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Publication Abstract from PubMed

The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) employs a MnIV/FeIII cofactor in each monomer of its beta2 subunit to initiate nucleotide reduction. The cofactor forms by reaction of MnII/FeII-beta2 with O2. Previously, in vitro cofactor assembly from apo beta2 and divalent metal ions produced a mixture of two forms, with Mn in site 1 (MnIV/FeIII) or site 2 (FeIII/MnIV), of which the more active MnIV/FeIII product predominates. Here we have addressed the basis for metal site-selectivity by solving X-ray crystal structures of apo, MnII, and MnII/FeII complexes of Ct beta2. A structure obtained anaerobically with equimolar MnII, FeII, and apo protein reveals exclusive incorporation of MnII in site 1 and FeII in site 2, in contrast to the more modest site-selectivity achieved previously. Site-specificity is controlled thermodynamically by the apo protein structure, as only minor adjustments of ligands occur upon metal binding. Additional structures imply that, by itself, MnII binds in either site. Together the structures are consistent with a model for in vitro cofactor assembly in which FeII specificity for site 2 drives assembly of the appropriately configured heterobimetallic center, provided that FeII is substoichiometric. This model suggests that use of an MnIV/FeIII cofactor in vivo could be an adaptation to FeII limitation. A 1.8 A resolution model of the MnII/FeII-beta2 complex reveals additional structural determinants for activation of the cofactor, including a proposed site for side-on (eta2) addition of O2 to FeII and a short (3.2 A) MnII-FeII interionic distance, promoting formation of the MnIV/FeIV activation intermediate.

Structural Basis for Assembly of the Mn/Fe Cofactor in the Class Ic Ribonucleotide Reductase from Chlamydia trachomatis.,Dassama LM, Krebs C, Bollinger JM, Rosenzweig AC, Boal AK Biochemistry. 2013 Aug 7. PMID:23924396^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.