Background: Old yellow enzyme (OYE) was the first flavoenzyme purified, but its function is still unknown. Nevertheless, the NADPH oxidase activity, the flavin mononucleotide environment and the ligand-binding properties of OYE have been extensively studied by biochemical and spectroscopic approaches. Full interpretation of these data requires structural information.
Results: The crystal structures of oxidized and reduced OYE at 2 A resolution reveal an alpha/beta-barrel topology clearly related to trimethylamine dehydrogenase. Complexes of OYE with p-hydroxybenzaldehyde, beta-estradiol, and an NADPH analog show all three binding at a common site, stacked on the flavin. The putative NADPH binding mode is novel as it involves primary recognition of the nicotinamide mononucleotide portion.
Conclusions: This work shows that the striking spectral changes seen upon phenol binding are due to close physical association of the flavin and phenolate. It also identifies the structural class of OYE and suggests that if NADPH is its true substrate, then OYE has adopted NADPH dependence during evolution.