Methylation of small molecules and macromolecules is crucial in metabolism, cell signaling, and epigenetic programming and is most often achieved by S-adenosylmethionine (SAM)-dependent methyltransferases. Most employ an S(N)2 mechanism to methylate nucleophilic sites on their substrates, but recently, radical SAM enzymes have been identified that methylate carbon atoms that are not inherently nucleophilic via the intermediacy of a 5'-deoxyadenosyl 5'-radical. We have determined the mechanisms of two such reactions targeting the sp(2)-hybridized carbons at positions 2 and 8 of adenosine 2503 in 23S ribosomal RNA, catalyzed by RlmN and Cfr, respectively. In neither case is a methyl group transferred directly from SAM to the RNA; rather, both reactions proceed by a ping-pong mechanism involving intermediate methylation of a conserved cysteine residue.

Radical S-adenosylmethionine (SAM) enzymes catalyze an astonishing array of complex and chemically challenging reactions across all domains of life. Of approximately 114,000 of these enzymes, 8 are known to be present in humans: MOCS1, molybdenum cofactor biosynthesis; LIAS, lipoic acid biosynthesis; CDK5RAP1, 2-methylthio-N(6)-isopentenyladenosine biosynthesis; CDKAL1, methylthio-N(6)-threonylcarbamoyladenosine biosynthesis; TYW1, wybutosine biosynthesis; ELP3, 5-methoxycarbonylmethyl uridine; and RSAD1 and viperin, both of unknown function. Aberrations in the genes encoding these proteins result in a variety of diseases. In this review, we summarize the biochemical characterization of these 8 radical S-adenosylmethionine enzymes and, in the context of human health, describe the deleterious effects that result from such genetic mutations.

RimO and MiaB are radical S-adenosylmethionine (SAM) enzymes that catalyze the attachment of methylthio (–SCH3) groups onto macromolecular substrates. RimO attaches a methylthio group at C3 of aspartate 89 of protein S12, a component of the 30S subunit of the bacterial ribosome. MiaB attaches a methylthio group at C2 of N6-(isopentenyl)adenosine, found at nucleotide 37 in several prokaryotic tRNAs. These two enzymes are prototypical members of a subclass of radical SAM (RS) enzymes called methylthiotransferases (MTTases). It had been assumed that the sequence of steps in MTTase reactions involves initial sulfur insertion into the organic substrate followed by capping of the inserted sulfur atom with a SAM-derived methyl group. In this work, however, we show that both RimO and MiaB from Thermotoga maritima (Tm) catalyze methyl transfer from SAM to an acid/base labile acceptor on the protein in the absence of their respective macromolecular substrates. Consistent with the assignment of the acceptor as an iron–sulfur (Fe/S) cluster, denaturation of the SAM-treated protein with acid results in production of methanethiol. When RimO or MiaB is first incubated with SAM in the absence of substrate and reductant, and then incubated with excess S-adenosyl-l-[methyl-d3]methionine ([methyl-d3]-SAM) in the presence of substrate and reductant, production of the unlabeled product precedes production of the deuterated product, showing that the methylated species is chemically and kinetically competent to be an intermediate.

A practical synthetic method for the annulation of benzo-rings by the intramolecular coupling of an aryl iodide and a methylene C−H bond is described. The palladium-catalyzed C−H functionalization is directed by an aminoquinoline carboxamide group, which can be easily installed and removed. High yields and broad substrate scope were achieved. An additive of ortho-phenyl benzoic acid, identified from a systematic screening, functions as a critical ligand for the catalytic process under mild condition, even at near room temperature.

Particulate emissions from two types of helicopter turboshaft engines operated with military JP-8 and paraffinic Fischer-Tropsch (FT) fuels were characterized as an objective of the field campaign held at the Hunter Army Airfield in Savannah, GA in June 2007. In general helicopter engines exhaust particles size distributions observed at the engine nozzle and 4.14 m downstream locations showing the geometric mean diameters smaller than 50 nm for all engine power settings investigated in this study. For both locations, the geometric mean diameter increased as the engine power setting increased; this trend also holds true for the emitted particle number concentration. The growth of particle geometric mean diameter was found significant, 7 nm, only in the case of the idle power setting.Sulfur-to-sulfate conversion was found to be independent of the engine power setting. Emissions of both sulfur and sulfate increased as the engine power increased. When JP-8 fuel was used, particles smaller than 7 nm were found to increase in samples taken at the downstream location. The number concentration in this tail increased as the power setting increased. No such observation was found when FT fuel was used implying that the increased formation of nuclei particles in the plume downstream was likely to be caused by the sulfur and aromatic compounds in the JP-8 fuel. Total particulate carbon emissions increased as the engine power setting increased. Use of FT fuel reduced the elemental carbon emissions as it compared to the JP-8 fuel, and organic carbon emission at idle power but not at the higher powers. The reduction of elemental carbon by the FT fuel was attributed to the absence of aromatics (soot precursors) in the fuel. The OC/EC ratio was found to be in the range of 3-50 depending on the engine power setting. The aircraft emitted OC/EC was found to decrease as the engine power increased.