Histidine overview:

Question 1

Histidine

Answer 1

His or H

Question 2

Structure:

Answer 2

• It is a little bit aromatic
• It is not particularly picky about its secondary structure
• It is often seen in active sites.

Question 3

Properties:

Answer 3

• R-group = weak base, reversible (+) charge

-It is hydrophilic due to the two nitrogens in its side chain.

-Both nitrogens can titrate (the first one at pH 6.2).

-It is neutral at physiological pH, but it can easily become positive,
and occasionally even negative.

-It can bind metal ions (mainly Zn, Ni, Cu).

-Serine protease catalytic triad

Question 4

Is…

Answer 4

• Catalytically, most
  important type of residue
• Full of surprises
• Name sounds like base
• Actually acid (pK = 6.1)
• But (of course) catalyzes like base
  – Usually proton acceptor

Question 5

Most common acid-base..

Answer 5

catalyst.

-pK is well matched with many substrates and
with water

• Rapid protonation and deprotonation
  – Reaction and regeneration
• Only small perturbations are needed
  – Base  Acid
• Many examples: e.g. Lactate dehydrogenase