Biochemistry - Chemical Reactions in Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Other Types of Light-Induced Reactions
Phytochrome

Organisms respond to light in a great variety of ways [79]. Many species—ranging from Bacteria to higher plants—exhibit phototaxis, meaning they can move toward a light source or orient themselves in a specific way relative to it. In higher plants, METABOLISM/14.html">Chloroplasts are oriented to maximize Light absorption. As they grow, plants reach toward the light (phototaxis), whereas certain organisms, conversely, avoid illuminated areas. Chlorophyll synthesis in plants, much like tanning in humans, is driven by photochemical reactions. Circadian cycles in Living organisms are frequently established under The Influence of light, which also governs seed germination and the flowering of many plants.

In 1951, it was discovered that a brief exposure of dark-adapted plants to red light (with a maximum effect at λ = 660 nm) triggers a wide range of physiological responses [155, 155a], including flowering, seed germination (particularly in lettuce seeds), and leaf development in dark-grown pea seedlings. Of particular interest is the fact that the effect induced by a short flash of red light is completely reversed by a subsequent flash of far-red light (730 nm). This discovery led to the isolation in 1959 of the chromoprotein phytochrome—a molecular "switch" of sorts that initiates a cascade of far-reaching processes in plants [155–157]. The photoconversion of phytochrome is fully reversible [Scheme (13-37)], allowing the "switch" to be toggled back and forth multiple times using a series of rapidly alternating light flashes.

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Phytochrome has a Molecular Weight of ~120,000, but it can be cleaved by Proteolytic Enzymes into photosensitive fragments. Its chromophore was found to be an open-chain tetrapyrrole, structurally related to phycocyanobilin. The putative Structure OF THE tetrapyrrole is shown in Fig. 13-22, although the true structure and conformation of this chromophore remain unknown. If the structure depicted in the figure corresponds to the red light-sensitive form of phytochrome, sensitivity to far-red light may arise from the conversion described below:

A shift in the absorption maximum could then be caused by an increase in the degree of conjugation resulting from photoinduced isomerization. However, it has been demonstrated that conversion (13-37) is accompanied by The formation of intermediates. In the P_R→P_FR direction, a bleached form P_HL is produced, whereas the reverse pathway proceeds via the formation of another intermediate, A:

It has been suggested that the bleached intermediate forms only if the reaction involves cis-trans isomerization, and a structure for this compound has also been proposed [158]. It is possible, however, that no significant conformational changes occur and that light simply induces a rotation around a single bond in the open-chain tetrapyrrole molecule, leading to the Formation of the P_BL form.

Regardless of the Chemical Nature of the processes accompanying the structural Modification of the phytochrome-bound chromophore, the fundamental question remains: How is the biological response triggered? Here again, as in the case of rhodopsin, one can hypothesize that the photoreaction induces major conformational or chemical Changes in the protein, which in turn lead to various physiological responses. Slow responses to changes in phytochrome state may be linked to Gene Transcription processes.

At the same time, one of the processes controlled by phytochrome is the folding of Mimosa leaves upon darkness. The entire process takes place within 5 minutes—far too short a time for transcriptional control to operate. This fact, along with the observation that a fraction of phytochrome is tightly bound to membranes, suggests that the primary action of phytochrome involves altering membrane properties. Which form—P_R or P_FR—is responsible for this effect is not entirely clear, though P_FR appears to be the more probable candidate for the "active form." A recently proposed hypothesis suggests that plastid-bound phytochrome facilitates the release of Gibberellins sequestered within the Plastids [158a].