Compound phosphatidate is demonstrated

The different parts of phosphoglycerides - fatty acids, phosphate and base - are capable of turning over independently. Thus, to study phospholipid synthesis we must first learn the origin of each constituent and then how they are welded together. Eighteen years after Hevesy's demonstration of the rapid rate of phospholipid turnover, came the first real understanding as to how complete phospholi-pids are built up.

Two American biochemists, Kornberg and Pricer, found that a cell-free enzyme preparation from liver would activate fatty acids by forming their coenzyme A esters. They then went on to demonstrate that these activated fatty acids could be used by an acyl transferase to esterify 3-sn-glycerol phosphate forming l,2-diacyl-3-sn-glycerol phosphate (phosphatidate). We now know that there are two distinct acyl-CoA:glycerol phosphate-O-acyl-transferases, specific for positions l and 2 (Section 3.4.1). Furthermore, organisms containing the Type II fatty acid synthases (Section 2.2.3.2), which form acyl-ACP products, use these as substrates for the acyltransferases. Such organisms are bacteria like Escherichia coli, cyanobacteria, algae and higher plants. In fact, plant cells produce phosphatidate within their plastids using acyl-ACPs but also contain acyl-CoA:glycerol phosphate acyl-transferases on the endoplasmic reticulum for the synthesis of extra-chloroplast lipids such as phos-phatidylcholines or triacylglycerols.

Phosphatidate is the parent molecule for all gly-cerophospholipids and was at first thought not to be a normal constituent of tissue lipids. Later studies have shown it to be widely distributed, but in small amounts. In fact, it is very important for the concentration of phosphatidate to be carefully regulated because of its function as a signalling molecule (Section 7.11).

Thus glycerol phosphate is one of the building units for phospholipid biosynthesis. It is mainly derived from the glycolytic pathway by reduction of dihydroxyacetone phosphate, though other methods are used to various extents by different organisms or tissues. Likewise, phosphatidate can also be produced by direct phosphorylation of diacylglycerol using diacylglycerol kinase.

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