# Aspartyl Proteases: Acid-Base Chemistry and Protein Interactions ## A) Percentage of Deprotonated Aspartic Acid Side Chains at pH 6. Aspartic acid has three ionizable groups with pKa values: - α-carboxyl: **pKa = 2.** - R group (side chain) carboxyl: **pKa = 4.1** - α-amino: **pKa = 10.** We're interested in the **side chain (R group) carboxyl**. ### Step 1: Use the Henderson-Hasselbalch Equation \[ \text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \] Where: - \( [\text{A}^-] \): deprotonated form (COO⁻) - \( [\text{HA}] \): protonated form (COOH) ### Step 2: Plug in Values \[ 6. = 4.1 + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \] \[ 6. - 4.1 = \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \] \[ 1.9 = \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \] ### Step 3: Solve for the Ratio \[ \frac{[\text{A}^-]}{[\text{HA}]} = 10^{1.9} \] \[ 10^{1.9} \approx 79.4 \] ### Step 4: Calculate Percentage Deprotonated Let total = \( [\text{A}^-] + [\text{HA}] \). Fraction deprotonated: \[ \text{Fraction deprotonated} = \frac{[\text{A}^-]}{[\text{A}^-] + [\text{HA}]} \] Plug in the ratio: \[ \text{Fraction deprotonated} = \frac{79.4}{79.4 + 1} = \frac{79.4}{80.4} \approx .9875 \] \[ \text{Percentage deprotonated} = .9875 \times 100\% = 98.75\% \] **Final Answer:** > **At pH 6., approximately \(\boxed{98.75\%}\) of aspartic acid side chains are deprotonated (negatively charged).** --- ## B) How Can One Asp Be Protonated and the Other Deprotonated at the Same pH? Despite the above prediction, **not all Asp residues behave identically within a protein's active site**. This is due to their *microenvironment*: ### Key Points: - **Local Environment Effects:** The pKa of an Asp side chain can be shifted by its environment. For example, being near a positive charge, being buried in a hydrophobic pocket, or being involved in hydrogen bonds. - **Hydrogen Bonding:** If an Asp forms a strong hydrogen bond (as donor or acceptor), it can stabilize either the protonated or deprotonated state. - **Electrostatic Effects:** Nearby residues (e.g., Lys, Arg, His) can stabilize a particular ionization state, raising or lowering the pKa. - **Substrate/Water Positioning:** The presence of substrate or water molecules can also affect pKa. ### Specific to Aspartyl Proteases: - **One Asp is in an environment that raises its pKa** (e.g., buried, hydrogen bonded to a polar group), so it remains protonated at pH 6. - **The other Asp is in a typical or lowered pKa environment** (e.g., exposed to solvent, near basic residues), so it is deprotonated at pH 6. **Conclusion:** > **The protein's active site microenvironment selectively shifts the pKa values of each Asp, allowing one to be protonated and one deprotonated at the same pH.** --- ## C) Three Types of Noncovalent Interactions in Proteins These interactions stabilize protein structure and mediate binding events. 1. **Hydrogen Bonds** - Between backbone amide and carbonyl groups, or between side chains and substrates. 2. **Ionic (Electrostatic) Interactions** - Between oppositely charged side chains (e.g., Asp/Glu and Lys/Arg). 3. **Van der Waals (London Dispersion) Forces** - Weak attractions between nonpolar atoms in close proximity. **Other possible answers:** - **Hydrophobic interactions** - **π-π stacking** (aromatic rings) --- ### Summary Table | Question | Key Point | Final Answer | |----------|-----------|--------------| | (A) | % Asp deprotonated at pH 6. | 98.75% | | (B) | Different Asp pKas in active site | Microenvironment shifts pKa | | (C) | Noncovalent interactions | Hydrogen bonds, ionic, van der Waals |