1.D.22 The Proton-Translocating Carotenoid Pigment, Zeaxanthin (Zeaxanthin) Family
The polar carotenoid pigment zeaxanthin (beta,beta-carotene-3,3'-diol), incorporated into planar lipid membranes formed with diphytanoyl phosphatidylcholine, increases the specific electric resistance of the membrane. Zeaxanthin incorporated into the lipid membrane at 1 mol% has a very small effect on the overall membrane resistance but facilitates equilibration of the transmembrane proton gradient. The equilibration process may be associated with a symport/antiport activity or with a transmembrane transfer of the molecules of acid (Kupisz et al., 2008). Zeaxanthin spans the membrane and tends to have its two hydroxyl groups anchored in the opposite polar zones of the membrane. The polyene chains of carotenoids may be able to form weak hydrogen bonds with water molecules. Possible molecular mechanisms responsible for proton transport by polyenes are discussed, including direct involvement of the polyene chain in proton transfer and indirect effects of the pigment on the physical properties of the membrane (Kupisz et al., 2008).
The reaction catalyzed by zeaxanthin is:
H+ (out) ⇌ H+ (in)