Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 68

Bacteriochlorophylls c, d, e, and f will be considered as a group, because they are found only in green photosynthetic bacteria, organisms that contain the antenna complex known as a chlorosome. They are also unusual among chlorophylls in that they are invariably found as complex mixtures of closely related compounds instead of as a single compound of unique structure. Several distinct structural features are found in these pigments, whose structures are shown in Fig. 4.4. Ring B contains a C‐7–C‐8 double bond, as in chlorophylls, making these pigments chlorins instead of bacteriochlorins. They also have a hydroxyethyl substituent at the C‐3¹ position in ring A. This functional group is essential to the aggregation of these pigments in the chlorosome, which will be discussed in Chapter 5. The C‐3¹ carbon is chiral, and both R and S diastereomers are found in cells (the C‐17 and C‐18 chiral carbons are stereochemically pure). These pigments also have hydrogens at the C‐13² position, instead of the bulky carboxymethyl substituent found in all other chlorophylls. This change allows the chlorin rings to pack together more closely. These pigments are structurally programmed for aggregation, and indeed, in the chlorosome, they are found as large oligomeric complexes with little protein.

The differences among the bacteriochlorophylls c, d, e, and f occur primarily in the C‐20 methine bridge position, where bacteriochlorophylls c and e have a methyl substituent, and at the C‐7 position, where bacteriochlorophylls e and f have a formyl substituent, like chlorophyll b. These changes tune the light absorption properties of these pigments, with the wavelength of maximum absorption decreasing as one goes from bacteriochlorophyll c to f. Other differences are found at the C‐8 and C‐12 positions, where a complex variety of substituents can occur, even in a single organism. The tails of these bacteriochlorophylls are also different from those of most other chlorophylls. The bacteriochlorophylls c, d, and e found in the green sulfur bacteria contain a farnesol tail instead of a phytol. This is one isoprene unit shorter than phytol. The filamentous anoxygenic phototrophic bacteria, which contain only bacteriochlorophyll c (as well as bacteriochlorophyll a) primarily utilize the 18‐carbon straight‐chain stearol substituent, although a variety of other tails are found in varying amounts.

Bacteriochlorophyll f has never been found in nature. The compound that is known as bacteriochlorophyll f has the C‐7 formyl substituent of bacteriochlorophyll e, as well as the C‐20 H of bacteriochlorophyll d. It is thus the logical completion of this set of pigments. However, organisms that contain bacteriochlorophyll f have been created by inactivating the methylase enzyme that adds the methyl group to C‐20 (Vogl et al., 2012).