Choline oxidase catalyzes oxidation of choline into glycine betaine through a two-step reaction
pathway employing flavin as the cofactor. On the light of kinetic
studies, it is proposed that a hydride ion is transferred from α-carbon
of choline/hydrated-betaine aldehyde to the N5 position of flavin
in the rate-determining step, which is preceded by deprotonation of
hydroxyl group of choline/hydrated-betaine aldehyde to one of the
possible basic side chains. Using the crystal structure of glycine
betaine–choline oxidase complex, we formulated two computational
systems to study the hydride-transfer mechanism including main active-site
amino acid side chains, flavin cofactor, and choline as a model system.
The first system used pure density functional theory calculations,
whereas the second approach used a hybrid ONIOM approach consisting
of density functional and molecular mechanics calculations. We were
able to formulate in silico model active sites to study the hydride-transfer
steps by utilizing noncovalent chemical interactions between choline/betaine
aldehyde and active-site amino acid chains using an atomistic approach.
We evaluated and compared the geometries and energetics of hydride-transfer
process using two different systems. We highlighted chemical interactions
and studied the effect of protonation state of an active-site histidine
base on the energetics of transfer. Furthermore, we evaluated energetics
of the second hydride-transfer process as well as hydration of betaine
aldehyde.