Scientists characterized the steady-state kinetics of cholesterol oxidase from B

Question

Scientists characterized the steady-state kinetics of cholesterol oxidase from Brevibacterium sterolicum by measuring the rate of the reaction as a function of cholesterol and oxygen concentrations at pH 7.25. Three series of measurements were performed by varying the concentration of cholesterol at three different fixed levels of oxygen (80 M, 140 M, and 260 M). The reaction velocity was recorded via a colorimetric assay in which the hydrogen peroxide that formed in the reaction was subsequently reacted with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) in the presence of horseradish peroxidase and the absorbance at 405 nm was recorded. When the results were plotted in double-reciprocal coordinates, a nice parallel line pattern was evident in the Lineweaver–Burk plot. What is the correct conclusion from this data?

Select one:

a. During its catalytic cycle, cholesterol oxidase releases a molecule of 5-cholesten-3-one before a molecule of oxygen binds to the reduced form of the enzyme.

b. The data is flawed, straight parallel lines were observed because the absorbance was measured at 405 nm rather than at 340 nm where NADH absorbs.

c. Oxygen acts as an un-competitive inhibitor of cholesterol oxidase.

d. Oxygen acts as a competitive inhibitor of cholesterol oxidase.

e. During its catalytic cycle, cholesterol transfers a pair of electrons directly to bound molecular oxygen

f. The data us flawed, straight parallel lines were observed because 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) irreversibly inactivates cholesterol oxidase.

Explanation / Answer

The formation of parallel lines indicate the increase in the inhibition with the concentration of oxygen . These parallel lines represents the uncompetative inhibition.

So, the correct answer is ,

c. Oxygen acts as an un-competitive inhibitor of cholesterol oxidase.

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