Plastid-associated endoplasmic reticulum

For the sake of study, the biosynthesis of carotenoid plant pigments can be divided into parts involving enzymes and their associated activities as listed in Table 5.3.1 and further detailed in Figure 5.3.1 through Figure 5.3.4. Some of the parts have common enzymatic mechanisms and may also be in distinct subcellular compartments such as cytoplasm, endoplasmic reticulum, or plastid thylakoid. 

Further desaturation and acyl chain elongation of fatty acids are generally accepted to involve cytosolic membranous systems associated with the endoplasmic reticulum (Stumpf, 1989). A critical importance, therefore, would appear to exist for a detailed understanding of the mechanics by which the fatty acids are exported from the plastid. At present, there is little available information on the precise transport system or its regulation. Although fatty acids are presumed to be released on hydrolysis of acyl-ACPs by specific thioesterases, it is not known whether an initial formation of acyl-CoA is required prior to transport across the plastidic membrane in association with carnitine or some other system. A direct transacylation between ACP and CoA or some carrier system would be less expensive energetically than a process involving hydrolysis and synthesis. 

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