  We study a minimal extension of the worm-like chain to describe polypeptides
having alpha-helical secondary structure. In this model presence/absence of
secondary structure enters as a scalar variable that controls the local chain
bending modulus. Using this model we compute the extensional compliance of an
alpha-helix under tensile stress, the bending compliance of the molecule under
externally imposed torques, and the nonlinear interaction of such torques and
forces on the molecule. We find that, due to coupling of the ``internal''
secondary structure variables to the conformational degrees of freedom of the
polymer, the molecule has a highly nonlinear response to applied stress and
force couples. In particular we demonstrate a sharp lengthening transition
under applied force and a buckling transition under applied torque. Finally, we
speculate that the inherent bistability of the molecule may underlie protein
conformational change \emph{in vivo}.
