The gateway to fatty acid synthesis is generally considered the plastidial acetyl coenzyme A carboxylase (ACCase), which converts acetyl-CoA to malonyl-CoA. In all plants studied other than grasses, the plastidial form of the enzyme involved in fatty acid synthesis has four dissociable subunits. A biotin carboxylase subunit first affixes a carboxyl group to the biotin of a second subunit, biotin carboxyl carrier protein (BCCP), using bicarbonate and ATP as substrates. The resulting conformational change brings the biotin arm to a carboxyltransferase domain formed by the remaining two subunits, where the biotin donates the carboxyl group to acetyl-CoA (Cronan and Waldrop, 2002; Nikolau et al., 2003). Grass ACCases possess the same activities as the multisubunit form, but combine them into a multifunctional homodimer that is the primary target of herbicides targeting weedy grasses (Zagnitko et al., 2001).
ACCase is considered to be the rate-limiting step in fatty acid synthesis. The multisubunit ACCase is light-activated by reduction of the carboxyltransferase subunits via the thioredoxin pathway, and is subject to feedback inhibition by oleic acid (Kozaki et al., 2001; Shintani and Ohlrogge, 1995). Although the β-carboxyltransferase is plastid-encoded while the remaining subunits are imported to the plastids, all four subunits are normally coordinately expressed (Ke et al., 2000). Attempts to upregulate fatty acid synthesis by manipulating individual subunits of the heteromeric ACCase have had mixed results. Increased biotin carboxylase has little effect, and overexpression of BCCP actually decreased fatty acid synthesis, perhaps due to incorporation of unbiotinylated enzyme into ACCase (Shintani et al., 1997; Thelen and Ohlrogge, 2002). However, Madoka et al. reported that transformation of tobacco with the plastidial carboxyltransferase subunit raised overall yield of seed oil by increasing seed production, although oil per seed remains constant (Madoka et al., 2002). Alternatively, introduction of homomeric ACCase to rapeseed plastids increased ACCase activity and, to a lesser extent, seed oil (Roesler et al., 1997).
The availability of bicarbonate and particularly of acetyl-CoA for ACCase can also impact overall fatty acid synthesis. Reduced carbonic anhydrase activity inhibited fatty acid synthesis in cotton embryos, presumably by decreasing local bicarbonate supplies (Hoang and Chapman, 2002). The sources of acetyl-CoA for ACCase probably vary between tissues and stages of development. In castor seed endosperm, malate generated by a specific phosphoenolpyruvate carboxylase isoform appears to be the major source of carbon for fatty acids (Blonde and Plaxton, 2003). In rapeseed embryos, on the other hand, malate does not contribute significantly; instead, carbon flows primarily from glycolysis, entering the plastid via transporters for glucose-6-phosphate, dihydroxyacetone phosphate, and especially phosphoenolpyruvate (Kubis and Rawsthorne, 2000; Schwender and Ohlrogge, 2002). There is also potential for increasing flow of carbon into seed oil via alternative sources of acetyl-CoA. For example, introduction of ATP:citrate lyase from rat into tobacco plastids increased total leaf fatty acids 16% (Rangasamy and Ratledge, 2000).
© 2018 Biocyclopedia | All rights reserved.