Probing interactions from solvent-exchangeable protons and monovalent cations with the 1,2-propanediol-1-yl radical intermediate in the reaction of dioldehydrase

Monovalent Cation Activation of the Radical SAM Enzyme Pyruvate Formate-Lyase Activating Enzyme

Pyruvate formate-lyase activating enzyme (PFL-AE) is a radical S-adenosyl-l-methionine (SAM) enzyme that installs a catalytically essential glycyl radical on pyruvate formate-lyase. We show that PFL-AE binds a catalytically essential monovalent cation at its active site, yet another parallel with B₁₂ enzymes, and we characterize this cation site by a combination of structural, biochemical, and spectroscopic approaches. Refinement of the PFL-AE crystal structure reveals Na⁺ as the most likely ion present in the solved structures, and pulsed electron nuclear double resonance (ENDOR) demonstrates that the same cation site is occupied by ²³Na in the solution state of the as-isolated enzyme. A SAM carboxylate-oxygen is an M⁺ ligand, and EPR and circular dichroism spectroscopies reveal that both the site occupancy and the identity of the cation perturb the electronic properties of the SAM-chelated iron–sulfur cluster. ENDOR studies of the PFL-AE/[¹³C-methyl]-SAM complex show that the target sulfonium positioning varies with the cation, while the observation of an isotropic hyperfine coupling to the cation by ENDOR measurements establishes its intimate, SAM-mediated interaction with the cluster. This monovalent cation site controls enzyme activity: (i) PFL-AE in the absence of any simple monovalent cations has little–no activity; and (ii) among monocations, going down Group 1 of the periodic table from Li⁺ to Cs⁺, PFL-AE activity sharply maximizes at K⁺, with NH₄⁺ closely matching the efficacy of K⁺. PFL-AE is thus a type I M⁺-activated enzyme whose M⁺ controls reactivity by interactions with the cosubstrate, SAM, which is bound to the catalytic iron–sulfur cluster.

Cobalamin- and corrinoid-dependent enzymes

This chapter reviews the literature on cobalamin- and corrinoid-containing enzymes. These enzymes fall into two broad classes, those using methylcobalamin or related methylcorrinoids as prosthetic groups and catalyzing methyl transfer reactions, and those using adenosylcobalamin as the prosthetic group and catalyzing the generation of substrate radicals that in turn undergo rearrangements and/or eliminations.

Dioldehydrase: an essential role for potassium ion in the homolytic cleavage of the cobalt-carbon bond in adenosylcobalamin

The complex of dioldehydrase with adenosylcobalamin (coenzyme B12) and potassium ion reacts with molecular oxygen in the absence of a substrate to oxidize coenzyme and inactivate the complex. In this article, high performance liquid chromatography and mass spectral analysis are used to identify the nucleoside products resulting from oxygen inactivation. The product profile indicates that oxygen inactivation proceeds by direct reaction of molecular oxygen with the 5'-deoxyadenosyl radical and cob(II)alamin. Formation of 5'-peroxyadenosine as the initial nucleoside product chemically correlates this reaction with aerobic, aqueous photoinduced homolytic cleavage of adenosylcobalamin (Schwartz, P. A., and Frey, P. A., (2007) Biochemistry, in press), indicating that both reactions proceed through similar chemical intermediates. The oxygen inactivation of the enzyme-coenzyme complex shows an absolute requirement for the same monocations required in catalysis by dioldehydrase. Measurements of the dissociation constants for adenosylcobalamin from potassium-free (Kd = 16 +/- 2 microM) or potassium-bound dioldehydrase (Kd = 0.8 +/- 0.2 microM) reveal that the effect of the monocation in stimulating oxygen sensitivity cannot be explained by an effect on the binding of coenzyme to the enzyme. Cross-linking experiments suggest that the full quaternary structure is assembled in the absence of potassium ion under the experimental conditions. The results indicate that dioldehydrase likely harvests the binding energy of the activating monocation to stimulate the homolytic cleavage of the Co-C5' bond in adenosylcobalamin.