Helix stability in succinylated and acetylated ovalbumins: effect of high pH, urea and guanidine hydrochloride.
Previous studies (Batra, P.P., Roebuck, M.A. and Uetrecht, D. (1990) J. Protein Chem. 9, 37-44) showed that succinylation or acetylation of 75% of the lysine residues has little effect on the secondary structure of ovalbumin. The acylation of the remaining 25% lysine residues, which apparently are partially buried, results in a substantial loss of the helical structure. These conformational changes may be due not only to electrostatic repulsions introduced by succinylation or acetylation of the positively charged epsilon-amino groups but also to steric hindrance, since an increase in the ionic strength failed to reverse the loss of the helical structure. An increase in pH to 12.2 results in a complete helix-to-coil transition in the maximally succinylated ovalbumin (but not in the partially succinylated or in any of the acetylated ovalbumins including the maximally acetylated derivative), perhaps because it is most expanded and its molecular interior most accessible to solvent as succinylation replaces +1 charge of epsilon-amino group with a -1 charge so that a net of -2 charge per succinyl group is placed on the protein molecule. This helix-to-coil transition in the maximally succinylated ovalbumin induced by high pH is fully reversed by increasing the ionic strength, indicating that only electrostatic effects are responsible for this disruption. Studies have also shown that although there is no loss of the helical structure until after the 75% surface lysine residues have been acylated, the helical structure does become progressively destabilized with increasing degree of modification, a conclusion drawn from urea unfolding curves. This destabilization of the helical structure is due primarily to electrostatic effects, as an increase in the ionic strength led to an increase in the urea transition mid-point. Unlike urea, the guanidine hydrochloride unfolding curves indicate that the transition mid-point for the native protein, as well as for the maximally succinylated and acetylated derivatives, is about the same, perhaps because the denaturant itself acts as an electrolyte.
Batra
P P
PP
Department of Biochemistry, School of Medicine and College of Science and Mathematics, Wright State University, Dayton, OH 45435.
Uetrecht
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D000107:Acetylation; D000215:Acylation; D000812:Anhydrides / Q000494:pharmacology; D002942:Circular Dichroism; D019791:Guanidine; D006146:Guanidines / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D010047:Ovalbumin / Q000378:metabolism; D011487:Protein Conformation; D011489:Protein Denaturation; D014508:Urea / Q000494:pharmacology
D000812:Anhydrides; D006146:Guanidines; D014508:Urea; D010047:Ovalbumin; D019791:Guanidine