Fig. 4.
Silicatein sequences and modeled active sites. (A) Multiple protein sequence alignment of silicatein parents (α and β) and progeny (X1 and XT). Stars indicate sequence changes that directly involve hydroxyl residues; asterisks indicate changes occurring immediately adjacent to hydroxyl residues. Yellow and blue highlights denote residues structurally conserved in a putative catalytic triad configuration as determined by homology modeling; gray highlights denote residues in the distorted active site region of silicatein XT (Fig. S7). (B) Homology-modeled active site of silicatein α overlaid with that of silicatein X1, showing the shared putative catalytic triad configuration. Native silicatein α has been previously proposed to operate through an SN2-type attack by the active site serine-25/26 on a silicon precursor center, leading to precursor hydrolysis and subsequent polycondensation (23).