5 '-deoxyadenosine contacts the substrate radical intermediate in the active site of ethanolamine ammonia-lyase: H-2 and C-13 electron nuclear doubleresonance studies

The mechanism of propagation of the radical center between the cofactor, su bstrate, and product in the adenosylcobalamin- (AdoCbl) dependent reaction of ethanolamine ammonia-lyase has been probed by pulsed electron nuclear do uble resonance (ENDOR) spectroscopy. The radical of S-2-aminopropanol, whic h appears in the steady state of the reaction, was used in ENDOR experiment s to determine the nuclear spin transition frequencies of H-2 introduced fr om either deuterated substrate or deuterated coenzyme and of C-13 introduce d into the ribosyl moiety of AdoCbl. A H-2 doublet (1.4 MHz splitting) was observed centered about the Larmor frequency of H-2. Identical ENDOR freque ncies were observed for H-2 irrespective of its mode of introduction into t he complex, A C-13 doublet ENDOR signal was observed from samples prepared with [U- C-13-ribosyl]-AdoCbl. The C-13 coupling tensor obtained from the E NDOR powder pattern shows that the C-13 has scalar as well as dipole-dipole coupling to the unpaired electron located at Cl of S-2-aminopropanol. The dipole-dipole coupling is consistent with a distance of 3.4 +/- 0.2 Angstro m between Cl of the radical and C5 ' of the labeled cofactor component. The se results establish that the C5 ' carbon of the 5 ' -deoxyadenosyl radical moves similar to7 Angstrom from its position as part of AdoCbl to a positi on where it is in contact with Cl of the substrate which lies similar to 12 Angstrom from the Co2+ of cob(II)alamin. These findings are also consisten t with the contention that 5 ' -deoxyadenosine is the sole mediator of hydr ogen transfers in ethanolamine ammonia-lyase.