Biomedical Engineering Reference
In-Depth Information
OH
O
O
HO
violaxanthin
OH
O
O
neoxanthin
HO
OH
HO
lutein
OH
HO
zeaxanthin
Fig. 3.3.
The three xanthophylls, namely lutein, violaxanthin, and neoxanthin,
contained by all plant species and to a large extent green algae in low light, and the
xanthophyll zeaxanthin, synthesized in high light
side
β
-carotene, the same three xanthophylls, namely lutein, violaxanthin,
and neoxanthin (Fig. 3.3) in low light; and that violaxanthin is deepoxidized
to zeaxanthin when light is in excess through the green lineage [23] is a clear
biological demonstration that each of these xanthophylls have a specific func-
tion. As mentioned earlier, xanthophylls in the thylakoid membrane are bound
to Lhc proteins. Moreover, each Lhc protein has several (2-4) xanthophyll-
binding sites. The a
nity of each site for xanthophyll species is summarized
in Fig. 3.4 for the major LHCII protein, which constitute by far the most
abundant xanthophyll-protein binding complex in the chloroplast. Detailed
biochemical and spectroscopic analysis has shown that the binding site L1 is
occupied by lutein in all Lhc proteins, site L2 can bind lutein or violaxan-
thin, site N1 is specific for neoxanthin, and site V1 bind violaxanthin [24-26].
Site L2 can exchange violaxanthin with zeaxanthin, produced in excess light,
and undergoes a conformational change, which increases heat dissipation and
decrease the lifetime of the
1
Chl
∗
-excited states [5, 6, 27]. As mentioned ear-
lier, xanthophylls have a light harvesting function whose importance can be
evaluated by spectroscopic methods. Figure 3.5 shows the deconvolution of
