I recently read a report from the journal Nature noting that the concentration of mercury dissolved in the oceans has more than tripled since the beginning of the Industrial Revolution. A number of questions immediately came to mind, such as, “How did this happen?” and “How concerned should we be?” And thus a Common Science® column was born. However, in the process of writing about mercury, I got off on a bit of a tangent on the critical importance of metals in maintaining good health and in the functioning of biological processes. That tangent eventually grew into this week’s column, which will serve as a prelude to the column on mercury next week.
To understand why metals are biologically important, we first need to review some chemistry. You should remember from high school chemistry that molecules are formed when atoms share electrons as part of a chemical bond.(1) In order for a molecule to participate in a chemical reaction, at least one of its chemical bonds must first be broken. In order for a bond to break, two molecules have to smash into each other with enough energy to break the bond – known as the activation energy barrier.
If you want to encourage a chemical reaction to occur or to make it go faster, there are two strategies that can be employed to overcome the activation energy barrier. The simplest approach is to heat up the reactants. Hot reactants move around faster, and thus smash into one another with greater enthusiasm. The other approach is to use a catalyst. Catalysts help chemical reactions to occur by reducing the amount of energy required for them to occur. Catalysts can be particularly useful if you want to induce a chemical reaction to occur but, due to some constraints, cannot add heat.
In both industrial and biological processes, catalysts often include metal atoms. The reason for this stems from the electrical properties of metals which allow them to either accept or donate electrons. As an example, let’s consider what is perhaps the most famous industrial reaction of all (at least to geeks like me): the reaction of nitrogen (N2) with hydrogen (H2) to make ammonia (NH3), the key component of industrial fertilizers, via the Haber-Bosch Process.(2) Nitrogen molecules consist of two nitrogen atoms bound together with a triple bond, making it one of the most stable and therefore unreactive molecules in the entire universe. Without the use of a catalyst, nitrogen will not react with hydrogen even at extremely high temperatures. Haber and Bosch found that if nitrogen molecules were first allowed to absorb on (stick to) osmium metal, they could be converted to ammonia at a high, but not unreasonable, temperature. The reason this works is that when nitrogen absorbs on osmium, some of the electrons in the nitrogen-nitrogen triple bond are pulled towards the metal, which weakens the bonds between the two nitrogen atoms. Now that this bond has been weakened by the catalyst, the activation energy barrier required to induce nitrogen to react with hydrogen is much lower. The development of the Haber-Bosch process, which today provides the fertilizer to generate approximately half of the food calories produced on Earth, is often rated as the greatest scientific achievement of the 20th century.
Now that we have finished our primer on how metals can catalyze chemical reactions, let’s turn to how that applies to people. The functioning of the human body involves a staggering number of chemical reactions, including the digestion of food, the repair of wounds and injuries, and the formation of new blood cells. Since the temperature of the human body hovers around 98.6 ºF, the only practical way to encourage chemical reactions in the body to go faster is to use a catalyst (3). In order to make sure that all of these needed chemical reactions occur, your body produces over 55,000 different catalysts. Catalysts in biologic systems are called enzymes. Many of the enzymes that your body produces include one or more metal atoms, such as iron, magnesium, manganese, zinc, and selenium. The metal atoms in these enzymes function in an analogous way to the osmium in the Haber-Bosch process. The need for your body to produce metal-containing enzymes is a primary reason that you need to eat a diet that contains a wide range of minerals in sufficient quantities and in the proper ratios. If your diet becomes deficient in minerals, a number of rather bad things can happen to you, including nerve damage, kidney stones, anemia, muscle weakness, birth defects, and, in the extreme, death.
If you have made it this far, you now have an understanding of how a catalyst functions, why metals make good catalysts, and that many processes in the human body rely on metal-containing enzymes. That information alone would constitute a reasonably complete Common Science® column. But I have yet to share with you the basis of my fascination with this topic. So pour yourself another cup of coffee and let’s delve into evolutionary development and biology.
The evolution of our species has occurred over the last 500,000 years. During this time, our bodies have evolved to respond to an impressive array of challenges, including wild swings of climate, devastating plagues, changes in food sources, and competition with other hominid species. While these past challenges were certainly significant, even events like the onset and retreat of ice ages occurred slowly over the course of thousands of years, which gave our ancestors time to react both strategically and biologically. Successful human evolution over the millennia proceeded in equilibrium with the available metal-containing minerals in our diets, which allowed us to create just the right balance of enzymes and go on to become the dominant species on Earth.
