Solvent inhibition profiles and inverse solvent isotope effects for enzymatic methyl transfer catalyzed by nicotinamideN‐methyltransferase

Abstract

AbstractNicotinamideN‐methyltransferase (NNMT) catalyzes the methyl transfer from universal methyl donor S‐adenosyl methionine (SAM) to nicotinamide (NA) to form theN‐methylnicotinamide. This important process is related to the level of nicotinamide adenine dinucleotide (NAD+) in vivo; thus, NNMT is regarded as an important drug target linked with various diseases. Although NNMT has been extensively studied in the area of biology and medicine, the detailed mechanism of NNMT is still not clear, especially in the aspect of the role of solvents (water) in the enzyme catalysis. Here, we have examined the effects of three common organic solvents (dimethyl sulfoxide [DMSO], methanol, and acetonitrile) and deuterated water to explore the enzymatic methyl transfer activity by NNMT (wild type [WT]) and its mutants. Competitive inhibition was detected for DMSO and acetonitrile as the solvent, and a subtle inhibitory effect was observed with methanol. Molecular docking suggested DMSO and acetonitrile compete with both cofactor and substrate. However, methanol is mainly bound to compete for the substrate. Furthermore, the activity increased when deuterated water was substituted for water with an inverse solvent isotope effectD2Okcat/Km= 0.49 ± 0.17 for WT andD2Okcat/Km= 0.32 ± 0.08 for Y20F. These effects in this enzymatic methyl transfer system without any metal ion or acid/base catalysis were due to the stabilization of protein provided by deuterated water.