Examining Product Specificity in Protein Arginine Methyltransferase 7 (PRMT7) Using Quantum and Molecular Mechanical Simulations

Protein arginine methyltransferase 7 (PRMT7) catalyzes the formation of monomethyl­arginine (MMA) but is incapable of performing a dimethylation. Given that PRMT7 performs vital functions in mammalian cells and has been implicated in a variety of diseases, including breast cancer and age-related obesity, elucidating the origin of its strict monomethylation activity is of considerable interest. Three active site residues, Glu172, Phe71, and Gln329, have been reported as particularly important for product specificity and enzymatic activity. To better understand their roles, mixed quantum and molecular mechanical (QM/MM) calculations coupled to molecular dynamics and free energy perturbation theory were carried out for the WT, F71I, and Q329S trypanosomal PRMT7 (TbPRMT7) enzymes bound with S-adenosyl-L-methionine (AdoMet) and an arginine substrate in an unmethylated or methylated form. The Q329S mutation, which experimentally abolished enzymatic activity, was appropriately computed to give an outsized ΔG^‡ of 30.1 kcal/mol for MMA formation compared to 16.9 kcal/mol for WT. The F71I mutation, which has been experimentally shown to convert the enzyme from a type III PRMT into a mixed type I/II capable of forming dimethylated arginine products, yielded a reasonable ΔG^‡ of 21.9 kcal/mol for the second turnover compared to 28.8 kcal/mol in the WT enzyme. Similar active site orientations for both WT and F71I TbPRMT7 allowed Glu172 and Gln329 to better orient the substrate for S_N2 methylation, enhanced the nucleophilicity of the attacking guanidino group by reducing positive charge, and facilitated the binding of the subsequent methylated products.