Computational Insights into the Structural Properties and Catalytic Functions of Selenoprotein Glutathione Peroxidase (GPx)

Selenium (Se) was discovered in 1817, and in mammals its deficiency has been associated with many fatal diseases, such as cancer, HIV, cardiovascular and Keshan-Back [1, 2]. In most selenoproteins discovered so far, Se is present as a selenocysteine residue that plays an important role in the catalytic activities of these enzymes [3]. One of the first selenoproteins to be discovered was Glutathione Peroxidase (GPx), which demonstrated a strong anti-oxidant activity [4, 5]. This protein reduces numerous reactive oxygen species (ROS), including hydrogen peroxide [4] and peroxynitrite [6], by utilizing various reducing substrates, and protects cell membranes and other cellular components against oxidative damage [4]. Four different classes of Se-dependent GPx have been classified: (1) cytosolic (GPx-1), (2) gastrointestinal tract (GPx-2), (3) extracellular (GPx-3) and (4) phospholipid hydroperoxide (GPx-4) [3]. All four types are easily reduced by glutathione while they also accept some other reducing substrates. For example, GPx-3 can be reduced by glutaredoxin and thioredoxin [7], and GPx-4 utilizes protein thiols as reducing substrates [8, 9]. The catalytic activity of GPx has been mimicked in various organoselenium compounds possessing a direct SeaN bond; the most prominent among them is an anti-inflammatory drug called ebselen [10–12]. The only crystal structures elucidated are of bovine erythrocyte (intracellular enzyme, GPx-1) and human plasma (extracellular enzyme, GPx-3) GPx, resolved at 2.0 and 2.9 Å resolution, respectively [13, 14]. The complete X-ray structure of human plasma GPx (2.9 Å) is available in the literature [4]. These enzymes are tetrameric, with two asymmetric units containing two dimers that exhibit half-site reactivity. The dimeric X-ray structure is shown in Figure 1.1 and each monomer contains a critical selenocysteine residue. The following experimental observations explicitly implicate this residue in the catalytic cycle: (a) Treatment 1