Interview with Dean Wilcox

Dean Wilcox is a Dartmouth professor of chemistry who has been a part of the Dartmouth College faculty since 1984. Wilcox has long been highly involved with chemistry.  He received his BS and MS degrees in Chemistry from the University of California, after which he did graduate research at MIT and Stanford University, obtaining a Ph.D. in Inorganic Chemistry from MIT in 1984.

Wilcox’s current research seeks to bridge the gap between inorganic chemistry and biochemistry, and focuses on understanding the interactions between metal ions and biological molecules, particularly proteins. The broad goal of his research, which spans numerous projects, is to determine the chemical basis for the biological roles and physiological effects of metal ions. Much of his current research focuses on the coordination chemistry of proteins that are rich in cysteine and histidine, amino acids that have great potential in binding metal ions. He has also been deeply interested in the role of metalloenzymes (enzymes that contain in their structure a bound metal ion that may be required for enzymatic activity) in the biological system. Another research project focuses on nitric oxide (NO) and its interaction with hemoglobin and other biological molecules. Wilcox is currently a part of the Dartmouth Superfund Basic Research Program, where the goal is to understand toxic metal interactions with proteins.

DUJS staff writer Shu Pang spoke with Wilcox on topics ranging from the motivation behind his current research, to his role in the Dartmouth Superfund Basic Research Program, to his thoughts concerning how evolution impacts the chemistry of metal ions.

DUJS: What motivated you to research metal ions in the biological system?

Dean Wilcox: Even as an undergraduate, one may have the opportunity to participate in research, because faculty like to encourage students to be involved in learning new things. I did my first research just after finishing organic chemistry, which was about all the chemistry that I knew. I was accepted into a summer undergraduate research program and I ended up working on a project involving organic and metallic chemistry, a project similar to what Professor Hughes was doing at Dartmouth at the time, unbeknownst to me since I was in California. When I decided to go to graduate school, I ended up attending the program at MIT, but by then I had left organic chemistry behind. The metals had lured me away. There were two faculty members at MIT who interested me — one was Mark Wrighton, now President of Washington University in St. Louis, and the other was Ed Solomen, who became my adviser. Solomon worked with metals bound to proteins, which is getting pretty far from organic chemistry. He showed me the most unusual absorption spectra of copper bound to a protein, which really peaked my curiosity. He lured me in with the interesting spectra and that’s how I started studying metals in biological systems. I did my Ph.D with him working on copper and other metals for five years.

DUJS: How important is the binding of metals in our body?

DW: There are several essential metals that are required in trace amounts and every one of them has a dose-response curve, which is a plot of the physiological response for different concentrations of the element. The lowest response is death and the maximum response is healthy life and in between is not so healthy life. A dose-response curve for an essential metal has a minimum response at low concentration, since we can’t survive without it. As the concentration increases we are healthier until a maximum plateau range is reached where we have the right amount. This establishes the dose that the government sets as our recommended daily allowance. But if we ingest too much, then we are unable to manage the higher levels of the metal and there will be toxic and eventually lethal consequences. So, each element has its own dose-response curve. The vertical axis of health and death does not change. What varies from metal to metal is the concentration scale. For instance, zinc, which is essential, has a very wide plateau range, and we can ingest lots of zinc before it becomes toxic. There is a belief that extra zinc from zinc lozenges will help prevent you from coming down with the common cold, but there is little concern about ingesting too much zinc. However, an element like selenium, which is also essential, has a very narrow dose-response curve, and it is toxic above a very small required amount. The chemistry of selenium is required for only a few key roles, and dangerous amounts can easily be ingested.

There are other elements that have no essential use at all. They are different in that their dose-response curve starts off high and we may survive small amounts of them, but the response falls off quickly, as there are only toxic effects of too much. But life has evolved in the presence of the elements it encountered on earth and we have developed some mechanisms to deal with low levels of toxic metals or metals that have no known use, like cadmium, mercury, arsenic, and lead. We have a protein known as metallothionein whose main role is to sop up those metals and prevent them from causing damage. 

DUJS: You have mentioned that your graduate research primarily involved copper. What is special about copper?

