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 Apoenzyme: Protein
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 Substrate binding Formation enzyme substrate complex
Production formation and dissociation
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