脂肪酸从头合成翻译

Abbreviations: ACP, acyl carrier protein; BC, biotin carboxylase domain; BCCP, biotin carboxyl carrier protein; CT, carboxyl transferase domain; DAGAT, diacylglycerol acyltransferase; DAP, day after pollination; DW, dry weight; EM, embryo morphogenesis; ENR, enoyl-ACP reductase; EREBP, ethylene responsive element- binding protein; FAE, fatty acid elongase complex; FAS, fatty acid synthase; HD, hydroxyacyl-ACP dehydratase; HtACCase, heteromeric acetyl-Coenzyme A carboxylase; KAR, 3-ketoacyl-ACP reductase; KAS, 3-ketoacyl-ACP synthase; LPD, lipoamide dehydrogenase; MAT, malonyltransferase; OPPP, oxidative pentose phosphate pathway; PDC, pyruvate dehydrogenase complex; PDH, pyruvate dehydrogenase; PEP, phosphoenolpyruvate; PKp, plastidic pyruvate kinase; PUFA, polyunsaturated fatty acid; SSP, seed storage protein; RuBisCO, ribulose-1,5- bisphosphate carboxylase/oxygenase; TAG, triacylglycerol; VLCFA, very long-chain fatty acid.缩写:ACP、酰基载体蛋白;BC,生物素羧化酶域;BCCP,生物素羧基载体蛋白;CT, 羧基转移酶域;DAGAT, 二酰基甘油酰基转移酶,DAP,授粉后天数;DW,干重,EM,胚胎形态发生;ENR, 烯酰-ACP还原酶;EREBP, 乙烯反应元件结合蛋白;FAE,脂肪酸延长酶复合体;FAS,脂肪酸合成酶,HD, 羟烷基-ACP脱水酶;HtACCase,杂聚肽乙酰辅酶A羧化酶;KAR,3-酮脂酰-ACP还原酶;KAS,3-酮脂酰-ACP合成酶;LPD, 硫辛酰胺脱氢酶;MAT，丙二酰转移酶;OPPP,氧化磷酸戊糖途径;PDC,丙酮酸脱氢酶复合体;PDH，丙酮酸脱氢酶;PEP,磷酸烯醇丙酮酸;PKp,质体丙酮酸激酶;PUFA,多不饱和脂肪酸;SSP,种子贮藏蛋白; RuBisCO,核酮糖-1,5 -二磷酸羧化酶/加氧酶;TAG,三酰甘油;VLCFA，极长链脂肪酸。

So far, the small size of A. thaliana seeds has prevented to characterise in a detailed manner the organic compounds delivered to the maturing embryos. In seeds of the related B. napus species, these compounds mainly consist of sucrose, glucose, Gln, Glu, and Ala [21–23].Once transported into embryo cells, incoming sucrose can be cleaved via two distinct pathways involving either invertase or sucrose synthase [24–26]. Cleavage of sucrose generates hexose- phosphates that are metabolised through the oxidative pentose phosphate pathway (OPPP) and the glycolytic pathway, providing precursors for fatty acid production in the form of acetyl-CoA.Glycolysis is considered as the predominant pathway for the production of these precursors. In oilseeds, maturing embryos do have a complete glycolytic sequence in the cytosol and in plastids [27–29]. The extent to which both glycolytic sequences are utilised in the conversion of hexose-phosphates into precursors of fattyacid biosynthesis is still a matter of debate. Sets of ESTs from developing A. thaliana seeds [29] and microarrays displaying seed-expressed genes [30,31] have been produced.The data sets thus generated suggest that a major route for the metabolism of hexose-phosphates involves the cytosolic glycolytic pathway up to phospho- enolpyruvate (PEP), this compound being massively imported into the plastid before conversion to pyruvate [32]Nevertheless, when considering the substrate specificities of the two PEP transporters identified in A. thaliana, namely AtPPT1 and AtPPT2 [33], it is tempting to speculate that low amounts of 2-phosphoglycerate and 3-phosphoglycerate may also be transported into the plastids (Fig. 2).T he ADP-dependent conversion of PEP to pyruvate and ATP is catalysed by plastidic pyruvate kinase (PKp).The enzyme prevalent in the plastids of developing seeds likely has a subunit composition of 4a4b1 [34], and the characterisation of Pkp-a–Pkp- b1 double mutants producing wrinkled seeds severely depleted in storage lipids has further established the importance of the plastid route in the conversion of PEP into precursors of fattyacid synthesis [35].The irreversible oxidative decarboxylation of pyruvate to produce acetyl-CoA, NADH, and CO2 is catalysed by the plastidform of pyruvate dehydrogenase complex (PDC) [36,37].This multien- zyme complex contains three components, namely E1 (pyruvate dehydrogenase, PDH, composed of a and b subunits), E2 (dihy- drolipoyl acetyltransferase), and E3 (dihydrolipoamide dehydro- genase, LPD) [38–41]During the formation of acetyl-CoA by the PDC complex, one-third of the carbon of precursors for fatty acids is released as CO2. A recent steady-state carbon flux analysis on cultured embryos of B. napus has led to propose that ribulose-1,5- bisphosphate carboxylase/oxygenase (RuBisCO) is able to refix this CO2 in green maturing embryos, thus increasing the efficiency of carbon use [42].This metabolic rechannelling involves the non- oxidative reactions of the OPPP, phosphoribulokinase, and the subsequent conversion of ribulose-1,5-bisphosphate and CO2 to 3-phosphoglycerate by RuBisCO (Fig. 2). Whereas the PDH bypass provided by the aerobic fermentation pathway that converts pyruvate to acetate and probably to fatty acids is important in vegetative organs