Structural evidence of the similarity of Sb(OH)3 and As(OH)3 with glycerol: implications for their uptake.

Recent experimental results suggest that As(III) and Sb(III) transport in prokaryotes and eukaryotes might be facilitated by aquaglyceroporins. GlpF, the glycerol facilitator in Escherichia coli was the first to be identified as a trivalent metalloid transporter. Quantum calculations have been performed to study the possible existence of common structural properties between As(OH) 3 and Sb(OH) 3 and glycerol. Because the mechanism of substrate migration is primarily related to the successive formation of hydrogen bonds between the substrate and the hydrophilic part of the channel wall, this study has focused on the structural, thermodynamic, and electrostatic comparison of the main As(III) and Sb(III) compounds present in aqueous solution at physiological pH values, As(OH) 3 and Sb(OH) 3, with the glycerol conformation closest to the structures of these As- and Sb-containing compounds. This particular glycerol conformation has then been compared to three known experimental conformations of glycerol present in the protein channel. Besides their stoichiometry and electroneutral condition, As(OH) 3 and Sb(OH) 3 show very strong similarities to both each other and the studied conformation of the glycerol molecule: Namely, they show a similar charge distribution and a slightly smaller volume than glycerol. Their smaller size can be an additional advantage for the transit through the narrowest region of the GlpF channel. However, the metalloid hydroxyl groups lack the flexibility of glycerol, which probably helps this molecule to adapt its conformation to the topology of the GlpF channel.