Zinc-dependent biochemical mechanisms that determine physiologic functions have received extensive study, but clear relationships with phenotype have not been defined. Zinc has a ubiquitous subcellular distribution, which hampers this situation. Consequently, zinc contrasts with iron, which exists in defined cellular components and has defined physiologic roles. Three general functional classes—catalytic, structural, and regulatory—define zinc’s role in biology (9
Zinc serves a catalytic role in enzymes from all six enzyme classes (10
). More than 300 zinc metalloenzymes have been identified. When the same enzyme identified in different species is counted only once, however, the number is much lower. How zinc is donated to apometalloenzymes has not been established. Zinc binding is a posttranslational protein modification, which likely requires a metal donor molecule or pH appropriate for zinc solubility, perhaps coordinated by events in the endoplasmic reticulum or a vesicular compartment. This process may require zinc transporter activity. An example is that a ZnT5/ZnT7 complex provides Zn2+
to activate tissue-nonspecific alkaline phosphatase (10
An enzyme is generally considered a zinc metalloenzyme if the removal of zinc causes loss of activity without irreversibly altering the protein, and selective reconstitution with zinc restores the activity. Examples are the nucleotide polymerases (RNA polymerases I, II, and III), alkaline phosphatases, and the carbonic anhydrases. Unequivocal evidence of a direct link between signs of zinc deficiency or toxicity and a specific metalloenzyme has not been shown in complex organisms, and it is most likely an overt physiologic defect would occur only if the zinc-requiring enzyme was rate limiting in a critical biochemical pathway. Older literature has examples of relationships among zinc, enzyme, and disease (e.g., alcohol dehydrogenase and liver disease and RNA polymerases and growth retardation). Such enzyme changes are no longer considered as representing a critical function for zinc. Reports documenting zinc-responsive control of enzymes for intermediary metabolism, perhaps operating through effects on intracellular zinc concentrations, have been published (11
). Demonstrated physiologic control of some zinc transporters offers a new appreciation of the way in which coordinated Zn2+
fluxes in cells could influence enzyme activity (reviewed in 13).
The structural function for zinc had its origin in 1985 with identification of a transcription factor having coordinating zinc-binding motifs (14
). These motifs (“zinc fingers”) use cysteine and histidine to form a tetrahedral Zn2+
coordination complex. These have the general structure -C-X2
-C-, where C designates cysteine or histidine and X designates other amino acids. Zinc fingers have two to four cysteines and up to two histidines. Removal of zinc from zinc finger proteins alters folding and results in loss of function and probably degradation. Classic examples of zinc finger transcription factors are the retinoic acid and calcitriol nuclear receptors. The human transcriptome has 2500 zinc finger protein genes (15
). This represents approximately 8% of the genome, a finding suggesting that a significant portion of the zinc requirement is allocated to maintain occupancy of zinc finger proteins. The mouse transcriptome has a comparable number of zinc finger genes (16
). Binding affinity (apparent stability constants) of the fingers varies widely (dissociation constant [Kd
] = 108
). By comparison, metallothionein (MT) binds zinc strongly (to a maximum Kd
). Both nitrosative stress and oxidative stress can disrupt zinc finger motifs and can cause loss of function, at least for oxidative stress (18
). Because zinc fingers exhibit a spectrum of binding affinities (19
), some sites may be particularly facile and potentially influenced by dietary zinc through zinc transporter activity.
Zinc finger proteins have a broad cell distribution and also bind RNA molecules and facilitate protein-protein interactions. These functions broaden their biologic role to include transcriptional and translational control, modulation of those processes, and signal transduction. Interest in zinc finger motifs is considerable because of their potential as targets for therapeutic interventions, including gene therapy.
