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

Abstract

Abstract 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 ( 203 Tl + , 205 Tl + ) do not interact with the unpaired electron in the cis ‐ethanesemidione radical at the active site of dioldehydrase. Pulsed‐EPR experiments implicate a 14 NH group, possibly of His 143 , interacting with the radical at the active site.