The composition of the diet we eat dictates the general metabolism of an organism. There is a need to process the major products in the diet (carbohydrate, lipid, and protein) into their basic components. These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. If the com position of the diet changes (eg, high carbohydrate vs low carbohydrate) metabolic pathways can adapt to metabolize the nutrient. In ruminants (and, to a lesser extent, other herbivores), dietary cellulose is fermented by symbiotic microorganisms to short-chain fatty acids (acetic, propionic, butyric), and metabolism in these animals is adapted to use these fatty acids as major substrates. The products of digestion when completely oxidized go to a common product, acetyl-CoA, which is then oxidized by the citric acid cycle (Figure 1).
Fig1. Outline of the pathways for the catabolism of carbohydrate, protein, and fat. All these pathways lead to the production of acetyl-CoA, which is oxidized in the citric acid cycle, ultimately yielding ATP by the process of oxidative phosphorylation.
Carbohydrate Metabolism Is Centered on Oxidation & Storage of Glucose
Glucose is metabolized by all tissues and is an important energy source for many (Figure2). Glucose is first metabolized to glucose-6-phosphate by hexokinase and from there it can go to many fates. It can be metabolized to pyruvate by the pathway of glycolysis . In aerobic tissues the pyruvate can be metabolized to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO2 and H2O. The citric acid cycle is linked to the formation of ATP in the process of oxidative phosphorylation. Glycolysis can also occur anaerobically (in the absence of oxygen) where pyruvate is converted to the end product lactate.
Fig2. Overview of carbohydrate metabolism showing the major pathways and end products. Gluconeogenesis is not shown.
Glucose and its metabolites also take part in other processes. Glucose can be stored as a polymer called glycogen in skeletal muscle and liver. It can be diverted to the pentose phosphate pathway, an alternative to part of the pathway of glycolysis . This pathway is a source of reducing equivalents (NADPH) for fatty acid syn thesis and the source of ribose for nucleotide and nucleic acid synthesis (. In the glycolytic pathway triose phosphate intermediates can give rise to the glycerol moiety of triacylglycerols. Pyruvate can provide for the synthesis of intermediates in the citric acid cycle that can provide the carbon skeletons for the synthesis of nonessential or dispensable amino acids . Acetyl-CoA derived from pyruvate is the precursor for the synthesis of fatty acids and cholesterol and hence of all the steroid hormones synthesized in the body. Some tissues can synthesize glucose from precursors such as lactate, amino acids, and glycerol by the process of gluconeogenesis. This is important for suppling glucose when dietary carbohydrate is low or inadequate.
Lipid Metabolism Is Concerned Mainly With Fatty Acids & Cholesterol
The long-chain fatty acids are either derived from dietary lipid or synthesized from acetyl-CoA derived from carbohydrate or amino acids (lipogenesis). Fatty acids may be oxidized to acetyl-CoA (β-oxidation) or esterified with glycerol, forming triacylglycerol as the body’s main fuel reserve. Stored triacyl glycerol (adipose tissue) can be mobilized (lipolysis) to release nonesterified fatty acids and glycerol.
Acetyl-CoA formed by β-oxidation of fatty acids may undergo three fates (Figure 3):
1. Oxidized to CO2 + H2O via the citric acid cycle
2. Synthesis of cholesterol and other steroids
3. Synthesize ketone bodies (acetoacetate and 3-hydroxybutyrate) in the liver
Fig3. Overview of fatty acid metabolism showing the major pathways and end products. The ketone bodies are acetoacetate, 3-hydroxybutyrate, and acetone (which is formed non enzymically by decarboxylation of acetoacetate).
Much of Amino Acid Metabolism Involves Transamination
The amino acids are required for protein synthesis (Figure 4). The essential or indispensable amino acids must be supplied in the diet, since they cannot be synthesized in the body. The nonessential or dispensable amino acids, which are supplied in the diet, can also be formed from metabolic intermediates by transamination using the amino group from other amino acids . If the carbon backbone is to be used for other processes the alpha amino nitrogen must be removed (deamination), metabolized in the liver to urea, and excreted by the kidney. The carbon skeletons that remain after transamination may (1) be oxidized to CO2 in the citric acid cycle, (2) be used to synthesize glucose (gluconeogenesis), fatty acids , or (3) form ketone bodies.
Fig4. Overview of amino acid metabolism showing the major pathways and end products.
Several amino acids are also the precursors of other com pounds, for example, purines, pyrimidines, hormones such as epinephrine and thyroxine, and neurotransmitters.
