Structural Modifications of an Amaranth Globulin Induced by pH and NaCl

Abstract

The influence of pH and NaCl on the structure of globulin-P, the polymerizable amaranth 11S type globulin, was studied by differential scanning calorimetry, gel filtration, and gradient sedimentation. At μ = 0.54, the protein is stable for pH ranging from 5 to 9 but becomes rapidly unfolded as pH decreases below 5. For pH values above 9, globulin-P denatures more gradually than in acidic medium, and it also dissociates into subunits, which are possibly less thermostable. At pH 6.5 or 8.5 and low sodium chloride concentrations (μ ≤ 0.01), dialyzed globulin-P destabilizes, yielding species of lower thermal stability. The increase in NaCl concentration up to 0.1 M induces folding of globulin-P toward a more stable structure. Above 0.1 M NaCl, increasing the ionic strength up to μ = 0.5 elevates the denaturation temperature (Td) and denaturation enthalpy (ΔH). From μ = 0.1 to 0.5 the content of soluble globulin-P polymers decreases, possibly owing to protein insolubilization. Above 0.5 M, NaCl shows a stabilizing effect reflected by increasing Td, whereas ΔH stays constant; this effect is similar to that found by other authors in some storage proteins. Keywords: Amaranth; globulin; protein structure; gel filtration; ultracentrifugation; pH effect; ionic strength effect