The metabolism of fats

As shown in Figure 5.21, fatty acids may be made available to cells in two ways:

  • In the fed state, chylomicrons assembled in the small intestine (section 4.3.3.2) and very low-density lipoproteins exported from the liver (section 5.6.2.2) bind to the cell surface, where lipoprotein lipase catalyses hydrolysis of triacylglycerols to glycerol and free fatty acids.
  • In the fasting state, hormone-sensitive lipase in adipose tissue is activated in response to falling insulin secretion or the secretion of adrenaline (section 4.3.2.2 and section 10.5.1) and catalyses the hydrolysis of triacylglycerol, releasing free fatty acids into the bloodstream, where they bind to albumin and are transported to tissues.

The glycerol is phosphorylated and converted to dihydroxyacetone phosphate (see Figure 5.10), which may be used either as a metabolic fuel (in the fed state) or for gluconeogenesis (in the fasting state).

The fatty acids are oxidized in the mitochondria by the ^-oxidation pathway, in which two carbon atoms at a time are removed from the fatty acid chain as acetyl CoA. This acetyl CoA then enters the citric acid cycle, together with that arising from the metabolism of pyruvate.

fed state fasting state tna""1"1"""'"1 ■■ ■■ ■ ase anc

fatty acids

fatty acyl CoA

fatty acyl CoA

Figure 5.21 An overview of fatty acid metabolism.

Figure 5.21 An overview of fatty acid metabolism.

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