Photosynthetic water oxidation in Synechocystis sp. PCC6803: mutations D1-E189K, R and Q are without influence on electron transfer at the donor side of photosystem II

Abstract

The oxygen-evolving manganese cluster (OEC) of photosynthesis is oxidised by the photochemically generated primary oxidant (P(+*)(680)) of photosystem II via a tyrosine residue (Y(Z), Tyr161 on the D1 subunit of Synechocystis sp. PCC6803). The redox span between these components is rather small and probably tuned by protonic equilibria. The very efficient electron transfer from Y(Z) to P(+*)(680) in nanoseconds requires the intactness of a hydrogen bonded network involving Y(Z), D1-His190, and presumably D1-Glu189. We studied photosystem II core particles from photoautotrophic mutants where the residue D1-E189 was replaced by glutamine, arginine and lysine which were expected to electrostatically differ from the glutamate in the wild-type (WT). Surprisingly, the rates of electron transfer from Y(Z) to P(+*)(680) as well as from the OEC to Y(ox)(Z) were the same as in the WT. With the generally assumed proximity between D1-His190 (and thus D1-Glu189) and Y(Z), the lack of any influence on the electron transfer around Y(Z) straightforwardly implies a strongly hydrophobic environment forcing Glu (acid) and Lys, Arg (basic) at position D1-189 into electro-neutrality. As one alternative, D1-Glu189 could be located at such a large distance from the OEC, Y(Z) and P(+*)(680) that a charge on D1-189X does not influence the electron transfer. This seems less likely in the light of the drastic influence of its direct neighbour, D1-His190, on Y(Z) function. Another alternative is that D1-Glu189 is negatively charged, but is located in a cluster of acid/base groups that compensates for an alteration of charge at position 189, leaving the overall net charge unchanged in the Gln, Lys, and Arg mutants.

No Evidence from FTIR Difference Spectroscopy That Glutamate-189 of the D1 Polypeptide Ligates a Mn Ion That Undergoes Oxidation during the S0 to S1, S1 to S2, or S2 to S3 Transitions in Photosystem II

“…This conclusion would be consistent with the earlier spectroscopic analyses of D1-Glu189 mutants (14,15,20), but presents another challenge: it requires that the proximity of D1-Glu189 to Mn in the X-ray crystallographic structural models be an artifact of the radiation-induced reduction of the Mn 4 cluster that occurred during the collection of the X-ray diffraction data. However, as noted earlier, recent X-ray absorption studies of PSII single crystals (21) and PSII membrane multilayers (22) show that the X-ray doses that were used in the crystallographic studies to irradiate the PSII crystals would have caused a surprisingly rapid reduction of the Mn 4 cluster's oxidized Mn(III/IV) ions to their fully reduced Mn(II) states and substantially modified the structure of the Mn 4 cluster.…”

“…The lower O 2 evolving activity of PSII in the D1-E189Q and D1-E189R mutants (60 − 70 % compared to wild-type) has been noted previously (14,15,46). Similarly, the lower O 2 evolving activity of PSII in the D2-H189Q mutant (40 − 50 % compared to wild-type) has been noted previously (39,50).…”

“…• during the S 1 → S 2 , S 2 → S 3 , and S 3 → S 0 transitions (15), (3) D1-E189K and D1-E189R PSII particles exhibit normal rates of electron transfer from Y Z to P 680 •+ and from the Mn 4 cluster to Y Z…”

“…It was proposed that this hydrogen bond network is maintained by Gln because of its hydrogen bonding characteristics and by Lys and Arg because of their flexibility and hydrogen bonding characteristics (14,16) and that this network compensates for the change in charge that is created when D1-Glu189 is changed to Gln, Lys, or Arg 2 (15).…”

Membrane-inlet mass spectrometry reveals a high driving force for oxygen production by photosystem II

“…O 2 -evolving PSII-cc were isolated from modified WT cells of Synechocystis sp. PCC6803 as described earlier (38). Typical rates of O 2 evolution under continuous illumination were 2,000-3,000 μmol ðO 2 Þ × mgðChlÞ −1 × h −1 (38) (see also SI Text).…”