Cu+ Contributes to the Orange Carotenoid Protein-Related Phycobilisome Fluorescence Quenching and Photoprotection in Cyanobacteria

Orange Carotenoid Protein in Mesoporous Silica: A New System towards the Development of Colorimetric and Fluorescent Sensors for pH and Temperature

Orange carotenoid protein (OCP) is a photochromic carotenoprotein involved in the photoprotection of cyanobacteria. It is activated by blue-green light to a red form OCPR capable of dissipating the excess of energy of the cyanobacterial photosynthetic light-harvesting systems. Activation to OCPR can also be achieved in the dark. In the present work, activation by pH changes of two different OCPs-containing echinenone or canthaxanthin as carotenoids-is investigated in different conditions. A particular emphasis is put on OCP encapsulated in SBA-15 mesoporous silica nanoparticles. It is known that in these hybrid systems, under appropriate conditions, OCP remains photoactive. Here, we show that when immobilised in SBA-15, the OCP visible spectrum is sensitive to pH changes, but such a colorimetric response is very different from the one observed for OCP in solution. In both cases (SBA-15 matrices and solutions), pH-induced colour changes are related either by orange-to-red OCP activation, or by carotenoid loss from the denatured protein. Of particular interest is the response of OCP in SBA-15 matrices, where a sudden change in the Vis absorption spectrum and in colour is observed for pH changing from 2 to 3 (in the case of canthaxanthin-binding OCP in SBA-15: λMAX shifts from 454 to 508 nm) and for pH changing from 3 to 4 (in the case of echinenone-binding OCP in SBA-15: λMAX shifts from 445 to 505 nm). The effect of temperature on OCP absorption spectrum and colour (in SBA-15 matrices) has also been investigated and found to be highly dependent on the properties of the used mesoporous silica matrix. Finally, we also show that simultaneous encapsulation in selected surface-functionalised SBA-15 nanoparticles of appropriate fluorophores makes it possible to develop OCP-based pH-sensitive fluorescent systems. This work therefore represents a proof of principle that OCP immobilised in mesoporous silica is a promising system in the development of colorimetric and fluorometric pH and temperature sensors.

Probing heavy metal binding to phycobiliproteins

Blue-green algae, also known as cyanobacteria, contain some of the most efficient light-harvesting complexes known. These large, colourful complexes consist of phycobiliproteins which are extremely valuable in the cosmetics, food, nutraceutical and pharmaceutical industries. Additionally, the colourful and fluorescent properties of phycobiliproteins can be modulated by metal ions, making them highly attractive as heavy metal sensors and heavy metal scavengers. Although the overall quenching ability metal ions have on phycobiliproteins is known, the mechanism of heavy metal binding to phycobiliproteins is not fully understood, limiting their widespread quantitative applications. Here, we show using high-resolution native mass spectrometry that phycobiliprotein complexes bind metal ions in different manners. Through monitoring the binding equilibria and metal-binding stoichiometry, we show in particular copper and silver to have drastic, yet different effects on phycobiliprotein structure, both copper and silver modulate the overall complex properties. Together, the data reveals the mechanisms by which metal ions can modulate phycobiliprotein properties which can be used as a basis for the future design of metal-related phycobiliprotein applications.

Elucidation of the essential amino acids involved in the binding of the cyanobacterial Orange Carotenoid Protein to the phycobilisome

The Orange Carotenoid Protein (OCP) is a soluble photoactive protein involved in cyanobacterial photoprotection. It is formed by the N-terminal domain (NTD) and C-terminal (CTD) domain, which establish interactions in the orange inactive form and share a ketocarotenoid molecule. Upon exposure to intense blue light, the carotenoid molecule migrates into the NTD and the domains undergo separation. The free NTD can then interact with the phycobilisome (PBS), the extramembrane cyanobacterial antenna, and induces thermal dissipation of excess absorbed excitation energy. The OCP and PBS amino acids involved in their interactions remain undetermined. To identify the OCP amino acids essential for this interaction, we constructed several OCP mutants (23) with modified amino acids located on different NTD surfaces. We demonstrated that only the NTD surface that establishes interactions with the CTD in orange OCP is involved in the binding of OCP to PBS. All amino acids surrounding the carotenoid β1 ring in the OCPR-NTD (L51, P56, G57, N104, I151, R155, N156) are important for binding OCP to PBS. Additionally, modification of the amino acids influences OCP photoactivation and/or recovery rates, indicating that they are also involved in the translocation of the carotenoid.

Binding of red form of Orange Carotenoid Protein (OCP) to phycobilisome is not sufficient for quenching

The Orange Carotenoid Protein (OCP) is responsible for photoprotection in many cyanobacteria. Absorption of blue light drives the conversion of the orange, inactive form (OCP^O) to the red, active form (OCP^R). Concomitantly, the N-terminal domain (NTD) and the C-terminal domain (CTD) of OCP separate, which ultimately leads to the formation of a quenched OCP^R-PBS complex. The details of the photoactivation of OCP have been intensely researched. Binding site(s) of OCP^R on the PBS core have also been proposed. However, the post-binding events of the OCP^R-PBS complex remain unclear. Here, we demonstrate that PBS-bound OCP^R is not sufficient as a PBS excitation energy quencher. Using site-directed mutagenesis, we generated a suite of single point mutations at OCP Leucine 51 (L51) of Synechocystis 6803. Steady-state and time-resolved fluorescence analyses demonstrated that all mutant proteins are unable to quench the PBS fluorescence, owing to either failed OCP binding to PBS, or, if bound, an OCP-PBS quenching state failed to form. The SDS-PAGE and Western blot analysis support that the L51A (Alanine) mutant binds to the PBS and therefore belongs to the second category. We hypothesize that upon binding to PBS, OCP^R likely reorganizes and adopts a new conformational state (OCP^3rd) different than either OCP^O or OCP^R to allow energy quenching, depending on the cross-talk between OCP^R and its PBS core-binding counterpart.

Prospects of fluorescence methods application for monitoring of cyanobacterial cultures in biotechnology

Cyanobacteria are photosynthetic microorganisms that possess a high potential for innovative applications in agriculture, food production, cosmeticals, wastewater remediation, biofuels, antioxidative enzymes production, etc. During the industrial cultivation of cyanobacteria several parameters as growth rate, physiological state and algological purity of the culture should be controled permanently. One of the methods that can provide on-line monitoring of cyanobacterial cultures and further process automatization is a fluorescence spectroscopy. In this work several fluorescence techniques are analysed and their possible adaptation for biotechnological applications is suggested.

Changing Color for Photoprotection: The Orange Carotenoid Protein

Under high irradiance, light becomes dangerous for photosynthetic organisms and they must protect themselves. Cyanobacteria have developed a simple mechanism, involving a photoactive soluble carotenoid protein, the orange carotenoid protein (OCP), which increases thermal dissipation of excess energy by interacting with the cyanobacterial antenna, the phycobilisome. Here, we summarize our knowledge of the OCP-related photoprotective mechanism, including the remarkable progress that has been achieved in recent years on OCP photoactivation and interaction with phycobilisomes, as well as with the fluorescence recovery protein, which is necessary to end photoprotection. A recently discovered unique mechanism of carotenoid transfer between soluble proteins related to OCP is also described.