FLAVOPROTEIN STRUCTURE AND MECHANISM .7. STRUCTURE AND MECHANISM OF THE IRON-SULFUR FLAVOPROTEIN PHTHALATE DIOXYGENASE REDUCTASE

Transfer of electrons between pyridine nucleotides (obligatory two-ele ctron carriers) and hemes or [2Fe-2S] centers (obligatory one-electron carriers) is an essential step mediated by flavins in respiration, ph otosynthesis, and many oxygenase systems, Phthalate dioxygenase reduct ase (PDR), a soluble iron-sulfur flavoprotein from Pseudomonas cepacia , is a convenient model for the study of this type of electron transfe r, PDR is folded into three domains; the NH2-terminal FMN binding and central NAD(H) binding domains are closely related to ferredoxin-NADP( +) reductase (FNR), The COOH-terminal [2Fe-2S] domain is similar to pl ant ferredoxins, and can be removed by proteolysis without significant ly altering the reactivity of the FNR-like domains, Kinetic studies ha ve identified sequential steps in the reaction of PDR with NADH that i nvolve pyridine nucleotide binding, hydride transfer to FMN, and intra molecular electron transfer from the reduced flavin to the [2Fe-2S] cl uster, Crystal structures of reduced and liganded PDR correspond to so me of the intermediates formed during reduction by NADH, Small structu ral changes that are observed in the vicinity of the cofactors upon re duction or NAD(H) binding may provide part of the reorganization energ y or contribute to the gating mechanism that controls intramolecular e lectron transfer.