Chlorophyll branch

Growing point sub-apical. Contains no chlorophyll. Branches arranged irregularly. 

The steps successive to protoporphyrin are unique to each branch of the pathway. Those unique steps were classified into the siroheme branch, the heme branch that includes phyrochromobilin, the chlorophyll branch, and the chlorophyll cycle that refers to the interconversion steps of chlorophyll a and b. The chlorophyll cycle was put into a separate category because this cycle is regulated by a distinct mechanism from those of that control the other branches. Moreover, the chlorophyll cycle has a unique feature distinct from the other branches. Not only is the chlorophyll cycle a part of the biosynthetic pathway but the latter two reactions of the cycle are also part of the chlorophyll degradation pathway. 

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. 

The biosynthesis of the tetrapyrrole macrocycle and its branches leading to haem and chlorophylls has been covered in detail in several reviews - - and will be concisely described in this section. Tetrapyrrole biosynthesis occurs entirely in the plastids and is composed of several enzymatic steps starting from 5-aminolevulinic acid (ALA), which is the key precursor of porphyrins and the source of their carbon and nitrogen. 

Up to this point, the biosynthesis steps are identical for both chlorophyll and haem, bnt depending on which metal is inserted in the center of the porphyrin, the pathway branches to form one or another. The insertion of iron is followed by... 

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

Phytene is an example of a diterpene. It is found as the phytyl side chain in chlorophyll a and vitamin K. Haslene is an example of a sesterpene. It is an imsaturated and branched simple lipid synthesized by marine pennate diatoms. One of the largest femilies of terpenes... 

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

Heme b is utilized for formation of hemoglobin, myoglobin, and many enzymes. It reacts with appropriate protein precursors to form the cytochromes c. Heme b is converted by prenylation to heme o405 and by prenylation and oxidation to heme fl.405a The porphyrin biosynthetic pathway also has a number of branches that lead to formation of corrins, chlorins, and chlorophylls as shown schematically in Fig. 24-22. 

These substances that are classified under the above definition of oil and grease belong to both the biological lipids and the petroleum hydrocarbons and include straight chain and branched hydrocarbons, fatty acids, and the esters of fatty acids. Certain organic dyes, sulfur compounds, and chlorophyll are also extracted, and contribute to the measurement. 

The earliest steps (MVA to GGPP) for polyisoprenoid biosynthesis are identical for all plants and animals (12,13). They involve the well-known diterpene pathway, MVA — MVAP — MVAPP — IPP + DMAPP — GPP — FPP — GGPP. The enzymes catalyzing these steps have been studied extensively, especially from animals (liver) and yeast, and to a more limited extent from higher plants. In some cases the enzymes have been purified to homogeneity most have been only partially purified. In both plants and animals a major branch at FPP leads to the production of squalene and the steroids. In plants, three major branches occur at GGPP, of which one leads to the carotenoids via phyto-ene, a second to the phytyl group of chlorophyll, and a third to the GAs. 

Milk fat contains small quantities of phytanic and pristanic acid. Phytanic acid is produced by bacterial cleavage of the phytol side chain of plant chlorophyll in the rumen. Some phytanic acid is converted to pristanic acid by a-oxidation in the liver. Both of these branched-chain acids are agonists for PPARa at physiological concentrations (Parodi, 2004). Milk fat from cows fed cannery fruit and vegetable waste no doubt contains other interesting phytochemicals with anti-cancer potential. 

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