A major pathway of tyramine and dopamine synthesis in insects is through the decarboxylation of tyrosine and DOPA, respectively. Although tyrosine decarboxylase (TDC) has been mentioned in some reports, it has never been critically analyzed. The high sequence identity shared by tyrosine decarboxylase and DOPA decarboxylase in insects, and the similar structures of the substrates, tyrosine and DOPA, raise the possibility that both tyrosine decarboxylase and DOPA decarboxylase (DDC) have activities to tyrosine and DOPA. In this study, after tyrosine decarboxylase and DOPA decarboxylase enzymes of Anopheles gambiae were expressed, their substrate specificities and biochemical properties were critically analyzed. My results provide clear biochemical evidence establishing that the mosquito tyrosine decarboxylase functions primarily on the production of tyramine with low activity to DOPA. In contrast, mosquito DOPA decarboxylase is highly specific to DOPA with essentially no activity to tyrosine. / Master of Science in Life Sciences