Cellular uptake of molybdenum and tungsten

Cells acquire molybdenum and tungsten as their highly soluble oxoanions, (MoO42-)-O-VI or (WO42-)-O-VI, which they internalize by means of an active (i.e. energy requiring) transmembrane importer, for subsequent conversion into the metalloenzyme cofactors Moco or Wco (and FeMoco in nitrogen fixers). This import system has been studied as one of the models for the functioning of the protein complex superfamily of ABC (ATP binding cassette) transporters, but its mechanistic details are presently not clear. The complex exhibits interesting variants, known as the microbial Mod, Tup, and Wtp system, and the - less well defined - eukaryotic MOT1 system, which mutually differ in oxoanion coordination chemistry and in the control of intracellular Mo/W levels. This evolutionary diversity of Mo/W transporters has resulted in confusing nomenclature whose rectification is here proposed. (C) 2011 Elsevier B.V. All rights reserved.