Dataset Information

Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase.

ABSTRACT: Selenium and sulfur are two closely related basic elements utilized in nature for a vast array of biochemical reactions. While toxic at higher concentrations, selenium is an essential trace element incorporated into selenoproteins as selenocysteine (Sec), the selenium analogue of cysteine (Cys). Sec lyases (SCLs) and Cys desulfurases (CDs) catalyze the removal of selenium or sulfur from Sec or Cys and generally act on both substrates. In contrast, human SCL (hSCL) is specific for Sec although the only difference between Sec and Cys is the identity of a single atom. The chemical basis of this selenium-over-sulfur discrimination is not understood. Here we describe the X-ray crystal structure of hSCL and identify Asp146 as the key residue that provides the Sec specificity. A D146K variant resulted in loss of Sec specificity and appearance of CD activity. A dynamic active site segment also provides the structural prerequisites for direct product delivery of selenide produced by Sec cleavage, thus avoiding release of reactive selenide species into the cell. We thus here define a molecular determinant for enzymatic specificity discrimination between a single selenium versus sulfur atom, elements with very similar chemical properties. Our findings thus provide molecular insights into a key level of control in human selenium and selenoprotein turnover and metabolism.

SUBMITTER: Collins R

PROVIDER: S-EPMC3266270 | biostudies-literature | 2012

REPOSITORIES: biostudies-literature

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Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase.

Collins Ruairi R Johansson Ann-Louise AL Karlberg Tobias T Markova Natalia N van den Berg Susanne S Olesen Kenneth K Hammarström Martin M Flores Alex A Schüler Herwig H Schiavone Lovisa Holmberg LH Brzezinski Peter P Arnér Elias S J ES Högbom Martin M

PloS one 20120125 1

Selenium and sulfur are two closely related basic elements utilized in nature for a vast array of biochemical reactions. While toxic at higher concentrations, selenium is an essential trace element incorporated into selenoproteins as selenocysteine (Sec), the selenium analogue of cysteine (Cys). Sec lyases (SCLs) and Cys desulfurases (CDs) catalyze the removal of selenium or sulfur from Sec or Cys and generally act on both substrates. In contrast, human SCL (hSCL) is specific for Sec although th ...[more]

PMID: 22295093

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Project description:Mammalian thioredoxin reductase (TR) contains a rare selenocysteine (Sec) residue in a conserved redox-active tetrapeptide of sequence Gly-Cys(1)-Sec(2)-Gly. The high chemical reactivity of the Sec residue is thought to confer broad substrate specificity to the enzyme. In addition to utilizing thioredoxin (Trx) as a substrate, other substrates are protein disulfide isomerase, glutaredoxin, glutathione peroxidase, NK-lysin/granulysin, HIV Tat protein, H(2)O(2), lipid hydroperoxides, vitamin K, ubiquinone, juglone, ninhydrin, alloxan, dehydroascorbate, DTNB, lipoic acid/lipoamide, S-nitrosoglutathione, selenodiglutathione, selenite, methylseleninate, and selenocystine. Here we show that the Cys(2) mutant enzyme or the N-terminal reaction center alone can reduce Se-containing substrates selenocystine and selenite with only slightly less activity than the wild-type enzyme, in stark contrast to when Trx is used as the substrate when the enzyme suffers a 175-550-fold reduction in k(cat). Our data support the use of alternative mechanistic pathways for the Se-containing substrates that bypass a critical ring-forming step when Trx is the substrate. We also show that lipoic acid can be reduced through a Sec-independent mechanism that involves the N-terminal reaction center. These results show that the broad substrate specificity of the mammalian enzyme is not due to the presence of the rare Sec residue but is due to the catalytic power of the N-terminal reaction center. We hypothesize that the N-terminal reaction center can reduce substrates (i) with good leaving groups such as DTNB, (ii) that are highly electrophilic such as selenite, or (iii) that are activated by strain such as lipoic acid/lipoamide. We also show that the absence of Sec only changed the IC(50) for aurothioglucose by a factor of 1.7 in the full-length mammalian enzyme (83-142 nM), but surprisingly the truncated enzyme showed much stronger inhibition (25 nM). This contrasts with auranofin, where the absence of Sec more strongly perturbed inhibition.

Dataset Information

Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase.

Publications

Biochemical discrimination between selenium and sulfur 1: a single residue provides selenium specificity to human selenocysteine lyase.

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