Deamination of 2-aminopropanol by ethanolamine ammonia-lyase, an AdoCbl-requiring enzyme. Kinetics and isotope effects for the R and S enantiomers of the substrate

Both (R)-2-aminopropanol and (S)-2-aminopropanol are substrates for ethanolamine ammonia-lyase, reacting to yield propionaldehyde and ammonia via a common enzyme-bound intermediate. (S)-Propanolamine shows a lower Km and a higher Vmax than the R enantiomer, indicating that the former is a substantially better substrate than the latter. From the isotope effects and other considerations, it appears that the rate-limiting step for (S)-propanolamine is the same as that for ethanolamine, namely, the transfer of hydrogen from the cofactor to the product. In contrast, isotope effects seen with (R)-propanolamine indicate that there are two rate-limiting steps, both located in the segment of the reaction mechanism which is unique to the R enantiomer. One of these is the transfer of hydrogen from the substrate to the cofactor. The other is a subsequent step whose rate is similar to that of the first hydrogen transfer step but which does not involve the rupture of a carbon-hydrogen bond. It is speculated that this second rate-limiting step may have to do with the migration of the amino group.