14463-33-3

EutT from Salmonella enterica is a member of a class of enzymes termed ATP:Co(I)rrinoid adenosyltransferases (ACATs), implicated in the biosynthesis of adenosylcobalamin (AdoCbl). In the presence of cosubstrate ATP, ACATs raise the Co(II)/Co(I) reduction potential of their cob(II)alamin [Co(II)Cbl] substrate by >250 mV via the formation of a unique four-coordinate (4c) Co(II)Cbl species, thereby facilitating the formation of a "supernucleophilic" cob(I)alamin intermediate required for the formation of the AdoCbl product. Previous kinetic studies of EutT revealed the importance of a HX11CCX2C(83) motif for catalytic activity and have led to the proposal that residues in this motif serve as the binding site for a divalent transition metal cofactor [e.g., Fe(II) or Zn(II)]. This motif is absent in other ACAT families, suggesting that EutT employs a distinct mechanism for AdoCbl formation. To assess how metal ion binding to the HX11CCX2C(83) motif affects the relative yield of 4c Co(II)Cbl generated in the EutT active site, we have characterized several enzyme variants by using electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies. Our results indicate that Fe(II) or Zn(II) binding to the HX11CCX2C(83) motif of EutT is required for promoting the formation of 4c Co(II)Cbl. Intriguingly, our spectroscopic data also reveal the presence of an equilibrium between five-coordinate "base-on" and "base-off" Co(II)Cbl species bound to the EutT active site at low ATP concentrations, which shifts in favor of "base-off" Co(II)Cbl in the presence of excess ATP, suggesting that the base-off species serves as a precursor to 4c Co(II)Cbl.

Ultrafast transient absorption spectroscopy was used to investigate the photochemistry of adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), and n-propylcobalamin (PrCbl) at pH 2 where the axial nitrogenous ligand is replaced by a water molecule. The evolution of the difference spectrum reveals the internal conversion process and spectral characteristics of the S1 excited state. The photolysis yield in the base-off cobalamins is controlled by competition between internal conversion and bond homolysis. This is in direct contrast to the process in most base-on alkylcobalamins where primary photolysis occurs with near unit quantum yield and the photolysis yield is controlled by competition between diffusive separation of the radical pair and geminate recombination. The absence of the axial nitrogenous ligand in the base-off cobalamins modifies the electronic structure and opens a channel for fast nonradiative decay. This channel competes effectively with the channel for bond dissociation, dropping the quantum yield for primary radical pair formation from unity in base-on PrCbl and AdoCbl to 0.2 ± 0.1 and 0.12 ± 0.06 in base-off PrCbl and AdoCbl, respectively. The photolysis of base-off MeCbl is similar to that of base-off AdoCbl and PrCbl with competition between rapid nonradiative decay leading to ground state recovery and formation of a radical pair following bond homolysis.

Vitamin B12r (cob(II)alamin), the Co(II) derivative of vitamin B12, reduces ClO3- , BrO3-, IO3-, and ClO2- to the corresponding halide ions in acid solution and o-iodosobenzoic acid to the o-iodo acid. Specific rates for these reductions have been measured and their acid dependencies examined. In each case, the overall rate is determined by the first step in the reaction sequence, the initial 1e reduction of the oxyhalogen. Each of the B12r-halate reactions is first order in [H+]; the B12r-ClO2- reaction exhibits both an [acid]-independent and first-order-[H+] term, whereas reduction of iodosobenzoic acid features an inverse-[H+] term. The B12r-IO3- reaction, but none of the others, is strongly autocatalytic, probably reflecting the reaction of the product I- with IO3- to form the very reactive oxidant, I2. Vitamin B12s, the Co(I) derivative of B12, reduces ClO3- about 104 times as rapidly as B12r, a rate ratio too small to be compatible with the 0.78-V difference in the formal potentials of the two reductants on the basis of the Marcus model for outer-sphere electron-transfer processes. It is proposed that the B12r-ClO3- reaction (and, by implication, the reactions with BrO3-, IO3-, and ClO2-) proceeds mainly by an inner-sphere route that utilizes an oxygen bridge between Co(II) and halogen. The path for the B12s-ClO3- reaction is uncertain, nor can we say whether this reduction involves transfer of a single electron or whether it is initiated by a 2e transaction, forming of a Co(III) intermediate that undergoes comproportionation with B12s to form 2 units of Co(II).

Vitamin B12s (cob(I)alamin), the Co(I) derivative of vitamin B12, reduces nitrate rapidly and cleanly to NH4+ at pH 1.5-2.5. The overall rate for this net transfer of eight electrons is determined by the initial step. The observed rate law, rate = k[Co1][NO3-][H+] (k = 2.1 × 104 M-2 s-1; 25°C; μ = 0.11), is consistent with initiation either by reaction of the nonprotonated form of B12s with molecular HNO3 or by hydride transfer to NO3- from the protonated form of B12s. Observed variation of the Co(II) (B12r) spectrum with acidity leads to a calculated pKA of 1.96 for the latter system.