Consider a histidine residue within a protein. Based on the histidine side-chain pKa value of 6.0 for the free amino acid, would you expect the side-chain to be fully protonated at pH 7.4? What should the charge be? What would the charge be at pH 5?

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### Histidine Residue Considerations Within a Protein

#### Questions to Explore:

- **Based on the histidine side-chain pKa value of 6.0 for the free amino acid, would you expect the side-chain to be fully protonated at pH 7.4?**
  - The pKa value of a side-chain is indicative of the pH at which the side-chain is 50% protonated. At pH values above the pKa, the side-chain tends to lose protons and thus deprotonate.

- **What should the charge be?**
  - At pH 7.4, which is above the pKa of 6.0, the histidine side-chain would be predominantly deprotonated. Deprotonation typically confers a neutral charge to the histidine side-chain.

- **What would the charge be at pH 5?**
  - At pH 5, which is below the pKa of 6.0, the histidine side-chain would be predominantly protonated, typically giving it a positive charge.

Understanding the protonation states of amino acids like histidine is crucial for predicting the behavior of proteins in different environments.
Transcribed Image Text:### Histidine Residue Considerations Within a Protein #### Questions to Explore: - **Based on the histidine side-chain pKa value of 6.0 for the free amino acid, would you expect the side-chain to be fully protonated at pH 7.4?** - The pKa value of a side-chain is indicative of the pH at which the side-chain is 50% protonated. At pH values above the pKa, the side-chain tends to lose protons and thus deprotonate. - **What should the charge be?** - At pH 7.4, which is above the pKa of 6.0, the histidine side-chain would be predominantly deprotonated. Deprotonation typically confers a neutral charge to the histidine side-chain. - **What would the charge be at pH 5?** - At pH 5, which is below the pKa of 6.0, the histidine side-chain would be predominantly protonated, typically giving it a positive charge. Understanding the protonation states of amino acids like histidine is crucial for predicting the behavior of proteins in different environments.
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