Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

29. Amino Acids

Acid-Base Properties of Amino Acids

Organic Chemistry

29. Amino Acids

Acid-Base Properties of Amino Acids

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Problem 21b

Textbook Question

Draw the pH–activity profile for an enzyme that has one catalytic group at the active site:
b. the catalytic group is a general-base catalyst with a pKa = 7.2.

Verified step by step guidance

Understand the concept of a pH–activity profile: This graph shows how the activity of an enzyme changes with pH. The activity is influenced by the ionization state of the catalytic group at the active site, which depends on its pKa value.

Identify the catalytic group and its role: The catalytic group in this case is a general-base catalyst. A general-base catalyst works by accepting a proton during the reaction, and its activity depends on whether it is in the deprotonated (active) or protonated (inactive) state.

Relate the pKa to the ionization state: The pKa of the catalytic group is 7.2. At pH values below the pKa, the group will be predominantly protonated (inactive). At pH values above the pKa, the group will be predominantly deprotonated (active). The enzyme activity will increase as the pH approaches and surpasses the pKa.

Sketch the pH–activity profile: The profile will show low activity at acidic pH (below 7.2), a peak in activity near the pKa (around pH 7.2), and a gradual decrease in activity at higher pH values due to potential denaturation or loss of enzyme structure.

Label the graph: On the x-axis, label the pH range (e.g., 0–14). On the y-axis, label the enzyme activity. Mark the pKa value (7.2) on the graph and indicate the region where the enzyme is most active (near pH 7.2).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH-Activity Profile

The pH-activity profile of an enzyme illustrates how the enzyme's activity varies with changes in pH. It typically shows a bell-shaped curve, indicating optimal activity at a specific pH range. This profile is crucial for understanding enzyme function, as it reflects the ionization states of amino acid residues in the active site that are essential for substrate binding and catalysis.

General-Base Catalysis

General-base catalysis involves the transfer of protons (H+) from a substrate to a base, which is often an amino acid side chain in the enzyme's active site. This process enhances the reaction rate by stabilizing the transition state. The effectiveness of a general-base catalyst is influenced by the pKa of the catalytic group, which determines its protonation state at a given pH.

pKa and Ionization

The pKa value of a functional group indicates the pH at which half of the molecules of that group are deprotonated. For a general-base catalyst with a pKa of 7.2, it will be predominantly deprotonated (active) at pH values above 7.2, allowing it to effectively accept protons from the substrate. Understanding the relationship between pH and pKa is essential for predicting enzyme activity and the shape of the pH-activity profile.

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