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Selenoenzyme Glutathione Peroxidase

Sam Pletz '13 and Jimmy Chapman '13


I. Introduction

Glutathione peroxidase belongs to the selenoprotein family and is observed in mammals, birds, and fish. This enzyme along with other members of the selenoprotein family have an integral role in the defense mechanisms of these organisms.

Specifically, the Selenoenzyme Glutathione Peroxidase from bovine plasma plays a role in catalyzing the reduction of hydroperoxides. This enzyme also protects biomembranes and cell structures from oxidative damage. Glutathione Peroxidase accomplishes this through the reduction of lipid hydroperoxides to their corresponding alcohols and through the reduction of free hydrogen peroxide to water using glutathione as the reducing substrate. The enzyme reduces lipid hydroperoxidases and hydrogen peroxide through the following proposed mechanism:


The crystal structure of Seleno-Glutathione Peroxidase from human plasma has been characterized and differs only slightly from that of Glutathione Peroxidase from bovine plasma (Rin et al. 1997). However, the physilogical role of Glutathione Peroxidase in humans is still unclear because of the low levels of reduced glutathione and the low reactivity of the enzyme (Rin et al. 1997).


II. General Structure

Glutathione peroxidase is a tetramer consisting of 4 identical single polypeptide chains each with 178 amino acid residues .Each monomer contains two parallel and two anti-parallel pleated β-sheets surrounded by four α-helices . α helices 1, 2, and 4, exist on one side of the β sheet complex whereas α4 exists on the other side . The catalyitically active selenocysteine is located near the first turn of α1 . The second region of the monomer consists of α3 and two anti-parallel pleated β-sheets, and several β turns. Hydrogen bonding between β1 and β3 stabilizes and completes the four stranded β complex.

III. Subunit Contacts

A unique feature of the protein is the tetrahedral quaternary structure that is a planar arrangement of four identical monomers . The contact regions of each subunit to another consists of 16 amino acid residues; both polar and non-polar amino acid residues can be found between them. Four hydrogen bonds can also be formed across the local axis by residues Glu-77, Arg-86 and the symmetry equivalent pair, Glu-277 and Arg-286 . (Epp et al. 1983). Therefore, each monomer has 20 identical interactions with each other.

Polar side chains and other backbone atoms contribute to more subunit contacts between each monomer. The interactions between each monomer decrease the accessible surface area and is the main source of free energy keeping the monomers together (Epp et al. 1983). Because each monomer is chemically identical, one would predict roughly identical values of the individual binding constants between the GSH peroxidase monomers (Epp et al. 1983).

IV. Active Centers

GSH peroxidase has high reaction rates. The reason for this is that the catalytically active selenocysteine residues on the molecular surface are exposed which allows for easy access for substrates . Aromatic residues in the immediated vicinity allow for hydrogen bonding with the selenocysteine . This helps to stabilize the active-site.

The selenocysteine lies in the area where the carboxy ends of two parallel Beta-sheets and the N-terminal end of one Alpha-helix 1 meet . This is the area where substrate binding occurs.The alpha1 helix combined with two adjacent parallel Beta strands form a Beta-Alpha-Beta substructure. Thus secondary structure formation is favorable due to the carboxy ends of the Beta strands near the binding region. The macrodipole created by alignment of the alpha helix parallel to the helix axis stabilizes the active-site selenolate.

When treated with cyanide, the charge distribution at the active site is shifted. The seleno-amino acid has a negative charge taken away. This results in a 0.4-nm shift of the Met-101 side chain located on the opposite surface of the monomer compared to the selenium site . The shift is consistent with the formation of hydrogen bonds of the selenoloate to 148-Trp N and to 70-Gln N. This results in stabilization of the active-site .

V. Substrate Binding

Highly oxidized Gluthatione peroxidase binds the donor substrate glutathione with high affinity (Ren et al. 1983). In contrast, reduced GSH peroxidase binds glutathione with low affinity. In an oxidized environment, GSH peroxidase binds four glutathione molecules per subunit by a selenosulfide linkage. The specific binding interaction between a subunit of the enzyme and glutathione can be characterized by several important amino acid interactions. Amino acids Arg-40 and Arg-167 form a salt bridge with the glutathione (Ren et al. 1997). Residue Gln-130 forms a hydrogen bond with the N-terminus of the ligand (Ren et al. 1997).

VI. References

Epp Otto, Ladenstein Rudolf, and Wendel Albrecht. 1983. The Refined Structure of the Selenoenzyme Glutathione Peroxidase at 0.2-nm Resolution. Biochemistry 133:51-69.

Ladenstein Rudolf, Epp Otto, Bartels Klaus, Jones Alwyn, Huber Robert and Wendel Albrecht. 1979. Structure Analysis and Molecular Model of the Selenoenzyme Glutathione Peroxidase at 2.8 Angstrom Resolution. Journal of Molecular Biology 134:199-218.

Ren Bin, Huang Wenhu, Akesson Bjorn, Ladenstein Rudolf. 1997. The Crystal Structure of Seleno-Glutathione Peroxidase from Human Plasma at 2.9 A Resolution. Journal of Molecular Biology 268:869-885.

Sharadamma C. K., Purushotham B., Radhakrishna M.P., Abhilekha M.P., and Vagdevi M.H. 2010. Role of Selenium in Pets Health and Nutrition: A Review. Asian Journal of Animal Sciences 5:64-70.