DW: Copper has wonderful and beautiful chemistry, as there are many unique things that copper can do. In our bloodstream we have a significant amount of iron that is part of hemoglobin that transports oxygen. About 65% of our iron is in our blood and iron is very important for our ability to live in an oxygen rich environment. But certain marine organisms, like arthropods and mollusks, have a different protein for transporting oxygen and it uses copper. If you take a live lobster and you rip off one of its legs, out comes blue blood, which is the oxygenated form. Our blood is red when oxygenated and blue when deoxygenated, but the blood of these creatures ranges from clear without oxygen to a beautiful blue color. We have essential enzymes that require copper and Solomon’s lab was studying many aspects of copper biochemistry.

DUJS: I understand your current research with metals involves the amino acids histidine and cysteine. Can you elaborate more on how these amino acids tie into your research?

DW: Proteins are the workhorse molecules of biochemical pathways in cells, and many of them, such as metalloenzymes, require metals. Most of the chemistry of life that involves metals relates to their interaction with proteins. A protein is a long coiled up polymer of a unique sequence of the twenty possible amino acids, but only a few of these are capable of binding metals, including histidine with its imidazole group [cyclic C₃H₄N₂] and cysteine with its sulfur. However, they have quite different properties. The nitrogen of the imidazole tends to bind metals like copper, while the sulfur of cysteine tends to bind metals like molybdenum. 

Histidine and cysteine are amino acids that have a high affinity for some of the metals that we study. If there is an evolutionary need for a protein to bind a metal, there is a limited set of amino acids that can serve this role. We can consider the many proteins that have metals bound to them and we find that predominantly histidine, cysteine, and the carboxylate groups of glutamic acid and aspartic acid are used to bind the metals. These are the four common amino acids that bind metals in the proteins that we study.

DUJS: You have already explained briefly how histidine and cysteine bind to metals. But how do the carboxylate groups of glutamic acid and aspartic acid do so?

DW: The carboxylate group is an anion, COO^–, at neutral pH but is protonated at lower pH. The proton is a Lewis acid that binds to a pair of electrons of the carboxylate. Metal ions are also Lewis acids that compete with protons and bind to carboxylates. There is also an electrostatic attraction between the carboxylate anion and the metal cation.  

DUJS: I understand you are involved with the Dartmouth Superfund Basic Research Program. Can you tell me more about the program?

DW: Many years ago the public and the government realized that there were many places all around the country where companies had dumped chemical waste into the ground and it was a real health hazard. So, Congress set up the Superfund program, and companies who generated the waste must contribute to a fund to clean up the mess they made. When the fund was set up, most of the money had to go to clean up these sites so they are no longer hazardous, but a portion of the money was set aside to study the effects of toxic chemicals on the environment and human health. Fifteen years ago, Karen Wetterhahn of Dartmouth’s chemistry department, whose own research examined how chromium can cause cancer, gathered a group to study the effect of toxic metals on human and environmental health and started the program at Dartmouth, which is called the Superfund Basic Research Program: Toxic Metals in the Northeast from Environmental Health to Human Health. The program involves epidemiologists who study the demographics and human health patterns, geologists who study how metals move through the ground water and whether they came from natural or man made sources, cell biologists who study how metals affect organisms, aquatic biologists who study how metals move through the biosphere and food chain, and chemists who study the fundamental chemistry of metals. Since arsenic is a fairly common toxic element in New Hampshire, it is one of the key elements we are studying. The rock formations under New Hampshire have a lot of arsenic and, depending on where you drill a well, you may get water contaminated with arsenic. The amount is not enough to kill you outright but over a long time of drinking the contaminated water, it can lead to a higher risk for serious health effects like diabetes, cancer and neurological and cardiovascular problems. Arsenic is not unique to New Hampshire but found in drinking water around the world. So, we have been studying arsenic, as well as mercury, lead and cadmium, from a variety of standpoints.

DUJS: Can you elaborate more on your research with the toxic metals you have just mentioned like arsenic, mercury, lead, and cadmium?

