The functions, purposes,
and roles of metallothioneins have been
the subject of speculations since the discovery of the protein over
60 years ago. This article guides through the history of investigations
and resolves multiple contentions by providing new interpretations
of the structure-stability-function relationship. It challenges the
dogma that the biologically relevant structure of the mammalian proteins
is only the one determined by X-ray diffraction and NMR spectroscopy.
The terms metallothionein and thionein are ambiguous and insufficient
to understand biological function. The proteins need to be seen in
their biological context, which limits and defines the chemistry possible.
They exist in multiple forms with different degrees of metalation
and types of metal ions. The homoleptic thiolate coordination of mammalian
metallothioneins is important for their molecular mechanism. It endows
the proteins with redox activity and a specific pH dependence of their
metal affinities. The proteins, therefore, also exist in different
redox states of the sulfur donor ligands. Their coordination dynamics
allows a vast conformational landscape for interactions with other
proteins and ligands. Many fundamental signal transduction pathways
regulate the expression of the dozen of human metallothionein genes.
Recent advances in understanding the control of cellular zinc and
copper homeostasis are the foundation for suggesting that mammalian
metallothioneins provide a highly dynamic, regulated, and uniquely
biological metal buffer to control the availability, fluctuations,
and signaling transients of the most competitive Zn(II) and Cu(I)
ions in cellular space and time.