The spectral diffusion of Zn-substituted cytochrome c in a glycerol/water host glass has been investigated on a time scale from picoseconds to milliseconds using photon echoes. The results are interpreted within the two-level system model and by using the concept of conformational protein substates. The experiments reveal a gap in the distribution function of two-level system relaxation rates between 1 MHz and 10 kHz. Evidence is provided for the existence of two distinct types of two-level systems.