Open AccessBiosynthesis of Oxetanocin-A Includes a B12-Dependent Radical SAM Enzyme That Can Catalyze both Oxidative Ring Contraction and the Demethylation of SAM
Author(s) -
Aoshu Zhong,
YuHsuan Lee,
Yung-Nan Liu,
Hungwen Liu
Publication year - 2021
Publication title -
biochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.43
H-Index - 253
eISSN - 1520-4995
pISSN - 0006-2960
DOI - 10.1021/acs.biochem.0c00915
Subject(s) - chemistry , aldehyde , demethylation , stereochemistry , methyltransferase , enzyme , nucleoside , biochemistry , purine nucleoside phosphorylase , methylation , catalysis , purine , dna , dna methylation , gene expression , gene
Oxetanocin-A is an antitumor, antiviral, and antibacterial nucleoside. It is biosynthesized via the oxidative ring contraction of a purine nucleoside co-opted from primary metabolism. This reaction is catalyzed by a B 12 -dependent radical S -adenosyl-l-methionine (SAM) enzyme, OxsB, and a phosphohydrolase, OxsA. Previous experiments showed that the product of the OxsB/OxsA-catalyzed reaction is an oxetane aldehyde produced alongside an uncharacterized byproduct. Experiments reported herein reveal that OxsB/OxsA complex formation is crucial for the ring contraction reaction and that reduction of the aldehyde intermediate is catalyzed by a nonspecific dehydrogenase from the general cellular pool. In addition, the byproduct is identified as a 1,3-thiazinane adduct between the aldehyde and l-homocysteine. While homocysteine was never included in the OxsB/OxsA assays, the data suggest that it can be generated from SAM via S -adenosyl-l-homocysteine (SAH). Further study revealed that conversion of SAM to SAH is facilitated by OxsB; however, the subsequent conversion of SAH to homocysteine is due to protein contaminants that co-purify with OxsA. Nevertheless, the observed demethylation of SAM to SAH suggests possible methyltransferase activity of OxsB, and substrate methylation was indeed detected in the OxsB-catalyzed reaction. This work is significant because it not only completes the description of the oxetanocin-A biosynthetic pathway but also suggests that OxsB may be capable of methyltransferase activity.