User:Melissa Siolin Martins/Sandbox 1

The Ohr protein (organic hydroperoxide resistance) is a thiol-dependent Cys-based peroxidase exclusive to bacteria, that participates in the antioxidant defense system of these organisms against damage induced by organic peroxides (OHPs), such as fatty acid peroxides and peroxynitrite ^[1][2]. This class of peroxides is toxic due to its ability to generate free radicals, highly reactive molecules containing an unpaired electron in an atomic orbital that can cause damage to important molecules such as DNA and proteins. Because of that, these substances are frequently used by the host defense against bacteria pathogens ^[3][4].

The bacterial defense system is composed of enzymes like Ohr, who are capable of detoxifying and neutralizing OHPs via a redox-active disulfide bond, transforming these peroxides into unreactive alcohols ^[5]. Therefore, this pathway against OHPs is crucial for their survival when faced with the host defense pathway during the infection.Ohr is unique when compared to other thiol based peroxidases, because it is very efficient in reducing OHPs but not so much when it comes to H2O2, while other peroxidases are broadspectrum, reducing OHP, H2O2 and other substrates such as fatty acid hydroperoxides and peroxynitrite with similar efficiencies. Therefore, Ohrs are specialized in OHPs, characteristic associated with the presence of a hydrophobic collar (HC) in their structure surrounding the active site, that interact more with hydrophobic substrates ^[2].

Ohr was described as a second system involved in bacterial resistance to OHPs by the discovery of the ohr gene from Xanthomonas campestris pv. phaseoli ^[2]. Furthermore, the Δohr mutant of X. campestris pv. phaseoli displayed a unique phenotype: increased and specific sensitivity to artificial OHPs but not to either H2O2 or superoxide generators. Ohr homologues from other bacteria such as Pseudomonas aeruginosa and Bacillus subtilis were also described as displaying a similar expression profile and role on OHP resistance ^[2].

This enzime is part of the Ohr/OsmC protein family. This group is characterized for having a Cys-based thiol-dependent peroxidase activity. Besides not having much sequence similarities, their protein structure is highly conserved, presenting two Cys residues (Cp and Cr) in their active site. The other catalytic residues (Rc and Ec) are conserved among all of the Ohr/OsmC protein family members, but were not found in Ohr-like enzymes ^[1][2][6].

In several bacterial genera with different lifestyles (pathogenic or non-pathogenic) other Ohr homologs and members of the Ohr/OsmC family have been found ^[2]. Recent studies revealed that they are not restricted to prokaryotes, being also present in eukaryotes, acquired possibly through lateral gene transfer. They found Ohr homologs in 217 species amongst non-vascular plants, animal pathogens and with especially high abundance in fungi (found in 186 species, mainly Ascomycota and Basidiomycota). However, they are absent in vertebrates and vascular plants. In most of the eukaryotic Ohr/OsmC proteins, there is a N-terminal signal sequence that predicts the cellular localization of the protein to organellar compartments such as mitochondria, peroxisome or chloroplasts. Because of that, it is proposed that the peroxidase activity of Ohr in eukaryotes is related to detoxification of endogenous sources of hydroperoxides, a different role from prokaryotes, where they are related to defense towards reactive oxygen species ^[6].

Curiosity: ohr is also present in Mycoplasma genitalium, which is the smallest self-replicating organism and lacks other antioxidant systems such as SODs and catalases.

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Structural highlights

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