hard · MCAT chem-phys
A biochemist studies a novel inhibitor of a metabolic enzyme by running steady-state assays at two fixed substrate concentrations, [S]_1 and [S]_2 (with [S]_2 > [S]_1), across a range of inhibitor concentrations. She plots 1/v against [I] (a Dixon plot) for each substrate concentration. The two resulting lines intersect at a single point located above the x-axis, at [I] = -2 mM.
Which conclusion about the inhibitor is best supported by this Dixon plot, and what would confirm it?
- The inhibitor is competitive with K_i = 2 mM; a secondary replot of the line slopes against 1/[S] should be linear and pass through the origin.
- The inhibitor is noncompetitive with K_i = 2 mM; a secondary replot of the line slopes against 1/[S] should be linear and pass through the origin.
- The inhibitor is uncompetitive with K_i = 2 mM; the two lines should instead be parallel, so this data must reflect assay error.
- The inhibitor is mixed-type with K_i = 2 mM; the intersection point's height above the x-axis cannot be predicted from K_i alone.
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