Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

Abstract

A photosystem I (PS I) complex containing plastoquinone-9 (PQ-9) but devoid of F X , F B , and F A was isolated and characterized from a mutant strain of Synechococcus sp. PCC 7002 in which the menB and rubA genes were insertionally inactivated. In isolated PS I trimers, the decay of P700؉ measured in the near-IR and the decay of A 1 ؊ measured in the near-UV were found to be biphasic, with (averaged) room temperature lifetimes of 12 and 350 s. The decay-associated spectra of both kinetic phases are characteristic of the oxidized minus reduced difference spectrum of a semiquinone, consistent with charge recombination between P700؉ and PQ-9 ؊ . The amplitude of the flash-induced absorbance changes in both the near-IR and the near-UV show that approximately one-half of the A 1 binding sites are either empty or nonfunctional. A spin-polarized chlorophyll triplet is observed by time-resolved EPR, and it is attributed to the 3 P700 product of P700 ؉ A 0 ؊ charge recombination via the T 0 spin level in those PS I complexes that do not contain a functional quinone. In those A 1 sites that are occupied, the P700 ؉ Q ؊ polarization pattern indicates that PQ-9 is oriented in a similar manner to that in the menB mutant. When excess 9,10-anthraquinone is added in vitro, it displaces PQ-9 and occupies the A 1 binding site more readily than in the menB mutant. This can be explained by a greater accessibility to the A 1 site in the menB rubA mutant due to the absence of F X and the stromal ridge polypeptides. The relatively low binding affinity of 9,10-anthraquinone allows it to be readily removed from the A 1 site by washing. However, all A 1 sites are shown to bind napthoquinones with high affinity and thus are proven to be functionally competent in quinone binding. The ability to readily displace PQ-9 from the A 1 site makes the menB rubA mutant ideal for introducing novel quinones, particularly anthraquinones, into PS I.Photosystem I is a multisubunit, pigment-protein complex that is found in the membranes of plants, algae, and cyanobacteria and that mediates the light-induced transfer of electrons from plastocyanin/cytochrome c 6 to ferredoxin/flavodoxin. According to current understanding, light-induced charge separation results in the oxidation of the primary electron donor P700 (E m Ј ϩ430 mV), a chlorophyll a/aЈ heterodimer located on the luminal (inner) side of the membrane, and the reduction of the primary electron acceptor A 0 (E m Ј approximately Ϫ1000 mV), a chlorophyll a monomer located in the interior of the membrane. The electron is passed to A 1 (E m Ј approximately Ϫ800 mV), an alkyl-substituted menadione (2-methyl-1,4-naphthalenedione); to F X (E m Ј Ϫ705 mV), an interpolypeptide (4Fe-4S) cluster; and finally to F A (E m Ј Ϫ520 mV) and F B (E m Ј Ϫ580 mV), which are (4Fe-4S) clusters bound to the extrinsic subunit PsaC located on the stromal (cytoplasmic) side of the membrane. In most organisms, including Synechocystis sp. PCC 6803, the quinone in the A 1 site is phylloquinone (2-methyl-3-phytyl-1,4-naphtho...

Vitamin K as a Diet Supplement with Impact in Human Health: Current Evidence in Age-Related Diseases

“…Nevertheless, several species of cyanobacteria and microalgae, such as the cyanobacteria Gloeobacter violaceus [172] and Synechococcus sp. PCC 7002 [173], the diatom Chaetoceros gracilis [174], and the red algae Cyanidium caldarium [175] have been shown to synthetize MK-4. Moreover, evidence from the literature describes several aquatic species with different content in vitamin K1 [169].…”

Recruitment of a Foreign Quinone into the A1 Site of Photosystem I

“…Thus, only the functional occupancy of the A 1 site can be estimated from the amplitude ratio of the 3 P 700 and P 700 ⅐ϩ AQ ⅐ Ϫ spectra. As shown in the accompanying paper (36), AQ has a lower affinity for the A 1 site compared with other quinones. Consistent with this, the 3 P 700 spectrum is not lost completely when extracted PS I is incubated with AQ.…”

“…The Role of F X in Determining the Redox Potential of the Quinone-As shown in the accompanying paper (36), AQ incorporation is also possible with PS I from the menB rubA double deletion mutant. The rubA deletion leads to assembly of PS I particles lacking all three FeS centers (66,67); hence, it provides an elegant way of investigating the role of F X in determining the quinone redox potential.…”

Overexpression of bifunctional fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase leads to enhanced photosynthesis and global reprogramming of carbon metabolism in Synechococcus sp. PCC 7002

“…Synechococcus 7002 is emerging as an important photosynthetic cell factory for metabolic engineering, because it is one of the fastest growing cyanobacteria with a doubling time of ~3.5h under photoautotrophic conditions (Sakuragi et al, 2005) and it is also capable of robust growth in salt water (e.g. sea water, coastal brackish water), which is a key requirement for large-scale productions of chemicals and fuels (Ruffing et al, 2016).…”