Multi-Substrate Enzyme Kinetics

Multi-substrate Enzyme Kinetics

Look at three common and easily understood types. We will use Cleland Nomenclature and "Kinetic mechanism diagrams."

·  Ordered Sequential Bi Bi mechanism (two on; two off); Note: A must bind first, Q is released last.

·  Ping Pong Bi Bi (one on, one off; one on, one off); Note: have some sort of modified enzyme intermediate (often covalent intermediate)

·  Random Sequential Bi Bi (two on; two off); Note: A or B may bind first, P or Q may be released last.

Two-substrate Enzyme Product Inhibition Patterns
(Based on: E. B. Cunningham, Biochemistry: Mechanisms of Metabolism. McGraw-Hill Book Company, New York (1978), and W. Cleland, "Substrate Inhibition: in Contemporary Enzyme Kinetics and Mechanism. (Daniel L. Purich, ed.) Academic Press, New york (1983))
Kinetic Mechanism / Variable Substrate / Product / Type of Inhibition

Ordered Sequential Bi Bi
Ordered Sequential Bi Bi / A / Q / Competitive
B / Q / Noncompetitive
A / P / Noncompetitive
B / P / Noncompetitive

Random Sequential Bi Bi
Random Sequential Bi Bi / A / Q / Noncompetitive
B / Q / Noncompetitive
A / P / Noncompetitive
B / P / Noncompetitive

Ping pong Bi Bi
Ping pong Bi Bi / A / Q / Competitive
B / Q / Noncompetitive
A / P / Noncompetitive
B / P / Competitive

Note that in each case we can predict/explain the pattern of inhibition on the basis of the substrate and inhibitor binding to the same "enzyme form." Thus for the Ordered Sequential mechanism only the first substrate and last product bind to the same form, in this case the free enzyme. Similarly for the Ping pong mechanism the first substrate and last product should be competitive as the both bind the free enzyme. In this case we also see a competitive inhibition between the second substrate and the first product, since they both bind to the E-X complex. The Random Sequential mechanism is a bit more subtle. Here we see across the board noncompetitive since in each case the substrates (and products) can each bind to more than one substrate form, so competitive inhibition will not be possible! (Think of the product as competing with one order of binding but not the other.)