Tetrahymena Glutathione Peroxidase Family: A Comparative Analysis of These Antioxidant Enzymes and Differential Gene Expression to Metals and Oxidizing Agents

Abstract

<jats:p>In the present work, an extensive analysis of the putative glutathione peroxidases (GPx) of the eukaryotic microorganism model Tetrahymena thermophila is carried out. A comparative analysis with GPx present in other Tetrahymena species and other very taxonomically diverse ciliates is also performed. A majority of ciliate GPx have replaced the selenocysteine (Sec) by Cys in its catalytic center, so they can be considered as phospholipid hydroperoxide glutathione peroxidases (PHGPx). Selenocysteine insertion sequence (SECIS) elements have been detected in several ciliate GPx that do not incorporate Sec in their amino acid sequences, and conversely, in other ciliate GPx with Sec, no SECIS elements are detected. These anomalies are analyzed and discussed. From the phylogenetic analysis using the ciliate GPx amino acid sequences, the existence of extensive intra- and interspecific gene duplications that produced multiple GPx isoforms in each species is inferred. The ancestral character of the selenoproteins is also corroborated. The analysis by qRT-PCR of six selected T. thermophila GPx genes has shown a quantitative differential expression between them, depending on the stressor (oxidizing agents, apoptotic inducer or metals) and the time of exposure.</jats:p>

Molecular Characterization, Protein–Protein Interaction Network, and Evolution of Four Glutathione Peroxidases from Tetrahymena thermophila

“…These putative regulatory elements are commonly found in the promoter regions of genes belonging to the antioxidant system and contribute to the overall regulation of transcription in the presence of ROS inducers [ 16 , 20 ]. These peculiar structural features, occurring only in Tt-GPx11 isoform, have suggested that this gene could originate from an ancestor with the complete catalytic tetrad and may be considered an apomorphic trait [ 32 ]. Based on the phylogenetic analysis results, Tt-gpx11 evolution does not follow the pathway of other T. thermophila NS-GPxs and presents some common features with the only Se-GPx (Tt-gpx12).…”

“…Gene expression analysis demonstrates, for the first time, that T. thermophila is able to biosynthesize a GPx with selenium-dependent activity. This is an important result because recent studies have only highlighted that mRNA for a Se-GPx is accumulated in the cell [ 32 ], and this observation does not certify the effective mRNA translation and protein synthesis. The demonstration obtained in the present work is of further interest if we consider that the mRNA sequence encoding Tt-GPx12 (the only Se-GPx of T. thermophila ) has no detectable selenocysteine insertion sequence (SECIS) element, which represent a sequence segment in the 3’UTR region considered essential for the insertion of a selenocysteine in the primary sequence of the protein.…”

Molecular Characterization, Protein-protein Interaction Network, and Evolution of four Glutathione Peroxidases from <em>Tetrahymena Thermophila</em>

“…These putative regulatory elements are commonly in the promoter regions of genes belonging to the antioxidant system and contribute to the overall regulation of transcription in the presence of ROS inducers [19,22,[36][37][38]. These peculiar structural features, occurring only in Tt-GPx11 isoform, have suggested that this gene could be originated from an ancestor with the complete catalytic tetrad and may be considered an apomorphic trait [39]. Based on the phylogenetic analysis results, Tt-gpx11 evolution does not follow the pathway of other T. thermophila NS-GPxs and present some common features with the only Se-GPx (Tt-gpx12).…”

Selenium as a Bioactive Micronutrient in the Human Diet and Its Cancer Chemopreventive Activity

“…Selenocysteine (Sec) is classified as the 21st protein amino acid, and its structure is similar to that of cysteine, with which it differs only in the presence of an Se atom instead of a sulphur atom in its structure. The residues of this amino acid are inserted into the proteins in the cotranslation process because it is coded by the UGA codon, which usually means the codon “STOP” and terminates protein synthesis [ 80 , 81 , 82 , 83 ]. In the human genome, 25 genes that encode selenoproteins have been discovered [ 81 , 84 ].…”

“…In the Se-dependent enzymes (GPx1-4 and GPx6), the selenocysteine residue is at the active site of the enzyme, whereas in the Se-independent enzymes (GPx5, GPx7, and GPx8), instead of selenocysteine, there is cysteine [2,85,86]. Their main function is to defend from cellular oxidative stress (reduction of peroxides, reactive oxygen, and nitrogen species) [80], whereas more detailed functions of particular Gpx are presented in Figure 3. There is a strong relationship between serum Se levels and cell redox balance-consumption of this micronutrient has a direct proportional impact on the activity of Se-dependent GPxs [82].…”