Study the given data and answer the questions that follow.
A sample of an enzyme called lactase was isolated from the intestinal lining of a calf. Assays were undertaken to evaluate the activity of the enzyme sample.
The substrate of lactase is the disaccharide lactose. Lactase breaks a lactose molecule in two, producing a glucose molecule and a galactose molecule.
Two assays were carried out.
`{:("Lactose concentration (% w/v)",15,15,15,15,15,15),("Concentration of enzyme smaple (%v/v)",0,5,10,15,20,25),("Reaction rate "mu"mole glucose "sec^(-1)mL^(-1),0,25,50,75,100,125):}`
`{:("Lactose concentration (% w/v)",0,5,10,15,25,30),("Concentration of enzyme smaple (%v/v)",5,5,5,5,5,5),("Reaction rate "mu"mole glucose "sec^(-1)mL^(-1),0,15,25,35,40,40):}`
Which of the following statements can be concluded from the two assays ?

Options

A. The reaction rate of the lactase assay is always proportional to the amount of enzyme present.
B. The amount of lactose in an assay has no effect on the rate of the reaction.
C. The reaction rate of the lactase assay is proportional to the amount of lactose present.
D. The reaction rate is proportional to the amount of enzyme present at a lactose concentration of 15%w/v.

Answer

D

Explanation

The reaction rate is proportional to the amount of enzyme present at a lactose concentration of `15% w//v`. The rate of reaction is proportional with the substrate concentration only to a limit, after which any increase in substrate concentration makes no difference in rate of reaction, the rate of reaction then remains constant.