CHAPTER 20
ENZYMES AND VITAMINS
A. Enzymes
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Are biological catalysts
Catalyze nearly all of the chemical reactions that take place in
the body
Enzymes increase the rate of a reaction, but are unchanged
themselves at the end of the reaction
An uncatalyzed reaction might eventually take place, but not at
a rate quickly enough to meet the body’s demands -- this is
why we need enzymes!
How Does An Enzyme Work?
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Lowers the activation energy for a reaction.
As a result, less energy is needed to convert reactants to
products. This allows more molecules to form product.
The enzyme does not affect the equilibrium position of the
reaction.
Enzymes Lower Activation Energy, But
Don’t Change Equilibrium Position
Names and Classification of Enzymes
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The enzyme name often describes the reaction taking place,
and the enzyme name always ends with the suffix -ase.
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Examples: oxidase catalyzes oxidation
lipid is hydrolyzed by lipase
What type of reaction would you think is catalyzed by a
hydrolase? An isomerase? An oxidoreductase?
While I don’t need you to memorize the classes and
subclasses in table 20.1, I could ask you to tell me the type of
reaction catalyzed by some of the more obvious classes on the
list.
B. Enzyme Action
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Each enzyme has a unique three-dimensional shape that binds
and recognizes a group of reacting molecules called substrates.
The active site of the enzyme is a small pocket to which the
substrate directly binds.
Some enzymes are specific only to one substrate; others can
bind more than one substrate.
Enzyme-Substrate Binding
Models of Enzyme Action
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Early theory: lock-and-key model. Active site (lock) had the
same shape as the substrate (key). Only the right shape key
could bind.
Current theory: induced fit model. Active site closely
resembles but does not exactly bind the substrate.
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Allows for more flexibility in type of substrate
Also explains how the reaction itself occurs. As the substrate flexes to
fit the active site, bonds in the substrate are flexed and stressed -- this
causes changes/conversion to product.
More Detail on Binding
An interactive animation on enzyme specificity and binding:
http://www.wiley.com/legacy/college/boyer/0470003790/animatio
ns/enzyme_binding/enzyme_binding.swf
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C. Factors Affecting Enzyme Activity
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Enzyme activity is defined as how fast an enzyme catalyzes its
reaction.
Many factors affect enzyme activity:
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Temperature: most have an optimum temp around 37oC
pH: most cellular enzymes are optimal around physiological pH, but
enzymes in the stomach have a lower optimum pH
Concentration of enzyme and substrate: have all of the enzyme
molecules been used up, even though substrate is still available?
Reaction Rate vs. Enzyme and Substrate
Conc.
D. Enzyme Inhibition
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Inhibitors stop the catalytic activity of the enzyme.
There are different methods of inhibition:
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Reversible: the inhibitor can be removed
Competitive inhibitors bind to the active site
Noncompetitive inhibitors bind somewhere other than the active site
and change the conformation of the active site
Irreversible: the inhibitor cannot be removed
Examples: toxins that form a permanent bond to the enzyme, antibiotics
(prevent bacterial cell wall formation)
How a Noncompetitive Inhibitor Works
Thinking about Inhibition…
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What kind of inhibitor competes with the substrate for the
active site?
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In what kind of inhibition does the addition of more substrate
reverse the inhibition?
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Competitive
Reversible, competitive
In what kind of inhibition is the structure of the inhibitor not
similar to that of the substrate?
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Noncompetitive
E. Control of Enzyme Activity
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We don’t always need high levels of products of enzymecatalyzed reactions around. What kind of control system is
used to regulate amounts of enzyme and products?
Two main methods: zymogens, and feedback control.
Zymogens
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Many enzymes are active as soon as they’re made.
However, some are made in an inactive form and stored. This
inactive form is called a zymogen or proenzyme.
To become active, the body needs only to cleave off a small
peptide fragment.
Many digestive enzymes are produced initially as zymogens…
why?
Feedback Control
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Some enzymes (allosteric enzymes) bind molecules called
regulators (different from the substrate) that can affect the
enzyme either positively or negatively
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Positive regulator: speeds up the reaction by changing the shape of the
active site -- substrate binds more effectively
Negative regulator: slows down reaction by preventing proper substrate
binding, again, by changing enzyme shape
Feedback control: the end product acts as a negative regulator.
If there is enough of the end product, it will slow down the
first enzyme in a pathway. Why does it slow down the first, and
not the third, or fourth?
Feedback Control
F. Enzyme Cofactors and Vitamins
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Many enzymes require small molecules or metal ions called
cofactors to catalyze reactions properly.
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Some metal ions (such as Fe2+ and Cu2+) participate in redox reactions
with oxidases
Other metal ions stabilize either the enzyme or substrate over the
course of the reaction
Vitamins: molecules essential for normal health that must be
obtained from the diet (body does not synthesize)
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Classified as either water-soluble (contain polar groups) or fat-soluble
(nonpolar compounds)
Vitamins
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Water soluble vitamins: not stored in the body, excess are
eliminated
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Many are enzyme cofactors (B vitamins, vitamin C)
Fat soluble vitamins: stored in the body and not eliminated -can be toxic if you take too much
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Not coenzymes or cofactors but play various important roles in the
body