Acid stabilization of insulin.
The effect of pH on the conformational stability of insulin was studied. Surprisingly, the Gibbs free energy of unfolding increased approximately 30% by acidification. pH titration of insulin's conformational stability is described by a transition involving a single proton with an apparent pK(a) of 7.0. The acid stabilization of insulin's conformation was attributed to the protonation of histidine at position 5 on the B-chain (HB5) as determined by 1H-NMR of the histidines, selective amino acid alteration, and enthalpies of ionization. Further acidification (at least to pH 2) does not decrease the free energy of unfolding. A conformational change in the tertiary structure, as indicated by the near-UV circular dichroism spectrum, accompanies this change in stability. We propose that this acid stabilization of insulin is physiologically important in maintaining insulin stability in the acid environment of the secretory/storage granules of the beta-cell of the pancreatic islets of Langerhans.
Bryant
C
C
Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46285.
Spencer
D B
DB
Miller
A
A
Bakaysa
D L
DL
McCune
K S
KS
Maple
S R
SR
Pekar
A H
AH
Brems
D N
DN
D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002942:Circular Dichroism; D004355:Drug Stability; D006639:Histidine / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D011522:Protons; D013816:Thermodynamics
D007328:Insulin; D011522:Protons; D006639:Histidine