Structural characteristics that stabilize or destabilize different assembly levels of phycocyanin by urea

Insights into the molecular mechanism of yellow cuticle coloration by a chitin-binding carotenoprotein in gregarious locusts

Carotenoids are hydrophobic pigments binding to diverse carotenoproteins, many of which remain unexplored. Focusing on yellow gregarious locusts accumulating cuticular carotenoids, here we use engineered Escherichia coli cells to reconstitute a functional water-soluble β-carotene-binding protein, BBP. HPLC and Raman spectroscopy confirmed that recombinant BBP avidly binds β-carotene, inducing the unusual vibronic structure of its absorbance spectrum, just like native BBP extracted from the locust cuticles. Bound to recombinant BBP, β-carotene exhibits pronounced circular dichroism and allows BBP to withstand heating (T0.5 = 68 °C), detergents and pH variations. Using bacteria producing distinct xanthophylls we demonstrate that, while β-carotene is the preferred carotenoid, BBP can also extract from membranes ketocarotenoids and, very poorly, hydroxycarotenoids. We show that BBP-carotenoid complex reversibly binds to chitin, but not to chitosan, implying the role for chitin acetyl groups in cuticular BBP deposition. Reconstructing such locust coloration mechanism in vitro paves the way for structural studies and BBP applications.

High Molecular Weight λ-Carrageenan Improves the Color Stability of Phycocyanin by Associative Interactions

Phycocyanin is a protein-chromophore structure present in Arthrospira platensis commonly used as a blue-colorant in food. Color losses of phycocyanin can be reduced by electrostatic complexation with λ-carrageenan. The aim of this study was to investigate the effect of molecular weight (MW) of λ-carrageenan on the color stabilization of electrostatic complexes formed with phycocyanin and λ-carrageenan. Samples were heated to 70 or 90°C at pH 3.0 and stored at 25°C for 14 days. The MW of λ-carrageenan was reduced by ultrasound treatments for 15, 30, 60, and 90 min. Prolonged ultrasonication had a pronounced effect on the Mw, which decreased from 2,341 to 228 kDa (0–90 min). Complexes prepared with low MW λ-carrageenan showed greater color changes compared to complexes prepared with high MW λ-carrageenan. The MW had no visible effect on color stability on day 0, but green/yellow shifts were observed during storage and after heating to 70°C. Medium MW showed less color stabilization effects compared to low MW when heated to 70°C. Moreover, for solutions prepared with ultrasonicated λ-carrageenan, significant hue shifts toward green/yellow, and precipitation were observed after a heat treatment at 90°C. In addition, the sizes of the complexes were significantly reduced (646–102 nm) by using ultrasonicated λ-carrageenan, except for the lowest MW λ-carrageenan when heated to 90°C. Overall, these findings demonstrated that decreasing the MW of λC had adverse effects on the color stability of PC:λC complexes heated to 70 and 90°C.

A study of the protein-protein interactions in the phycocyanin monomer from Synechocystis sp. PCC 6803 using a bacterial two-hybrid system

Investigations into the intramolecular interactions of the native protein in solution are important to understand its structural stability as well as its potential uses in future applications. In this study, we used a bacterial two-hybrid system to investigate the interaction between the phycocyanin α and β subunits that form the phycocyanin monomer. Key amino acid residues responsible for the interaction between the subunits were identified, providing direct experimental evidence for the intramolecular interaction.

Tuning C-Phycocyanin Photoactivity via pH-Mediated Assembly-Disassembly

Environment-triggered protein conformational changes have garnered wide interest in both fundamental research, for deciphering in vivo acclimatory responses, and practical applications, for designing stimuli-responsive probes. Here, we propose a protein-chromophore regulatory mechanism that allows for manipulation of C-phycocyanin (C-PC) from Spirulina platensis by environmental pH and UV irradiation. Using small-angle X-ray scattering, a pH-mediated C-PC assembly-disassembly pathway, from monomers to nonamers, was unraveled. Such flexible protein matrices impart tunability to the embedded tetrapyrroles, whose photochemical behaviors were found to be modulated by protein assembly states. UV irradiation on C-PC triggers pH-dependent singlet oxygen (¹O₂) generation and conformational changes. Intermolecular photo-crosslinking occurs at pH 5.0 via dityrosine species, which bridges solution-based C-PC oligomers into unprecedented dodecamers and 24-mers. These supramolecular assemblies impart C-PC at pH 5.0, which significantly enhanced ¹O₂ yield, fluorescence, and photostability relative to those at other pH values, a finding that makes C-PC appealing for tumor-targeted photodynamic therapy.

Biophysical and structural characterization of the small heat shock protein HspA from Thermosynechococcus vulcanus in 2 M urea

Small heat shock proteins (sHSPs) belong to the superfamily of molecular chaperones. They prevent aggregation of partially unfolded or misfolded client proteins, providing protection to organisms under stress conditions. Here, we report the biophysical and structural characterization of a small heat shock protein (HspA) from a thermophilic cyanobacterium Thermosynechococcus vulcanus in the presence of 2 M urea. HspA has been shown to be important for the protection of Photosystem II and the Phycobilisome antenna complex at elevated temperatures. Heterologously expressed HspA requires the presence of 1-2 M urea to maintain its solubility at concentrations required for most characterization methods. Spectroscopic studies reveal the presence of the β-sheet structure and intactness of the tertiary fold in HspA. In vitro assays show that the HspA maintains chaperone-like activity in protecting soluble proteins from thermal aggregation. Chromatography and electron microscopy show that the HspA exists as a mixture of oligomeric forms in the presence of 2 M urea. HspA was successfully crystallized only in the presence of 2 M urea. The crystal structure of HspA shows urea-induced loss of about 30% of the secondary structure without major alteration in the tertiary structure of the protein. The electron density maps reveal changes in the hydrogen bonding network which we attribute to the presence of urea. The crystal structure of HspA demonstrates a mixture of both direct interactions between urea and protein functionalities and interactions between urea and the surrounding solvent that indirectly affect the protein, which are in accordance with previously published studies.

Crystal Structure of Allophycocyanin from Marine Cyanobacterium Phormidium sp. A09DM

Isolated phycobilisome (PBS) sub-assemblies have been widely subjected to X-ray crystallography analysis to obtain greater insights into the structure-function relationship of this light harvesting complex. Allophycocyanin (APC) is the phycobiliprotein always found in the PBS core complex. Phycocyanobilin (PCB) chromophores, covalently bound to conserved Cys residues of α- and β- subunits of APC, are responsible for solar energy absorption from phycocyanin and for transfer to photosynthetic apparatus. In the known APC structures, heterodimers of α- and β- subunits (known as αβ monomers) assemble as trimer or hexamer. We here for the first time report the crystal structure of APC isolated from a marine cyanobacterium (Phormidium sp. A09DM). The crystal structure has been refined against all the observed data to the resolution of 2.51 Å to R_work (R_free) of 0.158 (0.229) with good stereochemistry of the atomic model. The Phormidium protein exists as a trimer of αβ monomers in solution and in crystal lattice. The overall tertiary structures of α- and β- subunits, and trimeric quaternary fold of the Phormidium protein resemble the other known APC structures. Also, configuration and conformation of the two covalently bound PCB chromophores in the marine APC are same as those observed in fresh water cyanobacteria and marine red algae. More hydrophobic residues, however, constitute the environment of the chromophore bound to α-subunit of the Phormidium protein, owing mainly to amino acid substitutions in the marine protein.