Acid stabilization of human growth hormone equilibrium folding intermediates.
Equilibrium denaturation experiments were performed on human growth hormone (hGH) under acidic conditions (pH 1.5-3.0) and different protein concentrations. At 0.1 mg/ml hGH using intrinsic tryptophan fluorescence and far-UV circular dichroism (CD) detection, midpoint values of 4.6 M GdnHCl were observed that are identical to those obtained at neutral pH. However, the delta G values were reduced at pH 2.5 relative to pH 8.0 (10.5 vs. 15 kcal/mol). Increasing the protein concentration to 1 mg/ml resulted in a biphasic denaturation profile by far-UV CD detection at 222 nm, while near-UV CD measurements at 295 nm yielded a cooperative transition with a midpoint value of 3.6 M GdnHCl. These results indicate that equilibrium intermediates having a propensity to aggregate are highly populated under acid conditions. Static light scattering measurements performed under partial unfolding conditions (4.5 M GdnHCl) at pH 2.5 confirmed the existence of a large molecular weight (congruent to 80 kDa) self-associated intermediate. No evidence of aggregation was found for hGH under acid conditions in the absence of denaturant, indicating that self-association results from the formation of an intermediate. Equilibrium GdnHCl concentration-jump experiments confirmed that association only occurs from an intermediate species and not from any other conformational state, and formation of the self-associated intermediate can lead to irreversible loss of protein due to precipitation. These results demonstrate that acid stabilizes equilibrium folding intermediates of hGH.
DeFelippis
M R
MR
Lilly Research Laboratories, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285.
Kilcomons
M A
MA
Lents
M P
MP
Youngman
K M
KM
Havel
H A
HA
D002942:Circular Dichroism; D005453:Fluorescence; D013006:Growth Hormone / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D011489:Protein Denaturation; D017510:Protein Folding; D013816:Thermodynamics
D013006:Growth Hormone