Zinc biochemistry: from a single zinc enzyme to a key element of life

Abstract

The nutritional essentiality of zinc for the growth of living organisms had been recognized long before zinc biochemistry began with the discovery of zinc in carbonic anhydrase in 1939. Painstaking analytical work then demonstrated the presence of zinc as a catalytic and structural cofactor in a few hundred enzymes. In the 1980s, the field again gained momentum with the new principle of "zinc finger" proteins, in which zinc has structural functions in domains that interact with other biomolecules. Advances in structural biology and a rapid increase in the availability of gene/protein databases now made it possible to predict zinc-binding sites from metal-binding motifs detected in sequences. This procedure resulted in the definition of zinc proteomes and the remarkable estimate that the human genome encodes ∼3000 zinc proteins. More recent developments focus on the regulatory functions of zinc(II) ions in intra- and intercellular information transfer and have tantalizing implications for yet additional functions of zinc in signal transduction and cellular control. At least three dozen proteins homeostatically control the vesicular storage and subcellular distribution of zinc and the concentrations of zinc(II) ions. Novel principles emerge from quantitative investigations on how strongly zinc interacts with proteins and how it is buffered to control the remarkably low cellular and subcellular concentrations of free zinc(II) ions. It is fair to conclude that the impact of zinc for health and disease will be at least as far-reaching as that of iron.

Figure 1

The three phases of discoveries in zinc biology. Top: Discoveries that led to the recognition of zinc as an essential nutrient began with Jules Raulin’s seminal investigations of the dependence of Aspergillus niger on zinc for growth and culminated in the discovery of zinc deficiency in humans by Ananda Prasad. Middle: Discoveries of zinc as a cofactor of proteins also has a defined beginning, namely, David Keilin’s finding of zinc in carbonic anhydrase, and reached a certain end point by Ivano Bertini’s estimates of the number of zinc proteins in zinc proteomes. Bottom: Discoveries related to the role of zinc(II) ions in regulation do not have well-defined beginnings and are open in terms of future implications.

Figure 2

Control and functions of free zinc(II) ion concentrations. Two pathways (left) increase the cytosolic free zinc(II) ion concentrations, [Zn²⁺]_i, which serve as zinc signals (right) that inhibit enzymes and induce metal-response element–binding transcription factor 1 (MTF-1)–dependent gene transcription, which includes the synthesis of zinc transporters (ZnT1), and thionein that activates zinc-inhibited enzymes. Buffering and muffling control the free zinc(II) ion concentrations. Zip, ZRT/IRT-like protein.

Figure 3

Quantitative aspects underlying cellular zinc biochemistry. Zinc (Zn), calcium (Ca), and magnesium (Mg), the three major redox-inert metal ions involved in cellular regulation, cover many orders of magnitude in concentrations and affinities. Among the three, free zinc(II) ions are controlled at the lowest concentrations. Other essential divalent transition metal ions, with the exception of cupric ions, bind less tightly than zinc to proteins and need to be controlled at specific concentrations that are determined by affinities following the Irving-Williams series. Fluctuations of cytosolic free zinc(II) ion concentrations cover a range that corresponds to the affinities of metallothionein (MT)-2 for zinc and are bordered by zinc enzymes with the highest affinity for zinc and proteins, which zinc may regulate, with lower affinity for zinc.