1. Enzymes
 Regulatory enzymes are usually the enzymes that are
the rate-limiting, or committed step, in a pathway,
meaning that after this step a particular reaction
pathway will go to completion
 There are five primary forms of enzyme regulation:
substrate availability, allosteric, post-translational
modification, interaction with control proteins

2. Properties of Enzymes
 In general, chemical reactions that release energy can
occur without input of energy
 The oxidation of glucose releases energy, but the
reaction does not occur without an input of energy
 Activation energy: the energy required to start such a
reaction
 Enzymes lower the activation energy so reactions can
occur at mild temperatures in living cells

3. Enzymes
 Provide a surface on which reactions take place
 Active site: the area on the enzyme surface where the
enzyme forms a loose association with the substrate
 Substrate: the substance on which the enzyme acts
 Enzyme-substrate complex: formed when the
substrate molecule collides with the active site of its
enzyme
 Enzymes generally have a high degree of specificity
 Endoenzymes (intracellular)/exoenzymes (extracellular)

4. Energy Requirements of a Chemical
Reaction
Figure 5.2

5. Enzyme Components
 Biological catalysts
 Specific for a chemical reaction; not used up in that
reaction
 Apoenzyme: Protein
 Cofactor: Nonprotein component
 Coenzyme: Organic cofactor
 Holoenzyme: Apoenzyme plus cofactor

6. The Parts of an Enzyme

7. Properties of Coenzymes and Cofactors
 Many enzymes can catalyze a reaction only if substances
called coenzymes, or cofactors are present
 Apoenzyme: protein portion of such enzymes
 Holoenzyme: nonprotein coenzyme or cofactor that is
active when combined with apoenzyme
 Coenzyme: nonprotein organic molecule bound to or
loosely associated with an enzyme
 Cofactor: an inorganic ion (e.g. magnesium, zinc) that
often improve the fit of an enzyme with its substrate

8. Components of a Holoenzyme
Figure 5.3

9. Important Coenzymes
 NAD+
 NADP+
 FAD
 Coenzyme A

10. mechanism
1. Substrate binding
2. Formation enzyme substrate complex
3. Production formation and dissociation
4. Enzyme recovery

11. The Mechanism of Enzymatic
Action
Figure 5.4a

12. Each substrate binds to an active site, producing
an enzyme-substrate complex. The enzyme helps
a chemical reaction occur, and one or more
products are formed

13. Enzyme Classification
 Oxidoreductase: Oxidation-reduction reactions
 Transferase: Transfer functional groups
 Hydrolase: Hydrolysis
 Lyase: Removal of atoms without hydrolysis
 Isomerase: Rearrangement of atoms
 Ligase: Joining of molecules, uses ATP

14. Factors Influencing Enzyme Activity
 Temperature
 pH
 Substrate concentration
 Inhibitors

15. Temperature and pH
 Enzymes are affected by heat and extremes of pH
 Even small pH changes can alter the electrical charges
on various chemical groups in enzyme molecules,
thereby altering the enzyme’s ability to bind its
substrate and catalyze a reaction
 Most enzymes have an optimum temperature, near
normal body temperature, and an optimum pH, near
neutral, at which they catalyze a reaction most rapidly
 The rate at which an enzyme catalyzes a reaction
increases with temperature up to the optimum T

16. Effect of Temperature on Enzyme
Activity
Figure 5.5a

17. Effect of pH on Enzyme Activity
Figure 5.5b

18. Effect of Substrate Concentration
on Enzyme Activity
Figure 5.5c

19. Enzyme Inhibition
 Competitive inhibitor: A molecule similar in
structure to a substrate can bind to an enzyme’s active
site and compete with substrate
 Noncompetitive inhibitors: attach to the enzyme at
an allosteric site, which is a site other than the active site
 noncompetitive inhibitors: distort the tertiary
protein structure and alter the shape of the active site
 Feedback inhibition: regulates the rate of many
metabolic pathways when an end product of a pathway
accumulates and binds to and inactivates the first
enzyme in the metabolic pathway

20. Enzyme Inhibitors: Competitive
Inhibition
Figure 5.7a–b

21. Competitive inhibition of enzymes

22. Allosteric regulation of enzyme
activity
 Allosteric regulation = the activation or inhibition
of an enzyme’s activity due to binding of an
effectors molecule at a regulatory site that is
distinct from the active site of the enzyme
 Allosteric regulators generally act by increasing or
decreasing the enzyme’s affinity for the
substrate

23. Enzyme Inhibitors: Noncompetitive
Inhibition
Figure 5.7a, c

24. Noncompetitive (allosteric) inhibition of
enzymes

25. modification enzymes
Can either activate it or inhibit it by altering the
conformation of the enzyme or by serving as a
functional group in the active site

26. denaturation

27. denaturation

28. Enzyme Inhibitors: Feedback
Inhibition
Figure 5.8