The enzyme tyrosine decarboxylase (EC 4.1.1.25) catalyzes the chemical reaction

L-tyrosine ${\displaystyle \rightleftharpoons }$ tyramine + CO₂

Hence, this enzyme has one substrate, L-tyrosine, and two products, tyramine and carbon dioxide.

This enzyme belongs to the family of lyases, specifically the carboxy-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is L-tyrosine carboxy-lyase (tyramine-forming). Other names in common use include L-tyrosine decarboxylase, L-(−)-tyrosine apodecarboxylase, and L-tyrosine carboxy-lyase. This enzyme participates in tyrosine metabolism and alkaloid biosynthesis. It employs one cofactor, pyridoxal phosphate.