Getting the adrenaline going: Crystal structure of the adrenaline-synthesizing enzyme PNMT

Background: Adrenaline is localized to specific regions of the central nerv ous system (CNS), but its role therein is unclear because of a lack of suit able pharmacologic agents. Ideally, a chemical is required that crosses the blood-brain barrier, potently inhibits the adrenaline-synthesizing enzyme PNMT, and does not affect other catecholamine processes. Currently availabl e PNMT inhibitors do not meet these criteria. We aim to produce potent, sel ective, and CNS-active PNMT inhibitors by structure-based design methods. T he first step is the structure determination of PNMT. Results: We have solved the crystal structure of human PNMT complexed with a cofactor product and a submicromolar inhibitor at a resolution of 2.4 Ang strom. The structure reveals a highly decorated methyltransferase fold, wit h an active site protected from solvent by an extensive cover formed from s everal discrete structural motifs. The structure of PNMT shows that the inh ibitor interacts with the enzyme in a different mode from the (modeled) sub strate noradrenaline. Specifically, the position and orientation of the ami nes is not equivalent. Conclusions: An unexpected finding is that the structure of PNMT provides i ndependent evidence of both backward evolution and fold recruitment in the evolution of a complex enzyme from a simple fold. The proposed evolutionary pathway implies that adrenaline, the product of PNMT catalysis, is a relat ive newcomer in the catecholamine family. The PNMT structure reported here enables the design of potent and selective inhibitors with which to charact erize the role of adrenaline in the CNS. Such chemical probes could potenti ally be useful as novel therapeutics.