Selenenic acids derived from selenocysteine are involved in cell signaling and certain enzymatic processes.  The best known selenoenzyme, glutathione peroxidase (GPx), catalyzes the reduction of peroxides by glutathione (GSH). The selenenic acid intermediate (E-SeOH) is formed upon oxidation of the catalytically active selenol (E-SeH) by hydrogen peroxide. This selenenic acid derivative of the peroxidase then reacts with a thiol-containing cofactor (GSH) to generate the key intermediate selenenyl sulfide (E-SeSG). This intermediate is subsequently attacked by a second GSH to regenerate the selenol and the glutathione cofactor is released in its oxidized form, GSSG. The catalytic mechanism of GPx, involves selenol (R-SeH), selenenyl sulfide (R1-SeS-R2), and selenenic acid intermediates.
:RSeH  +  HO   →  RSeOH + HO
:RSeOH + GSH   →   GS-SeR + HO
:GS-SeR + GSH   →   GS-SG + RSeH
In the absence of thiols, selenols tend to overoxidize to produce seleninic acids. Many organoselenium compounds (selenenamides, diaryl diselenides) contain "interesting" biological activities.  Their activity is attributed to their mimicry of glutathione peroxidase activity.  They reduce hydroperoxides that otherwise convert to toxic byproducts and/or reactive oxygen species that can cause further damage to the cell.