What is the pI of this alien amino acid that I invented?

In this alien amino acid, we find similarities to life on earth.

Why is it critical that the SH group has a pKa of 8 for biochemistry (similar to our cysteine)?

Question

What is the pI of this alien amino acid that I invented?

In this alien amino acid, we find similarities to life on earth.

Why is it critical that the SH group has a pKa of 8 for biochemistry (similar to our cysteine)?

BlackTom AI · Accepted Answer

pI = (pKa(carboxyl) + pKa(side chain thiol))/2 = (2 + 8)/2 = 5.0.

Relevant concepts:
- The isoelectric point (pI) of an amino acid with an ionizable side chain is the pH at which the net charge is zero; for residues whose side chain can be deprotonated to give a negative charge (like a thiol that becomes S–), the pI is the average of the two pKa values that flank the neutral (zwitterionic) form.  
- The Henderson–Hasselbalch relationship gives the fraction of deprotonated form at a given pH: [A–]/[HA] = 10^(pH − pKa).

Step-by-step reasoning and calculations:
1. Determine charge states as pH increases:
   - At very low pH: NH3+ (+1), COOH (0), SH (0) → net +1.
   - After COOH deprotonation (pKa = 2): NH3+ (+1), COO– (–1), SH (0) → net 0 (zwitterion).
   - After SH deprotonation (pKa = 8): NH3+ (+1), COO– (–1), S– (–1) → net –1.
   - After NH3+ deprotonation (pKa = 10): NH2 (0), COO– (–1), S– (–1) → net –2.
2. The zwitterion (net 0) exists between the pKa of the carboxyl (2) and the thiol (8), so pI = average of these two pKa values:
   - pI = (2 + 8)/2 = 5.0.

Why pKa ≈ 8 for the SH is critical:
- Catalytically active thiolate (S–) is the nucleophilic, reactive species in many enzyme mechanisms (covalent catalysis, nucleophilic attack, metal coordination, redox). At pH 7 and pKa 8, fraction deprotonated = 10^(7−8)/(1+10^(7−8)) = 0.1/1.1 ≈ 9.1% S–. This provides a significant population of reactive thiolate without being overwhelmingly deprotonated (which can be too reactive or unstable). If the thiol’s pKa were much higher (e.g., 9–10), the S– population at pH 7 would be tiny (≈1% or 0.1%), making thiolate-dependent catalysis ineffective under physiological conditions.

Why other options are incorrect (brief):
- Options giving pI = 6 or 9 are wrong because they average incorrect pKa pairs; the neutral zwitterion lies between pKa 2 and pKa 8, so pI is 5.  
- Statements tying higher pKa to higher catalytic activity are misleading: excessively high pKa reduces the fraction of reactive S– at physiological pH; very low pKa would make thiolate overly prevalent and potentially harmful.

Conclusion: The correct choice is the first option: pI = 5, and the SH pKa ≈ 8 is critical so that a useful fraction of thiolate (S–) is present near pH 7 to enable biochemical catalysis. This matches the provided answer.

What is the pI of this alien amino acid that I invented? In this alien amino acid, we find similarities to life on earth. Why is it critical that the SH group has a pKa of 8 for biochemistry (similar to our cysteine)? 单项选择题

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