115
Anatomy
As already said, the eyespot consists of a loose collection of globules situated on the dorsal side
of the reservoir, always in front of the photoreceptor. The pigments present in the eyespot globules
are carotenoids such as β-carotene, diatoxanthin, and diadinoxanthin. The absorption spectrum of
the eyespot shows a unique and large band centered at 460 nm (Figure 2.78, orange line).
Euglena photoreceptor is located near the base of the locomotory flagellum. Its regular structure has already been described as “Type I” crystal, that is, an ordered assemblage of stacked
membranes formed by 2D crystals of membrane proteins (Figure 2.73a). The spectral properties
of this photoreceptor are quite more complex than those of the eyespot and support the presence of
rhodopsin-like proteins. These proteins are characterized by optical bistability, that is, they possess
two isomeric forms A and B, which interconvert along a photocycling path photochemically but not
thermically. The absorption spectrum of A has a band centered at 498 nm (from now addressed as
A 498 ) (Figure 2.78, green line); this is the dominant form in the photoreceptor under physiological
conditions. The absorption spectrum of B has a band centered at 462 nm (from now addressed as
B 462 ) (Figure 2.78, blue line). The absorption spectrum of the eyespot perfectly matches the absorption spectrum of the B 462 form (Figure 2.78, orange line). A 498 is the nonfluorescent form (hence,
under physiological conditions, photoreceptor fluorescence is not observed), while B 462 is the fluorescent form, energetically lower, which can be considered the signaling state of the protein. The
presence of optically bistable proteins characterized by a fluorescent form is a common feature of
euglenoid photoreceptors. Figure 2.79 shows the photocycle recorded on a single cell of Euglena,
Phacus, and Trachelomonas. Optical bistability has recently been detected also in Chlamydomonas
rhodopsin, indicating that this could be a property of the photoreceptive protein present in the algae.
The functioning of Euglena photoreceptor in nature can be described as in the following: the
cells normally swim by rotating along a helicoidally path; during this motion, the photoreceptor
proteins are in a photodynamic equilibrium in which A 498 is the dominant isomer. The equilibrium
is interrupted when the eyespot comes between the incoming light and the photoreceptor, thus
screening the organelle (Figure 2.80). Owing to the superimposition of the absorption spectra of
the eyespot and the isomer form B 462 , only UV and green light illuminate the photoreceptor during
the screening period, and the only possible transition is that of A 498 to B 462 . In detail, the photoisomerizable isomeric form A 498 undergoes an intramolecular photoswitch, that is, A 498 becomes
B 462 through an excited state, and contemporaneously an intermolecular and unidirectional Forstertype energy transfer (FRET) occurs, that is, the newly formed fluorescent B 462 acts as an energy
donor for the nearby protein in the A 498 form, which acts as an energy acceptor. The intramolecular
1.0
0.8
0.6
0.4
Normalized absorbance
0.2
0.0
400
450
500
550
Wavelength (nm)
600
FIGURE 2.78 Absorption spectrum of eyespot of Euglena gracilis (orange line); absorption spectra of the A
isomer (green line) and of B isomer (blue line).
Anatomy
As already said, the eyespot consists of a loose collection of globules situated on the dorsal side
of the reservoir, always in front of the photoreceptor. The pigments present in the eyespot globules
are carotenoids such as β-carotene, diatoxanthin, and diadinoxanthin. The absorption spectrum of
the eyespot shows a unique and large band centered at 460 nm (Figure 2.78, orange line).
Euglena photoreceptor is located near the base of the locomotory flagellum. Its regular structure has already been described as “Type I” crystal, that is, an ordered assemblage of stacked
membranes formed by 2D crystals of membrane proteins (Figure 2.73a). The spectral properties
of this photoreceptor are quite more complex than those of the eyespot and support the presence of
rhodopsin-like proteins. These proteins are characterized by optical bistability, that is, they possess
two isomeric forms A and B, which interconvert along a photocycling path photochemically but not
thermically. The absorption spectrum of A has a band centered at 498 nm (from now addressed as
A 498 ) (Figure 2.78, green line); this is the dominant form in the photoreceptor under physiological
conditions. The absorption spectrum of B has a band centered at 462 nm (from now addressed as
B 462 ) (Figure 2.78, blue line). The absorption spectrum of the eyespot perfectly matches the absorption spectrum of the B 462 form (Figure 2.78, orange line). A 498 is the nonfluorescent form (hence,
under physiological conditions, photoreceptor fluorescence is not observed), while B 462 is the fluorescent form, energetically lower, which can be considered the signaling state of the protein. The
presence of optically bistable proteins characterized by a fluorescent form is a common feature of
euglenoid photoreceptors. Figure 2.79 shows the photocycle recorded on a single cell of Euglena,
Phacus, and Trachelomonas. Optical bistability has recently been detected also in Chlamydomonas
rhodopsin, indicating that this could be a property of the photoreceptive protein present in the algae.
The functioning of Euglena photoreceptor in nature can be described as in the following: the
cells normally swim by rotating along a helicoidally path; during this motion, the photoreceptor
proteins are in a photodynamic equilibrium in which A 498 is the dominant isomer. The equilibrium
is interrupted when the eyespot comes between the incoming light and the photoreceptor, thus
screening the organelle (Figure 2.80). Owing to the superimposition of the absorption spectra of
the eyespot and the isomer form B 462 , only UV and green light illuminate the photoreceptor during
the screening period, and the only possible transition is that of A 498 to B 462 . In detail, the photoisomerizable isomeric form A 498 undergoes an intramolecular photoswitch, that is, A 498 becomes
B 462 through an excited state, and contemporaneously an intermolecular and unidirectional Forstertype energy transfer (FRET) occurs, that is, the newly formed fluorescent B 462 acts as an energy
donor for the nearby protein in the A 498 form, which acts as an energy acceptor. The intramolecular
1.0
0.8
0.6
0.4
Normalized absorbance
0.2
0.0
400
450
500
550
Wavelength (nm)
600
FIGURE 2.78 Absorption spectrum of eyespot of Euglena gracilis (orange line); absorption spectra of the A
isomer (green line) and of B isomer (blue line).