Biochemistry, Vol. 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Digestion, nutrient transport, and metabolic interrelationships
There are five pathways of lipid transformation

As shown in Fig. 24-11, the Fatty acids incorporated into Lipids also undergo several Metabolic Pathways in the Liver.

a. Oxidation to CO₂ with ATP generation

Free fatty acids undergo activation and oxidation to yield acetyl-CoA and ATP (Section 18.8). The acetyl-CoA is subsequently oxidized via The Citric Acid Cycle, and Oxidative Phosphorylation generates ATP. Fatty acids serve as the primary substrate for METABOLISM/26.html">Energy Metabolism in the liver.

b. Formation of Ketone bodies

Excess acetyl-CoA released during Fatty acid oxidation that is not utilized by the liver is converted into the Ketone Bodies acetoacetate and D-β-hydroxybutyrate. These are transported via the bloodstream to peripheral Tissues, where they are oxidized through The Citric Acid cycle (Section 18.10). Ketone bodies can be regarded as a transport form of acetyl groups. Their contribution to the Energy supply of peripheral tissues can be substantial: in The Heart, for instance, they meet up to one-third of the energy demands.

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Fig. 24-11. Fatty acid metabolism in the liver.

c. Biosynthesis of Cholesterol and Bile acids

A portion of the acetyl-CoA derived from fatty acids (and from glucose) serves as the primary precursor for Cholesterol Biosynthesis, which in turn is a precursor for bile acids essential for fat Digestion AND ABSORPTION (Sections 21.17 and 24.1c).

d. Biosynthesis of plasma Lipoproteins

Fatty acids also serve as precursors in the Synthesis of the lipid moiety of plasma lipoproteins. Lipoproteins function as Lipid carriers to adipose tissue, where the lipids are stored as triacylglycerols.

e. Generation of free plasma fatty acids

Free fatty acids bind to serum albumin and are subsequently delivered via the bloodstream to The Heart and skeletal Muscles; these Organs take up and utilize fatty acids as their primary fuel.

Thus, hepatic metabolism exhibits exceptional metabolic flexibility and a broad dynamic range. The liver is exceptionally well-suited to act as the body's central metabolic clearinghouse. It ensures the delivery of required nutrient quantities to other organs, dampens metabolic fluctuations caused by intermittent food intake, and converts excess amino groups into urea and other excretion products eliminated by the Kidneys.

In addition to the conversion and distribution of CARBOHYDRATES, fats, and Amino Acids, the liver actively carries out the enzymatic detoxification of foreign Organic compounds, such as drugs, food additives, preservatives, and other potentially harmful substances of no nutritional value. Detoxification typically involves the enzymatic hydroxylation of relatively insoluble compounds, rendering them more soluble, more readily degraded, and easier for the body to excrete.