Page 210 - The Manga Guide to Biochemistry

Reaction rate (v) Allosteric enzymes
Although we’ve introduced enzyme reactions for which the Michaelis-Menten equation
applies, there are also many enzymes that exhibit activity for which the Michaelis-­
Menten equation does not apply.

One of these is a type of enzyme called an allosteric enzyme, which can respond to
environmental signals with changes in activity. This is referred to as an allosteric effect.
For example, a substrate that bonds to one subunit may bring about a change in the
three-dimensional structure of the enzyme, which causes the substrate to bond more
easily with another subunit.

In a case like that, the curve representing the relationship between the substrate
concentration and the reaction speed will be an S-shaped sigmoid curve rather than
the hyperbola that is typical of the Michaelis-Menten equation.

We’ll skip the details here, but not all enzyme reactions are simple ones that occur
according to certain fixed equations—in fact, in the body, most are extremely complex
and diverse.

Hyperbola of the Michaelis-Menten equation

Sigmoid curve of an allosteric enzyme

Substrate concentration (S)

196 Chapter 4