Histidine and Cysteine

[Mahaan]

Biochemistry textbooks extol the virtues of Histidine as an H+ donor and acceptor at physiological pH. The reasoning is that the imidazole ring has a pKa ~ 6.1, which is close to physiological pH --> thus significant amounts of both protonated and deprotonated species exist at pH 7.4 --> so, His can take up or give up H+ at this pH.

Using that logic, why not cysteine? It's side-chain pKa = 8 (according to my textbook); just as close to phys. pH.

What am I missing here?

[Geraldine639]

I do not really understand Jesus and look forward to the answer!

[jsabah]

Hi Mahaan!

Cysteine and histidine are 2 entirely different molecules.

Histidine is a basic molécule with an imidazole ring. His lateral chain has a pKa of 6. His lateral chain has an amine function inside of it, which can be ionised.

If pH<6 : NH3+ Function will dominate.
If pH = 7 : NH2 Function will dominate. At physiological pH, histidine gives an H+ to the environnement.

That's for Histidine, feel free to look at wikipedia's Histidine, it can help visualize what i'm explaining.

Concerning Cystein, it's entirely different :

Cystein has a lateral chain with a thiol function inside of it (SH). That chain has a pKa around 10. Which means :

If pH<10 : physiological pH (7) : SH will dominate.
If pH>10 : S- will dominate. (he will give a H+ to the environnement).

That's how i understand it.

If it's hard to understand, there's a basic rule in chemistry that explains that :

if pH<pKa : the molecule usually TAKES an H+ FROM the ENVIRONNEMENT.
if pH>pKa : the molecule usually GIVES an H+ TO the ENVIRONNEMENT.

Concrete Example : Let's take as an example H20. There are two forms of H20:

Either H20 takes an H+ from the acid environnement : he becomes H30+
Either H20 gives an H+ to a basic environnement : he becomes HO-

Basic pH Theory : Here's a little way to understand :

pH : 0 ------------------------------------- pKa ------------------------------------------ 14
ACID FORM (with H+) BASIC FORM (without H+)

You can also check wikipedia for pH, cysteine, and histidine for more explanation!

Hope it helped!

See ya.