Enzymes
Ck12 Science
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Printed: July 8, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
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Chapter 1. Enzymes
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Enzymes
Define enzyme.
Define active site.
Describe the process of an enzyme reaction.
Describe the process by which a competitive inhibitor alters the rate of an enzyme reaction.
Describe the process by which a non-competitive inhibitor alters the rate of an enzyme reaction.
Explain the role of cofactors in enzyme reactions.
What did he discover?
The first enzyme to be isolated was discovered in 1926 by American chemist James Sumner, who crystallized the
protein. The enzyme was urease, which catalyzes the hydrolytic decomposition of urea, a component of urine, into
ammonia and carbon dioxide.
urease
H2 NCONH2 (aq) + H2 O(l) −−−→ 2NH3 (g) + CO2 (g)
His discovery was ridiculed at first because nobody believed that enzymes would behave the same way that other
chemicals did. Sumner was eventually proven right and won the Nobel Prize in Chemistry in 1946.
Enzymes
An enzyme is a protein that acts as a biological catalyst. Recall that a catalyst is a substance that increases the rate
of a chemical reaction without itself being consumed in the reaction. Cellular processes consist of many chemical
reactions that must occur quickly in order for the cell to function properly. Enzymes catalyze most of the chemical
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FIGURE 1.1
The sequence of steps for a substrate
binding to an enzyme in its active site, reacting, then being released as products.
reactions that occur in a cell. A substrate is the molecule or molecules on which the enzyme acts. In the urease
catalyzed reaction above, urea is the substrate. The Figure 1.1 diagrams a typical enzymatic reaction.
The first step in the reaction is that the substrate binds to a specific part of the enzyme molecule. The binding of the
substrate is dictated by the shape of each molecule. Side chains on the enzyme interact with the substrate in a specific
way, resulting in the making and breaking of bonds. The active site is the place on an enzyme where the substrate
binds. An enzyme folds in such a way that it typically has one active site, usually a pocket or crevice formed by
the folding pattern of the protein. Because the active site of an enzyme has such a unique shape, only one particular
substrate is capable of binding to that enzyme. In other words, each enzyme catalyzes only one chemical reaction
with only one substrate. Once the enzyme/substrate complex is formed, the reaction occurs and the substrate is
transformed into products. Finally, the product molecule or molecules are released from the active site. Note that the
enzyme is left unaffected by the reaction and is now capable of catalyzing the reaction of another substrate molecule.
Inhibitors
An inhibitor is a molecule which interferes with the function of an enzyme, either by slowing or stopping the
chemical reaction. Inhibitors can work in a variety of ways, but one of the most common is illustrated in the Figure
1.2.
FIGURE 1.2
A competitive inhibitor is a molecule that
binds to the active site of an enzyme without reacting, thus preventing the substrate
from binding.
The inhibitor binds competitively at the active site and blocks the substrate from binding. Since no reaction occurs
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Chapter 1. Enzymes
with the inhibitor, the enzyme is prevented from catalyzing the reaction. Cyanide is a potent poison which acts
as a competitive inhibitor. It binds to the active site of the enzyme cytochrome c oxidase and interrupts cellular
respiration. The binding of the cyanide to the enzyme is irreversible and the affected organism dies quickly.
Non-competitive Inhibition
A non-competitive inhibitor does not bind at the active site. It attaches at some other site on the enzyme and changes
the shape of the protein. This shift in three-dimensional structure alters the shape of the active site so that the
substrate will no longer fit in the site properly (see Figure 1.3).
FIGURE 1.3
Non-competitive inhibition
Cofactors
Some enzymes require the presence of a non-protein molecule called a cofactor in order to function properly.
Cofactors can be inorganic metal ions or small organic molecules. Many vitamins, such as B vitamins, act as
cofactors. Some metal ions which function as cofactors for various enzymes include zinc, magnesium, potassium,
and iron.
Summary
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Enzyme is defined.
An overview of the enzyme reaction process is illustrated.
Competitive and non-competitive inhibitors are described.
The role of a cofactor is described.
Practice
Watch the animation at the site below and take the quiz:
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
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Review
Questions
1. What is the substrate?
2. How does a competitive inhibitor work?
3. How does a non-competitive inhibitor work?
• active site: The place on an enzyme where the substrate binds.
• enzyme: A protein that acts as a biological catalyst.
• inhibitor: A molecule which interferes with the function of an enzyme, either slowing or stopping the
chemical reaction.
• substrate: The molecule or molecules on which the enzyme acts.
References
1. Nobel Foundation. http://commons.wikimedia.org/wiki/File:James_Batcheller_Sumner.jpg .
2. User:TimVickers/Wikimedia Commons and User:Fvasconcellos/Wikimedia Commons. http://commons.w
ikimedia.org/wiki/File:Induced_fit_diagram.svg .
3. Jerry Crimson Mann (Wikimedia: Mcy jerry), User:TimVickers/Wikimedia Commons, and User:Fvasconcellos/Wikimedia
Commons. http://commons.wikimedia.org/wiki/File:Competitive_inhibition.svg .
4. Jerry Crimson Mann (Wikimedia: Mcy jerry). http://commons.wikimedia.org/wiki/File:Allosteric_competitiv
e_inhibition_3.svg .
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