Enzymes

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Chemistry 20
Chapter 15 Enzymes
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Enzyme
- Like a catalyst, they increase the rate of
reaction (biological reactions).
- But, they are not changed at the end of the
reaction.
- They are made of proteins.
- Lower the activation energy for the reaction.
- Less energy is required to convert reactants to
products.
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Enzyme

• Most of enzymes are globular proteins.
• Proteins are not the only biological catalysts.
• Most of enzymes are specific.
• (Trypsin cleaves the peptide bonds of
  proteins)
• Some enzymes are localized according to need.
• (digestive enzymes stomach)

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Names of Enzymes

• By replacing the end of the name of reaction or
  reacting compound
• with the suffix -ase .

Oxidoreductases oxidation-reduction reactions
(oxidase-reductase). Transferases transfer a
group between two compounds. Hydrolases
hydrolysis reactions. Lyases add or remove
groups involving a double bond without
hydrolysis. Isomerases rearrange atoms in a
molecule to form a isomer. Ligases form bonds
between molecules.
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Enzyme

• - Substrate the compound or compounds whose
  reaction an enzyme catalyzes.
• - Active site the specific portion of the enzyme
  to which a substrate binds during reaction.

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Enzyme catalyzed reaction

• An enzyme catalyzes a reaction by,
• Attaching to a substrate at the active site (by
  side chain (R) attractions).
• Forming an enzyme-substrate
• (ES) complex.
• Forming and releasing products.
• E S ES E P

Enzyme globular protein
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Lock-and-Key model

• Enzyme has a rigid, nonflexible shape.
• An enzyme binds only substrates that
• exactly fit the active site.
• The enzyme is analogous to a lock.
• - The substrate is the key that fits into the lock

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Induced-Fit model
1. Why Enzyme-Substrate Complex is not
stable? (no reason for the reaction to occur)
Problems
2. X-ray diffraction size and shape of the
actice site chanegs when a substrate enters.
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Induced-Fit model

• - Enzyme structure is flexible, not rigid.
• - Enzyme and substrate adjust the shape of the
  active site to bind substrate.
• - The range of substrate specificity increases.
• - A different substrate could not induce these
  structural changes and no catalysis would occur.

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Factors affecting enzyme activity
Activity of enzyme how fast an enzyme catalyzes
the reaction.
1. Temperature
2. pH
3. Substrate concentration
4. enzyme concentration
5. Enzyme inhibition
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Temperature

• Enzymes are very sensitive to temperature.
• At low T, enzyme shows little activity (not an
  enough amount of energy for
• the catalyzed
  reaction).
• - At very high T, enzyme is destroyed (tertiary
  structure is denatured).
• - Optimum temperature 35C or body temperature.

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pH

• Optimum pH is 7.4 in our body.
• Lower or higher pH can change the shape of
  enzyme.
• (active site change and substrate cannot fit
  in it)
• But optimum pH in stomach is 2.
• Stomach enzyme (Pepsin) needs an acidic pH to
  digest the food.
• - Some damages of enzyme are reversible.

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Substrate and enzyme concentration
Enzyme concentration ?
Rate of reaction ?
Substrate concentration ?
First Rate of reaction ?
End Rate of reaction reaches to its maximum all
of the enzymes are combined with substrates.
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Enzyme inhibition
Inhibitors cause enzymes to lose catalytic
activity.
Competitive inhibitor
Noncompetitive inhibitor
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Competitive Inhibitor

• Inhibitor has a structure that is so similar to
  the substrate.
• It competes for the active site on the enzyme.
• Solution increasing the substrate concentration.

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Noncompetitive Inhibitor

• Inhibitor is not similar to the substrate.
• It does not compete for the active site.
• When it is bonded to enzyme, change the shape
• of enzyme (active site) and substrate cannot
  fit in
• the active site (change tertiary structure).
• Like heavy metal ions (Pb2, Ag, or Hg2) that
• bond with COO-, or OH groups of amino acid
• in an enzyme.
• Penicillin inhibits an enzyme needed for
  formation
• of cell walls in bacteria infection is
  stopped.
• Solution some chemical reagent can remove the
• inhibitors.

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Competitive and Noncompetitive Inhibitor
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Enzyme cofactors
Metal ions bond to side chains.
obtain from foods. Fe2 and Cu2 are
gain or loss electrons in redox reactions.
Zn2 stabilize amino acid side chain during
reactions.
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Enzyme cofactors

• Enzyme and cofactors work together.
• Catalyze reactions properly.

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Vitamins and Coenzymes
Vitamins are organic molecules that must be
obtained from the diet. (our body cannot make
them)
Water-soluble vitamins have a polar group (-OH,
-COOH, or )
- They are not stored in the body (must be
taken). - They can be easily destroyed by heat,
oxygen, and ultraviolet light (need care).
Fat-soluble vitamins have a nonpolar group
(alkyl, aromatic, or )

• - They are stored in the body (taking too much
  toxic).
• A, D, E, and K are not coenzymes, but they are
  important
• vision, formation of bone, proper blood
  clotting.

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Enzyme Regulation

1. Feedback control
2. Proenzymes
3. Allosterism
4. Protein Modification
5. Isoenzymes

Enzyme regulation
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1. Feedback Control

• Feedback control reaction product of one
  enzyme controls the activity of another.

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2. Proenzymes (Zymogens)
Proenzyme (zymogen) an inactive enzyme that
becomes an active enzyme after a chemical change
(remove or change some polypeptides).
Trypsinogen (inactive enzyme) Trypsin (active
enzyme)
pH 5 - 6
pH 2
Digestive enzyme (hydrolyzes the peptide bonds of
proteins)
Why we do this process?
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3. Allosterism
Regulation takes place by means of an event that
occurs at the site other than the active site
but affects the active site.
Allosteric enzyme
Negative modulation inhibits enzyme
action Positive modulation stimulates enzyme
action
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4. Protein Modification
Usually a change in the primary
structure. (addition of a functional group by
covalent bond to the apoenzyme)

• pyruvate kinase (PK) is the active form of the
  enzyme
• it is inactivated by phosphorylation to pyruvate
  kinase phosphate (PKP).

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5. Isoenzymes
Enzymes that have different forms but they
catalyze the same reaction.
Different activities
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Enzymes in medicine

• - Most of enzymes are in cells.
• Small amounts of them are in body fluids (blood,
  urine,).

Level of enzyme activity can be monitored.
Find some diseases