Design and synthesis of C5 methylated L-arginine analogues as active site probes for nitric oxide synthase.

The role of nitric oxide (NO) as a biological signaling molecule is well established. NO is produced by the nitric oxide synthases (NOSs, EC 1.14.13.39), a class of heme proteins capable of converting l-arginine to NO and l-citrulline. Despite the large body of knowledge associated with the NOSs, mechanistic details relating to the unique oxidative chemistry performed by these enzymes remain to be fully elucidated. Furthermore, a number of disease states are associated with either the over- or underproduction of NO, making the NOS pathway an attractive target for the development of therapeutics. For these reasons, molecular tools capable of providing mechanistic insights into the production of NO and/or the inhibition of the NOSs remain of interest. We report here the stereospecific synthesis and testing of a number of new l-arginine analogues bearing a minimal substitution, methylation at position 5 of the amino acid side chain (such analogues have not been previously reported). The synthetic approach employed a modified photolysis procedure whereby irradiation of the appropriate diacylperoxide precursors at 254 nm gave access to the required unnatural amino acids in good yields. A heme domain construct of the inducible NOS isoform (iNOSheme) was used to assess the binding of each compound to the enzyme active site. The compounds were also investigated as either inhibitors of, or alternate substrates for, the inducible NOS isoform. The results obtained provide new insight into the steric and stereochemical tolerance of the enzyme active site. These findings also further support the role of a conserved active site water molecule previously proposed to be necessary for NOS catalysis.