Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
A number of carriers take part in photosynthetic electron transport

When the reaction center of Photosystem I, which is a complex of a chlorophyll a molecule and a specific protein, is raised to an excited state by light quanta received from antenna molecules, the absorbance of METABOLISM/14.html">Chloroplasts in the 700 nm region decreases. For this reason, the reaction center of photosystem I is conventionally designated as P700 (Fig. 23-12). The primary electron carrier in the transport chain leading from P700 to NADP⁺ is believed to be an iron-sulfur protein designated as P430. The secondary electron carrier is another iron-sulfur protein, ferredoxin. Spinach leaf ferredoxin, which has been obtained in crystalline form, has a Molecular Weight of approximately 10,700; its molecule contains two iron atoms bound to two acid-labile sulfur atoms. The iron atoms present in P430 and ferredoxin mediate a single-electron transfer coupled to the reversible valence change Fe(II)—Fe(III).

The Role of the third electron carrier is played by a flavoprotein called ferredoxin-NADP⁺ oxidoreductase. It transfers electrons from reduced ferredoxin (Fdred) to NADP⁺, reducing the latter to NADPH.

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In addition, there is another electron carrier chain through which electrons move "downhill" from the excited reaction center of Photosystem II to the electron holes of photosystem I (Fig. 23-12). In its oxidized state, the reaction center of photosystem II is characterized by an absorption maximum at 680 nm (hence its designation P680). We know very little about its nature; it is thought to be similar to the P700 reaction center of photosystem I, meaning it is also a chlorophyll-protein complex. Little is known about The Nature of the first electron carrier in this chain either; it is commonly designated as Z. From the reduced form of Z, electrons are passed "downhill" to plastoquinone, or PQ (Fig. 23-13), a lipid-soluble quinone with a long isoprenoid side chain, reminiscent of ubiquinone in the mitochondrial Respiratory Chain. The reduced form of plastoquinone, in turn, transfers electrons to a type b cytochrome known as cytochrome b₅₆₃. From there, they pass to cytochrome f (from the Latin frons, meaning leaf), which is closely related to mitochondrial cytochrome c. Cytochrome f transfers electrons to plastocyanin, a copper-containing blue protein. The actual electron carrier is the copper atom of this protein, which undergoes a reversible valence change of Cu(I)—Cu(II). Plastocyanin serves as the direct electron donor to the electron holes in the P700 reaction center of photosystem I.

Fig. 23-13 Plastoquinone A is the most abundant plastoquinone in Higher Plants and Algae. Other plastoquinones differ in the length of their side chain and the Nature of the substituents on the quinone ring. Plastol is the reduced form of plastoquinone.

The electron holes in the P680 reaction center of photosystem II are filled by electrons stripped from Water by an Mn²⁺-containing enzyme complex called H₂O dehydrogenase, which is still poorly understood.

It is worth emphasizing one very important point here. The entire sequence of reactions depicted in Fig. 23-12 is collectively termed the "light reactions" of Photosynthesis. This designation is convenient because it clearly distinguishes the energy-generating phase of photosynthesis from the dark reactions responsible for the reduction of CO₂ to glucose. However, the term "light reactions" is not entirely accurate. In reality, light is required for only two steps of these "light reactions," namely those that excite the two photochemical reaction centers (Fig. 23-12). Once electrons have absorbed light energy and been raised to a higher energy level, all subsequent steps of photosynthetic electron transport can proceed perfectly well in the dark.

Therefore, the term "light reactions" should always be used with a full understanding of the nature of the process involved.