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 pK
a of 6.04.
Histidine is a very catalytically active molecule. It plays critical roles in enzymes such as RNase and in Serine Proteases.