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LAB EXERCISE: Enzymes I Catechol Oxidase
Laboratory Objectives After completing this lab topic, you should be able to:
- Define enzyme and describe the activity of enzymes in cells.
- Differentiate competitive and noncompetitive inhibition.
- Discuss the effects of varying environmental conditions such as pH and temperature on the rate of enzyme activity.
- Discuss the effects of varying enzyme and substrate concentrations on the rate of enzyme activity.
- Discuss the scientific process, propose hypotheses, and make predictions based on hypotheses.
- Practice scientific persuasion and communication by constructing and interpreting graphs of enzyme activity.
Introduction Living cells perform a multitude of chemical reactions very rapidly because of the participation of enzymes. Enzymes are biological catalysts , compounds that speed up a chemical reaction without being used up or altered in the reaction. The material with which the catalyst reacts, called the substrate , is modified during the reaction to form a new product (Figure 1). But because the enzyme itself emerges from the reaction unchanged and ready to bind with another substrate molecule, a small amount of enzyme can alter a relatively enormous amount of substrate.
The active site of an enzyme will bind with the substrate, forming the enzyme-substrate complex. It is here that catalysis takes place, and when it is complete, the complex dissociates into enzyme and product or products.
Enzymes are, in part or in whole, proteins and are highly specific in function. Because enzymes lower the energy of activation needed for reactions to take place, they accelerate the rate of reactions. They do not, however, determine the direction in which a reaction will go or its final equilibrium.
Enzyme activity is influenced by many factors. Varying environmental conditions, such as pH or temperature, may change the three-dimensional shape of an enzyme and alter its rate of activity. Specific chemicals may also bind to an enzyme and modify its shape. Chemicals that must bind for the enzyme to be active are called activators. Cofactors are nonprotein substances that usually bind to the active site on the enzyme and are essential for the enzyme to work. Organic cofactors are called coenzymes , but other cofactors may simply be metal ions. Chemicals that shut off enzyme activity are called inhibitors , and their action can be classified as competitive or noncompetitive inhibition.
Figure 1. Enzyme activity. A substrate or substrates bind to the active site of the enzyme, forming the enzyme substrate complex, which then dissociates into enzyme and product(s). The enzyme may catalyze the addition or removal of a molecule or a portion of a molecule from the substrate to produce the product (a), or the enzyme may catalyze the splitting of a substrate into its component subunits (b).
Review Figure 1, illustrating enzyme activity. There are two ways to measure enzyme activity: (1) Determine the rate of disappearance of the substrate, and (2) determine the rate of appearance of the product.
In this laboratory, you will use both methods to investigate the activity of two enzymes, catechol oxidase and amylase. You will use an inhibitor to influence the activity of catechol oxidase and determine if it is a competitive or noncompetitive inhibitor. Additionally, you will investigate the effect of changing environmental conditions on the rate of amylase activity.
EXERCISE 1. The Action of Catechol Oxidase
Materials cut potatoes medium funnel cheese cloth 500ml beaker ice 200ml ice-cold distilled water test-tube rack pipette pump 3 small test tubes distilled or deionized (DI) water small ParafilmTM^ squares 3 calibrated 1-mL pipettes calibrated 5-mL pipette 1% catechol disposable pasteur pipettes disposable gloves (optional)
*Catechol is a poison! Avoid contact with all solutions. Do not pipette any solutions by mouth. Wash hands thoroughly after each experiment. If a spill occurs, notify the instructor.
Procedure
Preparation of Potato Extract
- Weigh 1 hand full of cut potatoes, and place in a blender.
- Add an equivalent amount of ice-cold distilled water. (For example if your hand full of potatoes weighs 200g, then add 200ml of water)
- Blend well.
- Strain through cheesecloth and be sure to keep extract on ice at all times!
Catechol Oxidase Activity
- Using Table 1, prepare the three experimental tubes. Note that all tubes should contain the same total amount of solution. Do not cross-contaminate pipettes! After each tube is prepared, use your finger to hold a ParafilmTM^ square securely over the tube mouth and then rotate the tube to mix the contents thoroughly. Use a fresh square for each tube.
