| Popis: |
Heme-based redox sensors are implicated in a number of important physiological processes such as nitrogen fixation, aerotaxis, and control of circadian cycles. These proteins often rely on proper ligand switching for functional activation. It is unclear how a protein's conformation in many of these heme-based sensors affects ligation at the heme and vice versa. GSU0935, a methyl-accepting chemotaxis sensor protein from Geobacter sulfurreducens, contains a periplasmic binding domain (PBD) with a c-type heme. Previous reports indicated that the heme iron switches its axial ligands from water to Met60 upon heme reduction. The heme iron ligation in the GSU0935 PBD was investigated in chemically-denatured protein samples to characterize the relationship between protein conformation and heme ligation using UV-visible absorbance spectroscopy. A red shift in the Soret band of GSU0935 was linked to misligation by deprotonated His169 at physiological pH under denaturing conditions. Stopped-flow studies showed that protein refolding results in rapid dissociation of His169 to be replaced by His54 as the distal heme ligand. His54 misligation acts as a kinetic trap during protein refolding and slows the formation of the native water-ligated heme. These results suggest that the heme domain of GSU0935 has a highly flexible N-terminal region and an exposed heme environment, which may be important for sensory function. |
