ENZYMES AND VITAMINS

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CHAPTER 20

• ENZYMES AND VITAMINS

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A. Enzymes

• 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 bodys demands -- this is why we need
  enzymes!

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How Does An Enzyme Work?

• 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.

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Enzymes Lower Activation Energy, But Dont Change
Equilibrium Position

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Names and Classification of Enzymes

• The enzyme name often describes the reaction
  taking place, and the enzyme name always ends
  with the suffix -ase.
• 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 dont 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.

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B. Enzyme Action

• 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.

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Enzyme-Substrate Binding

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Models of Enzyme Action

• 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.
• 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.

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More Detail on Binding

• An interactive animation on enzyme specificity
  and binding
• http//www.wiley.com/legacy/college/boyer/04700037
  90/animations/enzyme_binding/enzyme_binding.swf

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C. Factors Affecting Enzyme Activity

• Enzyme activity is defined as how fast an enzyme
  catalyzes its reaction.
• Many factors affect enzyme activity
• 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?

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Reaction Rate vs. Enzyme and Substrate Conc.

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D. Enzyme Inhibition

• Inhibitors stop the catalytic activity of the
  enzyme.
• There are different methods of inhibition
• 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)

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How a Noncompetitive Inhibitor Works

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Thinking about Inhibition

• What kind of inhibitor competes with the
  substrate for the active site?
• Competitive
• In what kind of inhibition does the addition of
  more substrate reverse the inhibition?
• Reversible, competitive
• In what kind of inhibition is the structure of
  the inhibitor not similar to that of the
  substrate?
• Noncompetitive

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E. Control of Enzyme Activity

• We dont always need high levels of products of
  enzyme-catalyzed reactions around. What kind of
  control system is used to regulate amounts of
  enzyme and products?
• Two main methods zymogens, and feedback control.

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Zymogens

• Many enzymes are active as soon as theyre 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?

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Feedback Control

• Some enzymes (allosteric enzymes) bind molecules
  called regulators (different from the substrate)
  that can affect the enzyme either positively or
  negatively
• 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?

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Feedback Control

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F. Enzyme Cofactors and Vitamins

• Many enzymes require small molecules or metal
  ions called cofactors to catalyze reactions
  properly.
• 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)
• Classified as either water-soluble (contain polar
  groups) or fat-soluble (nonpolar compounds)

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Vitamins

• Water soluble vitamins not stored in the body,
  excess are eliminated
• 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
• Not coenzymes or cofactors but play various
  important roles in the body