A calorimetric analysis of human plasma fibronectin: effects of heparin binding on domain structure

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Abstract
Fibronectin domain structure, as influenced by interaction with heparin, calcium, or chondroitin sulfate C, was analyzed by differential scanning calorimetry. A complex thermal denaturation transition was observed with a large sharp endotherm at 63.degree. C, a broad endotherm between 70 and 80.degree. C, and an exotherm at 80-90.degree. C. Analysis of the denaturation profiles revealed the existence of four thermal transitions, 59.1, 62.2, 67.3, and 74.3.degree. C, and an exotherm at 83.9.degree. C. The calorimetric enthalpies of the four endotherms are 1146 .+-. 259, 866 .+-. 175, 1010 .+-. 361, and 676 .+-. 200 kcal/mol, respectively. In all cases, the calorimetric to van''t Hoff enthalpy ratio was greater than 1.0. Computer analysis of the primary structure of fibronectin revealed 29 .+-. 8% homology among the type I homology units and 28 .+-. 7% homology among type III homology units, suggesting that different structural domains could arise from the same homology type. This may explain why more thermal transitions are observed for fibronectin than there are homology types. Addition of heparin to fibronectin in varying molar ratios, i.e., 10:1 to 30:1, resulted in a larger calorimetric enthalpy for the first type of structural domain (Tm = 59.1.degree. C) of fibronectin. At higher heparin to fibronectin ratios (40:1 or 75:1), the enthalpy of this domain decreased, while the others remained unchanged. In the presence of 5 mM calcium chloride, fibronectin thermal denaturation occurred at lower temperatures and was associated with precipitation of fibronectin. With both calcium chloride (5 mM) and heparin (20:1), a decrease in the total enthalpy of denaturation of fibronectin was observed, precipitation did not occur, and the Tm''s were comparable to fibronectin in the absence of Ca2+. Chondroitin sulfate C resulted in a large endotherm at 61.2.degree. C and aggregation above 65.degree. C, which was unlike the response to heparin. Thus, fibronectin has multiple heparin binding sites which differ in their affinities for heparin and sensitivity to calcium. These calorimetric assessments of the structural domains of fibronectin may provide insight into the structure-function relationships of the molecule.