RESUMO
The sperm whale apomyoglobin pH 4 folding intermediate exists in two forms, Ia and Ib, that mimic transient kinetic intermediates in the folding of the native protein at pH 6. To characterize the nature of the kinetic barrier that controls the formation of the earliest intermediate Ia, we have investigated the effects of small viscogenic cosolvents on its folding and unfolding kinetics. The kinetics are measurable by stopped-flow fluorescence and follow a cooperative two-state model in the absence and presence of cosolvents. Small cosolvents stabilize Ia, but, by applying the isostability test to separate the viscogenic effect of the cosolvent from its stabilizing effect, we found that, in both folding and unfolding conditions, the apparent rate constant decreases when solvent viscosity increases. The unitary inverse dependence of the apparent rate constant on solvent viscosity indicates a diffusion-controlled reaction. This result is consistent with the hypothesis that folding of the apomyoglobin pH 4 intermediate obeys a diffusion-collision model. Additionally, the temperature dependence of the reaction rate at constant viscosity indicates that the formation of Ia is also controlled by an energy barrier. Linear free energy relationships show that the transition state of the U <==> Ia reaction is compact and buries 45% of the surface area that is buried in native apomyoglobin. We conclude that the transition state of the U <==> Ia reaction resembles that for the formation of native proteins; namely, it is dry and its compactness is closer to that of the folded (Ia) form than of the unfolded form.
Assuntos
Apoproteínas/química , Apoproteínas/metabolismo , Mioglobina/química , Mioglobina/metabolismo , Desnaturação Proteica , Dobramento de Proteína , Animais , Apoproteínas/isolamento & purificação , Dicroísmo Circular , Difusão , Relação Dose-Resposta a Droga , Concentração de Íons de Hidrogênio , Cinética , Movimento (Física) , Mioglobina/isolamento & purificação , Ressonância Magnética Nuclear Biomolecular , Estrutura Secundária de Proteína , Estrutura Terciária de Proteína , Solventes/química , Espectrometria de Fluorescência , Espectrofotometria Ultravioleta , Cachalote , Sacarose/farmacologia , Temperatura , Termodinâmica , Ureia/farmacologia , ViscosidadeRESUMO
The work in the literature on apomyoglobin is almost equally divided between horse and sperm whale myoglobins. The two proteins share high homology, show similar folding behavior, and it is often assumed that all folding phenomena found with one protein will also be found with the other. We report data at equilibrium showing that horse myoglobin was 2.1 kcal/mol less stable than sperm whale myoglobin at pH 5.0, and aggregated at high concentrations as measured by gel filtration and analytical ultracentrifugation experiments. The higher stability of sperm whale myoglobin was identified for both apo and holo forms, and was independent of pH from 5 to 8 and of the presence of sodium chloride. We also show that the substitution of sperm whale myoglobin residues Ala15 and Ala74 to Gly, the residues found at positions 15 and 74 in horse myoglobin, decreased the stability by 1.0 kcal/mol, indicating that helix propensity is an important component of the explanation for the difference in stability between the two proteins.