L-acyl-glycerol-3-phosphate

Lecithins and related phospholipids usually contain a saturated fatty acid in the C-l position but an unsaturated acid, which may contain from one to four double bonds, at C-2. Arachidonic acid is often present here. Hydrolysis of the ester linkage at C-2 yields a l-acyl-3-phosphoglycerol, better known as a Iysophosphatidylcholine. The name comes from the powerful detergent action of these substances which leads to lysis of cells. Some snake venoms contain phospholipases that form Iysophosphatidylcholine. Lysophosphatidic acid (l-acyl-glycerol-3-phosphate) is both an intermediate in phospholipid biosynthesis (Chapter 21) and also a signaling molecule released into the bloodstream by activated platelets.15... 

Triacylglycerols arise not by acylation of glycerol itself but by a sequence of steps m which the first stage is acyl transfer to l glycerol 3 phosphate (from reduction of dihy droxyacetone 3 phosphate formed as described m Section 25 21) The product of this stage IS called a phosphatidic acid... 

Step 3 of Figure 29.3 Alcohol Oxidation The /3-hydroxyacyl CoA from step 2 is oxidized to a /3-ketoacyl CoA in a reaction catalyzed by one of a family of L-3-hydroxyacyl-CoA dehydrogenases, which differ in substrate specificity according to the chain length of the acyl group. As in the oxidation of sn-glycerol 3-phosphate to dihydroxyacetone phosphate mentioned at the end of Section 29.2, this alcohol oxidation requires NAD+ as a coenzyme and yields reduced NADH/H+ as by-product. Deprotonation of the hydroxyl group is carried out by a histidine residue at the active site. 

Figure 12.11 Phosphoglyceride structure. The members of this group are derivatives of the parent compound, l,2-diacyl-src-glycerol-3-phosphate (phosphatidic acid) in which X is a hydrogen atom. This is replaced by either an amino alcohol or a polyhydroxy residue. In phosphoglycerides derived from animal tissues R1 is usually a saturated acyl chain of between 16 and 20 carbon atoms and R2 is usually unsaturated. Polyunsaturated acyl chains containing 16 or 18 carbon atoms predominate in leaf phosphoglycerides and those of bacterial origin are often more complex.

This enzyme catalyzes the reaction of an acyl-CoA derivative with l-acyl-vn-glycerol 3-phosphate to generate coenzyme A and l,2-diacyl-5 n-glycerol 3-phosphate. The animal enzyme is reported to be specific for the transfer of unsaturated fatty acyl groups. Interestingly, the acyl-[acyl-carrier-protein] can also act as an acyl donor. 

This enzyme [EC 2.3.1.15] catalyzes the reaction of an acyl-CoA with xn-glycerol 3-phosphate to produce coenzyme A and a l-acyl-xn-glycerol 3-phosphate. The acyl-CoA derivatives contain an acyl group with a chain length of at least ten carbon atoms. In addition, the acyl-CoA can be substituted by an acyl-[acyl-carrier protein] derivative. 

This enzyme [EC 3.1.4.39], also known as alkylglycero-phosphoethanolamine phosphodiesterase, catalyzes the hydrolysis of l-alkyl-sn-glycero-3-phosphoethanolamine to produce 1-alkyl-xn-glycerol 3-phosphate and ethanol-amine. The enzyme will also act on the acyl and choline analogs of the lipid. 

The first steps of glycerophospholipid synthesis are shared with the pathway to triacylglycerols (Fig. 21-17) two fatty acyl groups are esterified to C-l and C-2 of L-glycerol 3-phosphate to form phosphatidic acid. Commonly but not invariably, the fatty acid at C-l is saturated and that at C-2 is unsaturated. A second route to phosphatidic acid is the phosphorylation of a diacyl-glycerol by a specific kinase. 

Fig. 5. Pathway depicting how flux through phosphatidylcholine (product of reaction 3) can promote acyl group diversity in plant triacylglycerols. Production of 18 2 (boxed) at the sn-2 position and its transfer to TG is used as a sample modification. Other fatty acid alterations may be substituted. Enzymes 1, glycerol-3-phosphate acyl-CoA acyltransferase and lysophosphatidic acid acyl-CoA acyltransferase 2, phosphatidic acid phosphatase 3, diacylglyceroliCDP-aminoalcohol aminoalcoholphosphotransferase 4, 18 l-desaturase or other fatty acid modifying enzyme 5, phosphlipid diacylglycerol acyltransferase 6, diacylglycerol acyltransferase 7, acyl-CoA phosphatidylcholine acyltransferase or phospholipase plus acyl-CoA synthetase.

DHAP can be diverted from its precursor role in ether lipid synthesis by conversion to jn-glycerol-3-phosphate by NADH glycerol-3-phosphate dehydrogenase. Another bypass that prevents the formation of alkyl-DHAP occurs if the ketone function of acyl-DHAP is first reduced by an NADPH-dependent oxidoreductase, since the product, l-acyl-2-lyso-in-glycerol-3-phosphate, can then be converted only to diacylglycerolipids. The metabolic removal and/or formation of fatty alcohols, DHAP, or acyl-DHAP from the ether lipid precursor pool may control ether lipid synthesis and accumulation. 

A solution of l-pahnitoyl-2-stearoyl-3-myristoylglycerol and phosphatidic acid (glycerol 3-phosphate with two fatty acyl moieties, attached to C-2 and C-3) dissolved in benzene is shaken with an equal volume of water. After the two phases separate, which Upid will be in the higher concentration in the aqueous phase, and why ... 

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