentrations. However, it assumes a 1:1 binding stoichiometry, which may not be the case in all situations.
(c) Y = [ P L ] /[ P ] + [ P L ]
This expression is similar to option (b), but it includes a term for [PL] as part of the denominator. However, this inclusion is unnecessary, and therefore, this expression is not correct.
(d) Y = 1 / 1 + 1 K A [ L ]
This expression represents the ratio of the fractional saturation Y of binding sites to the sum of 1 and the product of the dissociation constant K A and the ligand concentration [L]. This equation is determined based on the specific binding equilibrium between a protein and a ligand. Thus, this expression is the correct representation of fractional saturation.
Therefore, the correct expression for fractional saturation of binding sites in a protein is (d) Y = 1 / (1 + (1/K A ) [L]).