ABSTRACT
The limited set of structural and chemical functionalities represented in the side chains of the canonical amino acids necessitate post-translational processing of expressed protein products. Such processing combinatorially expands both the diversity and functional attributes of proteins. This suggests that while the extant set of twenty encoded amino acids can support the rudimentary functions of living systems, optimized function requires a larger set. Site-directed mutagenesis has proven to be an invaluable tool in tailoring structural features of enzymes (Zoller and Smith 1987) and has allowed detailed scrutiny of function and creation of enzymatic activities not found in nature. Nevertheless, the limited set of functional groups available constrains the range of possible applications. One could envision entirely new functions for proteins, or even the evolution of entire organisms endowed with extrabiological properties, if the set of functional groups present on the side chains can be expanded.
