The synthesis of triacyl glycerol in developing oilseeds is generally divided into four stages.
Stage I:
Synthesis of saturated acyl-ACP by stepwise elongation of acetyl-ACP to steoryl- ACP CoA residue is then condensed to the malonyl ACP, forming acetoacetate ACP. Reduction and dehydration is followed by a second reduction to form acyl-ACP.
Each of these reactions is catalysed by type I fatty acid synthase (Type I FAS), oleolyl ACP (18: 1 ACP) is synthesised from 18: 0 ACP by acyl ACP A 9 desaturase. The enzyme complex FAS is responsible for the synthesis of fatty acids like laurate (12), myristate (14), palmitate (C16), stearate (C18) and cleate (C18: 1).
Fatty acid elongation (acyl ACP) in the plant involves four reactions. The first condensation reaction between malonyl ACP and acetyl CoA is catalysed by ketoacyl synthase (XAS III). Condensation with malonyl ACP and butanoyl ACP (C4) catalysed by enzyme KAS II.
In addition, elongation of oleic acid (C18) to erucic acid (C22) is a major malonyl CoA dependent pathway in some oil seeds and their elongation takes place outside plastid (Fig. 17.1).
Stage II:
Fatty acid moieties are released into the cytoplasm where they are hydrolysed to free acids by acyl-ACP thioesterases. The specificity of the acyl-ACP thioesterase present in the developing seed determines the chain length of fatty acids released from acyl ACP elongation step.
Stage III:
Released fatty acid in cytoplasm undergoes modifications such as elongation, introduction of double bonds (desaturation and hydroxylation).
Stage IV:
Fatty acids are finally transported to the endoplasmic reticulum. Once enter inside ER, it is combined with glycerol-3. phosphate to form phosphatidic acid and subsequently forms triacyl glyceride. Incorporation of acyl-CoA into triacyl glycerol is carried out by acyl- CoA acyl transferase. The triacyl glycerate is then stored in lipid body.
In summarising the process, fatty acid biosynthesis takes place in the stroma of the plastids. Most of the modifications of the fatty acids take place in the cytoplasm as well as on the ER. Thus, the participation of cell compartments with the enzymes is crucial in the biosynthesis of plant oils. The triacyl glycerids are stored in oil bodies which are essentially oil droplets surrounded by a lipid monolayer.
During fatty acid synthesis in most oil producing crops-stepwise addition of 2-carbon units at the carbonyl end stops at the 16-carbon stage. The enzyme specific synthase can elongate palmitoyl ACP (16: 0) to steroyl ACP (18: 0). To produce a 16 or 18 carbon fatty acid from acetyl CoA and melonyl CoA at least 30 enzymatic reactions are required.
Further, conversion of steroyl ACP to oleolyl ACP by ∆ 9 steroyl desaturase enzyme due to the addition of double bonds. The process of fatty acid synthase once terminates are then exported into cytoplasm is converted to acyl-CoA esters.
These are then exported to endoplasmic reticulum, where stepwise acylation of glycerol-3 phosphate and further modifications also takes place including attachment of additional double bonds before trans triacyl glyc- erides (TAG). Fig. (17.2).
