Biological Catalysts 21.2 Names and Classification of Enzymes 21.3 Enzymes as Catalysts 21.4 Factors Affecting Enzyme Activity Chapter 21 Enzymes and Vitamins

2 Enzymes are Biological Catalysts Enzymes are proteins that: Increase the rate of reaction by lowering the energy of activation. Catalyze nearly all the chemical reactions taking place in the cells of the body. Have unique three- dimensional shapes that fit the shapes of reactants.

3 The name of an enzyme: Usually ends in –ase. Identifies the reacting substance. For example, sucrase catalyzes the reaction of sucrose. Describes the function of the enzyme. For example, oxidases catalyze oxidation. Could be a common name, particularly for the digestion enzymes such as pepsin and trypsin. Names of Enzymes

4 Enzymes are classified according to the reaction they catalyze. ClassReactions catalyzed OxidoreductasesOxidation-reduction TransferasesTransfer groups of atoms Hydrolases Hydrolysis LyasesAdd atoms/remove atoms to/from a double bond IsomerasesRearrange atoms LigasesUse ATP to combine molecules Classification of Enzymes

5 Classification of Enzymes: Oxidoreductases and Transferases

6 Classification: Hydrolases and Lyases

7 Classification: Isomerases and Ligases

8 The active site: Is a region within an enzyme that fits the shape of molecules called substrates. Contains amino acid R groups that align and bind the substrate. Releases products when the reaction is complete. Active Site

9 Enzymes may recognize and catalyze: A single substrate. A group of similar substrates. A particular type of bond. Enzyme Specificity

10 Lock-and-Key Model In the lock-and-key model of enzyme action: The active site has a rigid shape. Only substrates with the matching shape can fit. The substrate is a key that fits the lock of the active site.

11 Induced-fit Model In the induced-fit model of enzyme action: The active site is flexible, not rigid. The shapes of the enzyme, active site, and substrate adjust to maximum the fit, which improves catalysis. There is a greater range of substrate specificity.

12 Enzyme Catalyzed Reaction The proper fit of a substrate (S) in an active site forms an enzyme-substrate (ES) complex. E + S ES Within the ES complex, the reaction occurs to convert substrate to product (P). ES E + P The products, which are no longer attracted to the active site, are released. Overall, substrate is convert to product. E + S ES E + P

13 Example of An Enzyme Catalyzed Reaction

14 Diagnostic Enzymes The levels of diagnostic enzymes determine the amount of damage in tissues.

15 Isoenzymes Isoenzymes catalyze the same reaction in different tissues in the body. Lactate dehydrogenase, which converts lactate to pyruvate, (LDH) consists of five isoenzymes.

16 Enzymes: Are most active at an optimum temperature (usually 37°C in humans). Show little activity at low temperatures. Lose activity at high temperatures as denaturation occurs. Temperature and Enzyme Action

17 Enzymes: Are most active at optimum pH. Contain R groups of amino acids with proper charges at optimum pH. Lose activity in low or high pH as tertiary structure is disrupted. pH and Enzyme Action

18 Optimum pH Values Most enzymes of the body have an optimum pH of about 7.4. In certain organs, enzymes operate at lower and higher optimum pH values.

19 Enzyme Concentration The rate of reaction increases as enzyme concentration increases (at constant substrate concentration). At higher enzyme concentrations, more substrate binds with enzyme.

20 Substrate Concentration The rate of reaction increases as substrate concentration increases (at constant enzyme concentration). Maximum activity occurs when the enzyme is saturated.