Coenzyme B-12-dependent reactions. Part IV. Observations on the purification of ethanolamine ammonia-lyase

Evidence that a B12-adenosyl transferase is encoded within the ethanolamine operon of Salmonella enterica

Adenosylcobalamin (Ado-B12) is both the cofactor and inducer of ethanolamine ammonia lyase (EA-lyase), a catabolic enzyme for ethanolamine. De novo synthesis of Ado-B12 by Salmonella enterica occurs only under anaerobic conditions. Therefore, aerobic growth on ethanolamine requires import of Ado-B12 or a precursor (CN-B12 or OH-B12) that can be adenosylated internally. Several known enzymes adenosylate corrinoids. The CobA enzyme transfers adenosine from ATP to a biosynthetic intermediate in de novo B12 synthesis and to imported CN-B12, OH-B12, or Cbi (a B12 precursor). The PduO adenosyl transferase is encoded in an operon (pdu) for cobalamin-dependent propanediol degradation and is induced by propanediol. Evidence is presented here that a third transferase (EutT) is encoded within the operon for ethanolamine utilization (eut). Surprisingly, these three transferases share no apparent sequence similarity. CobA produces sufficient Ado-B12 to initiate eut operon induction and to serve as a cofactor for EA-lyase when B12 levels are high. Once the eut operon is induced, the EutT transferase supplies more Ado-B12 during the period of high demand. Another protein encoded in the operon (EutA) protects EA-lyase from inhibition by CN-B12 but does so without adenosylation of this corrinoid.

Stereochemistry of the rearrangement of 2-aminoethanol by ethanolamine ammonia-lyase

(1R)-2-Amino[1-2H1]ethanol and (1S,2RS)-2-amino[1,2-2H2]ethanols have been synthesised by decarboxylation of (2S,3R)-[3-2H1]serine and (2S,3S)-[2,3-2H2]serine respectively. The stereochemical integrity of these labelled 2-aminoethanols has been ascertained from the 1H-NMR spectra of their N,O-dicamphanoyl derivatives. This assay has also been used to confirm that samples of (2R)- and (2S)-2-amino [2-2H1]ethanols prepared from (2R)- and (2S)-[2-2H1]glycines are stereochemically pure. Ethanolamine ammonia-lyase rearranges (1R)-2-amino[1-2H1]ethanol to acetaldehyde at approximately the same rate as it rearranges unlabelled 2-aminoethanol, whilst (1S,2RS)-2-amino[1,2-2H2]ethanol is rearranged at the same rate as the (1,1-2H2)-labelled substrate. The isotope effect is approximately kH/k2H = 8. The 2H-NMR spectra of the 3,5-dinitrobenzoates of the ethanol produced by reduction in situ of the acetaldehyde formed in the rearrangements show that the 1-2H1 label migrates in (1S,2RS)-2-amino-[1,2-2H2]ethanol and 2-amino[1,1-2H2]ethanol but not in (1R)-2-amino[1-2H1]ethanol. The above results indicate that the adenosylcobalamin-dependent ethanolamine ammonia-lyase catalyses the rearrangement of 2-aminoethanol with migration of the 1-pro-S-hydrogen atom.

The extents of formation of cobalt(II)-radical intermediates in the reactions with different substrates catalysed by the adenosylcobalamin-dependent enzyme ethanolamine ammonia-lyase

1. The reactions of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase, with the 'good' and 'relatively poor' substrates 2-aminoethanol and (S)-2-aminopropanol respectively, under conditions of saturation with substrate were investigated by rapid freezing in conjunction with electron paramagnetic resonance (e.p.r.) spectroscopy and by stopped-flow spectrophotometry. 2. In disagreement with earlier reports [Babior et al. (1972) J. Biol. Chem. 247, 4389-4392], it was found that the reaction of 2-aminoethanol gave an e.p.r. signal observed in rapid freezing experiments characteristic of a coupled Co(II)-free radical system. This signal was similar to, though not identical with, that obtained with (S)-2-aminopropanol. The steady-state level of the signal with 2-aminoethanol as substrate was 0.56 of that attained with (S)-2-aminopropanol. 3. The results of these e.p.r. experiments were shown to be consistent with stopped-flow data obtained under closely similar reaction conditions, the latter indicating a corresponding ratio of 0.64. The results also are consistent with those of a rapid wavelength scanning, stopped-flow spectrophotometric study [Hollaway et al. (1978) Eur. J. Biochem. 82, 143-154].

The number of functional active sites per molecule of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase, as determined by a kinetic method

1. A kinetic approach to the determination of the number of functional active sites per molecule of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase, is described. 2. Time courses for formation and breakdown of a cob(II)alamin intermediate during reaction of the enzyme, fully saturated with adenosylcobalamin, with L-2-aminopropanol as substrate, were followed using a stopped-flow spectrophotometer under two conditions: (a) enzyme concentration much greater than that of substrate, (b) substrate concentration much greater than that of enzyme. 3. Results were analysed in terms of a three-step mechanism involving binding of substrate (k+1 step), cob(II)alamin formation (k+2 step) and cob(II)alamin breakdown (k+3 step). the kinetic scheme was shown to be sufficient to account for the observed time courses and rate constants of 80 s-1 (k+2) and 1.5 s-1 (k+3) were determined. 4. The number of active sites per enzyme molecule (n) was calculated from the kinetic data in three ways: (a) calculation from amplitude of absorbance measurement, (b) calculation from measurements of the values of rate constants and (c) analysis by computation of the kinetic data using the computer program FACSIMILE. A value for n close to 6 was calculated by each of these methods. This value is in disagreement with the literature value of about two sites per molecule but is consistent with the I6II6 subunit structure of the enzyme. 5. Kinetic analysis of data from experiments in which the adenosylcobalamin concentration was varied while substrate and enzyme concentrations remained constant showed that all the active sites function with identical rate constants. 6. The principle and mathematical basis of the kinetic method for determining the value of n is given as an Appendix.