P. Diziol, H. Haas, J. Rétey
Mar 3, 2005
Citations
0
Influential Citations
21
Citations
Journal
European journal of biochemistry
Abstract
1 The adenosylcobalamin-dependent ethanolamine ammonia-lyase from Clostridium sp. deaminates (2S)-2-aminopropanol to propionaldehyde about three times faster than it does (2R)-2-aminopropanol. 2 (2S)-2-Amino(1-2H2)propanol and (1S, 2S)-2-amino(1-2H1)propanol are deaminated by the enzyme with a large kinetic deuterium isotope effect (kH/k2H∼ 6), whereas (1R, 2S)-2-amino(1-2H1)-propanol behaves like the unlabelled substrate. 3 (2R)-2-Amino(1-2H2)propanol reacts about 1.7 times more slowly than unlabelled (2R)-2-aminopropanol or (1R, 2R)-2-amino(1-2H1)propanol. 4 From the above results it follows that the enantiomeric 2-amino(1-2H2)propanols react at the same rate, irrespective of their absolute configuration and the lyase reaction involves the 1-Hsi atom in both enantiomers. 5 (2S)-2-Amino[U-14C, 2-3H, 3-3H]propanol was prepared by a combination of enzymic and chemical reactions and the distribution of tritium between C-2 and C-3 was determined by its degradation to acetic acid. This doubly labelled substrate was converted on ethanolamine ammonia-lyase into (2R)-[U-14C, 2-3H, 3-3H]propionaldehyde. 6 On the other hand, the (2S) configuration was proved for all three samples of [2-3H]propionaldehyde recovered from incubations of [5′-3H]adenosylcobalamin with ethanolamine ammonia-lyase irrespective of whether the (2S) or (2R) enantiomer of 2-aminopropanol or propionaldehyde itself (in the presence of ammonium ions) were used as substrates. 7 Incubations with enzyme plus [5′-3H]adenosylcobalamin also yielded tritiated 2-amino-propanols. These were in turn incubated with horse liver alcohol dehydrogenase and diaphorase in deuterium oxide, which lead to exchange of the 1-HRe atom with deuterium. Retention of the tritium in the samples provided evidence for the (1S) configuration. By co-crystallization as the 4-methyl-oxazolidone derivative the (2S) configuration of the tritiated 2-aminopropanol samples was also proved. 8 These results show that ethanolamine ammonia-lyase deaminates (2S)-2-aminopropanol with retention, whereas (2R)-2-aminopropanol reacts with inversion of configuration. Tritium from [5′-3H]adenosylcobalamin is transferred in all cases to the 2-pro-S position of propionaldehyde and to the 1-pro-S position of 2-aminopropanol. In the case of (2R)-2-aminopropanol, tritium transfer is accompanied by inversion at the asymmetric centre. All these results, together with the previously reported racemization during the conversion of 2-aminoethanol, can be explained by a simple mechanistic scheme.