Suicide Inactivation of Dioldehydrase by 2‐Chloroacetaldehyde: Formation of the ‘cis‐Ethanesemidione’ Radical,and the Role of a Monovalent Cation

Dioldehydrase is an adenosylcobalamin‐dependent enzyme that catalyzes the dehydration of (R)‐ or (S)‐propane‐1,2‐diol to propanal. The reaction proceeds by a radical mechanism initiated by the homolytic scission of the covalent Co? C(5′) bond in the coenzyme to form cob(II)alamin and the 5‐deoxyadenosyl radical as transient intermediates. Dioldehydrase is subject to ‘suicide inactivation’ by substrate/product analogs. Inactivation by 2‐chloroacetaldehyde converts the inactivator into the ‘cis‐ethanesemidione’ radical. A mechanism for this process includes reaction of chloroacetaldehyde in the reverse of the normal catalytic process to a rearranged radical that eliminates HCl. K⁺ and other monovalent cations of similar size, including Tl⁺, are required for dioldehydrase activity and for suicide inactivation by glycolaldehyde or 2‐chloroacetaldehyde. A K⁺ ion is bound to propane‐1,2‐diol in dioldehydrase. Both EPR and pulsed‐EPR experiments show that the magnetic nuclei of thallous ions (²⁰³Tl⁺, ²⁰⁵Tl⁺) do not interact with the unpaired electron in the cis‐ethanesemidione radical at the active site of dioldehydrase. Pulsed‐EPR experiments implicate a ¹⁴NH group, possibly of His¹⁴³, interacting with the radical at the active site.