Abstract
The identity and rearrangements of substrate water molecules in photosystem II (PSII) water oxidation are of great mechanistic interest and addressed herein by comprehensive analysis of NH 4 + /NH 3 binding. Time-resolved detection of O 2 formation and recombination fluorescence as well as Fourier transform infrared (FTIR) difference spectroscopy on plant PSII membrane particles reveals the following. (1) Partial inhibition in NH 4 Cl buffer occurs with a pH-independent binding constant of ∼25 mM, which does not result from decelerated O 2 formation, but from complete blockage of a major PSII fraction (∼60%) after reaching the Mn(IV) 4 (S 3 ) state. (2) The non-inhibited PSII fraction advances through the reaction cycle, but modified nuclear rearrangements are suggested by FTIR difference spectroscopy. (3) Partial inhibition can be explained by anticooperative (mutually exclusive) NH 3 binding to one inhibitory and one non-inhibitory site; these two sites may correspond to two water molecules terminally bound to the “dangling” Mn ion. (4) Unexpectedly strong modifications of the FTIR difference spectra suggest that in the non-inhibited PSII, ammonia binding obliterates the need for some of the nuclear rearrangements occurring in the S 2 –S 3 transition as well as their reversal in the O 2 formation transition, in line with the carousel mechanism [Askerka, M., et al. (2015) Biochemistry 54, 5783]. (5) We observe the same partial inhibition of PSII by NH 4 Cl also for thylakoid membranes prepared from mesophilic and thermophilic cyanobacteria, suggesting that the results described above are valid for plant and cyanobacterial PSII.
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Claims 5
Claim · model-written summary
NH4Cl partly inhibited plant photosystem II with a pH-independent binding constant near 25 mM and blocked roughly 60% after the S3 state.
- Treatment
- NH 4 Cl buffere0
- Outcome
- Partial inhibitione0
- Direction
- inhibitione0
- Value
- ∼25 mMe0
- Qualifier
- complete blockage of a major PSII fraction (∼60%)e0
Not reportedTaxonComparator
Verbatim
e0“Partial inhibition in NH 4 Cl buffer occurs with a pH-independent binding constant of ∼25 mM, which does not result from decelerated O 2 formation, but from complete blockage of a major PSII fraction (∼60%) after reaching the Mn(IV) 4 (S 3 ) state.”
e1“plant PSII membrane particles”
Claim · model-written summary
The noninhibited plant PSII fraction continued through the reaction cycle with suggested altered nuclear rearrangements.
- Outcome
- modified nuclear rearrangementse0
Not reportedTaxon
Verbatim
e0“The non-inhibited PSII fraction advances through the reaction cycle, but modified nuclear rearrangements are suggested by FTIR difference spectroscopy.”
e1“plant PSII membrane particles”
Claim · model-written summary
Partial inhibition may reflect mutually exclusive ammonia binding to inhibitory and noninhibitory sites associated with two terminal water molecules.
Not reportedTaxon
Verbatim
e0“Partial inhibition can be explained by anticooperative (mutually exclusive) NH 3 binding to one inhibitory and one non-inhibitory site; these two sites may correspond to two water molecules terminally bound to the “dangling” Mn ion.”
e1“plant PSII membrane particles”
Claim · model-written summary
Ammonia binding appears to remove the need for some nuclear rearrangements during the S2–S3 and oxygen formation transitions.
Not reportedTaxon
Verbatim
e0“in the non-inhibited PSII, ammonia binding obliterates the need for some of the nuclear rearrangements occurring in the S 2 –S 3 transition as well as their reversal in the O 2 formation transition”
e1“plant PSII membrane particles”
Claim · model-written summary
The same partial PSII inhibition by NH4Cl occurred in mesophilic and thermophilic cyanobacteria.
- Taxon
- cyanobacteriaCyanobacteriotae0
- Treatment
- NH 4 Cle0
- Outcome
- partial inhibition of PSIIe0
Verbatim
e0“the same partial inhibition of PSII by NH 4 Cl also for thylakoid membranes prepared from mesophilic and thermophilic cyanobacteria”