  The theory of elastic rods can be used to describe certain geometric and
topological properties of the DNA molecules. A similar effective field theory
approach was previously suggested to describe the conformations and dynamics of
proteins. In this letter we report a detailed study of the basic features of a
version of the proposed model, which assumes proteins to be very long
continuous curves. In the most appealing case, the model is based on a
potential with a pair of minima corresponding to helical and strand-like
configurations of the curves. It allows to derive several predictions about the
geometric features of the molecules, and we show that the predictions are
compatible with the phenomenology. While the helices represent the ground state
configurations, the abundance of beta strands is controlled by a parameter,
which can either completely suppress their presence in a molecule, or make them
abundant. The few-parameter model investigated in the letter rather represents
a universality class of protein molecules. Generalizations accounting for the
discrete nature and inhomogeneity of the molecules presumably allow to model
realistic cases.
