Influence of human tryptophan hydroxylase 2 N- and C-terminus on enzymatic activity and oligomerization

Tryptophan hydroxylase (TPH) catalyses the first and rate limiting step in the biosynthesis of the neurotransmitter serotonin. There are two TPH isoenzymes in humans, encoded by two different genes: TPH1 and the recently described TPH2. We have expressed both human enzymes and various deletion mutants of TPH2 (Delta N44, Delta C17, Delta C19, Delta C51) in COS7 cells. TPH1 and 2 displayed different kinetic properties with a lower K-m value of TPH1. Removal of 44 amino acids from the N-terminus of TPH2 resulted in a 3-4-fold increased V-max, which indicates a strong inhibitory function of this part on the enzymes activity. TPH1 and 2 were able to form homooligomers and also heterooligomers with each other. The different deletion mutants (Delta C17, Delta C19 and Delta C51), which lack the putative C-terminal leucine zipper tetramerization domain, existed as monomeric enzymes. While short deletions (Delta C17 and Delta C19) hardly changed V-max values, the Delta C51 mutant lost 99% of TPH activity. These data identify a region between the C-terminal oligomerization domain and the catalytic domain, which is indispensable for TPH2 activity.