Plant aromatic amino acid decarboxylases (AAADs) are a group of economically important enzymes categorically joined through their pyridoxal 5'-phosphate (PLP) dependence and sequence homology. Extensive evolutionary divergence of this enzyme family has resulted in a selection of enzymes with stringent aromatic amino acid substrate specificities. Variations in substrate specificities enable individual enzymes to catalyze key reactions in a diverse set of pathways impacting the synthesis of monoterpenoid indole alkaloids (including the pharmacologically active vinblastine and quinine), benzylisoquinoline alkaloids (including the pharmacologically active papaverine, codeine, morphine, and sanguinarine), and antioxidant and chemotherapeutic amides. Recent studies of plant AAAD proteins demonstrated that in addition to the typical decarboxylation enzymes, some annotated plant AAAD proteins are actually aromatic acetaldehyde synthases (AASs). These AASs catalyze a decarboxylation-oxidative deamination process of aromatic amino acids, leading to the production of aromatic acetaldehydes rather than the AAAD derived arylalkylamines. Research has implicated that plant AAS enzymes are involved in the production of volatile flower scents, floral attractants, and defensive phenolic acetaldehyde secondary metabolites. Historically, the structural elements responsible for differentiating plant AAAD substrate specificity and activity have been difficult to identify due to strong AAAD and AAS inter-enzyme homology. Through extensive bioinformatic analysis and experimental verification of plant AAADs, we have determined some structural elements unique to given types of AAADs. This document highlights structural components apparently responsible for the differentiation of activity and substrate specificity. In addition to producing primary sequence identifiers capable of AAAD activity and substrate specificity differentiation, this work has also demonstrated applications of AAAD enzyme engineering and novel activity identification. / Ph. D.