Multiple Choice
The KI value for a certain competitive inhibitor is 10 mM. When no inhibitor is present, the Km value is 50 mM. Calculate the apparent Km when 40 mM inhibitor is present.
7. Enzyme Inhibition and Regulation
Apparent Km and Vmax
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Competitive inhibitor A at a concentration of 2 μM doubles the apparent K m for an enzymatic reaction, whereas competitive inhibitor B at a concentration of 9 μM quadruples the apparent Km. What is the ratio of the K I for inhibitor B to the K I for inhibitor A?
A
1.5
B
3
C
4
D
2/3
E
1/4
Verified step by step guidance1
Understand that competitive inhibitors increase the apparent Km of an enzyme without affecting Vmax. The relationship between the apparent Km (Km') and the inhibitor concentration [I] is given by the equation: Km' = Km(1 + [I]/KI), where KI is the inhibition constant.
For inhibitor A, the apparent Km is doubled, so Km' = 2Km. Substitute into the equation: 2Km = Km(1 + [2 μM]/KIA). Simplify to find the relationship: 2 = 1 + [2 μM]/KIA.
Solve the equation for KIA: [2 μM]/KIA = 1, which implies KIA = 2 μM.
For inhibitor B, the apparent Km is quadrupled, so Km' = 4Km. Substitute into the equation: 4Km = Km(1 + [9 μM]/KIB). Simplify to find the relationship: 4 = 1 + [9 μM]/KIB.
Solve the equation for KIB: [9 μM]/KIB = 3, which implies KIB = 3 μM. The ratio of KIB to KIA is KIB/KIA = 3 μM / 2 μM = 1.5.
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