User:Clara Costa D'Elia/Sandbox 1

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Function

Purple bacterial light-harvesting complexes modify the absorption properties of their chromophores to wavelengths such that the spectrum available to them in vivo is maximised; and the energy so absorbed is productively transferred and made available for photochemistry. The environment of the chromophores in the light-harvesting complex is governed by protein. The protein creates the conditions determining the relative disposition of the pigments, and contributes to the local environment that modulates their absorption spectra.

In order to increase the spectral cross-section of absorption, purple bacteria also produce light-harvesting complexes. In most cases a primary light-harvesting complex (LH1) and peripheral light-harvesting complexes (LH2) are synthesised

LH2 complexes are produced in variable amounts according to the available light levels, the absorbance range of the particular LH2 (800 and 850, 800 and 820 nm), the temperature, and the bacterial species and strain (Zuber & Brunisholz, 1991).

When purple bacteria are grown under anaerobic conditions they incorporate the photosynthetic apparatus described above into invaginated intracytoplasmic phospholipid membranes. RC, LH1 and LH2 are integral trans-membrane assemblies. After absorption of a photon by a pigment molecule all energy transfer processes occur within the membrane until energy is “trapped” by the RC complex

Each individual light-harvesting complex is composed of oligomers of short peptides (α and β) with associated pigments (Hawthornthwaite & Cogdell, 1991). αβ apoproteins with their non-covalently bound carotenoid and bacteriochlorophyll (Bchl ) pigments form the multi-subunit complexes LH1 and LH2

Secondary Structure

The differences between the LH1 and LH2 complexes reside in their protein/pigment stoichiometry and modes of oligomerization. Structural studies have shown that LH2 complexes are formed from eight or nine αβ subunit oligomers Each apoprotein possesses a long trans-membrane α-helix. In the β apoprotein the trans-membrane (8-9 turn) helix begins after a four-residue N-terminal extended section. The β apoprotein trans-membrane helix is slightly curved and is inclined to the C9 axis at an angle of approximately 15°. The membrane-spanning segment of the α apoprotein is, in the main, parallel with the C9 axis.

Tertiary structure

Relevance

Structural highlights

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