Biosynthesis of Mid-Chain Oxygenated Suberin Monomers

The mid-chain oxygenated fatty acids that are found in some suberin polymers are generated by the biosynthetic pathways shown in Fig. 6.4.10. Although these pathways have been demonstrated by studies with cell-free preparations of cutin-synthesizing systems, similar reactions are most probably involved in the biosynthesis of suberin monomers. 

One of the most commonly found mid-chain hydroxylated components is dihydroxyhexadecanoic acid, which has hydroxyl moieties at C-10, C-9, C-8 or C-7, and on C-16 (232, 243, 244). A crude cell-free preparation from excised epidermis of V. faba catalyzed C-10 hydroxylation of 16-hydroxyhexadecanoic acid (473). This hydroxylation reaction was also catalyzed by the endoplasmic reticulum fraction from the embryonic shoots of K faba. This preparation required O2 and NADPH to catalyze mid-chain hydroxylation and the activity was inhibited by the typical mixed-function oxidase inhibitors and also by CO (427). The inhibition by CO was photoreversible, as expected of a cytochrome P450 hydroxylase. 

Based on labeling studies, the members of the Cig family of mid-chain oxygenated acids were postulated to be derived from cu-hydroxyoctadecanoic acid as shown in Fig. 6.4.10 (255). The observed direct conversion of 18-hydroxy[18- H]-octadecanoic acid into 9,10-epoxy-18-hydroxyoctadecanoic acid and 9,10,18-tri-hydroxyoctadecanoic acid in plant tissue supported this predicted pathway (89). A 3000-g particulate fraction from a cell-free preparation of Spinacia oleracea leaves catalyzed the formation of c/5-9,10-epoxy-18-hydroxyoctadecanoic acid from 18-hydroxy[18- H]octadecanoic acid (90). This epoxidation reaction re- 

18-Trihydroxyoctadecanoic acid was generated from 9,10-epoxy-18-hy-droxyoctadecanoic acid by a 3000-g particulate fraction obtained from the skin of the young fruit of Malus pumila (91). This enzyme activity did not require any cofactors and showed fairly stringent substrate specificity. It catalyzed the hydration of the m-epoxide to the /ireo-product and this stereospecificity is consistent with the natural occurrence of the /Areo-product in cutin. The trihydroxy Cig acid thus generated probably is the biosynthetic precursor of 9,10-dihy-droxyoctadecanedioic acid, which has been found in several suberin polymers (244). 

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