We present a numerical study of the electronic structure of the cytochrome subunit of the photoreaction center of the bacterium Rhodopseudomonas viridis. The electronic structure is described by a tight-binding Hamiltonian, the resulting large secular matrix is diagonalized applying sparse matrix methods. In the highly structured density of states, localized chromophore eigenstates can be observed around the HOMO-LUMO gap. In the low energy range, optical excitations originate from transitions between these states. Eigenfunctions that are extended on a length scale of up to similar to 190 A can be found in the bulk of the valence band density of states. Stretched along alpha-helices, conduction band eigenstates are characterized by a cylindrical shape. We discuss the role of these states within electron transport and possible extensions of the model presented here.