Page 48 - Visions of the Future Chemistry and Life Science

Enzymology takes a quantum leap forward 37

observed rate the kinetic isotope effect is viscosity-dependent – as viscos-
ity increases the nuclear reorganisation step becomes rate limiting, and
thus the kinetic isotope effect tends to unity. In experimental studies,
measurements of (i) increased viscosity or (ii) decreased temperature
effects on the kinetic isotope effect may be used to discriminate between
these possible regimes, since both would be expected to selectively perturb
geometrical distortion of the protein.
The vibrationally enhanced ground state tunnelling theory assumes
that hydrogen transfer occurs entirely by quantum mechanical tunnelling.
The model is therefore appropriate for those enzymes catalysing ground
state tunnelling (see below). The model is likely to be incomplete for those
enzymes where tunnelling occurs just below the saddlepoint of the energy
surface (i.e. the reactant passes up the energy barrier before tunnelling) – in
these situations hydrogen transfer is likely to occur by a combination of
classical and quantum mechanical behaviour. In the case where hydrogen
transfer is by a combination of classical and quantum mechanical effects,
the activation energy will reflect partitioning of energy into a wide range
of modes within the protein, e.g. changes in protein geometry, bond angles
of reacting substrate etc., as well as thermal excitation of the reactive C–H
bond. However, experimental verification of the vibrationally enhanced
ground state tunnelling theory would demonstrate the importance of
protein dynamics in enzymatic hydrogen tunnelling. By analogy, therefore,
protein dynamics would also be expected to play a major role in those
enzymes where hydrogen tunnelling is not from the ground state, but from
an excited state of the substrate molecule. Experimental verification of a
role for protein dynamics is thus a key milestone in developing theories for
enzymatic hydrogen tunnelling – this verification is described below.

2.8 Experimental demonstration of vibration-driven tunnelling

Kinetic data for bovine serum amine oxidase were originally analysed in
terms of the tunnelling correction derivatives of transition state theory,
but the data are also consistent with – although not verification of – the
vibrationally enhanced ground state tunnelling theory. Alternatively, the
bovine serum amine oxidase data can also be interpreted in terms of a
hydrogen tunnelling reaction driven by substrate oscillations. Thus, ambi-
guity remains concerning the correct theoretical treatment of the bovine
serum amine oxidase kinetic data. This ambiguity arises because the