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Four proteins have been identified recently as diiron carboxylate proteins on the basis of conservation of six amino acids (four carboxylate residues and two histidines) constituting an iron-binding motif. Unlike previously identified proteins with this motif, biochemical studies indicate that each of these proteins is membrane bound, although homology modeling rules out a transmembrane mode of binding. Therefore, the predicted structure of each protein [the alternative oxidase (AOX), the plastid terminal oxidase (PTOX), the diiron 5-demethoxyquinone hydroxylase (DMQ hydroxylase), and the aerobic Mg-protoporphyrin IX monomethylester hydroxylase (MME hydroxylase)] is that of a protein bound monotopically to one leaflet of the membrane bilayer. Three of these enzymes utilize a quinol substrate, with two oxidizing the quinol (AOX and PTOX) and one hydroxylating it (DMQ hydroxylase). MME hydroxylase is involved in synthesis of the isocyclic ring of chlorophyll. Two enzymes are involved in respiration (AOX and, indirectly, the diiron DMQ hydroxylase through ubiquinone biosynthesis) and two in photosynthesis, through their roles in carotenoid and chlorophyll biosynthesis (PTOX and MME hydroxylase, respectively). We discuss what is known about each enzyme as well as our expectations based on their identification as interfacially bound proteins with a diiron carboxylate active site.
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Download Table: Characteristics of Cysteine Mutants of the Alternative Oxidase (PDF). Download Figure: Titration of the Alternative Oxidase with Activators (PDF). Sequence Alignments for Membrane-bound Diiron Proteins Download Alternative Oxidase (PDF). Download Plastid Terminal Oxidase (PDF). Download Diiron 5-DMQ Hydroxylase (PDF). Download Mg protoporphyrin IX monomethylester Hydroxylase (PDF).
Sequences were aligned with ClustalW using Biology Workbench. The red shading indicates complete conservation of the amino acid, and the yellow indicates more than 50% identity. The gray bars beneath the aligned sequence indicate the region of each of the four helices that bind the diiron center. The predicted iron-binding ligands are shown as black triangles. Due to the difficulty in predicting where each helix begins and ends, we have not precisely identified the end residues. Instead the uncertainty in helix length is indicated with a tapering of the gray bars.