I determined the binding constant of an amino acid substrate for an enzyme to be 5.0 mM, what does this number mean in terms of the affinity of the enzyme for the substrate?
What a loaded question. The answer Julio give is correct only with the steady state assumption and provided you are dealing with a uni uni reaction of the type
S +E ---->ES---->P+E If you are dealing with any thing more complex then the Kd may mean different things.
If you cannot make the steady state assumption and choose a rapid equilibrium model with a uni uni reaction then
Km=k-1/k1=Ks=Kd
Your answer also depends on how you derived the value. Was this number derived using the enzyme initial rate and modeling the Kd value?
My first impulse is to suggest as Julio does that the value 5mM is way too high and likely not biologically relevant. Typical biological Kd values are in the low micromolar range between 50 and 0.1 uM. The value 5mM suggests that you did something wrong but I cannot say what without more information.
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