at the extrinsic CF 1 portion [94]. The hypothetical mechanisms of ATP synthesis are proposed on the base
of data derived from investigations of ATP synthases from different sources.
Proton transport in CF 0 is supposedly mediated by the 12 copies of the proton-binding subunit III
(subunit c), which are arranged as a ring [69,84]. The association and dissociation of protons take place
at the strictly conserved carboxyl residues on subunit III (its localization was discussed before). Rotation
of the ring is driven by proton binding to the residue via a thylakoid lumen inlet channel supposedly
formed by subunit a (IV) [69,84]. The protonated binding site then moves from the stator interface to the
lipid phase of the membrane, where after 12 steps it reaches an outlet channel with access to the cyto-
plasmic, CF 1 binding side of the membrane. Agr210 on transmembrane helix 4 of subunit a (bacterial ho-
mologue of CF 0 subunit IV) is proposed to promote proton release to the outlet channel [84].
The andsubunits are proposed to remain fixed to the top of the subunit c (III) ring so that rota-
tion of the ring also drives rotation of the subunit within the 3 3 subunits of CF 1.
Although the detailed structures of the ring and stator portion of CF 0 (F 0 ) are under debate [95], there
is agreement that the translocation of one proton drives the ring and therefore the subunit around by
30° [89,96]. After four of these steps, i.e., after a turn of 120°, a newly synthesized ATP molecule is re-
leased from a binding site on CF 1 [94,95]. The subunit acts as an elastic element like a cylindrical tor-
sional bar that rotates eccentrically within the 3 3 subunits, causing conformational changes of the ac-
tive centers.
At any time all three pairs display different conformational stages, representing open (O), loosely
closed (L), and tightly closed (T) binding sites [94]. Substrate exchange with the medium is practically
restricted to the open site with competitive binding between ATP and ADP or Pi and random binding of
ADP and Pi. The rotation of the unit 120° within the 3 3 hexameric ring drives the concerted binding
change O →L→T→O. Conversion of ADP and Pi into ATP and H 2 O is a couplet with the L →T tran-
sition [89,94].
VI. THE LIPID MATRIX OF THE THYLAKOID MEMBRANE
Thylakoid lipids form about 50% of the mass of membranes and act as a fluid matrix for the functional
supramolecular complexes. The fatty acid tails form the hydrophobic, central core of the membrane and
hydrophilic heads of lipids are situated at the surface. The lipids are not equally distributed between the
two monolayers as well as in the lateral direction [97]. Because the thylakoid lipids are highly unsatu-
rated, the membranes are very fluid at physiological temperatures. Fluidity allows high lateral mobility
of pigment-protein complexes through the membranes. However, because of the high contents of proteins
(50% of the mass) the diffusion coefficient of individual molecules is limited to 10^10 –10^9 m^2 sec^1
[4,98].
Thylakoid lipids are a complex mixture containing about 80% galactolipids such as monogalacto-
syldiacyl glycerol (50 mol% total lipids) and digalactosyldiacyl glycerol (25 mol%), which are electri-
cally neutral. The remainder is mainly phosphatidylglycerol (10–15 mol%) and sulfoquinovosyl giacil-
glycerol (5–10 mol%), charged under physiological pH [97,99]. The chloroplast lipids are highly
unsaturated; the predominant fatty acid is linolenic (C18:3). However C16:3 fatty acids are also present
in some groups of plants [100]. A fatty acid specific to the thylakoid membranes is trans-3-hexadecanoic
acid (C16:1). It is a component of phosphatidylglycerol [97,99,100].
VII. TOPOLOGY OF THYLAKOID MEMBRANE
All photosynthetic organisms house an elaborately folded network of thylakoid membranes that convert
solar energy into biochemically useful forms [5]. In higher plants the continuous thylakoid membrane net-
work is differentiated into regular domains of closely appressed membranes in granal stacks that are in-
terconnected by nonappressed single membranes—the stroma thylakoids (Figure 6) [4,5]. The grana it-
self consists of an appressed central core domain, a curved domain such as the margins of grana lamellae,
and two end membranes, the outer membranes on terminal grana lamellae [4]. The stroma thylakoids are
planar membranes, although some authors distinguish the necklike connections between grana and stroma
lamellae as a separate domain [101]. All these domains differ in their enrichment of supramolecular com-
plexes and functions [4]. According to earlier models, PSII complexes are concentrated in grana when
292 DENEV AND MINKOV