1. 22 22-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March

2. 22 22-2 © 2003 Thomson Learning, Inc. All rights reserved Chapter 22 Enzymes

3. 22 22-3 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Catalysis Enzyme: Enzyme: a biological catalyst with the exception of some RNAs that catalyze their own splicing, all enzymes are proteins enzymes can increase the rate of a reaction by a factor of up to 10 20 over an uncatalyzed reaction some catalyze the reaction of only one compound others are stereospecific; for example, enzymes that catalyze the reactions of only L-amino acids others catalyze reactions of specific types of compounds or bonds; for example, trypsin that catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg

4. 22 22-4 © 2003 Thomson Learning, Inc. All rights reserved Classification of Enzymes Enzymes are commonly named after the reaction or reactions they catalyze example: lactate dehydrogenase, acid phosphatase Enzymes are classified into six major groups oxidoreductases:oxidoreductases: oxidation-reduction reactions transferases:transferases: group transfer reactions hydrolases:hydrolases: hydrolysis reactions lyases:lyases: addition of groups to a double bond, or removal of groups to create a double bond isomerases:isomerases: isomerization reactions ligases:ligases: the joining to two molecules

5. 22 22-5 © 2003 Thomson Learning, Inc. All rights reserved Terms in Enzyme Chem apoenzyme:apoenzyme: the protein part of an enzyme cofactor:cofactor: a nonprotein portion of an enzyme that is involved in a chemical reaction; examples are metallic ions such as Zn 2+ and Mg 2+ coenzyme:coenzyme: an organic cofactor; an example is heme substrate:substrate: the compound or compounds whose reaction an enzyme catalyzes active site:active site: the specific portion of the enzyme to which a substrate binds during reaction

6. 22 22-6 © 2003 Thomson Learning, Inc. All rights reserved Schematic of an Active Site

7. 22 22-7 © 2003 Thomson Learning, Inc. All rights reserved Terms in Enzyme Chem activation:activation: any process that initiates or increases the activity of an enzyme inhibition:inhibition: any process that makes an enzyme less active or inactive competitive inhibitor:competitive inhibitor: an substance that binds to the active site of an enzyme thus preventing binding of substrate noncompetitive inhibitor:noncompetitive inhibitor: any substance that binds to a portion of the enzyme other than the active site and thus inhibits the activity of the enzyme

8. 22 22-8 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Activity Enzyme activity: Enzyme activity: a measure of how much a reaction rate is increased We examine how the rate of an enzyme-catalyzed reaction is effected by enzyme concentration substrate concentration temperature pH

9. 22 22-9 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Activity Enzyme concentration

10. 22 22-10 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Activity Substrate concentration

11. 22 22-11 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Activity Temperature

12. 22 22-12 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Activity pH

13. 22 22-13 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action Lock-and-key model the enzyme is a rigid three-dimensional body the enzyme surface contains the active site

14. 22 22-14 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action Induced-fit model the active site becomes modified to accommodate the substrate

15. 22 22-15 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action the induced-fit model explains competitive inhibition the inhibitor fits into the active site, preventing the substrate from entering

16. 22 22-16 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action both the lock-and-key model and the induced-fit model emphasize the shape of the active site however, the chemistry of the active site is the most important just five amino acids participate in the active sites in more than 65% of the enzymes studies to date these five are His > Cys > Asp > Arg > Glu four these amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH)

17. 22 22-17 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action the mechanism of noncompetitive inhibition

18. 22 22-18 © 2003 Thomson Learning, Inc. All rights reserved Mechanism of Action we can distinguish between competitive and noncompetitive inhibition by the enzyme kinetics in the absence and presence of the inhibitor

19. 22 22-19 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Regulation Feedback control: Feedback control: an enzyme-regulation process where the product of a series of enzyme- catalyzed reactions inhibits an earlier reaction in a sequence the inhibition may be competitive or noncompetitive

20. 22 22-20 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Regulation Proenzyme (zymogen): Proenzyme (zymogen): an inactive form of an enzyme that must have part of its polypeptide chain cleaved before it becomes active an example is trypsin, a digestive enzyme it is synthesized and stored as trypsinogen, which has no enzyme activity it becomes active only after a six-amino acid fragment is hydrolyzed from the N-terminal end of its chain removal of this small fragment changes in not only the primary structure but also the tertiary structure, allowing the molecule to achieve its active form

21. 22 22-21 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Regulation Allosterism: Allosterism: a type of enzyme regulation based on an event occurring on the enzyme at a place other than the active site but that creates a change in the active site allosteric enzymean enzyme regulated by this mechanism is called an allosteric enzyme allosteric enzymes often have multiple polypeptide chains negative modulation:negative modulation: inhibition of an allosteric enzyme positive modulation:positive modulation: stimulation of an allosteric enzyme regulator:regulator: a substance that binds to an allosteric enzyme

22. 22 22-22 © 2003 Thomson Learning, Inc. All rights reserved The Allosteric Effect

23. 22 22-23 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Regulation Protein modification: Protein modification: the process of affecting enzyme activity by covalently modifying it the best known examples of protein modification involve phosphorylation/dephosphorylation example: pyruvate kinase (PK) is the active form of the enzyme; it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP)

24. 22 22-24 © 2003 Thomson Learning, Inc. All rights reserved Enzyme Regulation Isoenzyme: Isoenzyme: an enzyme that occurs in multiple forms; each catalyzes the same reaction example: lactate dehydrogenase (LDH) catalyzes the oxidation of lactate to pyruvate the enzyme is a tetramer of H and M chains H 4 is present predominately in heart muscle M 4 is present predominantly in the liver and in skeletal muscle H 3 M, H 2 M 2, and HM 3 also exist H 4 is allosterically inhibited by high levels of pyruvate while M 4 is not H4 in serum correlates with the severity of heart attack

25. 22 22-25 © 2003 Thomson Learning, Inc. All rights reserved Enzymes in Medicine Enzyme assays useful in medical diagnosis

26. 22 22-26 © 2003 Thomson Learning, Inc. All rights reserved End Chapter 22 Enzymes