Over the last two hundred years, with the advent of fossil fuel use, the growth of industrial agriculture, and the explosion of the human population, we have introduced environmental changes at a rate which is unprecedented in the history or our species. This includes some dramatic shifts in the mineral content of our diets. I have written several times about that fact that the mineral content in the U.S. food supply has fallen by around 30% since the 1940s. While it can be difficult to tease out all of the direct mechanisms, we know that these changes in the American diet over the past several decades have led to dramatic increases in tooth decay, heart disease and diabetes, processes which are all intertwined with the proper or improper functioning of metal-containing enzymes.
The dramatic rise in mercury concentration in the oceans represents a different sort of challenge to the human-metal interplay in our environment and diets. This time, rather than a developing deficiency, we are introducing an excess amount of a metal into our diets, a metal which, far from being essential for good human health, is poisonous to us. This is a development which deserves further exploration. But that will have to wait until next week.
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(1) I am aware that in many school districts (including, sadly, the Chapel Hill Carrboro City Schools) it is possible to receive a high school diploma without taking chemistry. This knowledge makes me sad, so I choose to repress it and continue to assume that all of you had chemistry in high school.
(2) If you want to know more about the Haber-Bosch process and its role in driving the human population explosion, please follow this link to Fun with Fritz and Carl.
(3) As you know, when the body is threatened by viruses and bacteria, it will respond by raising its temperature. This strategy is designed to make a number of disease-fighting chemical reactions go faster to defeat the intruder. However, if the body over-corrects and raises its temperature to 104 °F, it can start to cause collateral damage to other body systems by damaging beneficial enzymes. This is why fever of this magnitude requires intervention with medicine and/or physical cooling of the body. In a display of evolutionary solidarity, beneficial enzymes in plants also shut down at this temperature. Sadly, there is no aspirin for basil plants.http://chapelboro.com/columns/common-science/minerals-men
CHAPEL HILL – Researchers at the University Of North Carolina School Of Medicine have discovered a possible cause of autism.
A key group of enzymes, called topoisomerases, can have profound effect on the genetic factors behind brain development. Associate professor in the Department of Cell Biology and Physiology, Mark Zylka, says that these enzymes work to help keep DNA normal during developing times in a child’s life.
“These are enzymes that are called topoismerases, we like to think of them as the scissors and glue for DNA,” Zylka said “so DNA is a molecule that often gets tangled up inside of cells, and to relieve these tangles, these enzymes can cut the DNA, untangle it, and glue it back together.”
When topoisomerase inhibitors are present it may limit what genes are “untangled.” Zylka said that he found when these inhibitors are present long genes and genes related to autism are the most affected.
“So what we found was that these enzymes seem to play a very important role in neurons in the brain, these are brain cells, and in particular these enzymes seem to be important for allowing genes that are very long to be expressed, and in particular a large number of genes that have been linked to autism spectrum disorders” Zylka said.
These inhibitors that affect the enzyme topoisomerases are known to exist in chemo-therapeutic drugs and have been around for over 40 years. It was while studying these drugs that Zylka first began to study the effect the inhibitors would have on neurons. Zylka says they noticed that the drugs had effects on long genes, and that autism genes are also very long.
“So that’s when we sort of put two and two together and realized that inhibiting these enzymes could have a profound effect brain development” Zylka stated.
Discovering these enzyme inhibitors can lead to new discoveries for autism and diagnosing what exactly is happening. Zylka says that he thinks studying these inhibitors can help us identify what in nature may have inhibitors like these that could cause autism.
“We found that if you inhibit these enzymes, the expression of a lot of very long genes is impaired and so a lot of these genes are autism genes,” Zylka said “and so we think this could be used as a way to diagnose or to identify other factors or chemicals in the environment with similar effects.”
Currently the known inhibitors that Zylka is studying are in Chemo-therapeutic drugs and would only affect cancer patients that are going through Chemo-therapy.http://chapelboro.com/news/health/unc-researchers-find-possible-cause-for-autism