We characterized the alpha-to-beta transition in alpha-helical coiled-coil connectors of the human fibrin(ogen) molecule using biomolecular simulations of their forced elongation and theoretical modeling. The force (F)-extension (X) profiles show three distinct regimes: (1) the elastic regime, in which the coiled coils act as entropic springs (F < 100-125 pN; X < 7-8 nm); (2) the constant-force plastic regime, characterized by a force-plateau (F approximate to 150 pN; X approximate to 10-35 nm); and (3) the nonlinear regime (F > 175-200 pN; X > 40-50 am). In the plastic regime, the three-stranded a-helices undergo a noncooperative phase transition to form parallel three-stranded beta-sheets. The critical extension of the a-helices is 0.25 nm, and the energy difference between the alpha-helices and beta-sheets is 4.9 kcal/mol per helical pitch. The soft alpha-to-beta phase transition in coiled coils might be a universal mechanism underlying mechanical properties of filamentous alpha-helical proteins.