Structural change of ovalbumin-related protein X by alkali treatment

Chicken egg white protein ovalbumin (OVA) undergoes a conversion to a more thermostable form by alkali treatment, which is assumed to be involved in the physiological functions of OVA. Ovalbumin-related protein X (OVAX), a chicken egg white protein, has 77% sequence similarity to OVA and binds to heparin. In this study, structure characteristics and heparin-binding affinity of alkali-treated OVAX were investigated. Cation-exchange chromatography using SP Sepharose resin showed that alkali treatment (pH 10, 55°C) of OVAX induces the occurrence of a distinct OVAX form with a less positive-charge (acidic OVAX). Circular dichroism and tryptophan-fluorescence analyses showed that the newly-formed acidic OVAX form has an 8% lower α-helical content than its native counterpart, while there is no significant difference in steric environments around tryptophan residues between the 2 forms. The OVAX structure built by homology-modeling showed that OVAX possess a basic cluster domain with α-helix equivalent to 7% of total secondary structures, which does not contain any tryptophan residues. These results suggest that, during alkali treatment, OVAX undergoes mainly a conformational change of the α-helical basic cluster domain and thereby forms acidic OVAX. Acidic OVAX induced by alkali treatment exhibited weaker interactions with Heparin Sepharose resin than native OVAX did. Our results suggest that OVAX basic cluster domain is likely a specific binding site of heparin. Consequently, it is suggested that alkali treatment causes the collapse of the OVAX heparin binding site, which might participate in regulating the functions of heparin.