A Bacterial Tyrosine Aminomutase Proceeds through Retention or Inversion of Stereochemistry To Catalyze Its Isomerization Reaction

β-Amino acids are biologically
active compounds of interest
in medicinal chemistry. A class I lyase-like family of aminomutases
isomerizes (S)-α-arylalanines to the corresponding
β-amino acids by exchange of the NH2/H pair. This
family uses a 3,5-dihydro-5-methylidene-4H-imidazol-4-one
(MIO) group within the active site to initiate the reaction. The absolute
stereochemistry of the product is known for an MIO-dependent tyrosine
aminomutase from Chondromyces crocatus (CcTAM) that isomerizes (S)-α-tyrosine
to (R)-β-tyrosine. To evaluate the cryptic
stereochemistry of the CcTAM mechanism, (2S,3S)-[2,3-2H2]-
and (2S,3R)-[3-2H]-α-tyrosine
were stereoselectively synthesized from unlabeled (or [2H]-labeled) (4′-hydroxyphenyl)­acrylic acids by reduction with
D2 (or H2) gas and a chiral Rh-Prophos catalyst.
GC/EIMS analysis of the [2H]-β-tyrosine biosynthesized
by CcTAM revealed that the α-amino group was
transferred to Cβ of the phenylpropanoid skeleton
with retention of configuration. These labeled substrates also showed
that the pro-(3S) proton exchanges
with protons from the bulk media during its migration to Cα during catalysis. 1H- and 2H NMR analyses
of the [2H]-β-tyrosine derived from (2S)-[3,3-2H2]-α-tyrosine by CcTAM catalysis showed that the migratory proton attached to Cα of the product also with retention of configuration. CcTAM is stereoselective for (R)-β-tyrosine
(85%) yet also forms the (S)-β-tyrosine enantiomer
(15%) through inversion of configuration at both migration termini,
as described herein. The proportion of the (S)-β-isomer
made by CcTAM during steady state interestingly increased
with solvent pH, and this effect on the proposed reaction mechanism
is also discussed.