Silicon transporters

Pokrovski, G.S. and Schott, J., Experimental study of the complexation of silicon and germanium with aqueous organic species Implications for germanium and silicon transport and Ge/Si ratio in natural waters, Geochim. Cosmochim. Acta, 62, 3413, 1998. 

The results presented here suggest that a rich complex chemistry of pentacoordinate silicon with ligands derived from a-hydroxycarboxylic acids (including tartaric acid), hydroximic acids, and oxalic acid may be developed. As most of these ligands derive from natural products and as some of these X Si-silicon(IV) complexes were shown to exist in aqueous solution, compounds of this formula type are of particular interest it has been speculated in the literature [16] that silicon transport in biological systems might be based on higher coordinate Si species, and complexes such as the title compounds could be of interest as model systems in this respect. 

Hildebrand, M., Higgins, F., Busser, K., and Volcani, B. (1998). Characterization of a silicon transporter gene family in Cylindrothecafusiformis Sequences, expression analysis, and identification of homologs in other diatoms. Mol. Gen. Genet. 260, 480—486. 

Thamatrakoln, K., Alverson, A., and Hildebrand, M. (2006) Comparative sequence analysis of diatom sflicon transporters Toward a mechanistic model of silicon transport. J. Phycol. 42, 822-834. 

Heinen, W., 1965b. Time-dependent distribution of silicon in intact cells and cell-free extracts of Proteus mirabilis as a model of bacterial silicon transport. Arch. Biochem. Biophys., 110 137—149. 

Diatoms were also investigated for the mechanism of silicon transport that is an integral part of the silicification process. As the environmental concentrations of dissolved silicon are rather low, diatoms must have an efficient transport system. Orthosilicic acid must not only be transported into the cell, but also transported intracellularly into the SDV where silica formation occurs. A protein of the diatom C. fusiformis was characterized that transports silicon from seawater into the cell. This discovery was accomplished by cloning and characterizing the DNA that codes for this protein. However, silicon transporter proteins of this particular type are not necessarily involved in intracellular transport. 

Likhoshvai EV, Usol Tseva MV, Tikhonova IV, Adel Shin RV, Kler SA and Scherbakova TA (2002) Elements of the active center of silicon transporters in diatoms. Mol Biol 36 534-536. 

At low concentrations, Ge mimics the metabolic pathway of silicon, whereas at high concentrations it inhibits silicon transport by acting as a classical competitive inhibitor. This is true for all creatures for which silicon is essential or that need silifi-cation. In freshwater sponges, Ge acid (1-25 pg mL ) disrupts spirula formation (Simpson et al. 1983). Ge has also been shown to be toxic to marine gastropods (Rin-kevich 1986). It may also have an antimicrobial effect, with bacteria being more tolerant than yeast (Slawson et al. 1992). 

While the silicatein-mediated catalysis of silicon alkoxide condensation is interesting and potentially useful, it is unclear whether this reaction plays a significant role in biosilicification. This is largely beeause the identity of the proximate substrate for the biological synthesis remains unknown. Although it has been shown that the recently identified silicon transporter from diatoms mediates the uptake of dissolved silicic acid fi om the external environment [4], it is not yet clear whether this is the species that is transported into the SDV for condensation, or if that silicic acid is first conjugated or otherwise modified prior to condensation (cf. [13-15,22]). 

A probable mechanism of the refractory compounds synthesis is discussed. Hypothetical schemes of the carbon, boron and silicon transport in ionic-electronic melts are proposed. 

In Nature, specific silicon transport proteins (SITs) produced by diatoms are responsible for the uptake and delivery of orthosUicic acid (Si(OH)4, pIQ = 9.8) to the diatom. Within the cell, orthosihcic acid is then concentrated up to 1000-fold, resulting in the condensation of amorphous, hydrated sihca [45]. Strikingly, the... 

An HF acid test was made of the calcium carbonate matrix after mechanically separating it from the reacted quartz particles to determine if the initial portion of the reaction was caused by a rapid silicon transport into the calcium carbonate bulk and subsequent reaction. This analysis showed negligible silicon in the matrix for all the samples. This indicated that differences in the amount of the initial reaction for the various calcium carbonate matrix materials were not due to reaction away from the periphery of the quartz particles. 

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