Histidine is often found in the active site, its pka is 6 and hence, can act as an acid, base or as a nucleophile at physiological pH. Cystine has a Pka of 8 and hence cys should also be able to do the same job. Then why His and not cys? is it because Cys is prone to oxidation?
Yes, Cys is prone to autoxidation. Cys can undergo heavy metal-catalyzed autoxidation. Cys provides three metal ligands in a favorable geometric arrangement. Since autoxidation produces hydrogen peroxide, which is lethal to cells, Cys is a liability to aerobic cells and is generally maintained at a low concentration. However Cys also play a crucial role in redox-regulated proteins. Several redox proteins are involved in the regulation of different cellular processes.
Hope it helps!
So, José S. Dávila Costa, according to you lability of Cys to oxidation appears to be the major reason for its rejection as an active site residue
I wouldn´t say "the major reason", it could be just one reason. On the other hand, Cys can act as redox amino acid, then redox-regulated proteins have Cys in their active sites.
Hi Sanjeev,
"How proteins work" by Mike Williamson is a good source book(see page 191 for an answer to your question) to update on many of the emerging thoughts in protein science. The good old "Blind Watchmaker" by Richard Dawkins is excellent too. I will extend some of my thoughts in your aspartic acid vs glutamic acid question.
Cheers
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