The biotransformation of selenate into organic selenium starts with yeast detoxification by an excess of sodium selenate. The pathway through which this takes place is similar to that of sulfur metabolism for the production of sulfur-containing amino acids. In this pathway, selenium substitutes sulfur and is incorporated into the chemical structure of methionine and cysteine (Bierla et al. 2013).
Despite the homology between the gene sequences of these yeasts, S. boulardii has unique physiological and metabolic properties, such as resistance to temperature and acid stress (Fietto et al. 2004; Khatri et al. 2013). Similarly, hexose transporter genes (HXT11, HXT9), genes involved in asparagine catabolism (ASP3-1, ASP3-2, ASP3-3, ASP3-4), transporter gene ARN2, genes involved in thiamine or pyridoxin biosynthesis (SNZ2, SNZ3), and CUP1 metallothionein gene are absent from Saccharomyces boulardii (nom. inval.) ASM141397v1 (Khatri et al. 2017). However, none of these differences affects the capability of S. boulardii to biotransform inorganic selenium into organic selenium and its subsequent insertion in seleno proteins, seleno-nanoparticle production, and reduction to elemental selenium by detoxification processes.
Noting that biosynthesis starts with selenium detoxification and is carried out through a pathway similar to that of sulfur, we propose the beginning of absorption. Selenium could be absorbed in two different ways. The first one is through sulfur ABC membrane transporters, which are encoded by operon cysAWTP and where transport for selenium ions uses energy from hydrolysis of bound ATP. The second system is through the transport of selenium using sulfate permeases (Kieliszek et al. 2015) encoded by AB282_00450 and AB282_03394. These enzymes transfer selenate through the plasma membrane from the exterior.
Once the selenate is in the interior, the biotransformation process starts with the activation of selenate. This process is carried out through a sequence of two reactions. In the first one, the rest of the adenosyl-phosphoryl is transferred from ATP to selenate by the action of enzyme ATP sulfurylase encoded by AB282_02749. This produces adenylyl selenate, which is in turn phosphorylated to produce 3’phosphoadenylyl selenate through enzyme adenylyl-sulfate kinase (AB282_03058). Activated selenate is reduced to sulfite to carry out SeMet and SeCys biosynthesis. First, enzyme 3’-phosphoadenylsulfate reductase (AB282_05395) reduces it to adenosine 3’,5’-bisphosphate and free selenite, using reduced thioredoxin as substrate. Consecutively, the subunit alpha of assimilatory sulfite reductase (AB282_01793), turns selenite into hydrogen selenide. Selenide is transformed into selenohomocysteine by the action of O-acetylserine-O-acetylhomoserine sulfhydrylase (AB282_03569).
The biosynthesis reaction of SeMet from selenohomocysteine is catalyzed by enzyme cobalamin-independent methionine synthase (AB282_01662), where selenohomocysteine undergoes a methylation process to create SeMet. There is a dependence on cobalamin in the activation of methyltransferases, as in that of MetH isolated from E. coli (Thomas and Surdin-Kerjan 1997). Still, both in S. cerevisiae and S. boulardii, homocysteine methyltransferase is independent of cobalamin. This is verified since none require vitamin B12 as growth factor.
Additionally, SeMet creates S-adenosyl-selenomethionine through enzyme S-adenosylmethionine synthetase (AB282_03468/AB282_00999), the catalyzer when the adenosyl group of ATP is transferred to the selenium atom of methionine. There, selenohomocysteine is created again by enzyme S-adenosyl-L-homocysteine hydrolase encoded by AB282_01610, catabolizing S-adenosyl-L-homocysteine formed after the donation of the activated methyl group of S-adenosyl-L-methionine to a receptor. The substitution of methionine by SeMet in proteins does not significantly alter the kinetic properties of the enzymes (Kitajima and Chiba 2013).
On the other hand, the biosynthesis of SeCys from selenohomocysteine starts with the conversion of selenohomocysteine into selenocystathionine through the reaction catalyzed by the enzyme cystathionine β-synthase encoded by AB282_01996. The reaction is reversible by the action of the enzyme peroxisomal cystathionine β-lyase (AB282_02293) converting selenocystathionine into selenohomocysteine. A later step is the transformation of selenocystathionine into SeCys by the enzyme cystathionine γ-lyase (AB282_00053). In addition, SeCys is transformed into γ-glutamyl-selenocysteine, the first step in the biosynthesis of selenoglutathione. Finally, selenoglutathione is formed by the action of glutathione synthetase (AB282_04624), which catalyzes the synthesis of ATP-dependent selenoglutathione from γ-glutamyl-selenocysteine and glycine (Fig. 2).