Supercharge Operator of Hidden Symmetry in the Dirac Equation

We require that there exist some additional symmetry of the Dirac Hamiltonian in an arbitrary central potential which describes the degeneracy of spectrum with respect to the two signs of eigenvalues of famous Dirac's K operator. It is proved that this happens only for the Coulomb potential. In this particular case symmetry operator reduces to the well-known Johnson-Lippmann operator, which plays a role of supercharge in the Dirac-Coulomb problem and generates a Witten's superalgebra