For the last entry in our Black History Month series, we’re discussing Emmett Chappelle, the biochemist hired by NASA to develop tests and methods to identify microbial life on the surface of Mars. Of course, life has never been found on the red planet, but like many other innovations originally developed for space, Chappelle’s inventions have found new purpose here on Earth. One of his most notable discoveries was luciferase, the enzyme found in fireflies that glows when it reacts with the protein luciferin in the presence of ATP—the currency of life. Because luciferase only reacts with luciferin when there is enough ATP to power the reaction, it will only glow in the presence of a living cell. Chappelle used this bioluminescent property of luciferase to develop a sensitive assay for detecting life that is still used today throughout a variety of scientific fields.
Chappelle was born in 1925 in Phoenix, Arizona, where his parents were farmers on a small cotton farm. In 1942, right after graduating high school, Chappelle was drafted into the U.S. Army, where he served in Italy in the segregated 92nd Infantry. After the war, Chappelle returned to Arizona to attend Phoenix College, earning an associate’s degree in electrical engineering. With funding from the GI Bill, he then went to the University of California Berkeley to earn a bachelor’s degree in biology.
Even without any graduate training, Chappelle was hired to be a biochemistry instructor at the Meharry Medical College in Nashville, Tennessee, where he also started doing his own research on iron recycling by red blood cells. His research earned him recognition in the wider community of biologists, and in 1953, he accepted an offer to do graduate work at the University of Washington. He earned his master’s in biology in 1954 and immediately started working towards a Ph.D. at Stanford University. He spent four years at Stanford, but he did not end up finishing the degree. In 1958, he was offered a research position at the Research Institute for Advanced Studies in Baltimore, Maryland.
When Chappelle arrived in Baltimore in 1958, a lot of federal science funding was being dedicated to aerospace research as part of the ongoing Space Race. Chappelle’s research focused on using photosynthesis in plants to provide astronauts with a sustainable source of oxygen and reduce the risk of carbon monoxide poisoning. In 1963, he began working at Hazleton Laboratories under contract with NASA on developing ways to detect life on Mars. He began by working with NASA’s Viking probe to develop methods for scraping and collecting soil from the red planet for later testing. Later in 1966, Chappelle went to work for NASA directly at the Goddard Space Flight Center, where he invented sensitive biodetection assays and earned over fourteen patents.
Chappelle’s biodetection assays primarily relied on sensitive measurement of the cellular currency, Adenosine Triphosphate (ATP). If you remember from last summer’s post on mitochondria, ATP is the chemical fuel that cells use to store energy. Every living cell on Earth converts glucose to ATP and uses that ATP to fuel their chemical reactions. The energy in an ATP molecule comes from the highly energetic bonds between each of its three phosphate groups. These phosphate groups are much more stable on their own, so each of those bonds contains a certain amount of potential energy. That potential energy can be released—and used to power less favorable reactions—by breaking the phosphate bond and converting ATP to ADP (Adenosine Diphosphate). While there are many commonalities between cells on Earth, testing for ATP is one of the most reliable ways to detect active, living cells because it is a fundamental resource for pretty much every cellular process. In humans, a single living cell produces roughly a billion ATP molecules per second in order to meet all of its energetic demands.
While working for NASA, Chapelle set about developing an assay to detect the presence of ATP using an existing chemical system found in fireflies—bioluminescence. Bioluminescence is the chemical production of light in living systems. Many organisms across the natural world use bioluminescence for a variety of purposes—for example, fireflies use their glowing butts to warn off predators and attract mates. The bioluminescent reaction essentially requires three main ingredients: the enzyme luciferase, the protein substrate luciferin, and ATP. In the presence of oxygen and magnesium, luciferase binds to luciferin and uses the energy from ATP to turn it into oxyluciferin, releasing photons of light as a byproduct. This process is absolutely dependent on the presence of ATP in the solution, so the number of photons released is proportionally related to the number of ATP molecules available.
Using the luciferase reaction, Chappelle developed bioluminescent assays that could precisely count the number of bacteria cells in a sample of fluid, test the susceptibility of bacteria to antimicrobial agents, detect the presence of virus molecules, and identify cancerous tissue. In addition to developing and holding patents for each of these assays, Chappelle also held a patent for the instrument he used to precisely measure the amount of light emitted.
Today, bioluminescence is used in a variety of research contexts. It can be used to evaluate the metabolic activity of cells, which is indicative of their viability and abundance. Bioluminescence is also used in reporter gene assays to help researchers determine if a particular gene is active in a system they are studying. Bioluminescence can even be used in live animals to image molecular processes in real-time.
Chappelle retired from NASA in 2001 and received an Exceptional Scientific Achievement Medal for his work and mentorship of young scientists. In 2007, he was inducted into the National Inventors Hall of Fame. In 2019, at the age of 93, he died of renal failure at his home in Baltimore.
Check out last month’s series on the history and science of immunity and the development of the new Covid-19 vaccines. Science You Can Bring Home To Mom will be back in two weeks with a brand new series! Comment on this post or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!
