Shedding New Light on Photosynthetic Pigments

Jennifer Bridwell-Rabb has always felt inspired by light. In particular, she is amazed by the way different photosynthetic organisms rely on something so unpredictable to survive. Light-absorbing pigments capture radiation from the sun, allowing plants and some microbes to make usable energy in the form of carbohydrates. But how do they adapt to situations with variable light, and how do they protect themselves from damaging radiation? Bridwell-Rabb joined the Michigan Chemistry department in September to study these complex pigments and their assembly in nature. Her work will help harness the diversity of pigments themselves as well as the protein machinery that does the difficult chemistry of constructing them. This research will elucidate how organisms sense and adapt to light, presenting opportunities to engineer new photosynthetic pigments and sunscreens.

“I think I always wanted to do something math or science related,” Bridwell-Rabb said, but she had a wonderful high school Chemistry teacher who pushed her towards chemistry and biology. Still, she got a math degree at Central Michigan University in addition to her degrees in Chemistry and Biology. While doing research in the math department at Central Michigan, she thought she might want to be a biologist. When Bridwell-Rabb told a professor that biology just wasn’t detailed enough, her professor responded, “maybe you’re a chemist!”

She branched out to research in chemistry, and soon became fascinated by the mechanisms plants and microbes use to assemble complex light-absorbing pigments. These elaborate molecules are assembled by proteins in multiple steps, and they often need to absorb different types of light. Plants therefore require a variety of pigments, and she explains that “it takes nineteen steps just to assemble the scaffold of chlorophyll.” Because the chemical scaffolds require a significant energetic investment, the process of diversifying the structures must be carefully controlled. As Bridwell-Rabb puts it, “nature’s most efficient way of using that very expensive scaffold is to just tailor and tune it,” adding, “It’s a genius way to use an expensive molecule and make it adaptable.” While light can be harnessed as energy, radiation from the sun can also damage living things. In response, microbes have engineered sunscreen pigments to protect themselves, using similar tactics to make small changes to the large chemical framework. Bridwell-Rabb emphasizes that “simple tinkering can really change the wavelengths of light that they can use.” She wants to know how that tinkering is done. Ultimately, understanding how proteins make these subtly different pigments can have significant biomedical applications, including new sunscreens and photodynamic therapy, and advance our knowledge of biochemistry as a whole.

Jennifer Bridwell-Rabb’s research lab takes an interdisciplinary tactic to solving the mysteries of pigment assembly and diversification. “We combine a lot of different approaches.” She is trained in enzymology, and uses protein crystallography to “capture snapshots along the way.” For her lab’s work, it’s often important to know the structure of proteins down to the precise position of atoms, which can be accomplished by crystallizing the protein and imaging it with X-rays. A protein can be crystallized by itself, which shows how it might be able to facilitate a chemical reaction, or it can be caught in the act, holding onto the molecules the way they might be held during the ‘tinkering’ reactions.

To see what the pigment-making proteins are doing on the molecular level, Jennifer is making her own structural biology lab. “We’re putting a crystallization robot in a glove box.” Most of the proteins the lab works with are oxygen sensitive, so they must be kept in an anaerobic environment for the entire crystallography process. The ‘mosquito robot’, so named because of its small footprint, will go into a customized Coy chamber, essentially a stretchy glove box. Since the whole crystallography process needs to take place inside, the vinyl sides of the chamber will be able to flex when liquid nitrogen boils off during the final freezing step. The anaerobic crystallography chamber is one of the finishing touches on the Bridwell-Rabb lab space, and her lab members already have much of their work up and running. “I have two awesome students who have joined my lab who I’m so thankful for. They’re amazing, they’re fun, they’re hard working, and they support each other,” Bridwell-Rabb says. She’s excited to see how the diverse scientific backgrounds of her lab members come together to explore the incredible adaptability of photosynthetic organisms.