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Transcript
Chemistry 20
Chapter 15
Enzymes
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.
…
…
H2 + I2 
H…H
I …I
Eact
Eact
 2HI
- Less energy is required to convert reactants to products.
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)
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.
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.
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
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
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.
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.
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
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: 35°C or body temperature.
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.
Substrate and enzyme concentration
Enzyme concentration ↑
Substrate concentration ↑
Rate of reaction ↑
First: Rate of reaction ↑
End: Rate of reaction reaches
to its maximum: all of the enzymes
are combined with substrates.
Maximum activity
Enzyme inhibition
Inhibitors cause enzymes to lose catalytic activity.
Competitive inhibitor
Noncompetitive inhibitor
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.
Noncompetitive Inhibitor
- Inhibitor is not similar to the substrate.
Inhibitor
- 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.
Site
Competitive and Noncompetitive Inhibitor
Enzyme cofactors
Simple enzyme (apoenzyme)
protein
protein
protein
Metal ion
Organic
molecules
Enzyme + Cofactor
Enzyme + Cofactor (coenzyme)
(coenzyme)
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.
Enzyme cofactors
- Enzyme and cofactors work together.
- Catalyze reactions properly.
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.
Enzyme Regulation
1. Feedback control
2. Proenzymes
Enzyme regulation:
3. Allosterism
4. Protein Modification
5. Isoenzymes
1. Feedback Control
Feedback control: reaction product of one enzyme controls the
activity of another.
feedback inhibition
A
E1
B
E2
C
E3
D
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?
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
4. Protein Modification
Usually a change in the primary structure.
(addition of a functional group by covalent bond to the apoenzyme)
ATP
active
AD P
inactive
kinase
PK
PKP
phosphatase
Pi
H2 O
pyruvate kinase (PK) is the active form of the enzyme;
it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP).
5. Isoenzymes
Enzymes that have different forms;
but they catalyze the same reaction.
Different activities
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