Table of contents

1. Introduction to Biochemistry4h 34m
2. Water3h 23m
3. Amino Acids8h 10m
4. Protein Structure10h 4m
5. Protein Techniques14h 5m
6. Enzymes and Enzyme Kinetics13h 38m
7. Enzyme Inhibition and Regulation 8h 42m
8. Protein Function 9h 41m
9. Carbohydrates7h 49m
10. Lipids5h 49m
11. Biological Membranes and Transport 6h 37m
12. Biosignaling9h 45m
Review 1: Nucleic Acids, Lipids, & Membranes2h 47m
Review 2: Biosignaling, Glycolysis, Gluconeogenesis, & PP-Pathway3h 12m
Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism2h 26m
Review 4: Amino Acid Oxidation, Oxidative Phosphorylation, & Photophosphorylation1h 48m

2. Water

Acid Dissociation Constant

Biochemistry

2. Water

Acid Dissociation Constant

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Multiple Choice

Which statement correctly explains the protonation states of histidine residues at physiological pH (approximately 7.4) based on their acid dissociation constant ($pK_a \approx 6.0$)?

All histidine residues are fully deprotonated at physiological pH.

Most histidine residues are partially protonated, existing in equilibrium between protonated and deprotonated forms.

All histidine residues are fully protonated at physiological pH.

Histidine residues cannot act as proton donors or acceptors at physiological pH.

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Verified step by step guidance

Understand the concept of acid dissociation constant ($pK_a$): The $pK_a$ value of a functional group indicates the pH at which half of the molecules are protonated and half are deprotonated. For histidine, the $pK_a$ of its imidazole side chain is approximately 6.0.

Compare the $pK_a$ of histidine to the physiological pH: At physiological pH (7.4), the pH is higher than the $pK_a$ of histidine. This means that the environment favors the deprotonated form of histidine, but some molecules will still remain protonated due to the equilibrium.

Apply the Henderson-Hasselbalch equation: The equation $pH = pK_a + ext{log} rac{[ ext{A}^-]}{[ ext{HA}]}$ can be used to determine the ratio of protonated (HA) to deprotonated (A⁻) forms. At pH 7.4 and $pK_a$ 6.0, the ratio indicates that histidine exists in both forms, with a higher proportion in the deprotonated state.

Interpret the equilibrium state: Since histidine residues exist in equilibrium between protonated and deprotonated forms at physiological pH, they can act as proton donors or acceptors, making them versatile in biochemical reactions.

Conclude the correct explanation: Based on the analysis, the correct statement is that most histidine residues are partially protonated, existing in equilibrium between protonated and deprotonated forms at physiological pH.