Chemistry 20 Chapter 15 Enzymes

Enzyme H…H H2 + I2   2HI … I … I - Like a catalyst, they increase the rate of reaction (biological reactions). - But, they are not changed at the end of the reaction. Eact - They are made of proteins. - Lower the activation energy for the reaction.  2HI H2 + I2  H…H I … I … - Less energy is required to convert reactants to products.

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 with the suffix « -ase ». 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) 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 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. 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. 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 Metal ions: bond to side chains. (coenzyme) protein Metal ion Organic molecules (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 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 Feedback control Proenzymes Allosterism Protein Modification Isoenzymes Enzyme regulation:

1. Feedback Control Feedback control: reaction product of one enzyme controls the activity of another.

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 Negative modulation: inhibits enzyme action 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

Usually a change in the primary structure. 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).

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

- Most of enzymes are in cells. 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