Cobamides and ribonucleotide reduction. X. Electron paramagnetic resonance studies on cobalamin-dependent ribonucleotide reduction

Abstract

When ribonucleotide reductase from Lactobacillus leichmannii is incubated with its coenzyme (5 '-deoxyadenosylcobalamin), a thiol such as glutathione and certain nucleoside triphosphates, formation of paramagnetic products can be Dr. Lester J. Reed. The selenolipoate was reduced in the same manner as lipoate (Gunsalus and Razzell, 1957). The arabino analog of ATP (AraATP) and 2'-0-MeATP were synthesized 4696 BIOCHEMISTRY, VOL.

Reduction of Ribonucleotides

“…Even though this species only accumulates slowly during the reaction, its function as an intermediate appears possible (89). Similar doublets have been observed in other adenosylcobalamin-dependent reactions, and a seemingly satisfactory explanation has been given for these spectra: they arise from the interaction between an organic radical, probably on the substrate, and a low spin [Co(II)] complex (B12r) (90,91).…”

Radical catalysis of B 12 enzymes: structure, mechanism, inactivation, and reactivation of diol and glycerol dehydratases

“…It is true that most enzymatic reactions are catalyzed by ionic mechanisms, but it should be noted that radical mechanisms can also be possible for certain enzymes, because X-ray structures of enzymes have revealed that the active sites are rather hydrophobic. Since the detection of radical intermediates by EPR in the reactions catalyzed by AdoCbl-dependent enzymes [45,46,51,52] as well as non-B 12 ribonucleotide reductase [53] in 1972-1973, many enzymes have been established to catalyze by a radical mechanism and have been attracting general interest in this rapidly expanding field [54][55][56][57][58][59][60][61][62][63][64][65]. These enzymes utilize the high reactivity of free radicals to catalyze the reactions.…”

A physical explanation of the EPR spectrum observed during catalysis by enzymes utilizing coenzyme B

“…However, complete deuteration of a substrate of ribonucleotide reductase (guanosine triphosphate) yielded no evidence of spin density on the C-2 proton of this substance. Further, neither the use of deoxy [5', 5'-2H2 ] adenosyl cobalamin in place of the normally protiated coenzyme nor reduced seleno lipoate in place of reduced lipoate as the electron donor failed to induce any modification of the observed EPR spectrum [2]. Therefore, in two cases it has been demonstrated that electron density resides on the substrate and in the ribonucleotide reductase system 13C substitution in the ribose ring, particularly at the 2 and 3 positions, may yet yield evidence of spin density on substrate.…”

“…'/' and tan 20 {g2 --gl)flH (2) then the wavefunctions and energy levels for the coupled electron system are given as in Table II. Because the spin system interacts with an oscillating microwave field along the x-axis, the interaction inducing transitions between states is given by ----gl {JS1 x Hx --g2fJSu~Hx = ---fl(gl Sz x + g2S2~ )H~ and the major transitions induced are given b:~ AS~ = + 1 with relative intensities given by the square of the matrix elements as shown in Table III.…”