Unfolding of CBP21, a lytic polysaccharide monooxygenase, without dissociation of its copper ion cofactor

Chitin‐binding protein 21 (CBP21) from Serratia marcescens is a lytic polysaccharide monooxygenase that contains a copper ion as a cofactor. We aimed to elucidate the unfolding mechanism of CBP21 and the effects of Cu 2+ on its structural stability at pH 5.0. Thermal unfolding of both apo‐ and holoCBP21 was reversible. ApoCBP21 unfolded in a simple two‐state transition manner. The peak temperature of the DSC curve, t p , for holoCBP21 (74.4°C) was about nine degrees higher than that for apoCBP21 (65.6°C). The value of t p in the presence of excess Cu 2+ was around 75°C, indicating that Cu 2+ does not dissociate from the protein molecule during unfolding. The unfolding mechanism of holoCBP21 was considered to be as follows: N∙Cu 2+ ⇌ U∙Cu 2+ , where N and U represent the native and unfolded states, respectively. Urea‐induced equilibrium unfolding analysis showed that holoCBP21 was stabilized by 35 kJ mol −1 in terms of the Gibbs energy change for unfolding (pH 5.0, 25°C), compared with apoCBP21. The increased stability of holoCBP21 was considered to result from the structural stabilization of the protein‐Cu 2+ complex itself.