The free energy of dissociation of oligomeric structure in phycocyanin is not linear with denaturant

Non-conventional octameric structure of C-phycocyanin

C-phycocyanin (CPC), a blue pigment protein, is an indispensable component of giant phycobilisomes, which are light-harvesting antenna complexes in cyanobacteria that transfer energy efficiently to photosystems I and II. X-ray crystallographic and electron microscopy (EM) analyses have revealed the structure of CPC to be a closed toroidal hexamer by assembling two trimers. In this study, the structural characterization of non-conventional octameric CPC is reported for the first time. Analyses of the crystal and cryogenic EM structures of the native CPC from filamentous thermophilic cyanobacterium Thermoleptolyngbya sp. O–77 unexpectedly illustrated the coexistence of conventional hexamer and novel octamer. In addition, an unusual dimeric state, observed via analytical ultracentrifugation, was postulated to be a key intermediate structure in the assemble of the previously unobserved octamer. These observations provide new insights into the assembly processes of CPCs and the mechanism of energy transfer in the light-harvesting complexes.

Takuo Minato and colleagues determine the crystal and cryo-EM structures of the native C-phycocyanin (CPC) from the thermophilic cyanobacterium, Thermoleptolyngbya sp. O77, which was found to adopt both a conventional hexameric structure and a novel octameric assembly. These findings provide new insights into the assembly of CPCs and their mechanism of energy transfer.

Investigations of the Energy Transfer in the Phycobilisome Antenna of Arthrospira platensis Using Femtosecond Spectroscopy

Understanding the energy transfer in phycobilisomes extracted from cyanobacteria can be used for building biomimetic hybrid systems for optimized solar energy collection and photocurrent amplification. In this paper, we applied time-resolved absorption and fluorescence spectroscopy to investigate the ultrafast dynamics in a hemidiscoidal phycobilisome obtained from Arthrospira platensis. We obtained the steady-state and time-resolved optical properties and identified the possible pathways of the excitation energy transfer in the phycobilisome and its components, phycocyanin and allophycocyanin. The transient absorption data were studied using global analysis and revealed the existence of ultrafast kinetics down to 850 fs in the phycobilisome. The fluorescence lifetimes in the nanosecond time-scale assigned to the final emitters in each sample were obtained from the time-correlated single photon counting fluorescence experiments.

Structural integrity of Synechocystis sp. PCC 6803 phycobilisomes evaluated by means of differential scanning calorimetry

Phycobilisomes (PBSs) are supramolecular pigment-protein complexes that serve as light-harvesting antennae in cyanobacteria. They are built up by phycobiliproteins assembled into allophycocyanin core cylinders (ensuring the physical interaction with the photosystems) and phycocyanin rods (protruding from the cores and having light-harvesting function), the whole PBSs structure being maintained by linker proteins. PBSs play major role in light-harvesting optimization in cyanobacteria; therefore, the characterization of their structural integrity in intact cells is of great importance. The present study utilizes differential scanning calorimetry and spectroscopy techniques to explore for the first time, the thermodynamic stability of PBSs in intact Synechocystis sp. PCC 6803 cells and to probe its alteration as a result of mutations or under different growth conditions. As a first step, we characterize the thermodynamic behavior of intact and dismantled PBSs isolated from wild-type cells (having fully assembled PBSs) and from CK mutant cells (that lack phycocyanin rods and contain only allophycocyanin cores), and identified the thermal transitions of phycocyanin and allophycocyanin units in vitro. Next, we demonstrate that in intact cells PBSs exhibit sharp, high amplitude thermal transition at about 63 °C that strongly depends on the structural integrity of the PBSs supercomplex. Our findings implicate that calorimetry could offer a valuable approach for the assessment of the influence of variety of factors affecting the stability and structural organization of phycobilisomes in intact cyanobacterial cells.

