Divergent mechanisms of suicide inactivation for ethanolamine ammonia-lyase.
Ab initio molecular orbital calculations have been used to study the mechanism of suicide inactivation of ethanolamine ammonia-lyase induced by three different substrate analogues. Analysis of the normal catalytic mechanism with 2-aminoethanol (ethanolamine) as substrate predicts that both the hydrogen-abstraction and hydrogen-reabstraction steps involving the B(12)-cofactor are likely to be exothermic. On the other hand, the proposed inactivation mechanism for the first substrate analogue, glycolaldehyde, leads to a highly stabilized radical that results in a very endothermic (by ca. 90 kJ mol(-)(1)) hydrogen-reabstraction step, which is thought to halt the normal function of the enzyme. Curiously, the energy requirements for a catalytically imposed mechanism in the case of the second substrate analogue, 2-hydroxyethylhydrazine (HEH), parallel those for the catalytic substrate, despite the fact that HEH is found to inactivate EAL experimentally. However, further analysis reveals the presence of a lower energy pathway for HEH that leads to the formation of the highly stabilized hydrazinium radical cation. In a manner similar to when glycolaldehyde is the substrate analogue, this results in an endothermicity for the hydrogen-reabstraction step that is prohibitively large. In contrast to these related inactivation mechanisms, the third substrate analogue, 2-aminoacetaldehyde, apparently accomplishes the inactivation of EAL in an entirely different manner. A pathway for the experimentally observed formation of acetic acid and ammonium cation has been identified and appears catalytic in the sense that 5'-deoxyadenosyl radical is regenerated. However, mechanisms to account for the subsequent formation of 4',5'-anhydroadenosine and degradation of the corrinoid ring of the cofactor have not been elucidated.
Sandala
Gregory M
GM
School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia.
Smith
David M
DM
Radom
Leo
L
D000079:Acetaldehyde / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D003038:Cobamides / Q000737:chemistry / Q000378:metabolism; D004789:Enzyme Activation; D019856:Ethanolamine / Q000737:chemistry / Q000378:metabolism; D004981:Ethanolamine Ammonia-Lyase / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D006834:Hydrazines / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
D003038:Cobamides; D006834:Hydrazines; D019856:Ethanolamine; C015662:2-aminoacetaldehyde; C037834:2-hydroxyethylhydrazine; D004981:Ethanolamine Ammonia-Lyase; D000079:Acetaldehyde; D005998:Glycine; C010972:glycolaldehyde