Chlorophyll phytol chain

A hydrophobic phytol chain, which keeps the chlorophyll molecule embedded in the photosynthetic membrane. 

One of the simplest and most significant of the diterpenes is phytol, a reduced form of geranylgeraniol, which constitutes the lipophilic side-chain of the chlorophylls. Phytol also forms a part of vitamin E (tocopherols) and K molecules. Vitamin A is also a 20-carbon-containing compound, and can be regarded as a diterpene. However, vitamin A is formed from a cleavage of a tetraterpene. Among the medicinally important diterpenes, paclitaxel, isolated from Taxus brevifolia (family Taxaceae), is one of the most successful anticancer drugs of modern time. 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

Figure 9.37 Chemical structures of chlorophylls-a and b which contain a propionic acid esterified to a C20 phytol chlorophylls-cj and C2 have an acrylic acid that replaces the propionic acid. Also included are the pheopigments, the four dominant tetrapyrrole derivatives of chloropigments (pheopigments) found in marine and fresh-water/estuarine systems (chlorophyllide, pheophorbide, pheophytin, pyropheophorbide.) More specifically, chlorophyllase-mediated de-esterification reactions (loss of the phytol) of chlorophyll yield chlorophyllides. Pheophytins can be formed when the Mg is lost from the chlorophyll center. Pheophorbides are formed from removal of the Mg from chlorophyllide or removal of the phytol chain from pheophytin, and pyrolyzed pheopigments, such as pyropheophorbide and pyropheophytin, are formed by removal of the methylcarboxylate group (-COOCH3) on the isocylic ring from the C-13 propionic acid group.

Figure 19.5. Chlorophyll. Like heme, chlorophyll a is a cyclic tetrapyrrole. One of the pyrrole rings (shown in red) is reduced. A phytol chain (shown in green) is connected by an ester linkage. Magnesium ion binds at the center of the structure.

In 27 the carotenoid moiety is linked through an amide function which is derived from the carboxylic acid which bore the phytol chain of the original chlorophyll. There are a total of three saturated carbon atoms separating the n-electron systems. Although this structure could allow a folded con-... 

Very recently it was demonstrated that tocopherol moieties in kerogen are likely precursors of prist-l-ene (Figure 7) (30). This idea was supported by the fact that tocopherols are widely distributed in photosynthetic tissues and that they also occur as such in several recent sediments (31). It is tempting to conclude that during "natural pyrolysis" the generated pristene will be transformed to the well known component, pristane, in ancient sediments and oils. This example nicely illustrates that we have to be very careful when we conclude that acyclic isoprenoid hydrocarbons such as pristane originate from the chlorophyll side chain, phytol, based solely on structural similarities. 

Chlorophylls a and b are found in almost all photosynthesizing organisms. They possess a complex cyclic structure (called a porphyrin) with a magnesium atom at its center. Chlorophyll a possesses a methyl group attached to ring II of the porphyrin, whereas chlorophyll b has an aldehyde group attached to the same site. Pheophytin a is similar in its structure to chlorophyll a. The magnesium atom is replaced by 2 protons. Chlorophylls a and b and pheophytin a all possess a phytol chain esterified to the porphyrin. The phytol chain extends into and anchors the molecule to the membrane. Lutein and /5-carotene are the most abundant carotenoids in thylakoid membranes. 

Microbial degradation of isoprenoids in the rumen of ruminants, for example of the chlorophyll side chain formed by (2 ,3,7i ,lli ,15)-3,7,ll,15-tetramethylhexadec-2-en-l-ol (phytol), yields some more branched acids, such as (2RS,6R,10R)-tetramethylpentadecanoic (pristanic) or (3i S,7R,llR,15)-3,7,ll,... 

Green coloration, present in many vegetable oils, poses a particular problem in oil extracted from immature or damaged soybeans. Chlorophyll is the compound responsible for this defect. StmcturaHy, chlorophyll is composed of a porphyrin ring system, in which magnesium is the central metal atom, and a phytol side chain which imparts a hydrophobic character to the stmcture. Conventional bleaching clays are not as effective for removal of chlorophylls as for red pigments, and specialized acid-activated adsorbents or carbon are required. 

Although /3-oxidation is universally important, there are some instances in which it cannot operate effectively. For example, branched-chain fatty acids with alkyl branches at odd-numbered carbons are not effective substrates for /3-oxidation. For such species, a-oxidation is a useful alternative. Consider phy-tol, a breakdown product of chlorophyll that occurs in the fat of ruminant animals such as sheep and cows and also in dairy products. Ruminants oxidize phytol to phytanic acid, and digestion of phytanic acid in dairy products is thus an important dietary consideration for humans. The methyl group at C-3 will block /3-oxidation, but, as shown in Figure 24.26, phytanic acid a-hydroxylase places an —OFI group at the a-carbon, and phytanic acid a-oxidase decar-boxylates it to yield pristanie add. The CoA ester of this metabolite can undergo /3-oxidation in the normal manner. The terminal product, isobutyryl-CoA, can be sent into the TCA cycle by conversion to succinyl-CoA. 

From these stractural features it is interesting to note that each molecule of chlorophylls a and b consists of a hydrophilic part (tetrapyrrole macrocycle) and a hydrophobic portion (long terpenoid chain of phytol esterifying the acid group at C-17). Figure 2.1.2 shows the structures and nomenclature of chlorophylls a and b and their major breakdown derivatives. 

In plant plastids, GGPP is formed from products of glycolysis and is eight enzymatic steps away from central glucose metabolism. The MEP pathway (reviewed in recent literature - ) operates in plastids in plants and is a preferred source (non-mevalonate) of phosphate-activated prenyl units (IPPs) for plastid iso-prenoid accumulation, such as the phytol tail of chlorophyll, the backbones of carotenoids, and the cores of monoterpenes such as menthol, hnalool, and iridoids, diterpenes such as taxadiene, and the side chains of bioactive prenylated terpenophe-nolics such as humulone, lupulone, and xanthohumol. The mevalonic pathway to IPP that operates in the cytoplasm is the source of the carbon chains in isoprenes such as the polyisoprene, rubber, and the sesquiterpenes such as caryophyllene. 

The isoprenoid hycrocarbons pristane and phytane (derived from the phytol side chain of chlorophyll), as well as porphyrins, have been detected in organic extracts of the Nonesuch Shale of 1.1 billion year age [23]. Their presence points to the existence of photosynthetic pigments in the Precambrian era, but it is also possible that these extractable substances could have been contributed to the rock at a later time. However, in this instance contamination appears to be less likely on account of the large abundance of organic material in this shale. 

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