Glycerol, and phospholipid

Complex lipids, such as neutral fats (triacyl-glycerols), phospholipids, and glycolipids, are synthesized via common reaction pathways. Most of the enzymes involved are associated with the membranes of the smooth endoplasmic reticulum. 

Phosphatidic acids not only are intennediates in the biosynthesis of triacylglycerols but also are biosynthetic precursors of other members of a group of compounds called phosphoglycerides or glycerol phosphatides. Phosphorus-containing derivatives of lipids are known as phospholipids, and phosphoglycerides are one type of phospholipid. 

Triacylglycerols and some phosphoglycerols are synthesized by progressive acylation of glycerol 3-phosphate. The pathway bifurcates at phosphatidate, forming inositol phospholipids and cardiolipin on the one hand and triacylglycerol and choline and ethanolamine phospholipids on the other. 

When oxidized, 1 g of fat yields 9 kcal. Because of the caloric contribution from egg phospholipid and glycerol, caloric content of IVFE is 1.1 kcal/mL for the 10%, 2 kcal/mL for the 20%, and 3 kcal/mL for the 30% emulsions. 

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. 

General physiological roles for fatty acids in cellular lipids are caloric storage, membrane fluidity, and prostaglandin precursors. The first of these mainly involved the formation and hydrolysis of triacyl glycerols, transport and activation of non-esterified fatty acids, and other steps leading to energy conversion (110). The second role primarily involves activation and incorporation into 1- and 2- positions of different phospholipids which form a major part of membranes. The third role is linked to the requirement for certain unsaturated fatty acids in the diets of most animals (110). 

PA is the precursor of many other phosphoglycerides. The steps in its synthesis from glycerol phosphate and two fatty acyl CoAs were illustrated in Figure 16.14, p. 187, in which PA is shown as a precursor of triacylglycerol. [Note Essentially all cells except mature ery-. throcytes can synthesize phospholipids, whereas triacylglycerol synthesis occurs essentially only in liver, adipose tissue, lactating mammary glands, and intestinal mucosal cells.]... 

Two main types of lipids occur in biological membranes phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters of the three-carbon alcohol, glycerol. A typical structure is that of phosphatidylcholine (lecithin) ... 

In some cases the functions of phospholipases in cells are purely degradative and result in the release of the phospholipid components (fatty acids, glycerol, phosphate, and head-groups). But in many cases phospholipases have important roles in synthesis and regulation. For example, we have seen how phospholipase A2 catalyzes the first step in the remodeling of phosphatidylcholine to the surfactant... 

Since the synthesis of glycolipids involves the synthesis of both the lipid and the oligosaccharide portions, we shall also discuss that part of the recorded lipid synthetic work which has involved the use of carbohydrates. In fact, some of the first-recorded applications of carbohydrate molecules as chiral templates are to be found in the lipid field particularly with the use of 1,2 5,6-di-O-isopropylidene-D-mannitol [33] as a precursor of chiral glycerol derivatives for the synthesis of phospholipids and glycolipids based on glycerol and with the use of glucosamine derivatives for the synthesis of phytosphingosines [34] and since this area has not previously been reviewed, it will be treated with a more historical perspective. 

Lipids are of special concern in comparing plastic metabolism between the two groups of fish. Triacyl-glycerols, cholesterol ethers and non-esterified fatty acids, which are the direct sources of energy, have already been discussed in the previous section. We now turn to phospholipids and cholesterol, which are essential to the structure of cell membranes, and to polyunsaturated fatty acids, which determine to a large extent the functional activity of these membranes. 

Recruitment of a species depends on the abundance of the brood. Abundance, in its turn, is influenced by the fertility of the population and by the quality of the reproductive products. Such quality comprises specific features of the genome and the content of plastic (proteins, phospholipids and cholesterol) and energy (triacyl-glycerols, wax esters and glycogen)... 

The Atlantic cod offers an example (in a non-fatty fish) of extreme proportions of phospholipids and triacyl-glycerols. As much as 88% of the lipids of the white muscle (total lipid content 0.5% to 0.6%) consists of phospholipids and only about 1% is triacyl-glycerols. When the fish are starved, the minute proportion of triacyl-glycerols does not change, so in this situation a... 

---