Wax Esters

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These esters of long-chain fatty acids and long-chain fatty alcohols provide energy sources, as an alternative to TAG, in some animals, plants and micro-organisms.

Adipocytes And Lipoprotein Lipase

Fig. 3.12 The co-ordination of triacylglycerol synthesis and breakdown in adipose tissue. In the fed state, dietary fatty acids are delivered as chylomicron-triacylglycerol, which is hydrolysed by lipoprotein lipase (LPL) in adipose tissue capillaries to liberate fatty acids, that may be taken up into adipocytes and esterified for storage as triacylglycerol. Glycerol 3-phosphate for this process is provided by glycolysis, following glucose uptake by the insulin-regulated glucose transporter GLUT4. High circulating glucose concentrations in the fed state stimulate pancreatic secretion of insulin, which stimulates all aspects of the pathway of triacylglycerol storage via the cell-surface insulin receptor. In the fasted state or during exercise, low insulin concentrations remove the suppression of hormone-sensitive lipase (HSL), which may be stimulated by catecholamines (adrenaline and noradrenaline) acting through cell-surface P-adrenergic receptors. This process delivers non-esterified fatty acids to the circulation, for use as a fuel by other tissues including liver and skeletal muscle.

Fig. 3.12 The co-ordination of triacylglycerol synthesis and breakdown in adipose tissue. In the fed state, dietary fatty acids are delivered as chylomicron-triacylglycerol, which is hydrolysed by lipoprotein lipase (LPL) in adipose tissue capillaries to liberate fatty acids, that may be taken up into adipocytes and esterified for storage as triacylglycerol. Glycerol 3-phosphate for this process is provided by glycolysis, following glucose uptake by the insulin-regulated glucose transporter GLUT4. High circulating glucose concentrations in the fed state stimulate pancreatic secretion of insulin, which stimulates all aspects of the pathway of triacylglycerol storage via the cell-surface insulin receptor. In the fasted state or during exercise, low insulin concentrations remove the suppression of hormone-sensitive lipase (HSL), which may be stimulated by catecholamines (adrenaline and noradrenaline) acting through cell-surface P-adrenergic receptors. This process delivers non-esterified fatty acids to the circulation, for use as a fuel by other tissues including liver and skeletal muscle.

3.7.1 Occurrence and characteristics

Wax esters have the general formula R1COOR2 and are esters of long-chain fatty acids (R1COOH) with long-chain fatty alcohols (R2OH). They occur in some species of bacteria, notably the mycobacteria and corynebacteria. However, they are mainly important for providing a form of energy reserve that is an alternative to triacylglycerols in certain seed oils (e.g. jojoba), the oil of the sperm whale, the flesh oils of several deep-sea fish (e.g. orange roughy) and in zooplankton. Thus, a major part of jojoba oil consists of wax esters, 70% of sperm whale oil and as much as 95% of orange roughy oil. In global terms, however, the zooplankton of the oceans may be of greatest importance. At certain stages of their life cycles, the zooplankton may synthesize and store massive amounts of wax esters, as much as 70% of their dry weight.

The component fatty acids and alcohols of wax esters have an even number of between 10 and 30 carbon atoms. Normally, they are straight-chain saturated or monounsaturated chains of 16C-24C. Branched and odd-numbered chains are rare in both constituents except for the bacterial waxes. Wax esters from cold-water organisms exist as oils down to about 0oC and their physico-chemical properties differ from those of the TAG in that they are more hydrophobic and less dense, which may help to provide greater buoyancy for marine animals.

3.7.2 Biosynthesis of wax esters involves the condensation of a long-chain fatty alcohol with fatty acyl-CoA

It is concluded that zooplankton synthesize their own wax esters de novo since their primary food, the phytoplankton, are devoid of these lipids. Indeed, all marine organisms so far studied, mammals and fish as well as crustaceans, can synthesize wax esters de novo.

The fatty acyl components of wax esters are synthesized by the malonyl-CoA pathway (Section 2.2.3.4), although they can also be derived from dietary lipids. Because wax esters are often characterized by very long chain acyl groups, further elongation of the products of the fatty acid syn-thetase may be necessary, as described in Section 2.2.3.4. Waxes sometimes contain one or more keto groups that have escaped removal at the condensation stage of fatty acid synthesis (Section 2.2.3.2).

The fatty alcohol components are formed from acyl-CoAs by reduction, first by acyl-CoA reduc-tase to form a fatty aldehyde, and then by an aldehyde reductase to form the alcohol (Fig. 3.13). The enzymes are membrane-bound and require NADPH in the reduction; the aldehyde is normally a transient intermediate and does not accumulate. The final esterification is catalysed by acyl-CoA alcohol transacylase (Fig. 3.13).

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