Thermal Stability of α-Phycoerythrocyanin

The α-subunit of the phycobiliprotein, phycoerythrocyanin (α-PEC), from Mastigocladus laminosus shows photoreversible photochromism that is based on the Z ↔ E isomerization of the phycoviolobilin (PVB) chromophore. Thermal stability of the photochemistry and chromoprotein secondary structure have been studied by absorption and circular dichroism (CD) spectroscopy. Both photoisomers are stable and photoconvertible up to ∼ 70 °C. At T > 75 °C, photochemistry ceases because the E-state reverts rapidly thermally to the Z–state. The chromoprotein melts at 72 °C, the apoprotein already at 55 °C, indicating a considerable stabilization of the protein secondary structure by the chromophore.