Figure 1

Universal orders of metal preference and proposed buffered metal concentrations in cells. (a) The Irving-Williams series provides a preferred binding order for divalent metals. (b) An order of preference for thiolate-containing ligands of some nonessential, and some essential, metals. (c) How the occupancy of HpCdMT and HpCuMT might relate to the buffered concentrations of metals in a hypothetical cell. The total concentration of each metal in a cell (squares) is many orders of magnitude greater than the buffered concentration (circles), with the exception of bulk solutes such as sodium and potassium. The values have been adapted from a proposal of da Silva and Williams [4]. Proteins with affinities tighter than the buffered concentrations can acquire metals. HpCuMT (green triangles) is suggested to have an affinity for copper slightly tighter than the buffered concentration of copper found in many cell types, whereas HpCdMT (red triangles) is suggested to have a copper affinity too weak to compete effectively with the copper buffers of at least some cell types. In this scheme the situation is reversed for zinc. There is evidence that the relationship between the H. pomatia metallothioneins and buffers for non-cognate metals depart from this scheme in some cell types.