Case study- Enzyme Inhibition

A 54- year-old man complains of an acute onset of unilateral eye pain and reduction in the visual acuity. On physical examination, conjunctival injection (eye redness) and a mild-dilated and non reactive pupil is observed. Fundoscopic examination reveals cupping of the optic disc. Recognizing the signs and symptoms as glaucoma, the medication Acetazolamide is administered to decrease the production of aqueous fluid and lower the intraocular pressure. Acetazolamide is a non competitive inhibitor of carbonic anhydrase and therefore will cause:

A. An increase in Km

B. A decrease in Km

C. An increase in Vmax

D. A decrease in Vmax

E. A decrease in both Km and Vmax

Answer- D- A decrease in Vmax.

In noncompetitive inhibition, binding of the inhibitor does not affect binding of substrate. Formation of both EI (enzyme inhibitor complex) and EIS (enzyme inhibitor substrate) complexes is therefore possible. However, while the enzyme-inhibitor complex can still bind substrate, its efficiency at transforming substrate to product, reflected by Vmax, is decreased. Noncompetitive inhibitors bind enzymes at sites distinct from the substrate-binding site and generally bear little or no structural resemblance to the substrate, Km value thus remains unaffected (figure)

Non- competitive enzyme inhibition

Figure- In the presence of a non-competitive enzyme inhibitor the Vmax decreases but the Km remains the same.

Most frequently, in competitive inhibition the inhibitor, (I), binds to the substrate-binding portion of the active site and blocks access by the substrate. The structures of most classic competitive inhibitors therefore tend to resemble the structures of a substrate, and thus are termed substrate analogs. The effects of competitive inhibitors can be overcome by raising the concentration of the substrate. Thus, a competitive inhibitor has no effect on Vmax but raises Km for the substrate.

Hence option- A- Increase in Km could have been correct, if the mode of inhibition had been competitive.

A decrease in Km means increased affinity of the enzyme for its substrate which is not possible in non competitive inhibition. Km decreases in uncompetitive inhibition where the inhibitor binds essentially to enzyme substrate complex but in that case, Vmax is also lowered along with Km.

An increase in Vmax is impossible in the presence of an inhibitor.

A decrease in both Vmax and Km is observed in uncompetitive inhibition as mentioned above.

Thus the most suited and right option is- D- A decrease in Vmax

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