The following points highlight the three main metabolic pathways for intermediary metabolism. The metabolic pathways are: 1. Carbohydrate Metabolism 2. Lipid Metabolism 3. Amino Acid Metabolism.
Metabolic Pathway # 1. Carbohydrate Metabolism:
a. Pyruvate and lactate are formed in the mammalian cells as a result of the oxidation of glucose by glycolysis.
b. Glycolysis occurs in the cytoplasm of cells in absence of oxygen producing lactate only. s
c. Under aerobic condition, pyruvate is metabolized to acetyl-CoA which enters the citric acid cycle for complete oxidation to CO2 and H2O.
d. Glucose also takes part in other metabolic process as follows:
(i) It is converted to glycogen as a storage, particularly, in liver and skeletal muscle.
(ii) The HMP shunt or the pentose phosphate pathways arising from intermediates of glycolysis is a source of reducing equivalents (2H) for biosynthesis of fatty acids, cholesterol, etc. and it is a source of ribose which is important for nucleic acid formations.
(iii) Triose phosphate of glycolysis is a source of glycerol of fat.
(iv) Pyruvate and the intermediates of citric acid cycle form amino acids and acetyl-CoA is the building block for long-chain fatty acids and cholesterol, the precursor of all steroid hormones in the body.
Metabolic Pathway # 2. Lipid Metabolism:
a. The long chain fatty acids are synthesized form acetyl-CoA derived from carbohydrate or from dietary lipid.
b. In the tissues, fatty acids are oxidized to acetyl-CoA or esterified to acyl-glycerol to form fat which is the main caloric reserve of the body.
c. Acetyl -CoA formed by β-oxidation has the following significant roles in the body:
(i) It liberates CP2 and H2O and also yields high energy. Therefore, during the oxidation of fatty acids by β-oxidation for their complete oxidation, more energy is formed.
(ii) It is a source of cholesterol biosynthesis.
(iii) In the liver, it forms ketone bodies which are alternative water-soluble tissue fuels. These fuels become important sources of energy under certain conditions (e.g., starvation).
Metabolic Pathway # 3. Amino Acid Metabolism:
a. Amino acids are required for protein synthesis.
b. The essential amino acids must be supplied in the diet since these are not synthesized by the tissues.
c. Diet can supply the non-essential amino acids which are also formed from the intermediates of citric acid cycle by transamination.
d. Excess amino nitrogen as a result of deamination of amino acids is removed as urea and the carbon skeletons that remain after transamination give the following products:
(i) Carbon dioxide and water via the citric acid cycle.
(ii) Glucose (by gluconeogenesis).
(iii) Ketone bodies.
e. The amino acids are also the precursors of many other important compounds, e.g., purines, pyrimidine’s, and hormones such as epinephrine and thyroxine.