TITLE: Molecular Biophysics of Vitamin Function: Radical Pair Separation and Reactivity in B₁₂-Conducted Enzyme Catalysis

SPEAKER: Professor Kurt Warncke,

TIME: Thursday, November 12, 1998, starting at 4:00 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

An emergent theme in enzyme catalysis is use of extremely reactive, electron-deficient radical intermediates to perform biologically-challenging reactions. In the family of vitamin B₁₂ coenzyme (adenosylcobalamin) -dependent enzymes, thermal cleavage of the cobalt-carbon bond of B₁₂ generates low spin (S=1/2) Co(II) and the 5'-deoxyadenosyl organic radical. The deoxyadenosyl radical abstracts a hydrogen atom from a bound substrate molecule to initiate rearrangement to product. Our aims are to elucidate the fundamental molecular principles of efficient Co(II)-radical pair separation and stabilization, and to determine the mechanism of the ensuing rearrangment reaction, with an emphasis on how the protein guides these events. Static and time-resolved techniques of pulsed-electron magnetic resonance spectroscopy are used to reveal the electronic and nuclear structure of Co(II)-substrate biradical intermediate states in the enzyme, ethanolamine deaminase, from Salmonella typhimurium. Quantum-chemical computations are used to address elusive states that have not, as yet, been detected by experiment. Results showing the ligand state of Co(II), delocalization of unpaired spin density in the deoxyadenosyl radical, and rearrangment state and orientation of substrate radicals in ethanolamine deaminase provide insight into the mechanics of B₁₂-conducted catalysis.