A street interviewer asking strangers, “What do you know about zinc?” is likely to get puzzled looks and few quick answers. One respondent might recall hearing that zinc lozenges help reduce the severity of colds. One might say it’s an element, or a metal. Another might have seen zinc supplements in nutrition stores. Well, here is a new paper describing a previously unknown zinc transporter, offering a prompt and an opportunity to learn more. Also, I’m thoroughly enjoying biologist Michael Denton’s latest book, The Miracle of Man. And as it happens, zinc gives a perfect example of several themes he discusses in the book.
Zinc is element 30 in the periodic table, and the most abundant metal in the body after iron. Even so, we only carry about three grams of zinc by weight. That trace amount should not be disparaged: it is vital for 10 percent of the proteins and enzymes in our cells. In his earlier book The Miracle of the Cell (2020), Denton devoted two pages to zinc, listing the varieties of some 300 enzymes that rely on its unique properties. Last year, in an article about metals in proteins, Casey Luskin mentioned several important functions that zinc enzymes perform.
One important zinc enzyme Denton focuses on is carbonic anhydrase. It converts CO2 in our cells to bicarbonate, and then reverses the reaction in the lungs. As a result, the end product of oxidative metabolism — carbon dioxide — gets safely breathed out into the air for plants to take in. And that’s not all:
Carbonic anhydrase also aids in the regulation of fluid and pH balance and is involved in producing essential stomach acid. The enzyme also plays a role in vision. When it is defective, fluid can build up and cause glaucoma. The enzyme is one of the fastest known, catalyzing up to one million reactions per second. [Emphasis added.]
Clearly, we could not live without those three grams of zinc. But how does a metal as ideal as this get into the food chain?
Getting Zinc from Crust to Soil
First, it must be available in crustal rock. Zinc is not rare, but a kilogram of rock on average only has 70 milligrams of zinc (JLab Science Education), demoting it to the 23rd most abundant element in the crust (USGS). Hydrothermal vents and volcanoes transport zinc to the surface most often as sphalerite (Geology.com), a compound with iron and sulfur, which is mined throughout the world. While zinc has many applications for industry, our interest here is how it becomes available to living things. Sphalerite has cleavage planes and a relatively low hardness that make it easy to fragment, but it would never reach plant roots without help from the Earth’s amazing water cycle.
In Chapter 2 of The Miracle of Man, Denton waxes eloquent about the hydrological cycle. He relates some of the unique properties of water that make essential minerals bioavailable. These include water’s ability to exist in three states (gas, liquid, solid) at atmospheric temperatures, its low viscosity, its fame as a near-universal solvent, and its high surface tension. Because of these properties, water seeps into every crack in rock and breaks it apart as it freezes. Rivers carry dissolved minerals to the land, and glaciers grind bedrock into fine powder. This constant pulverizing and transporting of zinc brought to the surface by plate tectonics and volcanism is the start of the element’s journey into plants, but the job is not done.
If it were not for soils, with mineral-rich sands and clays that can store water, the minerals would still be unavailable for life. Denton sees a “wondrous synergy” of independent properties of water and earth’s crust that indicate “prior fitness” of the planet for complex life. He arranges these factors into a “teleological hierarchy” with design implications that suggest providence — not from theological arguments, but from the scientific evidence itself. It’s a “stunning ensemble” of factors that “stands as a monumental testimony that nature is indeed fine-tuned for terrestrial life.”
Getting Zinc from Soil to Cell
The “zinc cycle” continues with zinc transported by water to soil. Clay minerals, Denton notes, have a thousand-fold more surface area than sand. Because of their layered structure and electrical charges, clays attract water molecules and can hold them much longer, defying the gravity that would drain soil dry quickly. Roots from plants that penetrate soil’s sands and clays, however, depend on microbes able to take up minerals such as zinc and deliver them to the root hairs.
A paper in Frontiers in Soil Science explains how zinc-solubilizing bacteria (ZSBs) increase the bioavailability of dissolved zinc ions to the root hairs by adjusting the pH. This “benevolent job of microbes in the ecological nutrient cycle” again shows the partnership of “incredible bacteria” with higher organisms.
The precise effect of Zn mobilization that ZSB brings about is the deposition of organic acids (OAs), which acidify the surrounding soil environment and solubilize Zn due to the drop in pH. Other mechanisms include the secretion of chelating agents, like siderophores, which are believed to play a critical role in iron (Fe), Zn, and the solubilization of other micronutrients.
