Fluorescent biosensor tracks heme compound's activity in cells

The heme compound (which forms the nonprotein part of hemoglobin and some other biological molecules) works in cells as an essential catalyst called a cofactor and as a signaling molecule to trigger other processes. Seeking to find a way to monitor heme activity inside cells, a team of researchers at the Georgia Institute of Technology (Georgia Tech; Atlanta, GA) used a fluorescent biosensor to track those activities. Poor heme management can cause diseases like Alzheimer's, heart disease, and some types of cancer, so having biosensors that can monitor heme in cells could look at how cells make this essential toxin available in carefully sparse concentrations, according to Amit Reddi, a biochemist and assistant professor at Georgia Tech and prinicpal investigator of a paper describing the work.

In hemoglobin, the ionic iron in the heme molecule is what attracts the oxygen molecule and is embedded tightly in protein, rendering it non-toxic. Many scientists have long assumed that heme, even in other cells, is basically always static, held tight by the proteins it works with. However, in working with baker's yeast cells (which are eukaryotes, like human cells), the research team could observe heme being freed up to float around and participate in life processes.

Principal investigator Amit Reddi and lead researcher David Hanna observe an image of a baker's yeast cell taken under a microscope as it lights up green from the tailor-made fluorescent ratiometric biosensor they have infused it with.

Using the heme fluorescent biosensor, Georgia Tech graduate student Osiris Martinez-Guzman found an enzyme called GAPDH, known for its involvement in breaking down sugar, that the team observed helping buffer cellular labile heme (iron protoporphyrin IX), which got tied up in proteins, leaving only a limited amount free for biochemical reactions. When more labile heme is needed, nitric oxide (a signaling molecule) rapidly released heme from entangling proteins, so it could do jobs such as regulating gene expression.

The research team used a heme binding protein from bacteria and attached it to green fluorescent protein. Then, they used a blue laser to charge up the lamp part of the biosensor protein pair, allowing it to re-emit the green light. The green image disappears and reappears depending on how much heme is available, Reddi says, allowing them to see what is happening in real time.

Full details of the work appear in the Proceedings of the National Academy of Sciences; for more information, please visit http://dx.doi.org/10.1073/pnas.1523802113.