The competition plot: a simple test of whether two reactions occur at the same active site

Area of Science:

• Biochemistry
• Enzyme kinetics

Background:

• Enzyme active sites are crucial for catalytic activity.
• Distinguishing between single and multiple active sites is essential for understanding enzyme mechanisms.

Purpose of the Study:

• To introduce and validate the competition plot as a method for determining if two enzyme-catalyzed reactions share the same active site.
• To demonstrate the utility of the competition plot in distinguishing between different enzyme kinetic behaviors.

Main Methods:

• The competition plot method involves plotting total reaction rate against a variable 'p' that adjusts substrate concentrations.
• Analysis of the plot's shape (horizontal line, curve with maximum/minimum) indicates whether substrates compete for the same active site.
• Experimental validation using yeast hexokinase and galactokinase.

Main Results:

• The competition plot correctly identified glucose and fructose phosphorylation by yeast hexokinase occurring at the same active site.
• The plot differentiated glucose and galactose phosphorylation by a mixture of yeast hexokinase and galactokinase as occurring at different active sites.
• The method was adaptable for enzymes not strictly following Michaelis-Menten kinetics.

Conclusions:

• The competition plot is a reliable method for elucidating active site usage in enzyme-catalyzed reactions.
• This technique provides clear visual distinction between enzymes acting on the same or different active sites.
• The competition plot offers a valuable tool for biochemical research and enzyme mechanism studies.