  The hydrogen transfer reaction catalysed by soybean lipoxygenase (SLO) has
been the focus of intense study following observations of a high kinetic
isotope effect (KIE). Today high KIEs are generally thought to indicate
departure from classical rate theory and are seen as a strong signature of
tunnelling of the transferring particle, hydrogen or one of its isotopes,
through the reaction energy barrier. In this paper we build a qualitative
quantum rate model with few free parameters that describes the dynamics of the
transferring particle when it is exposed to energetic potentials exerted by the
donor and the acceptor. The enzyme's impact on the dynamics is modelled by an
additional energetic term, an oscillatory contribution known as "gating". By
varying two key parameters, the gating frequency and the mean donor-acceptor
separation, the model is able to reproduce well the KIE data for SLO wild-type
and a variety of SLO mutants over the experimentally accessible temperature
range. While SLO-specific constants have been considered here, it is possible
to adapt these for other enzymes.
