1-Acylglycerol 3-phosphate

A second esterification of this type leads to a phosphatidate (enzyme l-acylglycerol-3-phosphate acyltransferase 2.3.1.51). Unsaturated acyl residues, particularly oleic acid, are usually incorporated at C-2 of the glycerol. Phosphatidates (anions of phosphatidic acids) are the key molecules in the biosynthesis of fats, phospholipids, and glycolipids. 

Addition of -OH to the fatty acyl CoA, followed by loss of -SCoA from the tetrahedral intermediate, produces 1-acylglycerol 3-phosphate. 

The first acylation, catalyzed by acyl-coenzyme A-glycerol-3-phos-phate acyl transferase (GPAT), results in the formation of 1-acyl-glycer 01-3-phosphate (lysophosphatide, LPA). The LPA serves as the substrate for the second acylation reaction, catalyzed by acyl-CoA-l-acylglycerol-3-phosphate transferase (AGPAT), to form phosphatidic acid (PA). 

Hares, W. and Frentzen, M. (1991) Substrate specificities of the membrane-bound and partially purified microsomal acyl-CoA l-acylglycerol-3-phosphate acyltransferase from etiolated shoots of Pisum sativum (L.) Planta 185, 124-iM. 

Glycerol 3-phosphate acyl transferase (G3P-AT) and 1-acylglycerol 3-phosphate acyl transferase (LPA-AT), which catalyse the first two steps in course of de novo biosynthesis of glycerolipids, are known enzymic activities not only of plastids and microsomes (Frentzen 1986) but also of mitochondria from higher plants (Douce 1971, Vick and Beevers 1977, Sparace and Moore 1979). In mitochondria of castor bean endosperm the two enzymic activities were localized in both the inner and outer membrane (Sparace and Moore 1979), but little is known about the enzymic properties. 

Acyl-(acyl-carrier protein) glycerol-3-phosphate acyl transferase (glycero-P acyltransferasc) in higher-plant chloroplasts transfers the acyl group from acyl-(acyl-carrier protein) to the C-1 position of glycerol 3-phosphate to synthesize 1-acylglycerol 3-phosphate. Since this reaction is the first step of glycerolipid synthesis in the chloroplasts, it is of special interest to study this enzyme. 

The reactions indicated by broken arrows have not yet been experimentally demonstrated. Gly-3-P, glyoerol-3-phosphate LPA, l-acylglycerol-3-phosphate PA, phosphatidic acid DG, 1,2-diacylglycerol ACP, acyl-carrier protein X, unidentified galactose carrier. 

Fig. 1. Functional expression of the cDNA from Limnanthes douglasii in E. coli JC201. Activities of 1-acylglycerol-3-phosphate acyltransferase were determined with either oleoyl-CoA (light bars) or erucoyl-CoA (dark bars) in membrane fractions of JC201 cells harbouring the vector pQE60 or the construct pQEL21, respectively, as described before [5].

In summary, we succeeded in the isolation of a cDNA from Limnanthes douglasii which encodes an erucoyl-CoA specific l-acylglycerol-3-phosphate acyltransferase. The deduced amino acid sequence shows substantial sequence similarity to the respective acyltransferase of E. coli. Consequently, the isolated cDNA is suitable for transforming rape-seed with the aim to increase the content of erucic acid in the fatty acid composition of rapeseed oil and, thus, to improve its industrial applicability. 

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