Getting Zinc from Cell to Leaf
Plants require zinc for photosynthesis and other essential operations. Getting zinc from root hair to leaf is a study unto itself. A review article in the NIH journal Plants gives a brief overview:
Zn is an essential micronutrient for plant growth and development that is involved in several processes, like acting as a cofactor for hundreds of enzymes, chlorophyll biosynthesis, gene expression, signal transduction, and plant defense systems…. Plant Zn efficiency involves Zn uptake, transport, and utilization; plants with high Zn efficiency display high yield and significant growth under low Zn supply and offer a promising and sustainable solution for the production of many crops, such as rice, beans, wheat, soybeans, and maize. The goal of this review is to report the current knowledge on key Zn efficiency traits including root system uptake, Zn transporters, and shoot Zn utilization.
Further down, the paper tells how “In the uptake process, Zn2+ ions travel through the root epidermis, cortex, endodermis, pericycle, and xylem and are then translocated to the stem, leaves, phloem, and seeds.” The details are much too complicated for our present purposes here but can arouse awe at how many players and processes are involved in getting the zinc to the leaf.
Getting Zinc from Leaf to Human
Herbivores and carnivores benefit from the zinc in the leaves, fruits, and seeds of plants. Harvard Nutrition Source says that the trace amount of zinc in our bodies “is a major player in the creation of DNA, growth of cells, building proteins, healing damaged tissue, and supporting a healthy immune system.” And when sperm meets egg, zinc puts on a fireworks show!
Good plant sources of zinc include legumes, whole grains, and nuts. Meats, poultry, and seafood are rich animal sources of zinc. While zinc deficiency is rare in developed countries, it can cause loss of smell and taste, diarrhea, and other issues. Excess zinc can also be unhealthy, but the body normally regulates zinc homeostasis, maintaining the three-gram optimum.
That leads us finally to the news: a paper last month in Cell by Weiss et al. announced, “Zn-regulated GTPase metalloprotein activator 1 modulates vertebrate zinc homeostasis.” The prevention of zinc deficiency or overdose is regulated by a newly identified metalloprotein family, named ZNG1, that flies into action in situations of zinc starvation.
Using biochemical, structural, genetic, and pharmacological approaches across evolutionarily divergent models, including zebrafish and mice, we demonstrate a critical role for ZNG1 proteins in regulating cellular Zn homeostasis. Collectively, these data reveal the existence of a family of Zn metallochaperones and assign ZNG1 an important role for intracellular Zn trafficking.
In the accompanying news from Vanderbilt University, Erik Skaar says that “This is the first identified protein that puts zinc into other proteins” and an essential chaperone for responding to zinc deficiency.
We think that when the body is starved for zinc, ZNG1 ensures that zinc gets delivered to the most important zinc-containing proteins,” Skaar said. “This opens up an exciting new area of biology, where we have these regulatory factors controlling a number of different physiological processes through metal insertion.”
From Human Back to Significance
This short venture into the zinc cycle illustrates once again that the closer you look at intelligently designed biological systems, the more specified complexity you find. In The Miracle of the Cell, Michael Denton zoomed into the “prior fitness” of elements and molecules for their roles in complex life. In The Miracle of Man, he zooms out to the prior fitness of the planet as a whole, with its marvelous cycles, that work together to give man a habitat suited for the development of technology.
In this new book, Denton argues for a return to an older, human-centered view of the universe that was taken for granted until the Middle Ages but gradually lost when Copernicus, Vesalius, and ultimately Darwin seemed to demote man from any significance in the grand scheme to mere cosmic accidents in an uncaring cosmos. Some may find such a return to old ideas outrageous, Denton admits. But “While my conclusions are controversial, the evidences upon which they are based are not in the least controversial,” he says — right before plunging into chapter after chapter of uncanny coincidences in nature, without which complex human life would be impossible:
Humans are clearly no contingent cosmic afterthought. The exquisitely fine-tuned ensembles of environmental fitness described here, each enabling a vital aspect of our physiological design, amount to nothing less than a primal blueprint for our being written into the fabric of reality since the moment of creation, providing compelling evidence that we do indeed, after all, occupy a central place in the great cosmic drama of being.
Our brief look at just one element, zinc, should be sufficient to make us all reflect. Together, Denton’s combined evidence makes his conclusion — of an anthropocentric universe — nothing less than overpowering.