Composition of Thylakoid Membranes Lipids, Proteins, and Electron Carriers

Composition of Thylakoid Membranes - Lipids, Proteins and Electron Carriers 

In this section we will discuss the three major classes of the various constituents of the thylakoid membrane, namely lipids, proteins and electron carriers. All the essential constituents are listed in Tables 1 to 5. Since most ofthese constituents or complexes will be discussed in more detail in later chapters, the tables simply serve as a convenient listing ofthese components for easy reference. A new volume by Siegenthaler and Murata should be consulted for the structure and function of lipids. 

The galactolipids are known to be distributed asymmetrically in the lipid bilayer - about 60% of the galactoUpids are present in the outer leaflets and 40% in the inner leaflets of the thylakoid-membrane bilayer. The strong interaction between the head-groups of the galactolipid molecules determine then-packing properties and enhance the stability of the membrane. Formation of the bilayer structure of the thylakoid membrane also depends on the presence of proteins. Another characteristic of the thylakoid lipids is their high content ofthe trienoic acid, (C18 3) a-linolenic acid, which contributes to the fluidity of the membrane, necessary for the diffusion of lipophilic compounds such as plastoquinone, and the lateral diffusion of protein complexes. 

Protein comvlexes The presence of protein complexes in the thylakoid membrane has been deduced from freeze-fracture electron microscopy as well as from biochemical studies. Each of the five major complexes contains multiple proteins and prosthetic groups. The number of proteins range from three in the LHC 11 complex to five or more complexes in cytochrome fes/ nine in the ATP-synthase complex and a little over 20 each in the PS-11 and PS-1 complexes. Many of the protein components will be discussed in detail in later chapters. For the present, we merely list the currently known protein components ofthe five complexes in Tables 2 to 5. The common names, molecular mass in kilodaltons, encoding genes, and some remarks about their functions or factors are also included in the tables. In addition, 

Most proteins in the PS-11 complex are membrane spanning, but the three extrinsic proteins that are involved in oxygen evolution are located on the lumenal side of the thylakoid membrane. Although most proteins in the PS-1 complex are also membrane-spanning, a few are located toward the stromal side of the thylakoid membrane, e.g., the iron-sulfur proteins that contain FeS-A and FeS-B and the Fd-docking protein. On the other hand, some mobile electron-transfer proteins are present on one side or the other of the membrane, with Fd present on the stromal side and near the ferredoxin-NADP -reductase (FNR) and the FNR-binding protein, and the copper-protein electron carrier, plastocyanin (PCy), present on the lumenal side, close to the PS-1 primary donor P700. 

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