Blockage of the channel to heme by the E87 side chain in the GAF domain of Mycobacterium tuberculosis DosS confers the unique sensitivity of DosS to oxygen.

Two sensor kinases, DosS and DosT, are responsible for recognition of hypoxia in Mycobacterium tuberculosis. Both proteins are structurally similar to each other, but DosS is a redox sensor while DosT binds oxygen. The primary difference between the two proteins is the channel to the heme present in their GAF domains. DosS has a channel that is blocked by E87 while DosT has an open channel. Absorption spectra of DosS mutants with an open channel show that they bind oxygen as DosT does when they are exposed to air, while DosT G85E mutant is oxidized similarly to DosS without formation of an oxy-ferrous form. This suggests that oxygen accessibility to heme is the primary factor governing the oxygen-binding properties of these proteins.