From PNNL Lab: “New Protein Discovered Gives Insights to Iron’s Fate Underground”
Scientists identify molecule that steals electrons, leaving iron stuck in the mud
Results: It’s almost an evil twin story; a protein that steals electrons from iron in one microbe looks a lot like one that adds electrons in another microbe, according to scientists at Pacific Northwest National Laboratory and the University of East Anglia. Their survey of the genes of common groundwater bacterium Sideroxydans lithotrophicus ES-1, which removes electrons from iron, revealed that it contained genes in common with Shewanella oneidensis MR-1, which adds electrons to iron.
Proposed roles of MtoAB and CymAES-1 in Sideroxydans lithotrophicus ES-1-mediated extracellular Fe(II) oxidation. Decaheme c-Cyt MtoA, which is inserted into the porin-like, outer membrane (OM) protein MtoB, oxidizes Fe(II) directly on the bacterial surface and transfers the released electrons across the OM to the periplasmic proteins that have yet to be identified. The periplasmic proteins relay the electrons through the periplasm (PS) to the tetraheme c-Cyt CymAES-1. CymAES-1, located in the cytoplasmic or inner membrane (IM), reduces quinone to quinol. c-Cyts are labeled in red, and the direction of electron transfer is indicated by a yellow arrows. No image credit
Their results contribute to understanding of the molecular mechanisms by which microorganisms change the electron configuration of iron and, thus, change its mobility. The research was published in Frontiers in Microbiological Chemistry.
‘Recent studies indicate that aerobic Fe(II)-oxidizing bacteria, FeOB, would play a key role in niches having low levels of oxygen concentration, where microbial Fe(II)-oxidation can compete with the chemical oxidation of Fe(II),’ said PNNL biogeochemist Dr. Juan Liu, first author of the study paper.
Why It Matters: Science has realized the importance of microorganisms in research on processes such as carbon sequestration, the generation of new energy sources, and the movement and ultimate resting place of contaminants. Scientists are interested in the oxidation state, or loss of electrons, of iron because it dramatically affects the metal’s solubility in water, in which electron transfer proteins play critical roles. In contrast to Fe(II), trivalent iron, Fe(III), is not water soluble.
The difference in solubility between Fe(II) and Fe(III) also means that iron acquisition tends to be much more of a problem for organisms that use oxygen than for those that don’t, because anaerobic environments favor the more soluble Fe(II).
‘We have shown the generality of these reaction mechanisms in metal oxidizing and reducing bacteria,’ said Dr. Liang Shi, a PNNL microbiologist who led the study. ‘Whether it’s Fe(II) or Fe(III), iron’s solubility affects its accessibility to microorganisms. To access these different phases of Fe, some microorganisms seem to adopt a common mechanism.’”
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