New Insights into the Evolution of the Electron Transfer from Cytochrome f to Photosystem I in the Green and Red Branches of Photosynthetic Eukaryotes

Abstract

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin (Pc) alternatively replaces the heme-protein cytochrome c6 (Cc6) as the soluble electron carrier from cytochrome f (Cf) to photosystem I (PSI). The functional and structural equivalence of 'green' Pc and Cc6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce Pc, despite having evolved from green algae. On the other hand, Cc6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, Pc genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional Pc in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from Cf to PSI has been characterized in depth. The conclusion is that in the green lineage, this process involves strong electrostatic interactions between partners, which ensure a high affinity and an efficient electron transfer (ET) at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modeling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient ET, but favoring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type Pc with both Cf and PSI may not yet be optimized.

Keywords: Cytochrome c6; Cytochrome f; Electron transfer; Photosynthetic green and red lineages; Photosystem I; Plastocyanin.

Fig. 1

(A) Photosynthetic chain and electron transfer pathways (shown by arrows) based on the existence of alternative soluble electron carrier proteins. PQ, plastoquinone; PsaF, PSI subunit involved—in green algae and plants—in the electrostatic interaction with Pc and Cc₆, the two alternative soluble electron carriers between Cb₆f and PSI. Backbone representation of photosynthetic proteins of the green alga Chlamydomonas reinhardtii: (B) luminal exposed part of Cf in the Cb₆f complex (Protein Data Bank, PDB code, 1cfm); (C) Pc (PDB code, 2plt); (D) Cc₆ (PDB code, 1cyj); and (E) luminal exposed part of PsaF in PSI (PDB code, 6ijo). The copper (in Pc) and heme (in Cf and Cc₆) cofactors are highlighted.

Fig. 2

(A) Schematic representation of the evolution of the main lineages of photosynthetic eukaryotes and the presence of the electron carriers Pc and Cc₆. In the green lineage, plants (that only produce Pc) have evolved from green algae, which can produce Pc and Cc₆. In the red lineage, red algae only produce Cc₆ and are the origin, through secondary endosymbiosis events, of the red-type chloroplast of other secondary algae groups. (B) Electrostatic potential of the charged surface area—involved in the electrostatic interaction with its partners—of Cc₆, Pc and the PsaF subunit from the green alga C. reinhardtii, and of the Cc₆ (PDB code, 3dmi) and modeled PsaF (Bernal-Bayard et al. 2015) from the diatom P. tricornutum. The modeled acquired ‘green-type’ Pc of the diatom T. oceanica is also shown (in brackets). The proteins are oriented as shown in Fig. 1, with Pc and Cc₆ displaying their electrostatic areas in front and with the hydrophobic patches at the top of both proteins. Electrostatic potential values are shown on a scale from red to blue, corresponding to −10.0 and +10.0 kcal mol⁻¹, respectively, at 298 K. K_A and K_D, association and dissociation constants, respectively; k_ET, electron transfer rate constant. See the text for more details.

Fig. 3

Best-energy docking models for efficient ET (PsaA W652/PsaB W624 and Cc₆ heme groups at <3.0 Å distance) between P. tricornutum PSI and P. tricornutum or M. braunii Cc₆. Membrane lipids and PsaA/B subunits of PSI (in light grey), the PsaF subunit of PSI (in dark grey), P700 (in green) and Cc₆ of P. tricornutum (in blue) or M. braunii (in light brown) are depicted. PSI W652/W624 groups, located in the ET pathway between P700 and the cytochromes, are also shown. PSI:Cc₆ models correspond to the coordinates described in Bernal-Bayard et al. (2015).

Fig. 4

(Left) Backbone representation of the exposed luminal domain of Cf of the green alga C. reinhardtii. The view displays in front both the heme group and the bound Tyr1 (in green). (Right) Surface electrostatic potential distributions of C. reinhardtii Cf and the structural models of P. tricornutum and T. oceanica Cf. The proteins are oriented as shown in the backbone representation on the left, and electrostatic potential values are shown on the same scale as in Fig. 2. See the text and Supplementary Appendix S1 for more details.

Fig. 5

(A) Representative structure for the plant Cf:Pc complex (turnip Cf and spinach Pc; obtained from PDB code 2pcf) (Ubbink et al. 1998). Pc is colored in blue and the copper-bound His87 is shown. Best-energy docking models for efficient ET between Cf and Cc₆ (in red) of (B) the green alga C. reinhardtii (rank 1, docking energy −32.0 a.u., distance between Fe in Cf to heme in Cc₆ of 8.2 Å) and (C) the diatom P. tricornutum (rank 6, docking energy −24.7 a.u., distance between Fe in Cf to heme in Cc₆ of 8.0 Å, the shortest distance model). (D) Superimposed docking models of the P. tricornutum [Cf:Cc₆] complex (in C) and the model (in blue) corresponding to a truncated Cf without the small domain (rank 2, docking energy −31.0 a.u., distance between Fe in Cf to heme in Cc₆ of 8.4 Å). See Supplementary Appendix S1 for more details.

Fig. 6

(A–C) Representative best-energy docking models for efficient ET between the modeled Cf and Pc (in blue) of the diatom T. oceanica (selected by the shorter distances between Fe in Cf and Cu in Pc). The Cf heme, the iron-bound Tyr1 and the copper-bound His88 in Pc are shown. (A) Rank 62, docking energy −24.2 a.u., distance between Fe in Cf to Cu in Pc of 11.1 Å (the shortest distance model). (B) Rank 8, docking energy −31.1 a.u., distance between Fe in Cf to Cu in Pc of 12.7 Å. (C) Rank 16, docking energy −30.1 a.u., distance between Fe in Cf to Cu in Pc of 12.2 Å. (D) Best-energy docking model showing the Tyr84 group in Pc (rank 1, docking energy −38.4 a.u., distance between Fe in Cf to Cu in Pc of 20.5 Å). See the text and Supplementary Appendix S1 for more details.