High catalytic activity achieved with a mixed manganese–iron site in protein R2 of Chlamydia ribonucleotide reductase

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Ribonucleotide reductase (class I) contains two components: protein R1 binds the substrate, and protein R2 normally has a diferric site and a tyrosyl free radical needed for catalysis. In Chlamydia trachomatis RNR, protein R2 functions without radical. Enzyme activity studies show that in addition to a diiron cluster, a mixed manganese–iron cluster provides the oxidation equivalent needed to initiate catalysis. An EPR signal was observed from an antiferromagnetically coupled high-spin Mn(III)–Fe(III) cluster in a catalytic reaction mixture with added inhibitor hydroxyurea. The manganese–iron cluster in protein R2 confers much higher specific activity than the diiron cluster does to the enzyme.