<p>The enzyme tryptophan hydroxylase 2 (TPH2) hydroxylates L-tryptophan to L-5-hydroxytryptophan (5-HTP) in the presence of tetrahydrobiopterin (BH₄), iron ions (Fe²⁺), and molecular oxygen, the first and rate-limiting step in the synthesis of the neurotransmitter serotonin (5-HT) in the mammalian brain. We compared the effect of L-tryptophan and bi- and trivalent iron ions on the thermal stability of the mutant (mt) and wild type (wt) TPH2 molecules with those of BH₄. Mt TPH2 and wt TPH2 molecules were extracted from the midbrain of BALB/c and C57BL/6 mice, respectively. Thermal stability was examined using a thermal denaturation curve and assessed by T₅₀, the temperature at which half of the enzyme molecules are denatured. L-tryptophan at a concentration of 0.2 mM did not stabilize TPH2. BH₄ increased T₅₀ of the mt enzyme at concentration of 0.2 mM, but not 0.1 mM. It was shown for the first time that divalent and trivalent iron ions at concentrations of 0.01, 0.05, and 0.2 mM increased T₅₀ of the mt and wt enzyme significantly higher than BH₄. The results prompt using iron ions for elucidation of the molecular mechanisms of thermal stability of TPH2 in order to create pharmacological chaperones for correcting mutation-induced disturbances in TPH2 activity and the associated psychopathologies.</p>