Silicon transporter

A silicon transporter is a membrane protein that moves a dissolved silicon-containing solute across a cell membrane. In the diatom SIT family, the substrate is silicic acid, the soluble precursor used in building a silica-rich cell wall. The functional category need not imply that transporters in every lineage have the same structure or coupling mechanism.^[ref]

A purified-system experiment reconstituted SIT proteins into liposomes. An inward-directed sodium gradient drove uptake, detected by fluorescence from zinc-dependent complexes formed inside the liposomes. Protein-free and unrelated-transporter controls helped distinguish transport from mixing background. Selected transporters had apparent Michaelis constants near $20\mu \mathrm{M}$.^[ref]

Using that approximate scale in a simple Michaelis-Menten equation, $v/V_{\max }=C/(K_M+C)$ gives $1/2$ at $C=20\mu \mathrm{M}$ and $4/5$ at $C=80\mu \mathrm{M}$. This calculation illustrates saturation, not an additional experimental fit. A substrate-binding ratio measured at equilibrium does not by itself establish the number of sodium ions coupled to each transport event.