The PAS domain containing histidine kinase, Hpk2, is a heme binding sensor of oxygen levels.

Periodontal disease (PD) results from a shift in the composition of the microbial community of the subgingival crevice. As the bacterial population transitions from Gram- positive bacteria to predominantly Gram-negative anaerobes and spirochetes, dramatic changes occur in the physiological and immunological environment at diseased sites. thrives in periodontal pockets, indicating that it has a unique ability to adapt to the changing environmental conditions. Hpk2 (tde1970), a PAS domain containing histidine kinase (HK), is part of the Hpk2-Rrp2 (tde1969) two component regulatory (TCR) system. This TCR system is growth phase regulated and has been postulated to play a key role in adaptive responses. In this study, we employ predictive structural analyses and site-directed mutagenesis to investigate the functional role of specific amino acid (aa) residues located within the Hpk2 PAS domain. Specific substitutions impacted autophosphorylation (AP), phosphotransfer (PT), oligomerization, and hemin binding. The AP, PT, hemin binding and oligomerization potential of some mutated Hpk2 proteins differed under aerobic versus anaerobic reaction conditions. The data presented within suggest that the regulatory activity of Hpk2 is linked to diatomic gas levels. In a broader sense, this study highlights the importance of studying proteins produced by anaerobes under conditions that approximate the environment in which they thrive. Periodontal disease affects nearly 60% of the adult global population. Its costs to individuals, and society as a whole, are enormous. As periodontal disease develops, there is a shift in the composition of the oral microbial community. The bacteria that become dominant are able to cause significant damage to the tissues that support the teeth, leading to tooth loss. is one of the keystone pathogens associated with periodontal disease. An earlier study demonstrated that the Hpk2 and Rrp2 proteins play an important role in adaptive responses. Here, we explore the role of specific Hpk2 amino acids in environmental sensing and function using structural analyses and site-directed mutagenesis.