Understanding Trace Metal Management in Bacteria

Professor Arthur Glasfeld talks sabbatical research

Last Thursday, Sept. 17, Margaret Geselbracht Professor of Chemistry Arthur Glasfeld gave a chemistry seminar talk about his work this last spring. Glasfeld spent the spring semester on sabbatical at Durham University looking into zinc chaperones. This class of proteins help prevent improper interactions of zinc with other proteins in bacterial cells by binding with zinc and making it unavailable. Understanding these proteins is important because metal-ion physiology in bacteria is essential for bacteria to fight against our immune system, so understanding them may help our ability to fight bacterial infection. Manganese and iron are critical in defending bacteria against oxidative stress, one of our primary means of defending against them. Copper and zinc are bactericidal, and so we flood bacteria with those metals in order to kill them. Cobalt, another example given, is used in the production of vitamin B-12. Bacteria are the only organisms that produce this vitamin. Normally we get B-12 from the animal products we eat, so understanding the B-12 pathway can help ensure vegan diets have enough B-12, if plants can be engineered to uptake the vitamin. 

Glasfeld looked at a series of trace metals: manganese, iron, cobalt, nickel, copper, and zinc. Like hemoglobin and iron, there are specific proteins that bind exclusively to specific trace metals. The problem is that some metals are more likely to bind with proteins than others. Copper, for example, is one of the “stickiest” of those metals, and when all of the other metals are abundant, would be the one most likely to bind to the protein. This is a problem when there are specific proteins that require less strong metals like manganese. 

The sensitivity of these proteins are very low, but these metals as a whole are relatively abundant, so a large portion of them must be bound. The free metals can be difficult to access because they exist in astronomically tiny concentrations. This is why chaperone proteins are important; they help bring the trace metals that are free to where they are needed in the cell. Glasfeld normally uses crystallography to investigate these proteins, but the Robinson lab took a thermodynamics approach to understanding the correct metalation of given proteins by energetic matching of protein to metal ion within the cell. 

Glasfeld looked specifically at the yjiA protein in Salmonella, which had yet to be fully described. He used a dye competition assay to determine which metal it had the highest affinity for. This protein, however, is only competitive for zinc if GTP is present. Glasfeld hypothesized the mechanism that the protein will bind zinc in the presence of GTP, carry it around the cell until it finds where the zinc should go, and then it will hydrolyze the GTP into GDP and lose affinity for zinc and pass it on to where it needs to go. 

He then began to use crystals to investigate this mechanism. But after spending time creating and getting ready to analyze the crystallized proteins, Glasfeld broke his hand, and then the necessary machine broke, and then COVID-19 hit. Now his crystals are sitting frozen in Durham. 

This did not stop Glasfeld, however. Like the rest of the world, he turned to his computer. He began using modeling to make sense of the mechanism. This helped him realize that when the associated GTP is hydrolyzed into GDP, the protein changes shape dramatically and the binding site for zinc reverts to beta strands.

Eventually he will get the data from those crystals. Glasfeld said, on his estranged crystals but applicable to us all, “One of the things about science, the greater your hopes, the deeper your disappointment, but still, I really hope there’s something there.”

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