Figure 7.

Decarboxylation of orotidylate (OMP) via a nitrogen ylide, according to a mechanism proposed by Beak and Siegel (17). Because both the 2-syn and 2-anti isomers of BMP contain negative charges at positions corresponding to the 2- and 6-positons of the carbanionic intermediate, tight binding of either isomer of this potential transition state analogue would be compatible with the Beak–Siegel mechanism. The position of Lys-93 in the transition state, inferred from its position in the BMP complex, is shown on the right.