SAT Subject Biology Practice Question 294: Answer and Explanation
4. Consider the following graph of substrate concentration vs. product formation. Assume enzyme concentration to be constant. Why does the graph level off at high substrate concentrations?
A. All the enzyme is used up, and product formation cannot occur without it.
B. There is no more substrate to be converted into product.
C. Substrate concentration, exceeds enzyme concentration, and all active sites are saturated.
D. The reaction has run to completion.
E. An inhibitor has been added, and it has slowed down the rate of product formation.
Correct Answer: C
C When the concentration of substrate far exceeds the concentration of enzyme (remember, the question states that enzyme concentration is assumed to be constant), all the enzyme active sites are saturated with substrate, and the product is being formed at its maximum rate. The only way to increase product formation at this point is to increase the concentration of the enzyme. Note that enzymes should not be used up in the course of the reaction (A is wrong). Furthermore, product formation is still occurring, just at a stable rate (B and D are wrong). There is no reason to assume an inhibitor has been added; the rate of product formation remains constant.