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Function
light-harvesting complexes make possible for Purple bacterial to maximize the spectrum of light avaiable to them, modifyng the absorption properties of their chromophores; The energy absorbed is used in the bacteria photochemistry.
In the LHC The proteins determine the disposition of the pigments, therefore changing and influencing 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
Structural studies have shown that LH2 complexes are formed from nine ab subunit oligomers, organised in a ring of inner a and outer b-peptides. In between the b-peptides and close to the cytoplasmic surface are nine well-separated BChl a molecules absorbing at 800 nm.
Relevance
Structural highlights
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