Biomolecular Sciences Seminar Series: Dr. Martin Lawrence

Biomolecular Sciences Seminar Series

Dr. Martin Lawrence, Montana State University

Title: Catch and Release - Structural Insights into Prokaryotic Iron Homeostasis

Abstract: Iron is essential for almost all organisms, where it is incorporated into iron containing cofactors and metalloenzymes that catalyze essential cellular reactions. Its redox-active role is key to this chemistry, where Fe(II) can act as an electron donor and Fe(III) as an electron acceptor. However, when present in excess, free Fe(II) catalyzes the formation of destructive reactive oxygen species (Fenton reaction), while free Fe(III) is essentially insoluble and precipitates as ferric oxide or ferric oxyhydroxide. Iron is thus a double-edged sword, and iron homeostasis is critical. In this talk, I will present structural studies of two proteins involved in prokaryotic iron homeostasis. One is a miniferritin known as DPSL or thioferritin, that consumes hydrogen peroxide and free Fe(II) as it catalyzes the mineralization of ferrihydrite within its hollow core. Using Cryo-EM, we followed iron loading in thio-ferritin and determined high-resolution structures of the unmineralized state, as well as an early, iron-loading state that visualizes nucleation of ferric oxyhydroxide at the acidic 3-fold pores. Mechanistically, a conserved crucible of precisely positioned glutamates and unsaturated main chain carbonyls acts as a template to catalyze nucleation. We also determined a 2.4 Å structure at a later time point, revealing the role of a second constellation of main-chain carbonyls on the interior surface that subsequently supports crystalline mineral growth, which then proceeds into the center of the particle. Notably, the visualized mineral is consistent with one of two competing structural descriptions for ferrihydrite. This study provides the first pseudoatomic level observation of controlled mineral nucleation and growth in any member of the ferritin superfamily, informing general mechanisms of nucleation and biomineralization. The second protein is HmuS from the gastrointestinal bacterium Bacteroides thetaiotamicron (B. theta), an organism which can utilize heme as its sole iron source. How B. theta extracts iron from heme was unclear, since B. theta is an anaerobic organism, and previously known pathways require oxygen to cleave the protoporphyrin IX macrocycle to free the iron. The Cryo-EM structure of HmuS revealed structural similarity to the chelatase superfamily that inserts metals into porphyrin cofactors for the production of chlorophyll and vitamin B12. In contrast to these enzymes, we find that HmuS catalyzes the opposite reaction, working as a "dechelatase" to extract iron from heme within the human gut. Potential mechanisms for this reaction and its implications will be discussed.