Chemistry Reference
In-Depth Information
Scheme 4.3 Antibody 39-A11 catalyzes a Diels-Alder reaction be-
tween an electron-rich acyclic diene (8) and an N-aryl maleimide (9). It
was elicited with the bicyclo[2.2.2]octene hapten 7. The ethano bridge
locks the cyclohexene ring into the requisite boat conformation but has
no counterpart in the substrates or transition state.
hexachloronorbornene hapten was achieved by introducing two rare somatic muta-
tions into the germline scaffold, one at position L89 in the light chain (Ser
L89
Phe)
and another at H47 in the heavy chain (Trp
H47
Leu) [22]. These seemingly subtle sub-
stitutions, coupled with minor adjustments in CDR H3 residues, were sufficient to
convert a relatively non-specific, catalytically inactive germline antibody into a potent
catalyst.
This same family of germline antibodies has also given rise to a second Diels-Al-
derase, antibody 39-A11. In this case, the substituted bicyclo[2.2.2]octene derivative
7
served as hapten [24]. This antibody accelerates the reaction between an electron-rich
acyclic diene (
8
) and an N-aryl maleimide (
9
) to give a cyclohexene derivative (
10
)
(Scheme 4.3), but catalysis is relatively inefficient as judged by an EM of 0.35
M
. Ex-
amination of the 39-A11 structure points to an explanation for the low efficiency: the
large bicyclooctene unit used to mimic the reacting [4+2] system is relatively poorly
packed by the protein [31]. Most binding interactions in the complex are directed in-
stead to the aryl group shared by the substrate and transition state. The binding pocket
in which the cycloaddition takes place is simply too large to restrict all the degrees of
freedom available to the diene substrate, so low rates result. By improving packing in-
teractions with the cycloaddition transition state through introduction of large aromatic
groups at positions L91 and L96, it was possible to increase k
cat
by 5- to 10-fold [32].
These examples illustrate the important interplay between binding energy and cat-
alysis, underscoring the utility of complementary packing interactions at the transition
state. For 1E9, the mechanistic information contained in the transition state analog is
effectively mirrored in the induced binding pocket, allowing efficient catalysis. Even
so, 1E9 is not an evolutionary optimum with respect to activity. Despite the seemingly
