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General information

OxyR is a protein that functions as a transcription factor. The OxyR we are analyzing is that of Escherichia Coli. In this perspective, the protein works by regulating the transcription of specific genes, being sensitive to H2O2. This transcription factor belongs to the LysR family, with 34 kDa, which forms homotetramers. Under this focus, the members of this family present a domain linked to DNA in their N-terminal region, called helix-loop-helix, a central domain responsible for recognizing H2O2 that is sensitive to the regulation signal and, finally, a domain in C-terminal region responsible for activation and mutimerization.

Structure

OxyR activation

Protein activity in different forms

The OxyR transcription factor regulate the expression of genes involved in redox-homeostasis. OxyR protein is able to DNA-binding in its two different forms, oxidized and reduced. In your oxidized form, OxyR binds to four adjacent major grooves in DNA, while in the reduced form, OxyR binds to two pairs of adjacent major grooves separated by one helical turn (figure X). These two modes of binding allow OxyR to regulate different promoters, under different cell conditions. Both forms of OxyR act as repressors, regulating its own expression by oxyR transcription repression, while only oxidized OxyR is able to activate gene expression, inducing the expression of oxyS, hydroperoxidase I (katG), alkyl hydroperoxide reductase (ahpCF), glutathione reductase (gorA), glutaredoxin 1 (grxA) and other genes involved in redox-homeostasis, that protect the cell against oxidative stress.

The tetramers observed in crystals of the reduced (A) and oxidized (B) forms are shown with the DNA positioned on the tetramers with plausible orientations. The tetramers are shown as ribbons (red, green, blue, and yellow for each monomer) and the model of DNA is represented as helical coils (cyan).

Fig. 4. Model for OxyR activation and deactivation. Upon exposure to H,O, the Cyslg9 residue of OxyR is first oxidized to a sulfenic acid. This reactive intermediate subsequently reacts with CysZo8 to form a stable disulfide bond locking OxyR in an activated form. Oxidized OxyR is reduced by disulfide bond reduction by the glutaredoxin system. Because OxyR activates the transcription of grxA (Grxl) and gorA (glutathione reductase), the entire response is autoregulated.

Purification and crystallization

Protein crystallization is the process of formation of a regular array of individual protein molecules stabilized by crystal contacts. If the crystal is sufficiently ordered, it will diffract. Some proteins naturally form crystalline arrays, like aquaporin in the lens of the eye.[1] In the process of protein crystallization, proteins are dissolved in an aqueous environment and sample solution until they reach the supersaturated state.[2] Different methods are used to reach that state such as vapor diffusion, microbatch, microdialysis, and free-interface diffusion. Developing protein crystals is a difficult process influenced by many factors, including pH, temperature, ionic strength in the crystallization solution, and even gravity.[2] Once formed, these crystals can be used in structural biology to study the molecular structure of the protein, particularly for various industrial or medical purposes.[3][4]

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