Synthesis and decoding of selenocysteine and human health

Abstract

Selenocysteine, the 21st amino acid, has been found in 25 human selenoproteins and selenoenzymes important for fundamental cellular processes ranging from selenium homeostasis maintenance to the regulation of the overall metabolic rate. In all organisms that contain selenocysteine, both the synthesis of selenocysteine and its incorporation into a selenoprotein requires an elaborate synthetic and translational apparatus, which does not resemble the canonical enzymatic system employed for the 20 standard amino acids. In humans, three synthetic enzymes, a specialized elongation factor, an accessory protein factor, two catabolic enzymes, a tRNA, and a stem-loop structure in the selenoprotein mRNA are critical for ensuring that only selenocysteine is attached to selenocysteine tRNA and that only selenocysteine is inserted into the nascent polypeptide in response to a context-dependent UGA codon. The abnormal selenium homeostasis and mutations in selenoprotein genes have been causatively linked to a variety of human diseases, which, in turn, sparked a renewed interest in utilizing selenium as the dietary supplement to either prevent or remedy pathologic conditions. In contrast, the importance of the components of the selenocysteine-synthetic machinery for human health is less clear. Emerging evidence suggests that enzymes responsible for selenocysteine formation and decoding the selenocysteine UGA codon, which by extension are critical for synthesis of the entire selenoproteome, are essential for the development and health of the human organism.

Figure 1

Synthesis and co-translational incorporation of selenocysteine in humans. The cycle, which is conserved in archaea and eukaryotes, begins with a mischarging reaction in which seryl-tRNA synthetase attaches L-serine (L-Ser) to a non-cognate tRNA^Sec. A specific kinase, O-phosphoseryl-tRNA^Sec kinase (PSTK), phosphorylates the seryl group yielding a phosphoseryl (Sep)-tRNA^Sec intermediate. In the terminal synthetic reaction, O-phosphoseryl-tRNA^Sec:selenocysteinyl-tRNA^Sec synthase (SepSecS), catalyzes conversion of Sep-tRNA^Sec into selenocysteinyl (Sec)-tRNA^Sec by a mechanism that requires selenophosphate and a co-factor pyridoxal phosphate (PLP). Selenophosphate, the main selenium donor in man, is a product of the catalytic activity of selenophosphate synthetase (SPS2). Human SPS2 is a selenoenzymes that utilizes as a reaction substrate the final product of selenoprotein/selenocysteine degradation, selenide, and adenosine triphosphate (ATP). Finally, Sec-tRNA^Sec is targeted and delivered to the ribosome by a specialized elongation factor – EFsec. An auxiliary protein factor, SECIS-binding protein 2 (SBP2), is required for decoding of the selenocysteine UGA codon in all vertebrates, whereas a shorter ortholog is functional in invertebrates. Selenocysteine (green sphere) is inserted into the nascent protein (orange spheres) in response to a specific UGA codon. SECIS, an in-cis element in the selenoprotein mRNA located in the 3′-UTR, forms a stem loop structure and is required for decoding of the selenocysteine UGA codon. In bacteria, a single enzyme, SelA, converts Ser-tRNA^Sec to Sec-tRNA^Sec, elongation factor SelB binds directly to SECIS, which is, in turn, a part of the coding sequence.

Figure 2

PSTK and SepSecS recognize the distinct fold of tRNA^Sec by binding to different structural elements in tRNA^Sec (Left). Surface diagram of the human SepSecS tetramer (olive) complexed with human tRNA^Sec (red) shows that SepSecS binds the major groove of the extended acceptor-TΨC “helix” (based on PDB ID: 3HL2) (49) and it “measures” the distance between the variable arm and the CCA end. (Right) Archaeal PSTK (gray) binds the opposite side of tRNA^Sec (red) and ‘measures’ the distance between a longer D arm and the CCA end (based on PDB ID: 3ADD) (51). Acronyms explained in Figure 1 legend.

Figure 3

Point mutations in SepSecS give rise to progressive cerebello cerebral atrophy by affecting the tRNA^Sec-binding pocket and the active-site groove. (A) In the first mutant, alanine in position 239 (highlighted in red) in helix α8 is mutated into threonine. This substitution is likely to cause a change in positioning of helices α8 and α9. Because residues in helix α9 (Arg271 and Gln268) interact with the variable arm of tRNA^Sec (raspberry), any structural change in this part of the enzyme might reduce the binding affinity of the A239T SepSecS mutant for tRNA^Sec. (B) In the second mutant, a highly conserved tyrosine in position 334 is replaced with cysteine. The Y334C mutation would almost certainly remove an important hydrogen bond between the hydroxyl group in Tyr334 and the backbone carbonyl oxygen in a turn preceding Lys284 to which an obligatory co-factor PLP is covalently attached. Thus, it is likely that the orientation and position of PLP in the Y334C mutant be different than that in the wild-type enzyme. It is plausible that this mutation impairs the catalytic prowess of SepSecS. The enzyme is beige, tRNA^Sec is raspberry, and the important amino-acid residues are green stick-and-balls. The ribbon diagrams are based on PDBID 3HL2 (49). Acronyms explained in Figure 1 legend.

Figure 4

Mutations in components of the synthetic and decoding apparatus of selenocysteine have distinct effects on health of the human organism. A normal cycle supports synthesis of suitable levels of selenoproteins. Mutations (red star) in various components have been shown to cause disorders in humans. In particular, point mutations in SepSecS lead to severe neurological disorders in which the inflicted individuals cannot reach adulthood. Additionally, point mutations in the decoding apparatus (SBP2, SECIS, and SRE) cause a variety of disorders that are rarely lethal. The essentiality of the selenocysteine cycle has also been supported by the embryonic lethal phenotype of the mouse tRNA^Sec knockout mutant (blue star). Presumably, these mutations attenuate selenoprotein synthesis to a different degree, hindering development of the individual with differing levels of severity. Acronyms explained in Figure 1 legend.