Fatty acid biosynthesis dissociable/dissociated enzymes

The activities involved in yeast fatty acid biosynthesis are covalently linked as separate domains of two multifunctional polypeptides, a and p, encoded by the fas2 and fasl genes, respectively (Fig. 2) [57,58]. The functionalities associated with the 220 kDa a subunit include -ketoacyl synthase activity, -ketoacyl reductase activity, and an AGP domain which bears a phosphopantetheinylated serine. The 208 kDa -subunit has acetyl and malonyl CoA transacylase, palmi-toyl transferase, -hydroxyacyl-enzyme dehydratase, and enoyl acyl-enzyme reductase activities. The two subunits can be readily dissociated, and the individual activities maybe measured [57]. 

The de novo biosynthesis of fatty acids in plants occurs in the plastid and is cataiysed by a type II, dissociable fatty acid synthetase(FAS). Enoyl-ACP reductase (ER) catalyses the second reductive step in the synthetic cycle, converting a trans 2,3 moiety into a saturated acyl chain. The protein from B. napus has been purified[1], extensively sequenced[2] and the absolute mass of the enzyme in developing rape seed determined[3]. The cDNA which encodes ER has been cloned and fully sequenced Southern hybrisization with this cDNA shows the presence of four genes in B. napu A]. This study investigates the expression of ER mRNA and the steady-state levels of ER protein isoforms by Northern and two-dimensional Western analyses. 

The biosynthesis of fatty acids in plants Is catalysed by a type II, dissociable, fatty acid synthetase (FAS) made up of at least six catalytic polypeptides and a central, acyl carrier protein (ACP). There are three p Ketoacyl-ACP synthetases (KAS) KAS 1, catalysing the initial reaction between acetyl ACP and malonyl ACP KAS 2, catalysing the addition of acetyl ACP to the elongating fatty acid chain and the recently identified KAS 3. A further enzyme implicated in the initiation of FAS is acetyl CoA ACP transacylase (AC AT) [3]. AC AT catalyses the formation of acetyl ACP. KAS 3, however does not utilise acetyl ACP, instead the condensation occurs between malonyl-ACP and acetyl CoA. There has recently been much discussion about the possible roles of KAS 3 and ACAT which appear to have partially overlapping functions KAS 3 activity has been purified to homogeneity from spinach [1] and E. coli [4]. In both the purified enzyme has the ability to load an acetyl group from acetyl CoA, ACAT activity. Purification of ACAT from E. coli has been reported [3], but as this work predates the discovery of KAS 3 it is not clear whether this activity is resolvable from KAS 3. There is still confusion as to whether plants posses distinct ACAT activity since the assays that have been used for both enzymes contain ACP. 

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