Attached picture has data obtained from activity (initial velocity) measurement of an enzyme. This enzyme is homodimeric, dimer is more active than monomer. Each subunit has two obligatory Mg2+ at its active site, maybe there are extra binding sites for Mg2+ we don't know of yet.
At [Mg2+] = 0.5 there is this inflection point, which is reproduced in experiments, and I want to make an appropriate equation to fit the data, but I couldn't find the model.
Considering all this, what kind of magnesium binding can describe enzyme behavior giving such activity curve?
You could try a two-site equilibrium binding model. It might look like this:
Y = Ax/(B+x) + Cx/(D+x) where B and D are the dissociation constants of the two Mg2+ ions and A+C=100.
Alternatively, you could try a cooperative binding model (Hill equation):
Y = 100xn/(Kd + xn). In this case, the cooperativity looks like it will be negative, so n
Yes, I've derived an equation for two-site equilibrium binding model. Thank you!
I agree you need a two-site model as stated in the first answer. You can fit the model to the data using Excel's Solver tool. You should get two Kd (B and D in Shapiro's answer) and two amplitudes (A and C), but A and C might be equal. However, it doesn't look like a two-site isotherm. It looks like one-site with noise.
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