27 Enzyme Reaction Rates

Key Idea: Enzymes have a narrow range of conditions in which they work optimally. Outside this range, activity decreases and the enzyme may lose its structure.

In the graphs below, the rate of reaction (enzyme activity) is plotted against factors that affect enzyme performance.

Rate of reaction 1. (a) Describe the change in the reaction rate when enzyme concentration is increased (assuming substrate is not limiting):

With ample substrate (b) How does this allow a cell to change the rate of substrate use?

Rate of reaction Enzyme concentration

With fixed amount of enzyme

Substrate concentration 2. (a) Describe the change in the reaction rate when substrate concentration is increased (assuming a fixed amount of enzyme):

(b) Explain why the rate changes the way it does:

Rate of reaction

No inhibitor Competitive inhibitor 3. Study the graph comparing the effect of competitive and non- competitive enzyme inhibitors on reaction rate. (a) What is the general effect of an enzyme inhibitor:

Non-competitive inhibitor

Substrate concentration (b) How can the effect of a competitive inhibitor be overcome?

Enzyme activity

Too cold for the enzyme to operate Rapid denaturation at high temperature 4. Although an increase in temperature increases reaction rate, few enzymes remain functional at temperatures above 50 - 60°C. The rate at which enzymes are denatured increases with temperature. (a) What is meant by optimum temperature for enzyme activity?

0 01 02 03 04 05

Temperature (°C)

Enzyme activity

Pepsin Trypsin

Urease

1 2 3 4 5 6 7 8 9 10 Acid

pH Alkaline (b) Mark the optimum temperature on the graph left with a vertical line.

(c) What is the effect of denaturation on the enzyme's active site?

5. Like all proteins, enzymes are denatured by extremes of pH. Within these extremes, most enzymes have a pH range for optimum activity. (a) State the optimum pH for each of the enzymes:

Pepsin: Trypsin: Urease:

(b) Which one would you find in the stomach (pH 2)?