Ultrafast Photoreduction of Nitric Oxide Synthase by Electron Tunneling Wires

Citation

Bittner, Wendy Belliston (2005) Ultrafast Photoreduction of Nitric Oxide Synthase by Electron Tunneling Wires. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/e82v-jn92. https://resolver.caltech.edu/CaltechETD:etd-05192005-234308

Abstract

The Gray group has a long-standing interest in the study of methods for rapid delivery of electrons to enzyme active sites. This thesis describes picosecond to nanosecond reduction of the heme active site of the inducible nitric oxide synthase oxygenase domain (iNOSoxy) bound to Re- and Ru-diimine electron-tunneling wires. The Re wires have the form [(4,7-dimethylphenanthroline)ReI(CO)3L]+ where L is a perfluorinated biphenyl bridge connecting a rhenium-ligated imidazole or aminopropylimidazole to a distal imidazole (F8bp-im (1) and C3-F8bp-im (2)) or F (F9bp (3) and C3-F9bp (4)). All four bind tightly (micromolar to nanomolar Kd) in the active site channel of iNOSoxy. Upon excitation with 355 nm light, the bound rhenium of 1, 2, or 4 is quenched in fewer than 200 ps, possibly by electron donation from a nearby tryptophan residue. When a through-bond pathway from the rhenium to the heme iron exists, the active site Fe(III) is then reduced to Fe(II) within 300 ps, approximately ten orders of magnitude faster than the naturally occurring reduction. The Ru-diimine wire, [(4, 4’, 5, 5’-tetramethylbipyridine)2Ru(bpyF9bp)]2+ (5), also binds tightly to iNOSoxy. The binding of 5 is independent of tetrahydrobiopterin, arginine, imidazole, and 1, indicating that tmRu-F9bp resides on the surface of the enzyme. Reductive flash-quench studies have shown that the bound wire is capable of reducing the imidazole-bound active-site heme in approximately 50 ns, fully seven orders of magnitude faster than the comparable in vivo process. This work represents the first demonstration of electron-tunneling wires that specifically target and rapidly reduce an enzyme without blocking the active site channel.

Thesis Files