Question

Imagine that you are observing an enzyme-catalyzed reaction in the laboratory. The reaction is progressing as expected. As you periodically add more enzyme, the reaction increases proportionally until suddenly it stops increasing. At this point, no matter how much more enzyme that you add, the reaction rate does not change. Assuming no other chemicals have been added or changed, explain why the reaction rate has leveled off.

Answer 1

Upon the addition of enzyme, the rate of reaction increases proportionally, but upto a certain extent only. After a certain limit, reaction rate will not increase even after addition of more enzyme. This is because of the fact that when enzyme is present in the excess, the substrate becomes the limiting factor. If enzyme concentration is higher than the substrate concentration, then the substrate molecules won't be enough to bind with the excessive enzyme molecules. Reaction rate increases only till substrate molecules are available and enough in number to bind with enzyme. Once all the substrate molecules are bound to the enzyme, then no further increase in reaction rate will be observed, even if we keep on adding the enzyme, since all of the substrate molecules are exhausted and no more substrate is available to bind with the excess enzyme. So the reaction rate will become constant after this limit, and won't increase.

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