Calcium activation mechanism of a noncanonical aromatic L-amino acid decarboxylase from psilocybin mushroom Psilocybe cubensis

PcncAAAD is a noncanonical fungal aromatic L-amino acid decarboxylase (AAAD) featuring a unique appendage C-terminal domain (CTD) and two metal-binding sites. Unlike its mammalian and plant counterparts, PcncAAAD is activated by calcium, although the exact activation mechanism remains unclear. Here, we establish an in silico RMSD-based evaluation model through molecular dynamics simulations, validated by in vitro enzyme assays, to decipher the enzyme's calcium activation mechanism. The metal-binding site at the intra-monomer interface between the N-terminal domain and the CTD (site A) is found to play a primary role in the calcium activation of PcncAAAD, whereas the secondary site within the unique CTD (site B) contributes to the calcium-mediated stabilization of enzyme structure. Binding of calcium, but not sodium, exerts a profound influence on PcncAAAD activity by stabilizing a "lid-rim" structure underlying site A, which in turn maintains the integrity of the substrate-binding environment. In silico mutations disrupting site A or the lid-rim structure show severe structural distortion of the active site, leading to reduced or even eliminated activity as demonstrated by in vitro assays. These findings deepen our understanding of metal-activatable enzymes and hold promise for the rational design of engineered enzymes for the synthesis of aromatic amino acid derivatives.