The solution and redox properties of iron that make it the metal prosthetic group of choice for the activation of otherwise kinetically inert substrates, including dioxygen, also make ionic Fe cytotoxic to aerobic organisms. Eukaryotes from yeast to humans have to manage ferrous iron's inherent reactivity with dioxygen and ferric iron's instability in water;the oft-cited role of iron in human pathology from post-ischemic tissue damage to neurodegenerative disease is testament to the importance of managing ionic iron. We propose that the Fe-traffieking pathway that succeeds in suppressing Fe's abiologic side-reactions has two essential and inter-related features: sequential Fe-trafficking components are spatially contiguous and thereby are architecturally organized to support the channeling of ionic Fe species along the Fe-metabolic pathway. This model will be tested at three steps in the ionic Fe pathway in Saccaromyces cerevisiae, the most tractable eukaryotic cell for a systematic test of this Fe-metabolic model. These three steps are: at the plasma membrane where ferrireduction is coupled to iron permeation;in the cytoplasm where Fe is trafficked from the PM to protein acceptor sites;and in the vacuole where Fe-redox cycling is coupled to Fe-storage in reactions that precisely mirror those that occur in ferritin. A primary strategy to ascertain conformational contiguity of Fe-handling proteins will be fluorescence resonance energy transfer;we propose to use FRET to examine the spatial relationships between reductase and permease partners in the plasma and vacuolar membranes. A primary strategy in quantifying the relative partitioning of newly-arrived Fe will be the use of cells engineered to turn on or turn off production of these putative Fe-handling proteins. A new role in Fe- handling is proposed for the yeast HSP90 proteins, Hsp82 and Hsc82;in addition, we suggest that nitric oxide and glutathione combine in a dinitrosyldithiolato-Fe complex that plays a significant role in cytoplasmic Fe-handling. Outstanding progress has been made on the metabolism of Fe-prosthetic groups like heme and Fe/S clusters;ionic Fe is the precursor to these """"""""caged"""""""" Fe-species and is responsible for the """"""""corrosive chemistry"""""""" that characterizes the relationship between Fe and dioxygen. An understanding of how cells suppress this chemistry would make a significant contribution to our eventual elucidation of the molecular basis for the multitude of human pathologies often attributed in part to mismanaged ionic iron.
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