Spectroscopic characterization of rhino viral protease 2a: Zn is essential for the structural integrity

Recently, protease 2A of human rhinovirus 2 (HRV2 2A) was shown to require a zinc ion for the formation of an active enzyme although zinc is not involved mechanistically. The data presented clearly show that the zinc ion bound to a picornaviral‐specific motif represents an essential component of the native structure, probably representing a new Zn‐binding motif. This structure, containing mostly β ‐strand elements as shown by CD spectroscopy, changes drastically upon removal of zinc. The zinc‐depleted form does represent an intermediate with mostly unchanged secondary structure, but not a fully denatured random coil as obtained by guanidinium hydrochloride. This is indicated by the blue‐shifted fluorescence spectra and by CD. The native protein exhibited a cooperative phase transition at 53 °C. In contrast, the zinc‐depleted form did not show any transition at all, again demonstrating the stabilizing role of the zinc ion. A structural intermediate was observed during thermal and pH denaturation that may represent a molten globule, as suggested by its ANS binding.