C. Subunits of the LHCI Complex
Light-harvesting PSI complex has four distinct polypeptide components present in equimolar ratios [68].
All the proteins are products of nuclear genes (lhca1,lhca2,lhca3, and lhca4) with molecular masses of
22 kDa for Lcha1 and Lhca4, 23 kDa for Lhca2, and 25 kDa for Lhca3. The consensus membrane-fold-
ing pattern of all LHCI proteins revealed three transmembrane helices [69]. The polypeptides bind Chl a,
Chlb, and xanthophyll. In contrast to LHCII, proteins of LHCI are tightly bound to the PSI core complex,
forming dimers with two different emission maxima at 680 nm (Lhca2 and 3) and at 730 nm (Lhca1 and
4) [68]. The exact localization of LHCI on PSI is still not completely elucidated. More investigations are
also needed to determine the subunit composition and stoichiometry of the LCHI dimer as well as the
points of attachment of these dimers [68].
D. Electron Transport Within Photosystem I
Photosystem I functions as a plastocyanin:ferredoxin oxidoreductase. The primary electron donor of PSI
is P700, which is a chlorophyll adimer. Following excitation, P700 transfers an electron to the A 0 ac-
ceptor (chlorophyll amonomer) and is subsequently reduced by plastocyanin. The electron transport
within PSI is so fast that there is a little fluorescence from antenna chlorophyll at physiological tempera-
tures [5]. The next acceptor is A 1 , vitamin K 1 (phylloquinone); then the electron passes through the FX
electron acceptor, which is an iron-sulfur cluster (4Fe-4S), and by alternative working two iron-sulfur
clusters (4Fe-4S), FAand FB. The last step is reduction of ferredoxin docked on the stromal site of the
membrane [69]. Electrons from plastocyanin compensate the electron deficiency of P700. The plasto-
cyanin is a water-soluble 10.5-kDa protein and it is a mobile carrier of electrons between the two mem-
brane-embedded supramolecular complexes—cytochrome b 6 ƒ and PSI [4,81].
According to the available structural data, the existence of two probable electron transfer sites in
plastocyanin has been inferred. One site is the so-called hydrophobic patch around His87 (a copper-co-
ordinating residue on the “north pole” of the protein), which acts as the active site for redox interactions
with PSI [82]. The other one is referred to as the acidic patch around Tyr83 on the “east face” of the pro-
tein, which could act as the entry port of electrons coming from cytochrome ƒ [61]. PSI participates in
two types of electron transport, cyclic and noncyclic. It is generally thought that grana-localized PSI par-
ticipates in noncyclic electron transport from water via PSII, plastoquinone pull, cytochrome b 6 ƒ, and
plastocyanin to ferredoxin and consequently to NADPH. The stroma-lamellae–localized PSIs participate
in cyclic electron flow via reduced ferredoxin, cytochrome b 6 ƒ, plastocyanin, and back to P700 [4].
V. THE CHLOROPLAST ATP SYNTHASE
The chloroplast ATP synthase belongs to the family of F 1 -type adenosinetriphosphatases (ATPases),
which are also present in bacteria and mitochondria [83]. It generates ATP from ADP and inorganic phos-
phate (Pi) using energy delivered from a trans-thylakoid electrochemical proton gradient H[84].
Our view of the supramolecular structure of ATP synthase is based mainly on investigations of bac-
terial and mitochondrial ATP synthases. Because both the primary sequences of their subunits and their
functional characteristics are very conservative, it is reasonable to infer the supramolecular organization
of the chloroplast ATP synthase from data obtained with the enzyme from other sources [69].
The ATP synthase has long been described as the association of two distinct sectors: a membrane-
embedded CF 0 and a catalytic sector CF 1 located on the stromal surface of the thylakoid membranes. The
whole enzyme comprises nine subunits. The CF 0 is built up by four subunits (I, II, III, and IV) in an as-
sumed stoichiometry of 1:1(2):9–12:1. The CF 1 is built of five subunits: ,,^ ,, and in the stoi-
chiometry 3:3:1:1:1.
A. The CF 0 Supramolecular Organization
The most abundant subunit of CF 0 is subunit III. This is a small, hydrophobic (8 kDa), chloroplast-en-
coded (atpH) protein [69]. Based on the resolved structure of the bacterial homologue of subunit III, sub-
unit c, it was proposed that it has two membrane-traversing helices and a more polar loop region exposed
to the outer (stromal) site of the membrane [85]. At subunit c, Asp61 situated in the second transmem-
290 DENEV AND MINKOV