Answer :

Histidine retains a positive charge on its R group nitrogen when the environmental pH is below the imidazole side chain's pKa of approximately 6.0. At this pH, the imidazole ring is protonated, carrying a positive charge. The charge state of histidine is important for its role in enzyme active sites, particularly because its pKa is close to physiological pH, allowing it to act as a proton donor-acceptor.

This occurs when the pH of the environment is below the pKa of histidine's imidazole side chain, which is approximately 6.0. In such an acidic environment, the imidazole ring of histidine is mostly protonated, with the positive charge being equally distributed between the two nitrogens in the ring, making it a strong base at physiological pH.

As pH exceeds approximately 6, one of the protons is lost, and histidine transitions from a protonated, positively charged state to a neutral state consisting of two tautomeric forms, the N1-H, and the N3-H tautomers. In biological systems, histidine residues are often found in enzyme active sites, where they can act as proton donor-acceptor groups, playing a pivotal role in biochemical reactions.

It's important to understand that the pKa of the histidine side chain is close to physiological pH, which means even slight changes in pH can influence the charge state of the histidine residue. Therefore, at physiological pH, histidine can be present in both its protonated (positively charged) and deprotonated (neutral) forms, contributing to its versatility in enzymatic functions.

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