The chemical and biochemical properties of Mg present the cellular system with a significant challenge when transporting the ion across biological membranes. The dogma of ion transport states that the transporter recognises the ion then progressively removes the water of hydration, removing most or all of the water at a selective pore before releasing the ion on the far side of the membrane. Due to the properties of Mg, large volume change from hydrated to bare ion, high energy of hydration and very low rate of ligand exchange in the inner coordination sphere, these steps are probably more difficult than for most other ions. To date, only the ZntA protein of Paramecium has been shown to be a Mg channel. The mechanisms of Mg transport by the remaining proteins are beginning to be uncovered with the first three-dimensional structure of a Mg transport complex being solved in 2004.
The hydration shell of the Mg ion has a very tightly bound inner shell of six water molecules and a relatively tightly bound second shell containing 12–14 water molecules (Markham et al., 2002). Thus, it is presumed that recognition of the Mg ion requires some mechanism to interact initially with the hydration shell of Mg, followed by a direct recognition/binding of the ion to the protein. Due to the strength of the inner sphere complexation between Mg and any ligand, multiple simultaneous interactions with the transport protein at this level might significantly retard the ion in the transport pore. Hence, it is possible that much of the hydration water is retained during transport, allowing the weaker (but still specific) outer sphere coordination.
In spite of the mechanistic difficulty, Mg must be transported across membranes, and a large number of Mg fluxes across membranes from a variety of systems have been described. However, only a small selection of Mg transporters have been characterised at the molecular level.