Metabolic Pathways for Intermediary Metabolism (3 Pathways)

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.