Chemistry Reference
In-Depth Information
5
Protein-based Artificial Enzymes
Ben Duckworth and Mark D. Distefano
5.1
Introduction
The design of catalysts that rival the specificity and speed of natural enzymes is a chal-
lenging objective. Various approaches have been employed to create such molecules.
Polypeptides are a logical choice of possible framework for catalyst design because they
provide a simple means for generating complex structures. The catalyst structure can
be changed by altering the corresponding gene and expressing the desired protein in
bacteria. However, chemical methods can also be incorporated into such an approach
to greatly expand the range of possible reactions that can be studied. In such a che-
mogenetic strategy, a small molecule-based catalyst is coupled to a protein scaffold
devoid of catalytic activity. The small molecule provides intrinsic reactivity while
the protein component controls specificity; the protein can also be used to tune or
augment the reactivity of the small molecule catalyst. Such an approach provides en-
ormous flexibility in the design of new catalytic materials.
Early efforts in this field focused on using chemical modification to alter the type of
reaction catalyzed by existing enzymes or to change their substrate specificity. Thio-
subtilisin [1, 2], selenosubtilisin [3] and flavopapain [4] are all examples of proteins that
manifest altered reactivity resulting from chemical modification. Other work with sta-
phylococcal nuclease [5] and subtilisin [6, 7] employed chemical modification to mod-
ulate substrate selectivity. While that pioneering work served as the foundation for
protein-based catalyst design, these efforts were, notably, based on protein frameworks
that possessed native catalytic activity. Such proteins already contained highly evolved
substrate binding sites and/or catalytic machinery. The present chapter focuses on the
design of protein-based catalysts starting with polypeptides that have no intrinsic cat-
alytic activity. While this is an enormous challenge when compared with approaches
starting with existing enzymes, it opens up a plethora of exciting possibilities; if it
could be accomplished, the ability to create catalysts with enzyme-like properties
would have dramatic effects that would extend well beyond the field of chemistry. Sev-
eral recent articles provide a more comprehensive review of the field of protein design,
including work with existing enzymes [8-19].
