Stereochemistry of the acyl dihydroxyacetone phosphate acyl exchange reaction

Precursor of ether phospholipids is synthesized by a flavoenzyme through covalent catalysis

The precursor of the essential ether phospholipids is synthesized by a peroxisomal enzyme that uses a flavin cofactor to catalyze a reaction that does not alter the redox state of the substrates. The enzyme crystal structure reveals a V-shaped active site with a narrow constriction in front of the prosthetic group. Mutations causing inborn ether phospholipid deficiency, a very severe genetic disease, target residues that are part of the catalytic center. Biochemical analysis using substrate and flavin analogs, absorbance spectroscopy, mutagenesis, and mass spectrometry provide compelling evidence supporting an unusual mechanism of covalent catalysis. The flavin functions as a chemical trap that promotes exchange of an acyl with an alkyl group, generating the characteristic ether bond. Structural comparisons show that the covalent versus noncovalent mechanistic distinction in flavoenzyme catalysis and evolution relies on subtle factors rather than on gross modifications of the cofactor environment.

Facile syntheses of acyl dihydroxyacetone phosphates and lysophosphatidic acids having different acyl groups

In this study, we report novel and simple chemical syntheses of acyl dihydroxyacetone phosphate (DHAP) and 1-acyl glycero-3-phosphate [lysophosphatidic acid (LPA)], key intermediaries in the formation of glycerolipids containing ester and ether bonds. The synthesis of acyl DHAPs involved acylating the dimethyl ketal of DHAP by acid anhydride using 4-pyrrolidinopyridine as the catalyst, and the resulting product was deketalized by HClO(4) in acetone to produce acyl DHAP. The acid anhydride was either added directly or generated in the reaction mixture from the corresponding fatty acid using dicyclohexylcarbodiimide as the condensing agent. Using these methods, a number of acyl DHAPs having short-, medium-, and long-chain saturated and unsaturated acyl groups were synthesized, with overall yields from 37% to 75%. The activities of these acyl DHAPs as substrates for guinea pig liver peroxisomal acyl DHAP:NADPH reductase and alkyl DHAP synthase were then determined. Next, starting from these acyl DHAPs, a variety of LPAs were synthesized by chemical reduction of the ketone group. Biological activities of these LPAs were determined by measuring their relative abilities to release intracellular Ca(2+) via the LPA receptor. A combined chemical-enzymatic method is also described to prepare the natural LPA from the racemic mixture.

Alkyl-dihydroxyacetonephosphate synthase

Mammalian ether phospholipids are characterized by a glycero-ether linkage at the sn-1-position of the glycerol backbone. In humans this type of phospholipid species occurs mainly in the ethanolamine and choline phosphoglycerides comprising an estimated 15% of total phospholipids. The glycero-ether linkage is synthesized by replacement of the acyl chain in acyl-dihydroxyacetonephosphate by a long-chain alcohol that donates the oxygen for the ether linkage. Both the enzyme that forms acyl-dihydroxyacetone phosphate (see Chapter II of this volume) and the one that introduces the glycero-ether linkage. i.e. alkyl-dihydroxyacetonephosphate synthase, are located in peroxisomes. The deficiency of ether phospholipids in human inborn errors of metabolism, caused by defects in peroxisome biogenesis, has clearly delineated the indispensable role of peroxisomes in ether phospholipid synthesis. The most characteristic enzyme of ether lipid synthesis is alkyl-dihydroxyacetonephosphate synthase. Its discovery and some of its properties, including mechanistic studies, have been discussed in recent reviews. This review recapitulates these findings and focuses on the new insights into the structure and properties of the enzyme that have recently been obtained resulting from the purification and subsequent cloning and expression of the cDNA encoding this peroxisomal enzyme.