Multicopper oxidases (MCOs) couple the 4e- reduction of dioxygen to 2H2O with the oxidation of 4 equivalents of a 1-electron donor. A sub-family of these ubiquitous enzymes possesses specificity towards FeII that makes them essential to iron homeostasis in their respective organisms. Our hypothesis is that these ferroxidases function by channeling their FeIII product to a down-stream partner in their iron metabolic pathways. This program has delineated the structure and function in the Fet3p, Ftr1p high-affinity Fe-uptake complex in the Saccharomyces cerevisiae (Sc) plasma membrane (PM). The two fundamental questions addressed in this work are: 1) what structural motifs confer on an MCO this specificity for FeII as substrate;and 2) how is the Fet3p ferroxidase reaction coupled kinetically and physically to the membrane permeation of FeIII by Ftr1p. In this application we propose three specific aims.
In Aim I we will test our hypothesis of what structural motifs define a ferroxidase, specifically that a cohort of carboxylate side chains maximize three factors that determine e- transfer from FeII: FeII binding, FeII redox potential and electronic matrix coupling of the FeII and the type 1 CuII (T1Cu) in the ferroxidase. We will do this by interconverting laccase and ferroxidase enzymes based on our design principles. In addition, we will test our hypothesis that another cohort of carboxylate side chains stabilizes the increasing negative charge on dioxygen as it is reduced in 2, 2e- steps at the trinuclear cluster (TNC). Last we will test our hypothesis that the coordination changes associated with O2 reduction at the TNC trigger e- transfer from the T1 Cu via the canonical MCO His-Cys-His motif that connects the two.
In Aim II we propose to test our hypothesis that Fox1 in Chlamydomonas reinhardtii (Cr) is a human ceruloplasmin-like ferroxidase. We will characterize the Fox1 protein, demonstrating that it has the ferroxidase-specificity motifs that support both FeII oxidation and FeIII trafficking in a Fox1, Ftr1 complex in the Cr PM. We will construct mutants of both Fox1 and Ftr1 that we predict will be sensitive to a FeIII-chelator acting as a metabolite trap in Fe-trafficking between the two proteins in Fe-uptake;we have used this classic test of channeling in the Sc Fet3, Ftr1p complex. We propose that the Fe-trafficking between Fox1 and Ftr1 in Cr provides a realistic model of the putative FeIII-trafficking between hCp and transferrin.
In Aim III, we will test our hypothesis that the two human fungal pathogens, Candida albicans and Cryptococcus neoformans express an equivalent PM Fet, Ftr high-affinity Fe-uptake complex. We will quantify the 59Fe-uptake kinetics via these complexes both in situ and in recombinant form in Sc. Targeting specific ferroxidase and Fe-trafficking residues in the Ca and Cn proteins, we will test our hypothesis that these mutants exhibit the channeling defect exhibited by the Sc homologues. We propose that strains expressing these channeling mutants will exhibit a reduced virulence in vitro and in vivo. Last, using these chelator-sensitive clones, the NCI diversity collection will be mined for compounds with the potential as inhibitors of Fet, Ftr Fe-uptake in these fungal pathogens.
All oxygen-utilizing organisms from fungi to humans require the activity of a copper oxidase enzyme - a multicopper oxidase - to manage their metabolism of the essential nutrient, iron. Fungi from baker's yeast to the human pathogens C. albicans and C. neoformans use these enzymes to acquire the iron they need to thrive and survive. The goal of this research is to take a snap-shot of how these enzymes work and then to use this knowledge to block their trafficking of iron as way of suppressing the virulence of pathogenic fungi in both otherwise healthy and immunocompromised patients.
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