Pathways That Process The Major Products Of Digestion

The nature of the diet sets the basic pattern of metabolism. There is a need to process the products of digestion of dietary carbohydrate, lipid, and protein. These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. In ruminants (and to a lesser extent in 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. All the products of digestion are metabolized to a common product, acetyl-CoA, which is then oxidized by the citric acid cycle (Figure 15-1).

Carbohydrate Metabolism Is Centered on the Provision & Fate of Glucose (Figure 15-2)

Glucose is metabolized to pyruvate by the pathway of glycolysis, which can occur anaerobically (in the absence of oxygen), when the end product is lactate. Aerobic tissues metabolize pyruvate to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO2 and H2O, linked to the formation of ATP in the process of oxidative phosphorylation (Figure 16-2). Glucose is the major fuel of most tissues.

Carbohydrate

Protein

Fat

2CO2

Figure 15-1. Outline of the pathways for the catab-olism of dietary carbohydrate, protein, and fat. All the pathways lead to the production of acetyl-CoA, which is oxidized in the citric acid cycle, ultimately yielding ATP in the process of oxidative phosphorylation.

2CO2

Figure 15-1. Outline of the pathways for the catab-olism of dietary carbohydrate, protein, and fat. All the pathways lead to the production of acetyl-CoA, which is oxidized in the citric acid cycle, ultimately yielding ATP in the process of oxidative phosphorylation.

Diet

Glucose

Glucose

3CO2

Ribose — phosphate

Acylglycerols (fat)

2CO2

Figure 15-2. Overview of carbohydrate metabolism showing the major pathways and end products. Gluco-neogenesis is not shown.

Glycogen

3CO2

Ribose — phosphate

RNA DNA

Acylglycerols (fat)

2CO2

Figure 15-2. Overview of carbohydrate metabolism showing the major pathways and end products. Gluco-neogenesis is not shown.

Glucose and its metabolites also take part in other processes. Examples: (1) Conversion to the storage polymer glycogen in skeletal muscle and liver. (2) The pentose ph osphate pathway, an alternative to part of the pathway of glycolysis, is a source of reducing equivalents (NADPH) for biosynthesis and the source of ri-bose for nucleotide and nucleic acid synthesis. (3) Triose phosphate gives rise to the glycerol moiety of triacylglycerols. (4) Pyruvate and intermediates of the citric acid cycle provide the carbon skeletons for the synthesis of amino acids; and acetyl-CoA, the pre cursor of fatty acids and cholesterol (and hence of all steroids synthesized in the body). Gluconeogenesis is the process of forming glucose from noncarbohydrate precursors, eg, lactate, amino acids, and glycerol.

Lipid Metabolism Is Concerned Mainly With Fatty Acids & Cholesterol (Figure 15-3)

The source of long-chain fatty acids is either dietary lipid or de novo synthesis from acetyl-CoA derived from carbohydrate. Fatty acids may be oxidized to acetyl-CoA (P-oxidation) or esterified with glycerol, forming triacylglycerol (fat) as the body's main fuel reserve.

Acetyl-CoA formed by P-oxidation may undergo several fates:

(1) As with acetyl-CoA arising from glycolysis, it is oxidized to CO2 + H2O via the citric acid cycle.

Triacylglycerol

Steroids

Cholesterol

Triacylglycerol

Steroids

Cholesterol

Ketone bodies

2CO2

Figure 15-3. Overview of fatty acid metabolism showing the major pathways and end products. Ketone bodies comprise the substances acetoacetate, 3-hy-droxybutyrate, and acetone.

Ketone bodies

2CO2

Figure 15-3. Overview of fatty acid metabolism showing the major pathways and end products. Ketone bodies comprise the substances acetoacetate, 3-hy-droxybutyrate, and acetone.

(2) It is the precursor for synthesis of cholesterol and other steroids.

(3) In the liver, it forms ketone bodies (acetone, ace-toacetate, and 3-hydroxybutyrate) that are important fuels in prolonged starvation.

Much of Amino Acid Metabolism Involves Transamination (Figure 15-4)

The amino acids are required for protein synthesis. Some must be supplied in the diet (the essential amino acids) since they cannot be synthesized in the body. The remainder are nonessential amino acids that are supplied in the diet but can be formed from metabolic intermediates by transamination, using the amino nitrogen from other amino acids. After deamination, amino nitrogen is excreted as urea, and the carbon skeletons that remain after transamination (1) are oxidized to CO2 via the citric acid cycle, (2) form glucose (gluconeogenesis), or (3) form ketone bodies.

Several amino acids are also the precursors of other compounds, eg, purines, pyrimidines, hormones such as epinephrine and thyroxine, and neurotransmitters.

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