Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online

CHAPTER 2 - PROTEIN STRUCTURE 

A:  AMINO ACIDS

BIOCHEMISTRY - DR. JAKUBOWSKI

Last Update:  02/27/16

Learning Goals/Objectives for Chapter 2A:  After class and this reading, students will be able to

  • state the charge on amino acid side chains using the Henderson Hasselbach equation and the approximate charge by inspection at any given pH
  • draw mechanisms and identify products for the reaction of nucleophilic side chains Lys and Cys with common chemical modification agents and extend this understanding to reactions of His.
  • draw mechanisms for disulfide exchange reactions for sulfhydryls using them and oxidation numbers to explain redox reactions of cysteine/cystine.

A8.  Reactions of Histidine

Histidine is one of the strongest bases at physiological pH's. The nitrogen atom in a secondary amine might be expected to be a stronger nucleophile than a primary amine through electron release to that N in a secondary amine.  Opposing this effect is the steric hindrance by the two attached Cs of the N on attach on an electrophile .  However, in His, this steric effect is minimized since the 2Cs are restrained by the ring. With a pKa of about 6.5, this amino acid is one of the strongest available bases at physiological pH (7.0). Hence, it can often cross-react with many of the reagents used to modify Lys side chains. His reacts with reasonably high selectivity with diethyl pyrocarbonate.

Figure:  REACTIONS OF HISTIDINE

Figure:  Where is the H on His?  Where is the Charge?

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