Question

The oxidized cytochrome aa3 acts on a colorless substrate S and oxidizes it to a colored compound. The appearance of the latter is followed by a spectrophotometric assay.

The following results are obtained:

Vmax= 0.054 mmol/L/min and KM= 2.4 x 10^-4 mol/L

We then study the influence of cyanide ions (CN^-) on the kinetics of the reaction. The following results are then obtained:

Vmax= 0.0364 mmol/L/min and KM= 2.4 x 10^-4 mol/L

Choose all correct answers:

		The cytochrome aa3 has a better affinity for cyanide than for the substrate
		The cytochrome aa3 has a better affinity for the substrate than for the cyanide
		The cyanide exerts a competitive inhibition
		The cyanide exerts a non-competitive inhibition
		The cyanide exerts an activation

Answer 1

If we take a look at the information given in the table about the Vmax and Km before and after the addition of Cyanide to our reaction mixture we can see that the Vmax for normal (0.054) is higher than Vmax with cyanide (0.0364). So it means that addition of Cyanide ions have lower down the Vmax of the Cytochrome.

While if we look at the Km of both the reaction, it is similar for both of them.

This means that cyanide is affecting only the Vmax and isn't affecting Km at all.

This type of inhibition is called Noncompetitive inhibition.

Additionally in noncompetitive inhibition there's no issues regarding affinity of the substrate and affinity of the inhibitor because inhibitor in this type of inhibitor binds to a site other than the site where the substrate bind. This is called allosteric effect.

So the right option is Option (4) The cyanide exerts a non-competitive inhibition