of A. thaliana during plant growth, this pathway does not seem to be essential during the synthesis of bulk seed TAGs [43].A multisubunit heteromeric acetyl-CoA carboxylase (HtACCase) is generally considered to catalyse the first committed step of the fatty acid biosynthetic pathway in A. thaliana [44]. HtACCase contains three functional domains and catalyses the ATP-depen- dent formation of malonyl-CoA from acetyl-CoA and bicarbonate [45]. This reaction actually takes place in two st eps [46]. In the first one, carboxylation of a biotin cofactor covalently bound to the central-biotin carboxyl carrier domain (BCCP) is catalysed by the biotin carboxylase domain (BC). In the second one, catalysed by the carboxyltransferase domain (CT), the carboxyl group from carboxy-biotin is transferred to acetyl-CoA to form malonyl-CoA. Malonyl-CoA constitutes the central carbon donor for fatty acid synthesis. However, the malonyl group has to be transferred from CoA to a protein cofactor, or acyl carrier protein (ACP) before entering the fatty acid synthesis pathway [44]. This reaction is catalysed bya malonyl-CoA: acyl carrier protein malonyltransferase (MAT). Fatty acids are produced from acetyl-CoA and malonyl-ACP; acetyl-CoA is used as the starting unit and malonyl-CoA as the elongator.The malonyl-thioester enters into a series of condensa- tion reactions with acetyl-CoA, then acyl-ACP acceptors. Three separate condensing enzymes, or 3-ketoacyl-ACP synthases (KASI– III) are necessary for the production of an 18-carbon fatty acid. The initial condensation reaction is catalysed by KASIII, KASI is responsible for producing chain lengths from 6 to 16 carbons, and KASII finally elongates 16:0-ACP to 18:0-ACP [47]. Three additional reactions are required after each condensation step to obtain a saturated fatty acid that is two carbons longer than at the start of the cycle. These reactions are catalysed by 3-ketoacyl-ACP reduc- tase (KAR), 3-hydroxyacyl-ACP dehydratase (HD), and enoyl-ACP reductase (ENR) [48]The monofunctional enzymes involved in de novo fatty acid biosynthesis form an easily dissociable multisubunit complex referred to as fatty acid synthase (FAS) [49]. Some 16:0- ACP is released from the FAS, whereas molecules that are elongated to 18:0-ACP a re efficiently desaturated by a stromal stearoyl-ACP desaturase (SAD) [50]. The synthesis of long-chain fatty acids in the plastids is terminated when the acyl group is removed from ACP by specific acyl-ACP thioesterases (FAT) [51,52]. Once activated to CoA esters and transported to the ER, these fatty acid species can be modified (desaturated, elongated) in the so-called eukaryotic pathway. The formation of VLCFAs is catalysed by the fatty acid elongase (FAE) complex. This membrane-bound multienzyme complex involves four enzymatic reactions catalysed by 3-ketoacyl- CoA synthase (FAE1), 3-ketoacyl-CoA reductase, 3-hydroxyacyl- CoA dehydrase, and enoyl-CoA reductase [53–58]. Cycles of fatty acid elongation in the ER are very analogous to denovo fatty acid synthesis occurring in the plastids. The cytosolic malonyl-CoA species used for VLCFA biosynthesis are derived from citrate through the sequential action of A TP-citrate lyase [59] and homo- meric acetyl-CoA carboxylase [60–63 Interestingly, it has been shown in maturing embryos of rapeseed that mitochondrial metabolism is mostly devoted to cytosolic fatty acid elongation: flux around the TCA cycle is absent and mitochondrial substrate oxidation contributes little to ATP production while all the citrate formed is exported toward the cytosol and used for the production of malonyl-CoA [23]. TAG assembly can be considered as proceeding by distinct routes (reviewed in [64]). One of these, the Kennedy pathway, is relatively straightforward and relies on the sequential acylation of a glycerol-3-phosphate backbone by glyc- erol-3-phosphate acyltransferase, lysophosphatidic acid acyl- transferase, and diacylglycerol acyltransferase (DAGAT) [65–67]. TAGs are ultimately stored in oil bodies. Whereas the exact mech- anism of the biogenesis of these organelles remains a matter of debate [68,69], recent proteomic efforts have provided a detailed characterisation of the set of proteins embedded in the half membrane unit of oil bodies [70]This proteome includes, among other proteins, a steroleosin (HSD1) [71,72] and five distinct oleo- sins (S1–5) that cover much of the oil-body surface, determine the size of oil bodies, and prevent their coalescence as they are forced into contact with each other in the periphery of embryonic cells [73].到目前为止，A.南芥种子规模较小，无法将以有机化合物传递到成熟胚胎的详细方式定位。