Allosteric enzymes

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Allosteric enzymes

One of the assumptions of Michaelis-Menten kinetics is that there is no cooperativity. If the enzyme is multimeric, then binding of a substrate to one binding site should have no effect on the activity of neighboring sites. This assumption is often not true.

If binding of substrate to one binding site increases the activity of neighboring sites, the term positive cooperativity is used. Activity is related to substrate concentration by this equation:

When the variable h equals 1.0, this equation is the same as the Michaelis-Menten equation. With positive cooperativity, h will have a value greater than 1.0. If there are two interacting binding sites, h will equal two (or less, depending on the strength of the cooperativity). If there are three interacting binding sites, h will equal 3 (or less). Note that the denominator has the new variable K0.5 instead of KM.

To fit data to the equation for enzyme activity with positive cooperativity, use the equation below. For initial values, try these rules: Vmax=Ymax, K=.5*Xmid, and h=1.0

Y=Vmax*X^h/(K^h + X^h)

The variable h does not always equal the number of interacting binding sites (although h can not exceed the number of interacting sites). Think of h as an empirical measure of the steepness of the curve and the presence of cooperativity.

Enzyme kinetics in the presence of an inhibitor