Reciprocal, Copper-Responsive Accumulation of Plastocyanin and Cytochrome c 6 in Algae and Cyanobacteria: A Model for Metalloregulation of Metalloprotein Synthesis

On the basis of nutritional value and cytotoxicity, three broad categories of metals have been distinguished: metals that function as cofactors in enzymes and are toxic only at extremely high concentrations, metals that have no known metabolic function and are highly toxic, and metals that are essential for life but toxic at higher than threshold concentrations. Each of the three classes of metals presents a different biological problem with its own distinctive solution. The third situation, exemplified by copper and iron, is perhaps the most interesting to consider from the perspective of regulation, because the organism has to maintain metalloprotein levels and a relatively constant intracellular utilization pool, despite variation in the supply of metal nutrients and the potential for metal toxicity. The essence of the problem lies in the need for tight coordination and complementary regulation of uptake, utilization, and chelation. Elegant biological systems designed to address some aspects of this problem in eukaryotic cells, specifically the regulation of uptake and chelation, have been described in previous chapters. Another important topic is metal ion control of metal-utilizing pathways (exemplified by the biosynthesis of catalytic metalloproteins such as redox enzymes or electron transfer proteins). Recent experimental efforts in this area have emphasized metalloregulation of gene expression in microorganisms.