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