Chapter 15Enzymes.ppt

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

2、roteins.,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 na

3、me 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: rear

4、range 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 catalyz

5、es 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 e

6、xactly 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 subs

7、trate 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.,Factor

8、s 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 o

9、f energy for the catalyzed reaction). - At very high T, enzyme is destroyed (tertiary structure is denatured).- Optimum temperature: 35C 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 opt

10、imum 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 ,Rate of reaction ,Substrate concentration ,First: Rate of reaction ,End: Rate of reaction reaches to its maximum: al

11、l of the enzymes are 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.Solut

12、ion: 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 shapeof enzyme (active site) and substrate cannot fit inthe active site (change tertiary structure).Like hea

13、vy metal ions (Pb2+, Ag+, or Hg2+) thatbond with COO-, or OH groups of amino acidin an enzyme.Penicillin inhibits an enzyme needed for formationof cell walls in bacteria: infection is stopped.Solution: some chemical reagent can remove theinhibitors.,Competitive and Noncompetitive Inhibitor,Enzyme co

14、factors,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:

15、 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,an

16、d 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:,Feedback controlProenzymesAllosterismProtein ModificationIsoenzymes

17、Feedback control: reaction product of one enzyme controls the activity of another.,1. Feedback Control,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 (a

18、ctive 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: inhibi

19、ts enzyme actionPositive 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),pyruvate kinase (PK) is the active form of the enzyme;it is inactivated by phosphorylation to pyruvate k

20、inase 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,