PtdEtn and PtdCho are synthesised through the CDP-aminoalcohol pathway beginning with the activation of ethanolamine to phosphoethanolamine (P-Etn, Fig. 2), which forms the branch point between synthesis of the two lipids. The pathway from P-Etn to PtdEtn is direct via CDP-Etn (Fig. 2) but the pathways for PtdCho synthesis in plants vary depending on the tissue (Kinney 1993). The committed step of PtdCho synthesis is considered to be the methylation of P-Etn catalysed by P-Etn methyltransferase (Fig. 2, step 3) and the phospho-base methylation route via P-Cho and CDP-Cho is thought to carry the majority of the flux, at least in leaves (McNeil et al. 2000). PtdEtn is not itself methylated in plants but PtdmonomethylEtn (PtdMME) and PtddimethylEtn (PtdDME) may be methylated to form PtdCho (Fig. 2). Methylation of the free ethanolamine base has only been reported in endosperm of castor (Prudhomme and Moore 1992). The synthesis of P-Cho from P-Etn can be catalysed by a single phosphoethanolamine
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