Iron-sulfur cluster single-electron transfer

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

Flavins are unique coenzymes that are able to catalyze both one- and two-electron transfers. Because of this, many flavoproteins are involved in transferring electrons between other proteins. Often, flavoproteins are reduced by two-electron donors, such as pyridine nucleotides, and then pass those electrons one at a time to a single-electron acceptor, such as an iron-sulfur cluster in another protein. Conversely, some enzymes accept single electrons from reduced enzymes. In either case, the flavoenzymes are transferring single electrons thus, flavin semiquinone is frequently stabilized and observed during turnover. 

The reductase from M.capsulatus (Bath) is composed of a single polypeptide of Mr = 44.6 kDa and 1 mol of FAD and 1 mol of 2Fe2S cluster per mol of protein [24]. The reductase is responsible for an electron transfer from NADH to the hydroxylase [25]. All reductases from the other methanotrophs have the same cofactors, FAD and 2Fe2S cluster, and their molecular weights are about 40 kDa. Electrons from NADH are transferred to FAD to form fully reduced FADH2, which transfers one electron very rapidly to the iron-sulfur cluster to form a flavin semiquinone radical [15, 45]. The semiquinone radical is detected as a free radical signal at g = 2.004 by means of ESR spectroscopy [46]. Upon complete reduction of the reductase with sodium dithionite, the ESR spectrum of the reductase shows the g values at 2.04, 1.96, and 1.87 (gav = 1-96) characteristic of a reduced 2Fe2S cluster [46]. 

The initial 6 A-resolution X-ray work showed one of the [4Fe 4S] clusters is 15 A and the other 22 A from the preceding iron-sulfur electron acceptor, FeS-X. Discussion of the iron-sulfur center FeS-X itself will be deferred to Chapter 31, where the interaction of the FeS-X domain with PsaC (FeS-A/FeS-B) and its involvement in electron transfer to FeS-A/B will also be addressed. More recent work by the Berlin group at 4 A resolution has refined the distance measurements to 15.4 and 22.2 A, respectively. Fig. 12 (B) shows a three-dimensional model of PsaC based on low-temperature EPR measurements by Kamlowski and coworkers on PS-I single crystals. Their interpretation was made by using the bacterial ferredoxin model as a PsaC substitute in the PS-I reaction center matrix, and adjusting its orientation for the best agreement with the EPR data. The g-tensor orientation of EeS-A and FeS-B in the PS-I single crystals determines the PsaC orientation relative to the two clusters. 

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