Basic Amino acid. Positively charged at neutral pH. Side chain is a pentagon-shaped imidazole ring, with a partial positive charge.

Formula: C6H9N3O2
Molecular Weight: 155.15
Three-letter code: His
One-letter code: H
Histidine has the following structure:

    COOH
    |
H2N-C-H
    |
    CH2
    |
    C--NH
   ||    \
   ||    CH       
   ||   //
    C---Na
    |
    H
The nitrogen labled 'a' can be protonated, and has a pKa of 6.04.

Histidine is a very catalytically active molecule. It plays critical roles in enzymes such as RNase and in Serine Proteases.

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