title: An unconventional hexacoordinated flavohemoglobin from Mycobacterium tuberculosis.
abstract: Being an obligate aerobe, Mycobacterium tuberculosis faces a number of energetic challenges when it encounters hypoxia and environmental stress during intracellular infection. Consequently, it has evolved innovative strategies to cope with these unfavorable conditions. Here, we report a novel flavohemoglobin (MtbFHb) from M. tuberculosis that exhibits unique features within its heme and reductase domains distinct from conventional FHbs, including the absence of the characteristic hydrogen bonding interactions within the proximal heme pocket and mutations in the FAD and NADH binding regions of the reductase domain. In contrast to conventional FHbs, it has a hexacoordinate low-spin heme with a proximal histidine ligand lacking imidazolate character and a distal heme pocket with a relatively low electrostatic potential. Additionally, MtbFHb carries a new FAD binding site in its reductase domain similar to that of D-lactate dehydrogenase (D-LDH). When overexpressed in Escherichia coli or Mycobacterium smegmatis, MtbFHb remained associated with the cell membrane and exhibited D-lactate:phenazine methosulfate reductase activity and oxidized D-lactate into pyruvate by converting the heme iron from Fe(3+) to Fe(2+) in a FAD-dependent manner, indicating electron transfer from D-lactate to the heme via FAD cofactor. Under oxidative stress, MtbFHb-expressing cells exhibited growth advantage with reduced levels of lipid peroxidation. Given the fact that D-lactate is a byproduct of lipid peroxidation and that M. tuberculosis lacks the gene encoding D-LDH, we propose that the novel D-lactate metabolizing activity of MtbFHb uniquely equips M. tuberculosis to balance the stress level by protecting the cell membrane from oxidative damage via cycling between the Fe(3+)/Fe(2+) redox states.
authors.affiliation: Institute of Microbial Technology, Council of Scientific & Industrial Research, Sector 39 A, Chandigarh 160036, India.
authors.forename: Sanjay
authors.initials: S
authors.lastname: Gupta
authors.forename: Sudesh
authors.initials: S
authors.lastname: Pawaria
authors.forename: Changyuan
authors.initials: C
authors.lastname: Lu
authors.forename: Mangesh Dattu
authors.initials: MD
authors.lastname: Hade
authors.forename: Chaahat
authors.initials: C
authors.lastname: Singh
authors.forename: Syun-Ru
authors.initials: SR
authors.lastname: Yeh
authors.forename: Kanak L
authors.initials: KL
authors.lastname: Dikshit
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002462:Cell Membrane / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D006420:Hemeproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006860:Hydrogen Bonding; D015227:Lipid Peroxidation; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009169:Mycobacterium tuberculosis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D006420:Hemeproteins; D008565:Membrane Proteins; D011994:Recombinant Proteins; C086193:flavohemoprotein, Bacteria