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Publication Abstract from PubMed

Class II ribonucleotide reductases (RNR) catalyse the formation of an essential thiyl radical by homolytic cleavage of the Co-C bond in their adenosylcobalamin (AdoCbl) cofactor. Several mechanisms for the dramatic acceleration of Co-C bond cleavage in AdoCbl-dependent enzymes have been advanced, but no consensus yet exists. We present the structure of the class II RNR from Thermotoga maritima in three complexes: 1) with allosteric effector dTTP, substrate GDP and AdoCbl; 2) with dTTP and AdoCbl; 3) with dTTP, GDP and adenosine. Comparison of these structures gives the deepest structural insights so far into the mechanism of radical generation and transfer for AdoCbl-dependent RNR. AdoCbl binds to the active site pocket, shielding the substrate, transient 5'-deoxyadenosyl radical and nascent thiyl radical from solution. The e-propionamide side chain of AdoCbl forms hydrogen bonds directly to the alpha-phosphate group of the substrate. This interaction appears to cause a "locking-in" of the cofactor, and it is the first observation of a direct cofactor-substrate interaction in an AdoCbl-dependent enzyme. The structures support an ordered sequential reaction mechanism with release or relaxation of AdoCbl on each catalytic cycle. A conformational change of the AdoCbl adenosyl ribose is required to allow hydrogen transfer to the catalytic thiol group. Previously proposed mechanisms for radical transfer in B12-dependent enzymes cannot fully explain the mechanism in class II RNR, suggesting that it may form a separate class that differs from the well-characterised eliminases and mutases.

Structural basis for adenosylcobalamin activation in AdoCbl-dependent ribonucleotide reductases.,Larsson KM, Logan DT, Nordlund P ACS Chem Biol. 2010 Jul 30. PMID:20672854^[1]

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