A photosystem I reaction center driven by chlorophyll d in oxygenic photosynthesis

Abstract

A far-red type of oxygenic photosynthesis was discovered in Acaryochloris marina, a recently found marine prokaryote that produces an atypical pigment chlorophyll d (Chl d). The purified photosystem I reaction center complex of A. marina contained 180 Chl d per 1 Chl a with PsaA-F, -L, -K, and two extra polypeptides. Laser excitation induced absorption changes of reaction center Chl d that was named P740 after its peak wavelength. A midpoint oxidation reduction potential of P740 was determined to be +335 mV. P740 uses light of significantly low quantum energy (740 nm = 1.68 eV) but generates a reducing power almost equivalent to that produced by a special pair of Chl a (P700) that absorbs red light at 700 nm (1.77 eV) in photosystem I of plants and cyanobacteria. The oxygenic photosynthesis based on Chl d might either be an acclimation to the far-red light environments or an evolutionary intermediate between the red-absorbing oxygenic and the far-red absorbing anoxygenic photosynthesis that uses bacteriochlorophylls.

Figure 1

Chemical structure of chlorophylls in oxygenic photosynthesis of plants and cyanobacteria.

Figure 2

(A) Separation of the pigment–protein complexes from detergent-solubilized thylakoid membranes of A. marina by sucrose density gradient centrifugation. The membranes were solubilized at 4°C for 30 min with 0.8% β-dodecyl maltoside, 20 mM Bis-Tris (pH 7.0), 10% glycerol, 10 mM NaCl, 10 mM CaCl₂ and 2 mM EDTA-Na₂. The sucrose gradient centrifugation (198,600 × g for 16 h) yielded three pigment-protein bands (1–3 on the right). (B) Polypeptide composition of the thylakoids and purified PS I complex from A. marina. Materials were treated with SDS and resolved on a 16–22% SDS/urea-PAGE. Lanes: 1, molecular weight standards; 2, thylakoid membranes; 3, PS I reaction center complex. The gel was stained with Coomassie blue. Sizes of molecular weight standards are indicated on the left, and subunit identifications based on N-terminal amino acid sequencing analysis are indicated on the right.

Figure 3

(A) Absorption (ABS) spectrum (Upper) and (B) laser flash-induced difference absorption (ΔA) spectrum (Lower) at 15°C of A. marina PS I complex (solid line, filled circles). The PS I complexes (band 3 in Fig. 2A) was suspended in Tris⋅HCl buffer (pH 7.5) containing 100 mM NaCl and 0.3 mM sodium ascorbate. Absorption and difference absorption spectra of P700 of spinach PS I (dashed line) are also shown.

Figure 4

Redox titration of the primary donor (P740) of A. marina PS I. The initial extent of the absorption change (ΔABS) at 740 nm after a laser flash excitation was plotted against the redox potential of the medium. The midpoint redox potential value was estimated to be +335 mV as shown by a fitting with an one-electron Nernst’s curve by assuming 5% irreversible absorption change (solid line).

Figure 5

Absorption changes of P740 and Safranin-o after laser flash excitation in A. marina PS I complex. Absorption changes were monitored at 740 nm in traces a–c and at 518 nm in traces d and e. Traces: a, no addition; b, with 300 μM methyl viologen; c, with 300 μM methyl viologen and 2 mM dithionite; d, no addition; e, with 40 μM Safranin-o. Other experimental conditions were similar to those in Fig. 3.

Figure 6

Electron transfer in the PS I reaction center. (Left) Electron transfer pathways in PS I of plants, cyanobacteria, and A. marina. Light induces the electron transfer from the donor (P700 in plants or cyanobacteria, and P740 in A. marina) to the acceptor chlorophyll (A₀), quinone (Q), and the 4Fe4S clusters (F_X, F_B, and F_A). The acceptor side of A. marina PS I is not known. (Right) Schematic representation of the central portion of PS I reaction center complex modified from the x-ray structure of cyanobacteria (16). In plants and cyanobacteria the chlorophylls including P700 and A₀ are Chl a (9, 15, 16). In A. marina, P740 can be estimated to be a special pair of Chl d.