Effects of pH on the Conformation of Phycoerythrin from Nori Porphyra sp.

Abstract

Phycoerythrin is one of the algal chromoproteins similar to phycocyanin, obtained from nori (red algae, Porphyra sp.) Pycocyanin is known to undergo a reversible self-dissociation-associa-tion reaction in the pH range from 5.4 to 6.8 in which phycoerythrin fails to do so. We investigated physicochemical properties of phycoerythrin further more in wide pH region to reveal the con-formational changes in the phycoerythrin molecule. Optical density, intrinsic viscosity [η], and sedimentation constants (s) of phycoerythrin were nearly unchanged in the pH 5-9, i.e. [η] was 2.3×10-2-3.7×10-2 (10-1 dm3/g) and s were 12-13S. At less than pH 5, the optical density at 280 nm, [η] and s were increased with decreasing pH. At more than pH 9, [η] tended to increase with increasing pH and a small component having 4.1S appeared at pH 9 and then only 3.0S component remained at pH 11. However, these small com-ponents disappeared and 12S component reappeared after dialysis against the pH 6.8 buffer solution. We conclude that phycoerythrin takes three forms depending on pH changes: a moderately expanded form beyond pH 9, an entangled or agregated form below pH 5 and a compact form be-tween pH 5 and 9. Moreover, 3S component is considered to be the product from a dissociation of the phycoerythrin hexamer having 13S.