The secondary structure of human plasma fibronectin: conformational changes induced by acidic pH and elevated...

The secondary structure of native human plasma fibronectin, based on circular dichroic spectra, has been estimated to contain 79% beta sheet and 21% beta turn structures (Osterlund, E., Eronen, I., Osterlund, K. and Vuento, M. (1985) Biochemistry 24, 2661-2667). In this work changes in the secondary structure of the protein molecule are followed as a function of different temperatures and pH values by using circular dichroic spectroscopy in far- and near-ultraviolet regions. Conformational changes are reversible when raising the temperature quickly to 55 degrees C, and then cooling slowly to 20 degrees C. A few percent of alpha-helix is apparent, when the temperature is raised to 58.5 degrees C, but only about 9% random coil is formed, when the temperature is raised up to 70 degrees C. The largest conformational change is taking place, when fibronectin samples are heated from 57 to 58.5 degrees C. According to this study more than 90% of the secondary structure of the fibronectin molecule is preserved throughout the whole temperature range studied from 20 to 70 degrees C, and this is a fact even at pH as low as 3.0, when samples are fresh and not denatured by preparative procedures.