INTRODUCTION: SELENOPROTEINS 1. Definition of selenoprotein Selenoproteins are Selenocysteine (Sec) - containing proteins which have been identified in each of the 3 domains of life: eubacteria, archaea and eukaryotes. Sec, known as the 21st amino acid (aa), is encoded in these proteins by the codon UGA. Although it is normally a termination codon of protein synthesis, it is also used as a Sec codon. [6][7] Sec is an analog of cysteine (Cys), with the sulfur (S) atom present in the side chain of Cys being replaced by a Selenium (Se) atom. Se is an essential micronutrient and a deficiency may lead to disease, as for example the Keshan disease [8], but is also recognized as toxic in excess. Se is found in cells mostly in selenoproteins involved in redox systems and may have antioxidant protection capability [9].The set of selenoproteins in an organism is known as selenoproteome. [6] Figure 3. Structure of Selenocysteine. At the physiologic pH, its selenol is mostly deprotonated. Adapted from Kurokawa et al (2015)[9] 2. Biosynthesis of selenoproteins 2.1 Synthesis of selenocysteine In eukarya the first step of synthesis of Sec consists in the charge of a serine (Ser) to a tRNASec by the seryl-tRNA synthetase. The Ser-tRNASec is next converted into Sec-tRNASec by selenocysteine synthase, either directly or via a phosphoseryl intermediate produced by by the phosphoseryl-tRNA kinase, using monoseleno-phosphate as the substrate. [10] This compound (monoselenophosphate) is synthesized from selenite (SeO32-) or from an unstable selenide compound (Se2-) and ATP by selenophosphate synthetase 2 (SPS2) [7]. Nevertheless, the role of SPS1 is still not well established. The selenocysteine synthase activity could be borne by SLA/LP alone or by forming a complex with SECp43. Once the Sec-tRNASec is formed it can be incorporated creating a selenoprotein. Figure 4. Selenocysteine biosynthesis pathways in eukarya. Extracted from Allmang et al (2006)[10] 2.2 Incorporation of Sec into selenoproteins: SECIS In eukaryotes the Sec incorporation occurs at UGA codons via the interaction of two main different RNA- protein complexes. On the one hand, Sec-tRNASec has to be recruited by its specialized translation elongation factor, EFsec (eukaryotic Sec tRNA-specific elongation factor). On the other hand, the Sec insertion sequence (SECIS) has to be recognized by the SECIS binding protein 2 (SBP2)[11]. The SECIS element is a stem-loop structure located in the 3’-untranslated regions (UTRs) of selenoproteins genes in eukaryotes and archaea. [6] To describe the selenoproteome of Pogona vitticeps we will need to identify the selenoproteins as well as the proteins involved in their synthesis. Therefore, the proteins needed in the Sec-tRNASec formation and the SECIS recognition will have to be included in our genomic search. 3. Evolution of selenoproteins Selenoproteins exist in archaea, bacteria and eukaryotes, but not all species have them. For example, yeast and plants do not have selenoproteins but cysteine-containing homologues [12]. The predicted ancestral vertebrate selenoproteome is composed by 28 proteins, but several selenoproteins were lost after the terrestrial environment was colonized, providing us with the idea that tetrapods reduced their utilization of Sec compared with fishes [6]. During the speciation path, such as the one to P. vitticeps reptile, new selenoproteins were created by duplication of existing ones, some were lost and others replaced with Cys in the Sec position, as mentioned before. The function of more than half selenoproteins is still unknown, but there are some families that have been widely studied: glutathione peroxidase (GPx), thioredoxin reductase (TR) and iodothyronine deiodinase (DIO). They are involved in redox and catalytic reactions, and even if they have different enzymatic activities, all require reductants to provide the electrons for the catalytic redox reactions [6]. They will be studied with more detail in the discussion.