Iron site, formation, ribonucleotide reductase

Studies on three different iron-sulfur enzyme systems, which all require S-adenosyl methionine—lysine 2,3-aminomutase, pyruvate formate lyase and anaerobic ribonucleotide reductase—have led to the identification of SAM as a major source of free radicals in living cells. As in the dehydratases, these systems have a [4Fe-4S] centre chelated by only three cysteines with one accessible coordination site. The cluster is active only in the reduced... 

The site in the active Fe ribonucleotide reductase contains two Fe(III) ions 3.3 A apart, bridged by one carboxylate from a glutamate residue and a water-derived oxo bridge (57). The function of this iron center appears to be the formation and stabilization of a free radical on a tyrosine about 5 A away. This radical is formed by reaction of the reduced, diferrous center with 02, probably through peroxide and ferryl intermediates. This unusually stable tyrosyl radical is thought to partic-... 

Ribonucleotide reductase, an allosteric enzyme from Escherichia coli, converts ribonucleotide diphosphates to the corresponding deoxyribo-nucleotides, and therefore provides the necessary precursors for DNA synthesis (Reichard, 1967). During purification ribonucleotide reductase separates into two nonidentical subunits, proteins B1 and B2, each enzymatically inactive (Brown et a/., 1969a). The active enzyme is formed in the presence of magnesium ions and consists of a 1 1 complex of the two subunits (Thelander, 1973). Proton subunit B2 (mol. wt. 78,000), which participates in the formation of the catalytic site (Thelander et al, 1976), contains nonheme iron (Brown et al, 1969b) and an ESR-detectable organic... 

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