The enzyme Y catalyzes the elementary reaction AB→A+B

An enzyme concentration of 1.0 μM Y can convert a maximum of 0.5 μM AB to the productsA and B per second. Note: Since the concentrations of species in the cytoplasm of cells is small, the concentration unit of micromolar (μMor 10−6 M) is used for consistency with biochemical systems.

Six solutions are made, each with a Y concentration of 1.0 μM and varying concentrations of AB as shown below. Based on the concentrations, rank the solutions in decreasing order of reaction rate.

a) 0.2 uM AB

b) 0.3uM AB

c) 0.6 uM AB

d) 0.4 uM AB

e) 0.7uM AB

f) 0.5 uM AB

0.7μM = 0.6 μM = 0.5 μM > 0.4 μM > 0.3 μM > 0.2 μM

Explanation:

An enzyme solution is saturated when all the active sites of the enzyme molecule are full.  When an enzyme solution is saturated, the reaction is occurring at the maximum rate.

From the given information, an enzyme concentration of 1.0 μM Y can convert a maximum of 0.5 μM AB to the products A and B per second means that a 1.0 M Y solution is saturated when an AB concentration of 0.5 M or greater is present.

The addition of more substrate to a solution that contains the enzyme required  for its catalysis will generally increase the rate of the reaction. However, if the enzyme is saturated with substrate, the addition of more substrate will have no effect on the rate of reaction.

Therefore the reaction rates at substrate concentrations of 0.7μM, 0.6 μM, and 0.5 μM are equal. But the reaction rate at substrate concentrations of  0.2 μM is lower than at 0.3 μM, 0.3 μM is lower than 0.4 μM and 0.4 μM is lower than 0.5 μM, 0.6 μM and 0.7 μM.