Table 1. Contents of three experimental tubes Tube Distilled Water Catechol Potato Extract 1 5.5ml 0.5ml 2 5.5ml 0.5ml 3 5ml 0.5ml 0.5ml
- Explain the experimental design: What is the purpose of each of the three test tubes? Which is the control tube? Is more than one control tube necessary? Explain. Which is the experimental tube?
- Observe the reactions in the tubes, and record your observations.
EXERCISE 2. Inhibiting the Action of Catechol Oxidase
Materials test-tube rack pipette pump 3 small test tubes distilled water small ParafilmTM^ squares potato extract calibrated 5-mL pipette 1% catechol 4 calibrated 1-mL pipettes phenylthiourea (PTU) disposable pasteur pipettes disposable gloves (optional) spatula
Introduction This exercise will investigate the inhibition of enzyme activity by specific chemicals called inhibitors. The specific inhibitor used will be phenylthiourea (PTU). To be active, catechol oxidase requires copper as a cofactor. PTU is known to combine with the copper in catechol oxidase and inhibit its enzymatic activity. An inhibitor molecule affects an enzyme in one of two ways. Competitive inhibition takes place when a molecule that is structurally similar to the substrate for a particular reaction competes for a position at the active site on the enzyme. This ties up the enzyme so that it is not available to the substrate. Competitive inhibition can be reversed if the concentration of substrate is raised to sufficiently high levels while the concentration of the inhibitor is held constant (Figure 3). In noncompetitive inhibition , the inhibitor binds to a site that is not the active site. In doing so, it changes the nature of the enzyme so that its catalytic properties are lost. This can happen in two ways. Either the noncompetitive inhibitor itself physically blocks the access to the active site, or it causes a conformational change in the protein, thus inactivating the active site. In noncompetitive inhibition the inhibitor can become unbound, reversing the inhibition. However, unlike competitive inhibition, adding additional substrate will not reverse the inhibition (Figure 3).
In the following experiment, you will determine if PTU is a competitive or noncompetitive inhibitor.
Question Pose a question about the activity of PTU.
Hypothesis Hypothesize about the nature of inhibition by PTU.
Prediction Predict the results of the experiment based on your hypothesis (if/then).
EXERCISE 3. Examination of Catechol Oxidase Specificity
This exercise will investigate the specificity of enzyme activity by utilizing a structural isomer of catechol, known as hydroquinone (Figure 4).
Figure 4. Structural Isomers
Materials test-tube rack pipette pump 3 small test tubes distilled or deionized (DI) water small ParafilmTM^ squares 3 calibrated 1-mL pipettes calibrated 5-mL pipette 1% hydroquinone disposable pasteur pipettes disposable gloves (optional) potato extract
Question Pose a question about the specificity of catechol oxidase.
Hypothesis Hypothesize about the specificity of catechol oxidase.
Prediction Predict the results of the experiment based on your hypothesis (if/then).
Catechol Oxidase Specificity
- Using Table 3, prepare the three experimental tubes. Note that all tubes should contain the same total amount of solution. Do not cross-contaminate pipettes! After each tube is prepared, use your finger to hold a ParafilmTM^ square securely over the tube mouth and then rotate the tube to mix the contents thoroughly. Use a fresh square for each tube.
Table 3. Contents of three experimental tubes Tube Distilled Water Hydroquinone Potato Extract 1 5.5ml 0.5ml 2 5.5ml 0.5ml 3 5ml 0.5ml 0.5ml
- Observe the reactions in the tubes, and record your observations.
OH
OH
OH OH catechol
hydroquinone
BIO201 Enzymes I : Catechol Oxidase
Part 1: Action of Catechol Oxidase
- What is the purpose of each of the three test tubes? Which is the control tube? Is more than one control tube necessary? Explain. Which is the experimental tube?
- Design a simple table to record your results.
- Explain your results in terms of your hypothesis.
Part 2: Inhibition of Catechol Oxidase
- Why was the concentration of catechol increased in test tube 2?
- Why should the catechol be added to the test tube last?
