Chapter 15 Enzymes Chemistry 20. Enzyme E act - Like a catalyst, they increase the rate of reaction (biological reactions). - Lower the activation energy.
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Chapter 15
Enzymes
Chemistry 20
Enzyme
Eact
Eact
- Like a catalyst, they increase the rate of reaction (biological reactions).
- Lower the activation energy for the reaction.
2HIH2 + I2 H…H
I … I
… …
- Less energy is required to convert reactants to products.
- But, they are not changed at the end of the reaction.
- They are made of proteins.
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
Problems:
1. Why Enzyme-Substrate Complex is not stable?(no reason for the reaction to occur)
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
Maximum activity
Enzyme concentration ↑ Rate of reaction ↑
Substrate concentration ↑ First: Rate of reaction ↑
End: Rate of reaction reachesto its maximum: all of the enzymesare combined with substrates.
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.
- 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.
Inhibitor
Site
Competitive and Noncompetitive Inhibitor
Enzyme cofactors
protein
protein
protein
Metal ion
Organicmolecules
(coenzyme)
Simple enzyme (apoenzyme)
Enzyme + Cofactor
Enzyme + Cofactor (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
Water-soluble vitamins: have a polar group (-OH, -COOH, or …)
Vitamins are organic molecules that must be obtained from the diet.(our body cannot make them)
Fat-soluble vitamins: have a nonpolar group (alkyl, aromatic, or …)
- They are not stored in the body (must be taken).
- They can be easily destroyed by heat, oxygen, and ultraviolet light (need care).
- 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
Enzyme regulation:
1. Feedback control
2. Proenzymes
3. Allosterism
4. Protein Modification
5. Isoenzymes
Feedback control: reaction product of one enzyme controls the activity of another.
1. Feedback Control
A B C DE1 E2 E3
feedback inhibition
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)
Digestive enzyme (hydrolyzes the peptide bonds of proteins)
Why we do this process?
pH = 5 - 6
pH = 2
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 ADP
kinase
phosphatase
Pi
PK PKP
inactiveactive
H2O
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
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