Role of protein nanowires in metal cycling and mineralization

在铁和锰的循环过程中，地杆菌被公认为是重要的媒介。这些细菌“呼吸”铁和锰氧化物，这一过程需要细胞产生毛状细丝(菌毛)来结合矿物质，并向它们排放新陈代谢中产生的电子。这些生物“纳米线”的表面装饰着口袋，这些口袋可以捕获许多其他金属，特别是那些带正电荷的金属(阳离子金属)。这表明，地质杆菌可以循环和矿化的金属范围远远超过目前公认的范围。该项目将研究地杆菌细胞与其导电菌毛结合和矿化的金属光谱。这项研究将通过研究一种在自然界具有全球影响的新形式的微生物能量转导，来推进美国国家科学基金会“促进科学进步”的使命。这些研究的目标金属不仅天然丰富，而且是许多工业活动的副产品，以高流动性形式在有毒水平上积累，迅速进入食物链，增加暴露风险。研究其固定化的生物学机制将提供开发生物作用和生物修复技术所需的基本知识，从而满足国家需要。这项研究还与教育努力交织在一起，旨在培养生物学和地质学交界处的年轻专业人员，但对物理、化学和工程有深刻的理解。还将努力培训未来的教育工作者，激发他们参与促进科学交流和包容的外联项目的动机。该项目将侧重于地质杆菌，这是迄今为止所描述的唯一使用蛋白质纳米线作为细胞和细胞外金属电子受体之间的电子管道的微生物。每个菌毛纤维是由相同的多肽亚单位(菌毛素)组装而成，并在其表面暴露出许多羧基侧链配体，这些配体可以结合阳离子金属并使它们处于还原的最佳位置。研究人员将在一系列实验中测试这一点，评估金属陷阱的聚集度、菌毛导电性和电荷对结合和还原沉淀三价钴(Co3)、二价镉(CD2)和一价银(Ag)金属阳离子的影响。这些阳离子金属存在于地杆菌还原铁的环境中。是一个活跃的过程，是一个由蛋白质纳米线介导的过程，伴随着铁氧化物的还原而矿化。这表明，地杆菌菌毛还介导了可溶金属阳离子的还原沉淀，人们可以在结合细胞显微镜检查和X射线近边结构吸收(XANES)光谱分析的生物分析中对此进行评估。研究人员还将收集PILI伴生矿物的LIII边缘扩展X射线吸收精细结构(EXAFS)光谱，以模拟金属的原子配位。这将允许表征矿物相并识别对金属矿化负责的纳米线配体。

Geobacter bacteria are recognized as important agents in the cycling of iron and manganese. These bacteria "breath" iron and manganese oxides, a process that requires the cells to produce hair-like filaments (pili) to bind the minerals and discharge onto them electrons generated in their metabolism. The surface of these biological "nanowires" is decorated with pockets that could trap many other metals, particularly those that are positively charged (cationic metals). This suggests that the range of metals that Geobacter bacteria can cycle and mineralize is far greater than currently acknowledged. This project will investigate the metal spectrum that Geobacter cells can bind and mineralized with their conductive pili. This research will advance NSF's Mission "to promote the progress of science" by studying a novel form of microbial energy transduction that has global implications in nature. Not only are the metals targeted in these studies naturally abundant, they are also byproducts of many industrial activities and accumulate at toxic levels as highly mobile forms, which are rapidly introduced into the food chain and increase the risk of exposure. Investigating biological mechanisms for their immobilization will provide the fundamental knowledge needed to develop technologies for biomining and bioremediation, thus addressing a national need. The research is also intertwined with educational efforts directed at training young professionals at the interface of biology and geology, but with deep understanding of physics, chemistry, and engineering. Efforts are also aimed at training future educators and stirring their motivation to engage in outreach projects that promote science communication and inclusion.This project will focus on Geobacter bacteria, the only microorganisms described to date that use protein nanowires as electronic conduits between the cell and extracellular metal electron acceptors. Each pilus fiber is an assembly of the same peptide subunit (the pilin) and exposes on its surface many carboxyl side chain ligands, which could bind cationic metals and position them optimally for their reduction. The investigator will test this in a series of experiments that evaluate the effect of piliation, pilus conductivity, and charge of the putative metal traps to bind and reductively precipitate trivalent cobalt (Co3+), divalent cadmium (Cd2+) and monovalent silver (Ag+) metal cations. These cationic metals are found in environments where iron reduction by Geobacter spp. is an active process, a process mediated by protein nanowires, and mineralized concomitantly to the reduction of iron oxides. This suggests that the Geobacter pili also mediate the reductive precipitation of the soluble metal cations, which one can assess in biological assays coupled to microscopic examination of the cells and bulk X-ray Absorption Near Edge Structure (XANES) spectroscopic analyses. The researcher will also collect the LIII-edge extended X-ray absorption fine structure (EXAFS) spectra from the pili-associated mineral to model the atomic coordination about the metal. This will allow to characterize the mineral phase and identify the nanowire ligands that are responsible for metal mineralization.