 DW: Some of my own research has focused on the biochemistry of arsenic and we have done a bit of work on the protein metallothionein that sops up mercury, lead, cadmium, and possibly arsenic. In particular, we really want to understand: What proteins bind arsenic and how tightly does they bind? Can we understand the in vivo chemistry of arsenic? We are not making predictions about which proteins bind arsenic, but given some biological clues we may be able to correlate the health effects of arsenic to its interaction with some key protein in a biochemical pathway. We want to know how the chemical properties of arsenic can disrupt the function of target proteins. What are the thermodynamics of arsenic binding to the protein? What is the rate of the binding process? Others have discovered an enzyme that puts methyl groups on arsenic, adding one or two, and this was thought to be a detoxification process. But our research has shown that arsenic with one methyl group will bind to certain proteins more tightly than arsenic itself and may be more dangerous. However, the arsenic can’t hang on as tightly with a second methyl group so the protein’s affinity for arsenic goes down. Maybe in the end the methylation of arsenic is a detoxification process, but it looks like the first step creates something more dangerous.

DUJS: How likely is it for the second methyl group to be added to the toxic metal, which may be as you said the body’s detoxification process?

DW: Good question. I don’t know if the rate is higher, lower or the same as adding the first methyl group. It has been found that the urine of individuals who are drinking arsenic-contaminated water contains arsenic with zero, one, or two methyl groups. This suggests that there may be significant levels of the more dangerous monomethylated form. This is all part of the biochemistry of arsenic that we are trying to understand.

DUJS: I know you are also involved with a research project involving nitric oxide. Can you tell me more about how NO ties into your research?

DW: Soon after I arrived at Dartmouth, I met pharmacology professor Roger Smith who had been here for many years and taught pharmacology to hundreds of medical students. His area of research included compounds that are vasodilators, which relax blood vessels and lower blood pressure. A number of compounds can do this and he had been studying the reactions of some of them with hemoglobin. He had uncovered some unusual behavior and came to me about it. We use certain instruments in chemistry that can be of great help. Ultimately we, and others, realized that many of these vasodilators were releasing NO and Professor Smith was actually studying the effects of NO. Later, it became known that we have enzymes that catalyze the release NO that helps to help regulate our blood pressure. Beginning in the 1980s, there has been a lot of excitement about NO, not only its crucial therapeutic role but also its biosynthesis and in vivo roles. On a humorous note, Roger Smith is known as Dr. NO for all of his studies of how nitric oxide is a vasodilator. 

There has been a lot of interest in quantifying NO levels in vivo, which is not an easy thing to do. In this regard I have collaborated with Professor Hal Schwartz of the radiology department who is interested in quantifying oxygen in living organisms. The physician of a diabetic would like to know if they are getting enough oxygen to their feet. We have used the same approach that Professor Swartz uses to determine oxygen in tissue to measure NO levels in tissue. But right now our NO research is on a backburner. That’s the way with research — sometimes there’s a lot of excitement and flurry of activity and some things get figured out and then many move on to another area of research. Currently, our studies of metal-binding proteins are providing exciting results. We are making progress addressing interesting and important questions, and others are asking us to help solve their problems with our expertise. 

DUJS: To wrap up, you have mentioned evolution several times while speaking of your research. So, what role does evolution really play with metal ions and how have we evolved to have the interaction that we now observe with metal ions?

DW: There is definitely a lot of excitement involving metals and evolution. Early in the evolution of life, the earth did not have much oxygen. It was a reducing environment and iron was much more available. But now with considerable oxygen in the air, it is a very oxidizing environment and that turns out to be a problem. In the presence of oxygen, iron turns to rust, which is not very soluble or accessible. But life got addicted to iron back in the earlier era when there was a lot of available iron. So, we have evolved ways to attain enough iron, since we have important needs for it. On the other hand, there is nickel. At least for humans, we don’t have any known biochemical need for which nickel fits the bill. Some microorganisms and plants do require nickel and there are many questions about when and how nickel became part of their biochemical pathways.

It is believed that the protein metallothionein appeared to help organisms survive when they are exposed to mercury, lead, and cadmium, which disrupt essential biochemistry of organisms. So, those organisms that had metallothionein were able to survive and metallothionein is found throughout the animal kingdom. You can see this in plants too. When the soil has high levels of lead or cadmium, the plants that are able to survive have proteins that are equivalent to metallothionein. These plants are able to grow in the presence of toxic metals, whereas other plants wither and die. Metals have played important but still poorly understood roles in evolution, which I think is very exciting.