In the brain, the human
flavoprotein D-amino acid oxidase (hDAAO) is involved in the degradation of the gliotransmitter D-
serine, an important modulator of
NMDA-receptor-mediated neurotransmission; an increase in hDAAO activity (that yields a decrease in D-
serine concentration) was recently proposed to be among the molecular mechanisms leading to the onset of
schizophrenia susceptibility. This human flavoenzyme is a stable homodimer (even in the
apoprotein form) that distinguishes from known D-
amino acid oxidases because it shows the weakest interaction with the
flavin cofactor in the free form. Instead, cofactor binding is significantly tighter in the presence of an active site
ligand. In order to understand how hDAAO activity is modulated, we investigated the
FAD binding process to the
apoprotein moiety and compared the folding and stability properties of the
holoenzyme and the
apoprotein forms. The
apoprotein of hDAAO can be distinguished from the
holoenzyme form by the more "open" tertiary structure, higher
protein fluorescence, larger exposure of hydrophobic surfaces, and higher sensitivity to proteolysis. Interestingly, the
FAD binding only slightly increases the stability of hDAAO to denaturation by
urea or temperature. Taken together, these results indicate that the weak cofactor binding is not related to
protein (de)stabilization or oligomerization (as instead observed for the homologous
enzyme from yeast) but rather should represent a means of modulating the activity of hDAAO. We propose that the absence in vivo of an active site
ligand/substrate weakens the cofactor binding, yielding the inactive
apoprotein form and thus avoiding excessive D-
serine degradation.