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The process of enzyme catalysis of biological processes follows a simple pathway and involves three principal species.

The substrate, which we will represent as $S$, is the molecule that is to undergo reaction. The enzyme, $E$, acts as the catalyst, and the product, $P$, is the result of the reaction.

The substrate, $S$, attaches to a receptor site on the enzyme, $E$, where it is held in place as the reaction proceeds. Once the reaction is complete, the product, $P$, is released from the receptor site, along with any remaining portion of the substrate molecule.

The process can be represented as shown below, where $S-E$ represents the substrate, bound to the catalyst. As the product forms, the receptor site on the catalyst is again freed to accept another substrate molecule.

$$S+E\rightarrow \left[ S-E \right] \rightarrow P+E$$

The reaction rate can be investigated either in terms of decreasing concentration of substrate, or increasing appearance of the product.

As part of her senior-year STEM project, Athena designs an experiment to study a particular enzyme-catalyzed reaction. Since the product of the reaction is a colored species, she sets up a colorimetric interface to measure the rate of appearance of the colored product, in units of concentration of product over time: $\frac { mol\cdot { L }^{ -1 } }{ min } $, or simply, $\frac { M }{ min } $. The plot of product formation vs. time appears below.

John G Little. Created for Copyright 2016. All rights reserved.

By design, Athena's experiment began with a large excess of the substrate, $S$, relative to the enzyme, $E$.

Which of the following offers the BEST description of the results of her study?


After initially increasing rapidly, the rate of product formation slows as the system approaches and achieves equilibrium, as shown by the leveling of the graph. The steady level of product formation indicates the equilibrium state and can be expected to remain at that level indefinitely, barring a change in temperature.


Product formation rises rapidly up to the point at which not enough substrate molecules are available to bond to the enzyme's receptor sites. The rate of product formation levels as enzyme molecules await collision with substrate molecules. This level will begin to slowly diminish over time.


The reaction proceeds relatively rapidly until the point where the concentrations of enzyme and substrate level out. From there on, the reaction rate is steady and will remain so, as long as both substrate and enzyme remain in the system.


The reaction starts off slowly, as substrate and enzyme molecules collide and bind together. The rate of product formation increases as more and more of the receptor sites on the enzyme become occupied, and the reaction rate picks up.

Once all the enzyme receptor sites have been occupied, the reaction rate stabilizes since a particular receptor site only becomes available as the previous reaction at that site goes to completion. Over time, as more and more of the substrate is converted to product, the rate of formation can be expected to decrease.

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