Unusual dehydrations in anaerobic bacteria: considering ketyls (radical anions) as reactive intermediates in enzymatic reactions

Dehydratases have been detected in anaerobic bacteria which use 2-, 4- or 5-hydroxyacyl-CoA as substrates and are involved in the removal of hydrogen atoms from the unactivated beta- or gamma-positions. In addition there are bacterial dehydratases acting on 1,2-diols which are substrates lacking any activating group. These enzymes contain either FAD, or flavins + iron-sulfur clusters or coenzyme B12. It has been proposed that the overall dehydrations are actually reductions followed by oxidations or vice versa mediated by these prosthetic groups. Whereas the gamma-hydrogen of 5-hydroxyvaleryl-CoA is activated by a transient two-election alpha, beta-oxidation, the other substrates are proposed to require either a transient one-electron reduction or an oxidation to a ketyl (radical anion).