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 22

Textbook Question

The pH–activity profile for glucose-6-phosphate isomerase indicates the participation of a group with a pK_a = 6.7 as a basic catalyst and a group with a pK_a = 9.3 as an acid catalyst. Draw the pH–activity profile and identify the amino acids that participate in the catalysis.

Verified step by step guidance

Understand the pH–activity profile: The pH–activity profile represents the enzyme activity as a function of pH. It helps identify the pKa values of functional groups involved in catalysis. Here, the enzyme has two key groups with pKa values of 6.7 and 9.3, indicating their roles as a basic and acidic catalyst, respectively.

Interpret the pKa values: A pKa of 6.7 suggests a group that acts as a base at pH values above 6.7, while a pKa of 9.3 suggests a group that acts as an acid at pH values below 9.3. These values correspond to the ionization states of amino acid side chains in the active site of the enzyme.

Identify potential amino acids: Amino acids with side chains that match these pKa values are likely involved in catalysis. For pKa = 6.7, histidine (imidazole group) is a strong candidate due to its pKa being close to this value. For pKa = 9.3, lysine (ε-amino group) or tyrosine (phenol group) could be involved, as their pKa values are in this range.

Draw the pH–activity profile: The profile will show enzyme activity peaking at a pH range where both groups are optimally ionized for catalysis. At pH values below 6.7, the basic group is protonated and inactive. At pH values above 9.3, the acidic group is deprotonated and inactive. Between these pH values, the enzyme exhibits maximum activity.

Summarize the catalytic mechanism: The basic group (likely histidine) facilitates proton abstraction, while the acidic group (likely lysine or tyrosine) donates a proton during the reaction. These groups work together to stabilize the transition state and drive the isomerization of glucose-6-phosphate.

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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 illustrates how the activity of an enzyme varies with changes in pH. It typically shows peaks at optimal pH levels where the enzyme is most active, and declines at extreme pH values. Understanding this profile is crucial for identifying the ionization states of amino acid side chains that influence enzyme activity.

pKa and Ionization

pKa is the measure of the strength of an acid in solution, indicating the pH at which half of the species are deprotonated. In enzymatic reactions, amino acids with side chains that have pKa values near the physiological pH can act as proton donors or acceptors, thus playing critical roles in catalysis. The pKa values provided (6.7 and 9.3) suggest specific amino acids are involved in acid-base catalysis.

Amino Acid Catalysis

Certain amino acids in enzymes can act as acid or base catalysts during biochemical reactions. For instance, histidine often participates in catalysis due to its imidazole side chain, which can be protonated or deprotonated depending on the pH. Identifying these amino acids in the context of the given pKa values is essential for understanding the mechanism of glucose-6-phosphate isomerase.

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