Page 50 - Visions of the Future Chemistry and Life Science

Enzymology takes a quantum leap forward 39

potential energy barrier model for hydrogen tunnelling, reaction rates
are strongly dependent on temperature (apparent activation energy
1
45kJmol ) and, importantly, this activation energy was found to be
independent of isotope. These observations indicate that thermal distor-
tion of the protein scaffold – but not vibrational excitation of the substrate
– are required to drive hydrogen transfer. Thus, a fluctuating energy surface
is a feature of the tunnelling process. The vibrationally enhanced ground
state tunnelling theory equivalent of regime IV of the static barrier plot
(Figure 2.7) recognises that thermal motions of the protein molecule are
required to distort the protein scaffold into conformations compatible with
hydrogen tunnelling. Regime IV of the vibrationally enhanced ground state
tunnelling theory plot therefore has a nonzero value for the slope, the value
of which is the energy required to distort the protein into the geometry
compatible with hydrogen tunnelling. With methylamine dehydrogenase,
it has thus been possible to quantify the energy term associated with struc-
tural distortion of the protein during an enzyme catalysed reaction.
The temperature dependence in regime IV – i.e. ground state tunnel-
ling – for vibrationally enhanced ground state tunnelling theory contrasts
markedly with that for the static barrier model. Although there is a size-
able energy term in this regime for the vibrationally enhanced ground state
1
tunnelling theory model (apparent activation energy 45kJmol ), the
apparent linearity seen in the accessible temperature range for methyl-
amine dehydrogenase probably does not extend to lower temperatures. At
low temperatures, nuclear vibrations will be frozen, thus preventing dis-
tortion of the nuclear scaffold into geometries compatible with hydrogen
tunnelling. Thus, over a large temperature range, complex temperature
dependencies of the reaction rate are predicted.
Ground state tunnelling driven by protein dynamics (vibrationally
enhanced ground state tunnelling theory) is the only theoretical treatment
consistent with our work on methylamine dehydrogenase. As indicated
above, a prediction of vibrationally enhanced ground state tunnelling
theory is that ground state tunnelling may occur even when the kinetic
isotope effect
7 – a regime interpreted previously as indicating classical
behaviour. The kinetic isotope effect with methylamine dehydrogenase is
large ( 18), and thus the presence of tunnelling is predicted by current
dogma. In the case of sarcosine oxidase, our studies on hydrogen tunnelling
have shown that the kinetic isotope effect approaches the classical limit.
Furthermore, our recent analysis of hydrogen tunnelling in trimethylamine