Aromatase inhibitor acting by increasing km value

Case details

An 8-year-old girl presents with precocious puberty, short stature, and coffee-colored macules. The patient has been diagnosed with McCune-Albright syndrome that results from estrogen production due to excessive Aromatase activity from ovarian follicular cysts. To treat the precocious puberty and other symptoms, the patient has been prescribed with a drug that inhibits the Aromatase activity. The drug is known to act by binding to substrate binding site, producing an increase in km.

Which of the following choices best describes the mechanism of inhibition of the drug?

A. Competitive

B. Non Competitive

C. Allosteric

D. Suicidal

E. Feedback inhibition

Discussion

This is an example of competitive enzyme inhibition.

In competitive inhibition, an enzyme can bind substrate (forming an ES complex) or inhibitor (EI) but not both (ESI). The competitive inhibitor resembles the substrate and binds to the active site of the enzyme (Figure-1). The substrate is thereby prevented from binding to the same active site.

Competitive inhibition

Figure-1-The competitive inhibitor resembles the substrate and binds to the active site of the enzyme. The substrate is thereby prevented from binding to the same active site.

A competitive inhibitor diminishes the rate of catalysis by reducing the proportion of enzyme molecules bound to a substrate. The hallmark of competitive inhibition is that it can be overcome by a sufficiently high concentration of substrate. Under these conditions, the substrate “outcompetes” the inhibitor for the active site. However, the apparent value of Km  is altered; the effect of a competitive inhibitor is to increase the apparent value of Km. As the value of [I] increases, the value of Km increases (see Figure-2). In the presence of a competitive inhibitor, an enzyme will have the same max as in the absence of an inhibitor. The inhibitor does not bind covalently to the enzyme to bring about irreversible changes.

 

Graphical representation of competitive inhibition

Figure-2- As the concentration of a competitive inhibitor increases, higher concentrations of substrate are required to attain a particular reaction velocity. A sufficiently high concentration of substrate can completely relieve competitive inhibition.

As regards other options

In non competitive inhibition, the inhibitor binds to a site other than the active site; thus the km remains unaltered whereas the Vmax is decreased.

Allosteric inhibition is a physiological way of regulation of enzyme activity.

Binding of the inhibitor alters either the affinity of the enzyme for its substrate (in K type enzymes) or the reaction velocity (Vmax) is decreased (in V type enzymes) .

The V max is decreased in V type enzymes and Km is increased in K type enzymes.

Suicidal Inhibition

Also called Mechanism based inhibition, the inactive inhibitor gets activated by host enzyme to inhibit the subsequent enzyme.

In Feedback inhibition also called Product inhibition, the product of the reaction pathway inhibits the key regulatory enzyme.

 

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