Catalyzes the decarboxylation of tryptophan to tryptamine. Tryptamine is a neurotransmitter that induces the release of serotonin, which is suggested to modulate gastrointestinal motility. Therefore, the tryptophan decarboxylase from the gut bacteria Ruminococcus gnavus (strain ATCC 29149 / VPI C7-9) may influence host brain and behavior. Has weak activity with tyrosine and phenylalanine. H(+) + L-tryptophan = CO2 + tryptamine Inhibited by (S)-alpha-fluoromethyltryptophan. kcat is 4400 min(-1) with tryptophan as substrate. kcat is 230 min(-1) with phenylalanine as substrate. Homodimer. In vitro, tryptamine is excreted into the extracellular fluid, suggesting that the bacterium has the potential to excrete tryptamine in the host gut lumen. Belongs to the group II decarboxylase family.