Zinc is a trace mineral, meaning the human body requires only small amounts of it to function properly. It is the second most abundant trace element, found in every cell and necessary for countless biological processes. When involved in these processes, zinc exists as an ion, carrying a net electrical charge due to the gain or loss of electrons. In biological systems, zinc exists almost exclusively as a positively charged ion, which dictates its behavior and enables its numerous roles in health.
The Atomic Basis of the +2 Charge
The chemical behavior of zinc, and the reason it carries a +2 charge, is rooted in its atomic structure. Zinc is element number 30, meaning a neutral atom contains 30 protons and 30 electrons. The electron configuration shows that the zinc atom has two electrons in its outermost shell, the 4s orbital.
To achieve a more stable state, atoms tend to lose or gain electrons. Zinc achieves maximum stability by losing these two 4s electrons, which are the farthest from the nucleus and easiest to remove.
By shedding these two negatively charged electrons, the atom is left with 30 protons but only 28 electrons, creating a net positive charge of +2. The resulting ion, \(\text{Zn}^{2+}\), has a highly stable configuration due to a completely filled 3d subshell. This stable structure prevents the ion from losing further electrons, which is why zinc virtually always forms a \(\text{Zn}^{2+}\) ion.
How Charged Zinc Interacts with Molecules
The \(\text{Zn}^{2+}\) ion’s positive charge and stable electron configuration define its chemical function within the body. Because it is positively charged, it acts as a Lewis acid (an electron pair acceptor), allowing it to readily bind to electron-rich sites on biological molecules. The \(\text{Zn}^{2+}\) ion coordinates tightly with electron donor atoms found in proteins, such as the nitrogen atoms in histidine or the sulfur atoms in cysteine and methionine.
This coordination chemistry allows zinc to perform two major roles: structural stabilization and catalytic activity. For structural roles, the \(\text{Zn}^{2+}\) ion acts as a rigid chemical scaffold, holding parts of a protein in a precise three-dimensional shape. A common example is the zinc finger motif, where the ion stabilizes a structure that allows the protein to recognize and bind to DNA.
The catalytic role involves the \(\text{Zn}^{2+}\) ion participating in the active site of over 300 enzymes. Here, the positive charge pulls electron density away from a substrate molecule, polarizing it and making it more reactive. In the enzyme carbonic anhydrase, for instance, the \(\text{Zn}^{2+}\) ion coordinates a water molecule, enabling the enzyme to quickly convert carbon dioxide to bicarbonate.
Essential Functions in Human Health
The unique chemical properties of the \(\text{Zn}^{2+}\) ion translate directly into its necessity for maintaining human health. One of its most recognized roles is supporting the immune system, where it is required for the development and proper functioning of immune cells, including T-cells and natural killer cells. Even a mild deficiency can impair the activity of these infection-fighting lymphocytes and macrophages.
Zinc’s role as a structural stabilizer is important for the processes of growth and repair throughout the body. It is required for DNA synthesis and protein production, which are fundamental to cell growth and division. This makes \(\text{Zn}^{2+}\) important during periods of rapid growth, such as childhood and pregnancy, and for the repair of damaged tissue during wound healing.
The ion also plays a direct role in sensory perception, specifically taste and smell. Zinc is a component of gustin, a protein involved in taste bud function, and is necessary for an enzyme that helps process compounds related to smell. Furthermore, zinc is involved in cell signaling pathways and maintaining healthy vision and neurological function.
Dietary Intake and Bioavailability
Since the body does not have a specialized system for storing large reserves of zinc, a regular dietary intake is required to ensure a constant supply of the \(\text{Zn}^{2+}\) ion.
Sources of Zinc
The best sources of highly bioavailable zinc are animal proteins, such as red meat, poultry, and seafood, especially oysters. Dairy products and eggs also contribute to intake. Zinc is also found in plant-based foods like legumes, nuts, seeds, and whole grains.
Bioavailability Challenges
The bioavailability, which is the fraction of ingested zinc that the body can actually absorb and use, is often lower from plant sources. This reduction in absorption is primarily due to the presence of phytates (phytic acid), which are the main storage form of phosphorus in many plants. Phytates bind to the positively charged \(\text{Zn}^{2+}\) ions in the gut, forming an insoluble complex that the body cannot easily absorb. This is a concern for individuals who consume high amounts of legumes and grains, such as vegetarians and vegans, who may have lower zinc levels. The absorption process happens in the small intestine, where the \(\text{Zn}^{2+}\) ion requires specific transport proteins to move from the gut into the bloodstream.