Mechanistic investigation of transcutaneous protein delivery using solid-in-oil nanodispersion: A case study with phycocyanin

Phycocyanin (PC), a water-soluble protein-chromophore complex composed of hexameric (αβ)₆ subunits, has important biological functions in blue-green algae as well as pharmacological activities in biomedicine. We have previously developed a solid-in-oil (S/O) nanodispersion method to deliver biomacromolecules through the skin, although the transcutaneous mechanism has not yet been fully elucidated. To study the mechanism of transcutaneous protein delivery, we therefore enabled S/O nanodispersion by coating PC with hydrophobic surfactants and evaluated how the proteinaceous macromolecules formulated in an oil phase might permeate the skin. The extent of S/O nanodispersion of PC was dependent on the type of surfactant, suggesting that the selection of a suitable surfactant is crucial for encapsulating a large protein having a subunit structure. By measuring the intrinsic fluorescence of PC, we found that S/O nanodispersion facilitated the accumulation of PC in the stratum corneum (SC) of Yucatan micropig skin. Furthermore, after crossing the SC layer, the fluorescent recovery of PC was evident, indicating the release of the biologically active form of PC from the SC into the deeper skin layer.

C-Phycocyanin as a tumour-associated macrophage-targeted photosensitiser and a vehicle of phthalocyanine for enhanced photodynamic therapy

C-Phycocyanin was first reported as a tumour-associated macrophage targeted photosensitiser and a vehicle of zinc phthalocyanine for enhanced photodynamic therapy.

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

Phycocyanin is one of the two phycobiliproteins always found in the Phycobilisome antenna complex. It is always situated at the ends of the peripheral rods, adjacent to the core cylinders composed of allophycocyanin. The basic phycocyanin monomer is an (αβ) dimer of globin-like subunits with three covalently linked phycocyanobilin cofactors. Monomers assemble further into trimers, hexamers, and rods which include non-pigmented linker proteins. Upon isolation in low ionic strength solution, rods quickly disintegrate into phycocyanin trimers, which lose contacts with other phycobiliproteins and with the linker proteins. The trimers, however, are quite stable and only the presence of high concentrations of chaotropic agents (such as urea), very acidic solutions, or elevated temperatures induces monomerization, followed by separation between the subunits. We have recently determined the crystal structures of phycocyanin from the thremophilic cyanobacterium Thermosynechococcus vulcanus in the presence of 2 or 4 M urea, and shown that 4 M urea monomerizes the phycocyanin trimers. In this paper, we will describe the phycocyanin structures in 2 and 4 M urea more completely. By mapping out the urea positions, we describe the structural elements within the trimeric interaction interface that may be interrupted by the presence of 4 M urea. In addition, we also identify what are the structural characteristics that prevent 4 M urea from inducing subunit dissociation.

Recent developments in production and biotechnological applications of C-phycocyanin

An extensive range of pigments including phycobiliproteins are present in algae. C-phycocyanin (C-PC), a phycobiliprotein, is one of the key pigments of Spirulina, a microalgae used in many countries as a dietary supplement. Algal pigments have massive commercial value as natural colorants in nutraceutical, cosmetics, and pharmaceutical industries, besides their health benefits. At present, increasing awareness of harmful effects of synthetic compounds and inclination of community towards the usage of natural products have led to the exploitation of microalgae as a source of natural pigments/colors. This review describes recent findings about the sources and production of C-PC, with emphasis on specific techniques for extraction and purification, along with potential industrial applications in diagnostics, foods, cosmetics, and pharmaceutical industries.

Unfolding of C-phycocyanin followed by loss of non-covalent chromophore-protein interactions 1. Equilibrium experiments

Optical spectroscopic properties of the covalently linked chromophores of biliproteins are profoundly influenced by the state of the protein. This has been used to monitor the urea-induced denaturation of C-phycocyanin (CPC) from Mastigocladus laminosus and its subunits. Under equilibrium conditions, absorption, fluorescence and circular dichroism of the chromophores were monitored, as well as the circular dichroism of the polypeptide. Treatment of CPC trimers (alphabeta)3 resulted first in monomerization (alphabeta), which was followed by a complex unfolding process of the protein. Loss of chromophore fluorescence is the next process at increasing urea concentrations; it indicates increased flexibility of the chromophore while maintaining the native, extended conformation, and a less compact but still native-like packing of the protein in the regions sampled by the chromophores. This was followed by relaxation of the chromophores from the energetically unfavorable extended to a cyclic-helical conformation, as reported by absorption and CD in the visible range, indicating local loss of protein structure. Only then is the protein secondary structure lost, as reported by the far-UV CD. Sequential processes were also seen in the subunits, where again the chromophore-protein interactions were reduced before the unfolding of the protein. It is concluded that the bilin chromophores are intrinsic probes suitable to differentiate among different processes involved in protein denaturation.