5‘-Deoxyadenosine Contacts the Substrate Radical Intermediate in the Active Site of Ethanolamine Ammonia-lyase:  2H and 13C Electron Nuclear Double Resonance Studies

The mechanism of propagation of the radical center between the cofactor, substrate, and product in the adenosylcobalamin- (AdoCbl) dependent reaction of ethanolamine ammonia-lyase has been probed by pulsed electron nuclear double resonance (ENDOR) spectroscopy. The radical of S-2-aminopropanol, which appears in the steady state of the reaction, was used in ENDOR experiments to determine the nuclear spin transition frequencies of 2H introduced from either deuterated substrate or deuterated coenzyme and of 13C introduced into the ribosyl moiety of AdoCbl. A 2H doublet (1.4 MHz splitting) was observed centered about the Larmor frequency of 2H. Identical ENDOR frequencies were observed for 2H irrespective of its mode of introduction into the complex. A 13C doublet ENDOR signal was observed from samples prepared with [U-13C-ribosyl]-AdoCbl. The 13C coupling tensor obtained from the ENDOR powder pattern shows that the 13C has scalar as well as dipole−dipole coupling to the unpaired electron located at C1 of S-2-aminopropanol. The dipole−dipole coupling is consistent with a distance of 3.4 ± 0.2 Å between C1 of the radical and C5‘ of the labeled cofactor component. These results establish that the C5‘ carbon of the 5‘-deoxyadenosyl radical moves ∼7 Å from its position as part of AdoCbl to a position where it is in contact with C1 of the substrate which lies ∼12 Å from the Co2+ of cob(II)alamin. These findings are also consistent with the contention that 5‘-deoxyadenosine is the sole mediator of hydrogen transfers in ethanolamine ammonia-lyase.