Pleiotropic Role of Recombinant Silaffin‐Like Cationic Polypeptide P5S3: Peptide‐Induced Silicic Acid Stabilization, Silica Formation and Inhibition of Silica Dissolution

Abstract

Silica-mineralizing organisms such as diatoms manage several aspects of silica chemistry when polymerizing monomeric silicic acid into amorphous silica. Silicic acid is undersaturated in the diatoms' habitats and mechanisms of enrichment and prevention of uncontrolled mineralization are not well understood. Diatom-biosilica is associated with organic compounds, including polycationic, post-translationally modified peptides termed silaffins, which induce the condensation of silicic acid under supersaturated conditions. Here, we report the pleiotropic action of the designed silaffin-like peptide P5S3, which (i) stabilizes 4-8x silicic acid (in supersaturated conditions of 8.3 mm), (ii) decelerates silica hydrolysis in non-saturated conditions (1 mm) and (iii) enhances silica precipitation at 15-30x supersaturation (30 mm), forming a composite precipitate. Abundant cationic amino acids in P5S3 can interact with silica, though blocking experiments with selected anions also indicate a role of non-electrostatic (e. g. amide-silica) interactions. This shows how silaffin-like peptides could potentially contribute to both keeping silica soluble, and forming peptide-silica precipitates.

Secondary structure and dynamics study of the intrinsically disordered silica‐mineralizing peptide P5S3 during silicic acid condensation and silica decondensation

“…As shown before, the pleiotropic effects of P 5 S 3 on the formation or hydrolysis of silica involves the interaction of the peptide with silica or oligomers of silicic acid. We used UV‐CD to assess the impact of such interactions on the secondary structure of the peptide.…”

“…Moreover, for the highest silicic acid concentrations, we can exclude that the different extents of structural changes at pH 7.5 and 9.5 are due to some P 5 S 3 staying in solution, because in similar assays (pH 7.5, with 0.1 mM P 5 S 3 and 30 mM silicic acid (molar ratio 1:300), P 5 S 3 quantitatively co‐precipitated with silica (see Figure B (no NaCl)). At pH 5.5 and below, no precipitate (neither silica, nor P 5 S 3 ) occurred in earlier experiments …”

“…Molybdenum blue analysis was used to quantify the silicic acid in the pellet following hydrolysis in 1 M NaOH for 1 h at room temperature. For localization of P 5 S 3 , NaOH was added to the supernatant to achieve a concentration of 1 M, the peptide concentrated and desalted by trichloroacetic acid (TCA) precipitation, and analyzed by SDS‐PAGE.…”

“…At less supersaturated concentrations of silicic acid (that is, 8.3 mM, 4‐ to 8‐fold supersaturated) the peptide impeded the silicic acid polymerization to silica. At globally undersaturated (that is, 1 mM) silicic acid concentrations, P 5 S 3 slowed down the dissolution of silica . Here, we report on the conformation of the peptide in the presence or absence of silica as derived from NMR and CD 19, spectroscopy.…”

“…The R5 and PL12 peptides are partial sequences of Cylindrotheca fusiformis Sil1 2 and Thalassiosira pseudonana Sil3, respectively. Conversely, P 5 S 3 is a tailored peptide that can be produced by recombinant bacterial expression and is active in silicic‐acid mineralization and stabilization . Basic amino acids are highlighted in blue (underlined) [Color figure can be viewed at http://wileyonlinelibrary.com]…”