Why is lead so bad anyway?

June 10th 2016

Why is lead so bad anyway?

The recent water crisis in Flint, Michigan occurred when government officials switched from the Detroit Water system to the Flint River without proper corrosion control measures. This caused corrosion that leached high levels of lead into the drinking water, effectively poisoning an entire community. Lead exposure earned its foreboding colloquialism “poisoning” for a reason. It is particularly hazardous for the 6,000–12,000 children who were exposed because it is absorbed faster in their continually developing bodies. Even in adults, it can affect every organ system and cause a host of long term problems including kidney failure, irritability, learning disabilities and behavioral problems.

In this post, I will discuss some of the fundamental reasons why a single element can have such a profoundly negative effect across so many different systems in the body.

It all starts with atoms

I will return to lead in a minute, but I want to first discuss the chemical idea of polarizability because it ultimately plays a major role in how lead interacts with proteins in the body.

Atoms are the fundamental building blocks of everything around us and the type of atom dictates the number of electrons that it possesses. We can divide the electrons into two main categories: core and valence, or outermost, electrons. Let’s compare the electronic configuration of two different elements: oxygen and sulfur.

Oxygen vs Sulfur Atoms

A comparison of oxygen versus sulfur atoms. Sulfur atoms have a large radius because they are one row lower on the periodic table

When we look at the periodic table we can see that they are in the same column which tells us that both elements have the same number of valence electrons, in this case 6 (shaded in green above). The total number of core electrons between oxygen and sulfur, however, are different because sulfur is 1 row lower on the table. Oxygen has 2 electrons in its core, while sulfur has 10 (shaded in orange). Since electrons repel each other because negative charges repel other negative charges, the cloud of core electrons will be large for the sulfur compared to the oxygen (see above image). Since the core is larger for sulfur the outermost valence shell will be further away from the nucleus. This means that there will be less of an attractive force between the nucleus and the outermost sulfur electrons compared to oxygen where the electrons are much closer. Polarizability of Oxygen vs Sulfur Atoms

The attraction between the nucleus and the outermost electrons is stronger for oxygen than sulfur due to oxygen’s smaller radius which puts the electrons closer to the positively charged core. This stronger attraction of the electrons to the nucleus make it less susceptible to to polarization from an external charge (dark red point charge on left of each atom) than sulfur.

Since the interaction between the nucleus and the valence electrons are less for sulfur than for oxygen, it is easier to “polarize” or influence the spatial position of the electrons within that shell if you had a neighboring atom nearby. We call this chemical hardness and it means that an element like oxygen will have outermost electrons that are closer to that of a billiard ball- tight, compact, and immovable, while sulfur will have much more diffuse electrons that are closer to that a sponge- squishy and movable depending on the environment that they are in.

We’ve noticed that chemical bonds, which result in the sharing of electrons between two atoms, tend to form when they both have a similar chemical hardness. Now that we understand better how the polarizability of an electron cloud affects the bonds that an atom will form, let’s turn to biochemistry to understand how it plays a role when lead is introduced into the body.

Structure is everything- a tour of proteins

Proteins control a vast array of necessary tasks within the human body. They are composed of a chain of amino acids in a specific ordering which spontaneously folds in on itself to form a three dimensional structure that then will perform a very specific task using its “active site” (see image below). A key step in the folding process is the formation of sulfur bridges. If we compared protein folding to origami folding, most of the structure is driven by the attractive but relatively weak van der Waal or electrostatic forces (which is similar to a fold in origami) while sulfur bridges would be the equivalent of strategically applying tape in areas of the model that you’d really like to stay together.

Sulfur bridges play an absolutely critical role in allowing the protein to do its job because the bridges are only in places of the protein where you really need the stability.

Lead Disrupts Protein Structures

Proteins are initially synthesized as a single strand of amino acid units and some of the units have sulfur species called thiols. After formation, the sulfurs can chemically bind together to form sulfur-sulfur bridges which then help direct the formation of the final folded protein. Lead has a similar chemical hardness to sulfur and it can nestle into the protein and disrupt the active site which deactivates the protein.

Unfortunately, for the protein, sulfur and lead have a very similar chemical hardness. This means that if a protein encounters lead floating around in the body, the lead has this nasty tendency to nestle in to the sulfur bridges and break them apart. Once these bonds have been broken the protein will fall apart, and it can no longer do its job. If proteins can’t do their job, it will wreak all sorts of havoc in millions of chemical processes that are needed in your body.

One particular protein that is significantly affected by lead is Porphobilinogen synthase. This protein is a step in a production line for making porphyrin. Porpyrin is the scaffold that holds the iron atom in hemoglobin which is the active protein in red blood cells. This iron atom is what oxygen molecules from your lungs bind to in order to be transported all over your body. Clearly anything that would inhibit the machinery that is responsible for the uptake and delivery of oxygen would have enormous consequences for someone affected by lead poisoning.

Lead exposure isn’t just a concern for the residents of Flint. The internet has a variety of resources to help you assess the risk levels in your own community, but be advised - it’s a nationwide problem. The next time you reach for the tap, think about your sulfur bridges and maybe reach for the Brita instead.

References

How to test for lead in your home water supply, CNN

Got lead in your water?, USA Today

Pocket Scholar

Pocket Scholar teaches undergraduate science courses through short videos and interactive exercises that you can do from the comfort of your browser.

Share

About the Author

Stephen Jensen

Founder of Pocket Scholar, Ph.D. Surface Scientist and Clean Energy Aficionado.