Adjacent amino acids affect Cys behavior in disulfide?

sjszarka at ACS.UCALGARY.CA sjszarka at ACS.UCALGARY.CA
Thu Sep 12 11:36:00 EST 1996

The protein we are working on appears to have a predisposition for negatively
charged amino acids adjacent to some of the Cys residues involved in disulfide
bond formation.

Since the proximity of the negatively charged residues may destabilize the
ionized form of the Cys sulfur, hence increase the pKa, would changing these
residues alter the folding and cross-linking of this protein?

Are these Possibilities?

1) The repulsion from the adjacent side chains help to rotate the ionized Cys
residue with respect to the peptide backbone. Possibly putting the sulfur in
closer proximity with the other Cys side group involved in the disulfide bond.

2) If adjacent negative charges make ionization of the Cys sulfur less likely
(ie. increae the pKa), would this property afffect the isomerization behavior of
the disulfide bonds in this protein?

3) If these negative charges increase the pKa of adjacent Cys residues (from the
literate value of approx. 8.3), then to achieve a higher percentage of
ionization of the sulfur atoms and potentially greater reativity for disulfide
bond formation; would it be better to carry out in vitro refolding experiments
at a pH of 9 or 10?

Any comments or suggestions would be greatly appreciated.

Steven Szarka, Ph.D.          Molecular Biology Division      
                              Dept. of Biological Sciences
                              University of Calgary
                              Calgary, AB, Canda
                              T2N 1N4

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