Probing pH sensitivity of αC-phycoerythrin and its natural truncant: A comparative study

• Effect of pH on the two natural variants of αC-PE was studied.
• Both variants maintain their structure within the pH range of pH 5.5–8.75.
• Both variants lost their structure at extremes of pH.
• pH-Induced denaturation of both forms of C-PE are reversible in the pH range 2.5–11.0

Cyanobacterial phycoerythrin (αC-PE) from Phormidium tenue exists in two natural forms named as full length (FL-αC-PE) and truncated (Tr-αC-PE). FL-αC-PE and Tr-αC-PE are produced when cyanobacterium is grown in the optimal medium and nutrient deficient medium, respectively. Despite of N-terminal deletion, both proteins show similar spectroscopic properties. In this study, different optical properties of these two natural variants of C-PE were measured in the pH range 1.0–12.0 (1.0 ≤ pH ≤ 12.0). It was observed that: (i) their absorption, fluorescence and CD spectra remain unchanged within the range adjacent to neutral pH, 5.5–8.75, (ii) at pH values higher than 8.75 and lower than 5.5 their absorption, fluorescence and CD spectral signatures are changed significantly, and (iii) emission spectra of the covalently linked tetrapyrrole chromophores and Trp residue are perturbed at extreme pH values in the range 8.75 < pH < 5.5. Refolding experiments further suggest that pH-induced denaturation of both forms of C-PE is reversible in the pH range 2.5–11.0, but irreversible beyond this range on both sides of pH extremes. The pH-induced denaturation of both the full length and truncated αC-PEs follows a two-state mechanism.