Chlorophyll degradation pathways

FIGURE 4.1.1 Possible chlorophyll degradation pathways in plant tissues or in processed foods. Pheophorbide a monooxygenase is specific for pheophorbide a. RCC = red chlorophyll catabolite. FCC = fluorescent chlorophyll catabolite. NCC = non-fluorescent chlorophyll catabolite. 

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. 

Takamiya, K., Tsuchiya, T., and Ohta, H., Degradation pathway(s) of chlorophyll What has gene cloning revealed Trends Plant Sci., 5, 426, 2000. 

In addition to those reactions described above, it appears that chlorophyll can be degraded by yet another pathway. Chichester and McFeeters (1971) reported on chlorophyll degradation in frozen beans, which they related to fat peroxidation. In this reaction, lipoxidase may play a role, and no... 

However, this accumulation has not been unequivocally proven. The recent identihcation of urobilinogenoidic linear tetrapyrroles in extracts from primary leaves of barley indicated that further degradation of the v-NCC 1 can take place. While the monoxygenation of pheophorbide a in the earlier phases of chlorophyll breakdown in higher plants appears to be a remarkably stringent entry point, the rather diverse structures of NCCs may indicate that the later phases of the detoxi-hcation process follow less strictly regulated pathways." ... 

A small proportion of our diet consists of very-long-chain fatty acids (20 or more carbons) or branched-chain fatty acids arising from degradative products of chlorophyll. Very-long-chain fatty acid synthesis also occurs within the body, especially in cells of the brain and nervous system, which incorporate them into the sphin-golipids of myelin. These fatty acids are oxidized by peroxisomal p- and a-oxidation pathways, which are essentially chain-shortening pathways. 

Two of the most common branched-chain fatty acids in the diet are phytanic acid and pristanic acid, which are degradation products of chlorophyll and thus are consumed in green vegetables (Fig.23.15). Animals do not synthesize branched-chain fatty acids. These two multi-methylated fatty acids are oxidized in peroxisomes to the level of a branched C8 fatty acid, which is then transferred to mitochondria. The pathway thus is similar to that for the oxidation of straight very-long-chain fatty acids. 

The non-specific lipoxygenases can cooxidize carotenoids and chlorophyll and thus can degrade these pigments to colorless products. This property is utilized in flour bleaching (cf. 15.4.1.4.3). The involvement of LOX in cooxidation reactions can be explained by the possibility that the peroxy radicals are not as rapidly and fully converted to their hydroperoxides as in the case of specifically reacting enzymes. Thus, a fraction of the free peroxy radicals are released by the enzyme. It can abstract an H-atom either from the unsaturated fatty acid present (pathway 2a in Fig. 3.30) or from a polyene (pathway 2b in Fig. 3.30). 

---