Ethanolamine ammonia lyase

And yet a rational explanation can be found that is also consistent with and complementary to the stereochemical and mechanistic data on other AdoCbl-dependent rearrangements already discussed in this chapter. Once again a loss of stereospecificity in the enzymic reaction points to the transient existence of a trigonal intermediate the faces of which are quasi-homotopic because rotation about the C—C bond is faster than the subsequent hydrogen-atom transfer step (Fig. 31). 

The correctness of this hypothesis could be tested since both enantiomers of 

Before one tries to construct a steric picture of the events that occur at the active site of the lyase, it is necessary to discuss the history of the re-isolated substrate, which also became labelled with tritium during the above experiments. In order to locate the tritium in these 2-aminopropanols, they were dissolved in deuterium oxide and incubated with HLADH and diaphorase [33]. Although NMR monitoring 

It is worth noting here that in contrast to the enantiomeric propanediols used in the dioldehydratase reaction, the two migrating groups here (1-H and -NH2) should have a syn-clinal arrangement if the coenzyme is to abstract and deliver the hydrogen atom on the same side of the molecule. 

---

Ethanolamine ammonia lyase, L-/ -lysine mutase, D-a-lysine mutase and ornithine mutase are representative of cobamide enzymes in which transfer of hydrogen occurs with cleavage of the C—N bond (Fig. 16). 

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... 

Spin labeled 5 -deoxyadenosylcobinamide has been used as a cofactor for ethanolamine-ammonia-lyase and the ESR spectrum followed during catalysis (123). This spin labeled coenzyme is biologically active in this enzyme. Enzyme kinetics showed this derivative to have the same Vmax as the cofactor 5 -deoxyadenosylcobinamide, but it has a higher Km value of 5.1 X 10-6 M compared to 4.6 X 10-6 for 5 -deoxyadenosylcobinamide (123). When the spin labeled coenzyme was incubated with apoenzyme to give the enzyme-coenzyme complex, the nitroxide ESR spectrum resembled that of free spin label but the lines are broadened significantly. 

Fig. 29. Decrease in intensity of nitroxide ESR signal npon addition of deuterated ethanolamine to ethanolamine ammonia lyase containing spin labeled cobinamide coenzyme. The two curves are for different concentrations of coenzyme to enzyme. The arrows indicate the point at which alcohol dehydrogenase and NADH was added to remove acetaldehyde from the enzyme. Note that full intensity is regained...

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. 

ETHANOLAMINE AMMONIA-LYASE ETHANOLAMINE KINASE CHOLINE KINASE... 

Abeles and associates showed that when dioldehydratase (Table 16-1) catalyzes the conversion of l,2-[l-3H]propanediol to propionaldehyde, tritium appears in the coenzyme as well as in the final product. When 3H-containing coenzyme is incubated with unlabeled propanediol, the product also contains 3H, which was shown by chemical degradation to be exclusively on C-5 . Synthetic 5 -deoxyadenosyl coenzyme containing 3H in the 5 position transferred 3H to product. Most important, using a mixture of propanediol and ethylene glycol, a small amount of inter-molecular transfer was demonstrated that is, 3H was transferred into acetaldehyde, the product of dehydration of ethylene glycol. Similar results were also obtained with ethanolamine ammonia-lyase 399... 

Ethanolamine, pKa value of 99 Ethanolamine ammonia-lyase 871 Ether lipids 382 Ether phosphatides 383s Ethidium 223s Ethidium bromide 221, 222 Ethylenediamine, binding constants to metal ions 311... 

Fig. 32. A rationalization of the results from the transformation of stereospecifically labelled propanol-amines on ethanolamine ammonia lyase.

Faust, L. P., Connor, J. A., Roof, D. M., Hoch, J. A., and Babior, B. M., 1990, Cloning, sequencing, and expression of the genes encoding the adenosylcobalamin-dependent ethanolamine ammonia-lyase of Salmonella typhimurium. J. Biol. Chem. 265 12462n 12466. 

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. 

Hollaway, M. R., White, H. A., Joblin, K. N., Johnson, A. W., Lappert, M. F., and Wallis, O. C., 1978, A spectrophotometric rapid kinetic study of reactions catalysed by coenzyme-B 12-dependent ethanolamine ammonia-lyase. Eur. J. Biochem. 82 1439154. 

Tan, S. L., Kopczynski, M. G., Bachovchin, W. W., Orme-Johnson, W. H., and Babior, B. M., 1986, Electron spin-echo studies of the composition of the paramagnetic intermediate formed during the deamination of propanolamine by ethanolamine ammonia-lyase, and AdoCbl-dependent enzyme, J. Biol. Chem. 261 348303485. 

Dial Dehydratases, Glycerol Dehydratase, and Ethanolamine Ammonia Lyase... 

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. 

If ES involves a radical pair, the recombination rate of ES fe) is possible to be influenced by an external magnetic field. On the other hand, ki and k should be independent of the field. Harkins and Grisssom [4] studied MFEs on the conversion of unlabeled and deuterated ethanolamine to acetaldehyde and ammonia in bacteria by ethanolamine ammonia lyase. In this reaction, AdoCbP acts as a coenzyme and a radical pair is easily generated through the enzyme-induced homolysis of the C-Co bond. The escape 5 -deoxyadenosyl radical from the pair initiates the conversion reaction. They measured MFEs on the Vmax and Vmax/Km valucs at 25°C and obtained the results as shown in Fig. 15-4. The Vmax value was independent of B up to 0.25 T. This is reasonable because kj should be independent of B. On the other hand, the Vmax/ m values of the unlabeled and deuterated systems exhibited decreases of 25 % (at 0.1 T) and 60 % (at 0.15 T), respectively. These magnetically induced deceases can be explained by the HFCM, where k2 should be increased by such low fields as 0.1-0.15 T. At higer fields, the values were found to increase from their minimum... 

Observed MFEs on the Vmax and Vmax/ m values for the conversion of unlabeled and deuterated ethanolamine to acetaldehyde and anunonia in bacteria by ethanolamine ammonia lyase at 25°C. 

---