Zn/S clusters, such as those in MT, may act as low redox potential units (17
). These zinc thiol clusters, when oxidized by cellular oxidants (including oxidative and nitrosative stressors), results in zinc release. The glutathione/glutathione disulfide (GSH/GSSG) redox couple and some selenium compounds influence zinc release, which potentially integrates MT into cellular redox mechanisms. Nitric oxide (NO) may also mobilize zinc from this protein’s thiolate clusters (20
). This mobilization may be limited to the protein’s beta domain (3 Zn cluster), whereas the alpha domain (4 Zn cluster) is viewed as having a detoxifying role. Increased oxidative and nitrosative stress that accompanies zinc deficiency (22
) may be explained in part by induction of NO synthase (24
A hybrid function between structure and regulatory is that movement of large quantities of zinc are associated with insulin secretion by pancreatic β cells, zinc metallodigestive enzyme secretion by pancreatic acinar cells, and acid secretion by parietal cells of the stomach. In the first two instances, Zn2+
has a stabilizing role during the secretory process, whereas Zn2+
may replace hydrogen ion (H+
) during gastric acid release (26
). The transporters
ZnT8, ZnT2, and ZIP11 are the likely major players for these functions.
Regulation of gene expression is a biochemical role for zinc. Originally identified as an active component of the metalloregulatory mechanism for MT
gene regulation, the metalresponse element (MRE)-binding transcription factor 1 (MTF1) now is believed to provide zinc responsiveness to many genes (28
), including acting as a master regulatory transcription factors (30
) for miRNA genes involved in gene repression. Null mutation of the MTF1
gene produces embryonic lethality in mice, a finding indicating importance in animals. On zinc occupancy, MTF1 translocates to the nucleus, where it participates in chromatin binding through MREs of the gene promoter. Polymorphisms in the zinc finger domain of the human MTF1
) suggest the possibility of genetic variation in the response of MTFl-regulated genes to dietary zinc intakes. A homologous transcription factor MTF2, documented to be involved in stem cell development (32
), may repress genes during normal zinc status and produce activation on zinc depletion. Reciprocal expression of zinc-responsive genes, including zinc transporters, that maintain zinc homeostasis may be regulated by these and other transcription factors that provide opposite responses to zinc status.
The second regulatory role executed by zinc is as a regulator of cell signaling pathways. This places Zn2+
in an intracellular role that is analogous to calcium (Ca2+
), except at a finer level of control. A primary mode of action is through regulation of kinase and phosphorylase activity (33
is a powerful inhibitor of phosphatases in the low micromolar range. Such control of phosphorylation and dephosphorylation could explain many of the effects attributed to zinc on activity of phosphorylated transcription factors, cell surface receptor binding of growth factors and cytokines, and activity of key phosphorylated substrates within cells. The profound effects of zinc on the immune system may thus be traceable to effects of Zn2+
availability to indirectly regulate transcription factors such as the STATs, NFAT and CREBP, as well as the phosphatase, calcineurin and the cytoplasmic protein tyrosine kinase, lymphocytespecific protein tyrosine kinase. The coordination of such activities may relate to any one or several of the 24 zinc transporters that exhibit differing specificity of expression in various cell types. Relevant examples are the influence of ZIP8 on interferon gamma (IFN-γ) production by T cells, and regulation by ZIP6 of lipopolysaccharide-induced histocompatibility complex in dendritic cells (36
Zinc is abundant in the central nervous system. A considerable portion is in the form of ionic Zn2+
, in concentrations referred to as [Zn2+
]i from the picomolar up to the millimolar range in synaptic vesicles (38
). Zinc affects activity of N
-methyl-D-aspartate and γ-aminobutyric acid receptors to influence synaptic transmission.
Neuronal [Zn] is tightly controlled by a brain-specific MT and members of both the ZnT and ZIP transporter families. The mechanism of zinc transport across the blood-brain barrier has not been established. Dietary zinc deficiency has been shown to alter brain zinc homeostasis (39
). Cerebral ischemia leads to Zn2+
release, which participates in activation of downstream signaling cascades (particularly P13K/Akt) and oxidative and nitrosative stress that leads to necrotic, apoptotic, and autophagic death of neuronal and glial cells (38