Abstract
In oxygenic photosynthesis there are two 'light states'-adaptations of the photosynthetic apparatus to spectral composition that otherwise favours either photosystem I or photosystem II. In chloroplasts of green plants the transition to light state 2 depends on phosphorylation of apoproteins of a membrane-intrinsic antenna, the chlorophyll-a/b-binding, light-harvesting complex II (LHC II), and on the resulting redistribution of absorbed excitation energy from photosystem II to photosystem I. The transition to light state 1 reverses these events and requires a phospho-LHC II phosphatase. Current structures of LHC II reveal little about possible steric effects of phosphorylation. The surface-exposed N-terminal domain of an LHC II polypeptide contains its phosphorylation site and is disordered in its unphosphorylated form. A molecular recognition hypothesis proposes that state transitions are a consequence of movement of LHC II between binding sites on photosystems I and II. In state 1, LHC II forms part of the antenna of photosystem II. In state 2, a unique but as yet unidentified 3-D structure of phospho-LHC II may attach it instead to photosystem I. One possibility is that the LHC II N-terminus becomes ordered upon phosphorylation, adopting a local alpha-helical secondary structure that initiates changes in LHC II tertiary and quaternary structure that sever contact with photosystem II while securing contact with photosystem I. In order to understand redistribution of absorbed excitation energy in photosynthesis we need to know the structure of LHC II in its phosphorylated form, and in its complex with photosystem I.
Highlights
Bennett (1977) reported that chloroplast membrane polypeptides become labelled with 32P, either from 32P-orthophosphate supplied to a suspension of isolated, intact pea chloroplasts, or from [γ –32P]ATP present in a suspension of pea thylakoid membranes
Variation in room temperature fluorescence yield is a property of photosystem II, so light-harvesting complex II (LHC II) phosphorylation was assumed to divert absorbed excitation energy away from photosystem II
Phosphorylation of LHC II is accompanied by changes in low-temperature (77 K) fluorescence emission spectra that indicate re-distribution of absorbed excitation energy to photosystem I at the expense of photosystem II (Bennett et al 1980b)
Summary
The light-dependency of LHC II phosphorylation was suggested to be a consequence of photosynthetic electron transport, and a reasonable expectation at the time was that the LHC II kinase is activated, like other chloroplast components, by thioredoxin and reduced ferredoxin, accepting electrons from photosystem I (Bennett 1979b). In ‘light 1’, photosystem I runs faster than photosystem II, plastoquinone becomes oxidized, the kinase is inactive, and the light-independent phospho-LHC II phosphatase activity predominates, returning a proportion of absorbed excitation to photosystem II at the expense of photosystem I This general mechanism, outlined, is consistent with redox effects on the ATP-induced fluorescence quenching and 32P-labelling of LHC II (Allen and Horton 1981, Allen et al 1981, Horton et al 1981). Plastoquinone redox control allows state transitions eventually to move light-harvesting pigment molecules between the two photosystems in proportion to the capacity of the two reactions centres to use the absorbed excitation energy in photochemistry. It seems to be generally agreed for algae and green plants (Wollman 2001, Allen 2003b, Goldschmidt-Clermont and Bassi 2015, Nawrocki et al 2016) that light-states 1 and 2 are states of physiological adaptation that maximize quantum yield of photosynthesis at limiting light intensity (Bonaventura and Myers 1969, Murata 1969, Myers 1971)
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