Breakdown of fat in adipocytes requires catalytic action of three enzymes, hormone sensitive triacylglycerol lipase (called LIPE) to remove the first fatty acid from the fat, diglyceride lipase to remove the second one and monoglyceride lipase to remove the third. Of these, only LIPE is regulated and it appears to be the rate limiting reaction. Synthesis of fat starting with glycerol-3-phosphate requires action of acyl transferase enzymes, such as glycerol-3-phosphate acyl transferase, which catalyze addition of fatty acids to the glycerol backbone.
Figure 6.13.1: Synthesis of Fat
Interestingly, there appear to be few controls of the metabolism of fatty acids. The primary control of their oxidation is availability. One way to control that is by control of the breakdown of fat. This process, which can be stimulated by the epinephrine kinase cascade, is controlled through LIPE, found in adipocytes (fat-containing cells). Breakdown of fat in apidocytes requires action of three enzymes, each hydrolyzing one fatty acid from the glycerol backbone. As noted earlier, only HSTL, which catalyzes the first hydrolysis, is regulated.
Synthesis of fat requires glycerol-3-phosphate (or DHAP) and three fatty acids. In the first reaction, glycerol-3-phosphate is esterified at position 1 with a fatty acid, followed by a duplicate reaction at position 2 to make phosphatidic acid. This molecule, which is an intermediate in the synthesis of both fats and phosphoglycerides, gets dephosphorylated to form diacylglycerol before the third esterification to make a fat.
Figure 6.13.2: Activation of Fat Hydrolysis
Phosphatidic acid, as noted above, is an important intermediate in the metabolism of glycerophospholipids. These compounds, which are important membrane constituents, can be synthesized in several ways.