Ethanolamine ammonia lyase mechanism

Ethanolamine ammonia lyase has a molecular weight of 520,000 and consists of 8 or 10 subunits. Two 5 -deoxyadenosylcobalamin molecular bind per enzyme molecule, and recent kinetic studies by Babior show that these two molecules carry out catalysis independently. Evidence is available that this enzyme functions by a radical mechanism since both spin labeling and Co(II) esr experiments indicate that Co(II) is an intermediate during H-transfer. Also, 5 -deoxyadenosine has been detected as a product of oxygenation of the enzyme-substrate complex (99—101). 

The most extensive and informative enzyme work with B12 spin labels has been carried out on the enzyme ethanolamine ammonia-lyase 123). This work has employed six-coordinate 4-hydroxy-2,2,6,6-tetra-methylpiperidine-N-oxyl-5 -deoxyadenosylcobinamide. Ethanolamine ammonia lyase uses 5 -deoxyadenosylcobalamin as cofactor. Spin labeled 5 -deoxyadenosylcobinamide was used in order to determine the nature of the first step in the mechanism of action of ethanolamine ammonia lyase by determining the manner in which the Co—C bond is broken. Spin labeled 5 -deoxyadenosylcobinamide was synthesized by taking reduced diaquocobinamide and reacting it with an excess of 5 -tosyl-adenosine to give 5 -deoxyadenosylcobinamide. This was stirred for three days with a 20 fold excess of 4-hydroxy-2,2,6,6-tetramethylpiperidine... 

The application of magnetic resonance techniques to biological systems is a relatively new approach for the study of macromolecules. In this review we have presented the different approaches which have been made to study Bi2-enzymes. Clearly some progress has been made particularly from the application of ESR to a study of the enzymes ethanolamine ammonia-lyase and ribonucleotide reductase. Although 13C NMR is well in its developmental stages it is obvious that this technique will prove to be very useful for the examination of coenzyme-enzyme interactions. Studies of how corrinoids bind in enzymes and how sulfhydryl containing proteins are involved in enzyme catalysis comprise two major problems which must be overcome before realistic mechanisms can be presented for this group of enzymes. 

Harkins, T. T., and Grissom, C. B., 1994, Magnetic field effects on B]2 ethanolamine ammonia lyase evidence for a radical mechanism. Science 263 9589960. 

Ethanolamine ammonia lyase. EAL converts ethanol-amine to acetaldehyde, with loss of ammonia. EAL depends upon adenosylcobamides, such as coenzyme B12 (3), but a range of other adenosylcobamides are also accepted as cofactors, while cobalamins with /3-ligands other than the 5 -deoxy-5 -adenosyl group (of AdoCbl) are inhibitors. Active EAL is multimeric and has an apparent molecular mass of about 560 600kDa. Similar to the mechanism of DD, a radical mechanism is proposed for the isomerization of the vicinal amino-alcohol substrates (ethanolamine, (/f)- and (5)-aminopropanol) by EAL, starting with the abstraction of an H atom from the C-1 position of the substrates. 

Weisbiat, D. a., Babior, B. M. (1971) The mechanism of action of ethanolamine ammonia-lyase, a Bi2-dependent enzyme. Vlll. Further studies with compounds labeled with isotopes of hydrogen Identification and some properties of the rate-limiting step, J. Biol. Chem. 246, 6064-6061. 

Wetmore, S. D., Smith, D. M., Bennett, J. T., Radom, L. (2002) Understanding the mechanism of action of Bi2-dependent ethanolamine ammonia-lyase Synergistic interactions at play, /. Am. Chem. Soc. 124, 14054-14065. 

The small value of k 10" s" for the coenzyme is noteworthy in comparison to values of 2xl0 s" estimated for the rate-determining step in the diol dehydrase and ethanolamine ammonia lyase systems. This shows one of the problems in using the radical mechanism. This difficulty usually is explained by assuming that the enzyme somehow distorts the coenzyme so that the homolysis is about 15 kcal mol" more favorable in the holoenzyme. This distortion may destabilize the reactant and/or stabilize the transition state in order to hasten homolysis in the holoenzyme. For some time, it was thought that the axial base might promote these distortions, but Hay and Finke found that... 

Ethanolamine ammonia lyase, a coenzyme Bi2-requiring enzyme, catalyzes the following reaction. Propose a mechanism for this reaction. 

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