A graphical method for determining inhibition constants

Figures & data

Figure 1.  Quotient velocity plot for competitive inhibition. The lines were drawn in accordance with Equation (3). The following values of parameters were used: K_m = 1 and K_i = 2. The substrate concentration is indicated by each line.

Figure 2.  Quotient velocity plot for noncompetitive inhibition. The lines were drawn in accordance with Equation (4). The following values of parameters were used: K_m = K’_m = 1 and K_i = K’_i = 2. The substrate concentration is indicated by each line.

Figure 3.  Quotient velocity plot for uncompetitive inhibition. The lines were drawn in accordance with Equation (5). The following values of parameters were used: K_m = 1, K’_i = 4, K’_m = 0, and K_i = ∞. The substrate concentration is indicated by each line.

Figure 4.  Quotient velocity plot for mixed type inhibition. The lines were drawn in accordance with Equation (6). The following values of parameters were used: K_m = 1, K_i = 2, and K’_i = 4. The substrate concentration is indicated by each line.

Table 1.  Values of intersection points and slopes of quotient velocity plots.

Inhibition type	Intersection of lines at two or more values of [S]	Slope
Competitive (Km ≠ K’m)	–Ki at abscissa	Km/[S]·1/Ki
Noncompetitive (Km = K’m, Ki = K’i)	(–Ki, −1) third quadrant	(Km/[S] + 1)/Ki
Uncompetitive (Ki = ∞, K’m = 0)	Parallel	1/K’i
Mixed type (Km ≠ K’m, Ki ≠ K’i)	(–K’i, –Ki/K’i) third quadrant	(Km/[S]·1/Ki + 1/K’i)