Cells perform their functions (growth, reproduction, specific metabolic activities, etc.) by utilizing various  types of chemical reactions.  These reactions must be carried out in an orderly fashion (when to begin, when to stop, rate of reaction, etc.) for a cell to function efficiently.  To perform these orderly processes cells regulate  chemical reactions through the use of enzymes.  Enzymes are biological (protein) catalysts.  Catalysts are molecules which speed up the rate of a reaction without itself binding used up or permanently altered. Enzymes couple reactants together to form products, driving energy requiring endergonic reactions with the energy release by exeronic reactions. Write in your notes definitions of endergonic reactions and exergonic reactions.  In this section you are going to learn how enzymes work. To understand this concept you need to focus on the following principles concerning enzymes. Write these in your notes. 

Principles: 

1. Enzymes are molecules with  a specific three dimensional shape. 
2. Enzymes are specific for which reaction it catalyzes. 
3. Enzymes may be used over and over. 
4. Changing the three dimensional shape of the enzyme will destroy enzyme 
    activity. 
 
 
  
 
 

Refer to the figure above  showing the cycle of enzyme-substrate interactions while reading the material explaining how an enzyme functions. First, you will need to learn the chemical makeup of enzymes. Enzymes consist of protein; therefore, you may want to review protein structure presented in miniunit Alpha. ( click here ) 

In addition to concepts concerning protein structure in Alpha  you need to learn the four levels of protein structure - primary, secondary, tertiary and quaternary. Study behavior objective 27  and read the pages indicated by B-22.  Write in your notes a description for the four levels of protein structure and draw figures representing each level ( use the following terms - amino acid sequence, helix, pleated sheet,  folded peptides, aggregation of two or more peptides). 
 

PROTEIN STRUCTURE                DESCRIPTION                          DRAWING 

primary
secondary
tertiary
quaternary

 

All proteins  have primary, secondary and tertiary structures. Quaternary level is not present in all proteins. These four levels of protein structure will produce a specific three-dimensional structure. (Principle 1: Enzymes are molecules with  a specific three dimensional shape.) The way an enzyme functions is that the enzyme becomes intimately linked to this three- dimensional structure. Note the three dimensional shape of the enzyme in the figure below. 
 
 

                          / 
Enzymatic reaction 

To understand principle 2 read the pages indicated by B-23 and study behavioral objectives 28, 29 and 30. 

Principle 2:  Enzymes are specific for which reaction it catalyzes. In the figure above only sucrose will fit into the active site. Maltose has a different shape and will not be acted on by the enzyme. 

In your notes draw a figure representing an enzyme showing its active site. The active site is that portion of an enzyme where the reaction occurs. The active site has a specific three-dimensional shape which will allow only specific molecules (substrates) enter. These substrate will then react with one another forming a product. In your notes draw and label a cycle of enzyme-substrate interactions (study figure in the above readings). Use the following molecules as an example: substrates = two glucose molecules, show enzyme-substrate complex, products = maltose and water, show active site. 
 

Principle 3:  Enzymes may be used over and over. Note that after sucrose has been hydrolyzed and the products glucose and fructose are released the active site has not changed. 
 

1. After  the products are formed they drop out of the active site has the active site been changed? 

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2. Can additional substrates enter the active site? 

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3. Can the enzyme be used over and over? 

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Principle 4: Changing the three dimensional shape of the enzyme will destroy enzyme activity. 

4. If the three dimensional shape of the an active site was changed, could the enzyme still catalyze a reaction? 

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5.  At this point the enzymatic reaction would stop. Why? 

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Read the pages indicated by B-24  and study behavioral objective 31 to learn how the activity of enzyme is influenced by their environment.  In your notes explain how the following conditions effect enzyme activity. 

6. Placing enzyme-catalyzed reactions in a warm environment. 

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7. Placing enzyme-catalyzed reactions in a cold  environment 

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8. Placing enzyme-catalyzed reactions in a freezing environment. 

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9. Placing enzyme-catalyzed reactions in a hot (boiling or above) environment. 

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At high temperatures protein becomes denatured. Denatured protein is where a protein's three- dimensional shape changes. Usually it is a permanent change.  Egg albumin is clear and slimy in a fresh egg and becomes white and solid after it has been heated to a high temperature (fried eggs). 

10.. Adding a poison (chemical which binds with the active site of an enzyme). 

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How does a cell regulate the amount of a product formed and the rate activity  by enzymes? 

This is accomplished several ways. 

1. The cell may regulate the synthesis of enzymes. This mechanism will be studied in miniunit Delta. 

2. Enzymes may be synthesized in an inactive form  and activated when needed. 

3. The cell has means of activating or inhibiting enzymes  depending on the need of the product formed. 

In your notes explain the following terms in relation to enzymes: feedback inhibition, allosteric regulation, competitive inhibition. These may be found in your readings. 

11. Which of the following types of inhibitors would stop an enzymatic reaction the quickest? 

 a) allosteric inhibition 
 b) competitive inhibition 
 c) feedback inhibition 

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You must answer the following question correctly to move to the next lesson. 

12. Which of the following statements is (are) true concerning enzymes? 

  a ) Enzymes react with specific reactants (substrates). 
  b ) Enzymes three dimensional shapes are closely related to their activities. 
  c ) Enzymes change the shape of the reactants. 
  d ) Enzymes have active sites. 
  e ) all of the above 
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