Enzymes and the Rate of Reaction
Prelab
Enzymes are large protein molecules that act as biological catalysts—they speed up biochemical reactions
in the body. The molecule with which an enzyme reacts is called a substrate. Enzymes are very specific;
they react only with their unique substrate. When enzymes were discovered, fast chemical reactions in
the body (which took place slowly in test tubes) could be explained. Enzymes speed up biochemical
reactions by lowering the energy of activation of their substrates.
Catalase
Catalase is an enzyme that converts hydrogen peroxide into water and oxygen. The substrate for
catalase—hydrogen peroxide (H2O2)—is a product of cellular metabolism, which can poison cells.
H2O2
catalase
H2O + O2
If hydrogen peroxide were allowed to build up even over a short period of time it would kill the cell.
Catalase has to work very quickly to ensure that hydrogen peroxide concentrations remain low.
The Rate of Reaction
The rate of reaction of an enzyme can be measured to determine how fast the enzyme is converting
substrate into product. A rate is the speed at which something occurs during a certain time period. An
example would be 20 mL per second (20 mL/sec).
Rates are calculated using the formula (which is the same as finding the slope):
y2 - y1
x2 - x1
Example
Sucrase is an enzyme that converts sucrose into glucose and fructose. The following data for glucose
were taken in an experiment. What is the reaction rate for sucrase between 30 and 45 seconds?
Prelab Questions
1. Using the graph above for sucrase, during which 15-second period is the rate of reaction the
greatest? Calculate this rate.
2. Hypothesize why the rate of reaction for sucrase is zero between 90 and 120 seconds.
3. What is an enzyme?
4. a. What is catalase?
b. What is the substrate for catalase?
c. What are the products of the catalase reaction?
5. What factors do you predict will have an affect on the rate of reaction for this experiment? Name
two.