Structural characterization of human tryptophanhydroxylase 2 reveals L-Phe as the superior regulatorydomain ligand relevant for serotonin biosynthesis

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in the biosynthesis of serotonin in the brain. Consequently, regulation of TPH2 is relevant for serotonin related diseases, yet, the regulatory mechanism of TPH2 is poorly understood and structural as well as dynamical insights are missing. Here, we use NMR spectroscopy to determine the structure of a 47 N-terminally truncated variant of the regulatory domain (RD) dimer of human TPH2 in complex with L-Phe, and show that L-Phe is the superior RD ligand compared to the natural substrate, L-Trp. Using cryo-EM we obtain a low-resolution structure of a similarly truncated variant of the complete tetrameric enzyme with dimerized RDs. The cryo-EM 2D class averages additionally indicate that the RDs are dynamic in the tetramer and likely exist in a monomer-dimer equilibrium. Our results provide structural information on the RD both as an isolated domain and in the TPH2 tetramer, which will facilitate future elucidation of TPH2s regulatory mechanism affecting serotonin regulation.