Recombinant mFVIIa chimeras bind to msEPCR in contrast to mFVIIa. Isothermal titration calorimetry was used to study the interaction of (A) mFVIIa, (B) mFVIIa-FMR, and (C) mFVIIa-(1-43 mPC) with msEPCR. Upper portion of each panel: heat flow tracings were obtained upon successive injections (2 µL) of msEPCR into the sample cell containing 25 µM mFVIIa or mFVIIa chimeras in dialysate at 25°C. The first injection (0.5 µL) was eliminated from the analysis. The heat flow profile from an identical injection of msEPCR into dialysate was used as a reference and is shown offset (top of upper panel) for clarity. Bottom portion of each panel: corrected integrated heats (kilocalories per mole of injectant) were analyzed. The line is drawn according to parameters listed in Table 1.