Computational Studies Devoted to the Catalytic Mechanism of Threonine Aldolase, a Critical Enzyme in the Pharmaceutical Industry to Synthesize β‑Hydroxy-α-amino Acids

The
catalytic mechanism of threonine aldolase (TA) was herein studied
in atomic detail employing the computational ONIOM hybrid QM/MM methodology.
TA is a PLP-dependent enzyme that catalyzes the retro-aldol cleavage
of threonine into glycine and acetaldehyde, as well as the reverse
reaction. This enzyme is currently seen as the optimal approach for
the regioselective synthesis of β-hydroxy-α-amino acids
(HAAs), which are very difficult to obtain by standard methods. The
results obtained herein show that the catalytic mechanism of TA occurs
in three steps: (i) deprotonation of the hydroxyl group of EA1, (ii)
covalent bond cleavage, and (iii) hydrolysis. According to the Gibbs
free energy profile, the rate-limiting step of the catalytic process
is the covalent bond cleavage, which results in the formation of acetaldehyde.
The calculated energy barrier for this step is 16.7 kcal mol–1, which agrees very well with the kinetic data available in the literature
(17.4 kcal mol–1). All these results can now be
used for the optimization of the synthesis of HAAs that serve as building
blocks of several commercial drugs, such as antibiotics, immunosuppressants,
and the anti-Parkinson’s disease drug l-threo-3,4-dihydroxyphenylserine.