Carotenoid pigments, molecular structures

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

Mimuro M, Nagashima U, Takaichi S, Nishimura Y, Yamazaki 1 and Katoh T (1992) Molecular structure and optical properties of carotenoids for the in vivo transfer function of the algal photosynthetic pigment system. Biochim Biophys Acta 1098 271-274... 

Polar carotenoids present in membranes have been shown to limit molecular oxygen penetration into lipid bilayer as demonstrated by the pigment-related decrease of the oxygen diffusion-concentration product (Subczynski et al., 1991). This effect, being most probably a direct consequence of the influence ofthe carotenoids on molecular dynamics and structure of lipid membranes, appears particularly important taking into consideration the deleterious role of active oxygen species with respect to biomembranes. 

The minimum structural unit of each ofthe three light-harvesting pigment-protein complexes consists of two BChl molecules and one carotenoid molecule, each noncovalently bound to two low molecular-weight (5-7 kDa) polypeptides designated as a and (3. In the core antenna complex B1015 of Rp. viridis and other BChl / -containing species, however, there is also a y-polypeptide. 

---