| N-methyl nicotinamide transferase（NNMT）is a cytoplasmic enzyme and mainly expressed in liver and adipose tissues.NNMT catalyzes methyl transfer from cofactor S-adenosine-L-methionine（SAM）to nicotinamide（NA）with products of S-adenosine-L-homocysteine（SAH）and N-methyl nicotinamide（MEN）.This process plays an important role in maintaining human health and is related to some diseases,such as metabolic disorders,obesity,cardiovascular disease,cancers,and Parkinson’s disease.Although NNMT has been extensively studied in the area of biology and medicine,detailed mechanism of NNMT is still fuzzy,especially in the role of solvents（water）in the enzyme catalysis.To explore the mechanism of interaction between enzyme activity sites and solvent molecules,we have examined the effects of three common organic solvents（dimethyl sulfoxide,methanol and acetonitrile）and deuterium water on the methyl transfer catalyzed by NNMT（WT）and its mutants.The kinetic data shows that NNMT（WT）has similar results with its mutant:A competitive inhibition was detected for DMSO and acetonitrile as solvent and a subtle inhibitory effect was observed with methanol.The docking results demonstrate that five solvents（DMSO,methanol,CH3CN,H2O and D2O）are able to dock with both NNMT active site and surface,indicating that the selected solvents could spread to the whole protein to affect its function.All these docking results suggest that the solvent molecules（DMSO,Me CN and Me OH）are able to inhibit the activity of NNMT,either by competing the cofactor/substrate domain affecting the binding of corresponding cofactor and/or substrate,or changing the micro-environment of surrounding cofactor,substrates or even enzyme.With the kinetic and docking approaches reported here,it will not only help to understand the interaction of environment with the protein,but also help to screen potential inhibitor by choosing appropriate solvent. |
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