enzyme kinetics Enzyme kinetics is the study of the rates of enzyme catalysis, enzyme-catalysed chemical reactions. In enzyme kinetics, the reaction rate is measured and the effects of varying the conditions of the reaction are investigated. Studying an enzyme' ...

, a secondary plot uses the intercept or slope from several Lineweaver–Burk plots to find additional kinetic constants. For example, when a set of v by curves from an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

with a ping–pong mechanism (varying substrate A, fixed substrate B) are plotted in a Lineweaver–Burk plot, a set of parallel lines will be produced. The following Michaelis–Menten equation relates the initial reaction rate ''v''₀ to the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

concentrations and :

\begin 
\frac &= \frac+\frac+\frac
\end

The y-intercept of this equation is equal to the following: :

\begin
\mbox = \frac+\frac
\end

The y-intercept is determined at several different fixed concentrations of substrate B (and varying substrate A). The y-intercept values are then plotted versus 1/ to determine the Michaelis constant for substrate B,

K_M^B

, as shown in the Figure to the right.The Horseradish Peroxidase/ o-Phenylenediamine (HRP/OPD) System Exhibits a Two-Step Mechanism. M. K. Tiama and T. M. Hamilton, ''Journal of Undergraduate Chemistry Research'', 4, 1 (2005). The slope is equal to

K_M^B

divided by

v_\max

and the intercept is equal to 1 over

v_\max

Secondary plot in inhibition studies

A secondary plot may also be used to find a specific inhibition constant, K_I. For a competitive

enzyme inhibitor An enzyme inhibitor is a molecule that binds to an enzyme and blocks its Enzyme activity, activity. Enzymes are proteins that speed up chemical reactions necessary for life, in which Substrate (biochemistry), substrate molecules are converted ...

, the apparent Michaelis constant is equal to the following: :

\begin
\mbox K_m=K_m\times \left(1+\frac\right)
\end

The slope of the Lineweaver-Burk plot is therefore equal to: :

\begin
\mbox =\frac\times \left(1+\frac\right)
\end

If one creates a secondary plot consisting of the slope values from several Lineweaver-Burk plots of varying inhibitor concentration the competitive inhbition constant may be found. The slope of the secondary plot divided by the intercept is equal to 1/K_I. This method allows one to find the K_I constant, even when the Michaelis constant and v_max values are not known.

References

{{refend Enzyme kinetics