Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 60

Chlorophyll a is found in all known eukaryotic photosynthetic organisms. Among prokaryotes, it is found in large quantities only in the cyanobacteria (including the prochlorophytes), although traces of chlorophyll a or minor variants are found in some anoxygenic bacteria, where it is thought to have an important function as an intermediate in the electron transport chain. Some prochlorophytes contain divinyl chlorophyll a, in which the substituent at the C‐8 position on ring B is vinyl instead of ethyl.

Table 4.1 Distribution of chlorophylls and bacteriochlorophylls

Type of organism	Chl a	Chl b	Chl c	Chl d,f	BChl a	BChl b	BChl c,d,e	BChl g	Carotenoids	Bilins
Purple bacteria					+a	+a			+
Green sulfur bacteria					+		+		+
Filamentous anoxygenic phototrophs					+		+		+
Heliobacteria								+	+
Cyanobacteria	+	+b	+b	+b					+	+
Green algae	+	+							+
Diatoms	+		+						+
Brown algae	+		+						+
Dinoflagellates	+		+						+
Cryptomonads	+		+						+	+
Red algae	+								+	+
Plants	+	+							+

^a Purple a or b, but not both in the same species.

^b Most cyanobacteria contain Chl a as their only chlorophyll‐type pigment. Prochlorophytes contain in addition Chl b. Some types also contain Chl c, d, or f.

An important variant of chlorophyll a is chlorophyll a′. This pigment differs from chlorophyll a only in the stereochemistry at the C‐13² position. It is found in small but reproducible amounts in photosystem I complexes, where one molecule forms half of P700, the special pair of pigments that is the primary electron donor (see Chapter 7). The spectral and redox properties of chlorophyll a′ are very similar to those of chlorophyll a. Current evidence suggests that chlorophyll a′ is made from pre‐existing chlorophyll a, although the putative C‐13² invertase enzyme has not been identified.