ABSTRACT
The first 2Fe-2S ferredoxins to be discovered were the so-called plant-type ferredoxins in spinach chloroplasts [18]. They are found in many oxygenic photosynthetic organ isms, occur frequently in isoforms, and serve as terminal electron acceptors to photo system I (PSI). Their Fe2S2 cluster (with a redox potential of about —400 mV) is reduced by one electron from an Fe4S4 cluster of the PSI subunit PsaC and trans ferred to ferredoxin-NADP+ reductase. Besides their role in photosynthesis, they serve as electron carriers in a variety of metabolic pathways, such as nitrite reduction, nitrogen fixation, sulfite reduction, glutamate synthesis, thioredoxin oxidoreduction, and lipid desaturation. In bacterial dioxygenase systems, 2Fe-2S ferredoxins are the electron shuttle between reductase flavoproteins and oxygenase. Many redox active enzymes contain domains or subunits similar to 2Fe-2S ferredoxins, e.g., bacterial aromatic di-and monooxygenases, fumarate reductase, eukaryotic succinate dehydro genase, xanthine dehydrogenase, phenol hydroxylase, phthalate dioxygenase reduc tase, and cytoplasmic Fe-only hydrogenases. The soluble adrenodoxin-type 2Fe-2S ferredoxins (redox potential about -270 mV) occur in oxygenase systems and can be divided into the vertebrate-type and Pseudomonas putida-type ferredoxins, with about 50% sequence similarity between these subgroups. Vertebrate ferredoxins are found in mitochondrial monooxygenase systems of different tissues where they trans fer single electrons from NADPH:ferredoxin reductase to membrane-bound cyto chrome P450 enzymes involved in steroid hormone biosynthesis and other metabolic processes. The bacterial electron carriers putidaredoxin and terpredoxin transfer electrons from NADH-dependent ferredoxin reductases to soluble cyto chrome P450, e.g., P450cam (catalyzing the first step of camphor metabolism) in the case of putidaredoxin.
