An Unprecedented Ring-Contraction Mechanism in Cobalamin-Dependent Radical S-Adenosylmethionine Enzymes.

A unique member of the family of cobalamin (Cbl)-dependent radical S-adenosylmethionine (SAM) enzymes, OxsB, catalyzes the ring constriction of deoxyadenosine triphosphate (dATP) to the base oxetane aldehyde phosphate, a crucial precursor for oxetanocin A (OXT-A), which is an antitumor, antiviral, and antibacterial compound. This enzyme reveals a new catalytic function for this big family that is different from the common methylation. On the basis of density functional theory calculations, a mechanism has been proposed to mainly include that the generation of 5'-deoxyadenosine radical, a hydrogen transfer forming 2'-dATP radical, and a Cbl-catalyzed ring contraction of the deoxyribose in 2'-dATP radical. The ring contraction is a concerted rearrangement step accompanied by an electron transfer from the deoxyribose hydroxyl oxygen to CoIII without any ring-opening intermediate. CoIICbl has been ruled out as an active state. Other mechanistic characteristics are also revealed. This unprecedented non-methylation mechanism provides a new catalytic repertoire for the family of radical SAM enzymes, representing a new class of ring-contraction enzymes.

How does Mo-dependent perchlorate reductase work in the decomposition of oxyanions?

Mo-Dependent perchlorate reductase (PcrAB), responsible for the ending step of anaerobic respiration of perchlorate reducing bacteria, catalyzes the conversion of perchlorate to chlorate and subsequently chlorite and is also able to deoxidate bromate, iodate, and nitrate. Herein, the reaction mechanisms for the PcrAB-catalyzed decomposition of these oxyanions have been investigated using density functional calculations and a chemical model constructed from the X-ray crystal structure. It is revealed that the reactions of halogen oxyanions proceed through a very fast O-X (X = Cl, Br, and I) heterolytic cleavage activated by the MoIV center, followed by a rate-limiting reduction of the resulting MoVI[double bond, length as m-dash]O back to MoIV dominated by the slow proton-coupled electron transfer (PCET). However, the O-N bond heterolysis in the nitrate decomposition has a barrier (16.2 kcal mol-1) comparable to the PCET-dominating reduction of MoVI[double bond, length as m-dash]O. This heralds an exciting future where a proper mutation of electron/proton transfer passage of perchlorate reducing bacteria may lead to a decomposition preference for halogen oxyanions rather than non-toxic nitrate, providing a friendly bioremediation method. Other open mechanistic questions are also addressed, where in particular an O-O rebound mechanism without PCET has been ruled out.