Probing the structural stability of R-phycocyanin under pressure

TL;DR

This study investigates the structural stability of R-phycocyanin under high pressure, revealing its multiphase unfolding process and demonstrating pressure's potential to preserve its colour and bioactivity better than heat treatment.

Contribution

It provides new insights into the high-pressure-induced unfolding mechanism of R-phycocyanin using combined spectroscopic and scattering techniques.

Findings

R-PC undergoes conformational changes, oligomer dissociation, and monomer unfolding under pressure.

Partial refolding occurs when pressure is released, retaining 50% of original colour.

High pressure preserves R-PC's colour and bioactivity better than heat treatment.

Abstract

The red macroalgae Porphyra, commonly known as Nori, is widely used as food around the world due to its high nutrient content, including the significant abundance of coloured phycobiliproteins (PBPs). Among these, R-phycocyanin (R-PC) stands out for its vibrant purple colour and numerous bioactive properties, making it a valuable protein for the food industry. However, R-PC's limited thermal stability necessitates alternative processing methods to preserve its colour and bioactive properties. Our study aimed to investigate the in-situ stability of oligomeric R-PC under high pressure (HP) conditions (up to 4,000 bar) using a combination of absorption, fluorescence, and small-angle X-ray scattering (SAXS) techniques. The unfolding of R-PC is a multiphase process. Initially, low pressure induces conformational changes in the R-PC oligomeric form (trimers). As pressure increases above 1,600 bar, these trimers dissociate into monomers, and at pressures above 3,000 bar, the subunits begin to unfold. When returned to atmospheric pressure, R-PC partially refolds, retaining 50% of its original colour absorbance. In contrast, heat treatment causes irreversible and detrimental effects on R-PC colour, highlighting the advantages of HP treatment in preserving both the colour and bioactive properties of R-PC compared to heat treatment. SIGNIFICANCE: HP is a powerful probe that reveals intermediate states of proteins through subtle structural changes not accessible by other denaturation methods. By combining HP-small-angle-Xray scattering with HP-absorption and fluorescence spectroscopy, we elucidate the multiphase unfolding process of R-phycocyanin. This process includes: 1) conformational changes, 2) oligomer dissociation at moderate pressures, and 3) monomer unfolding. Our approach provides new opportunities for the structural determination of protein intermediates and oligomers using HP.