Metabolic Pathway :Specific Types of Metabolic Reactions

Metabolic Pathway

Chemical reactions normally occur in a series of step wise reactions with the final product(s) referred to as end products and the substances in between called intermediates. This chain of reactions can be represented symbolically in the following ways: A + X ® B ® C ® D + Y A ®  B ®  C ®  D ^ ¯ X Y In this metabolic pathway B + C are intermediates and D + Y end products.

Specific Types of Metabolic Reactions 1. Hydrolysis and Condensation Reactions 2. Phosphorylation and Dephosphorylation Reactions 3. Oxidation-Reduction Reaction

1. Hydrolysis and Condensation Reactions      Hydrolysis (hydro -water; lysis - splitting) involves the breaking of covalent bonds with water molecules. This can be represented as: A¾B + H2O ¾® ¬ A¾OH + H¾B      Note that the bond between A and B is broken by attaching the hydroxyl group to A and the H to B.

Condensation - The reverse of hydrolysis. The reaction above can go in reverse with A- OH reacting with B-H to form a larger molecule B-A with the release of H2O.

A¾B + H2O ® ¬¾ A¾OH + H¾B

2. Phosphorylation and Dephosphorylation Reactions      Phosphorylation involves the addition of a phosphate group, HPO4-2 or H2PO4-1, to a molecule. Phosphate groups can be represented in equations as Pi. Hence, a generalized equation for a phosphorylation is: A + Pi ¾® ¬ A--P

Dephosphorylation results in the removal of a phosphate group from a molecule. A + Pi ® ¬¾ A--P

3. Oxidation-Reduction Reaction      Oxidation involves the removal of electrons from any molecule. Reduction involves the addition of electrons. The two occur together in oxidation-reduction reactions with the molecule losing electrons being oxidized and the molecule gaining electrons being reduced.      The terms oxidation and reduction can have more subtle meanings: 1. Oxidation can refer to the reaction of a molecule with molecular oxygen (O2) even though oxygen may actually form a covalent bond (sharing electrons). As a result of the reaction the oxygen nucleus attracts the electrons more strongly and in this sense "removes electrons".  For example, here is the equation for the oxidation of glucose: C6H12O6 + 6O2 ® 6CO2 + 6H2O 2. Oxidation can also refer to the removal of hydrogen atoms. Although a hydrogen atom is removed it carries with it an electron and this lose of a hydrogen atom from a molecule is considered equivalent to the lose of an electron. The following generalized equation illustrates this kind of oxidation: HA-BH ® A-B + 2H      Conversely, the addition of hydrogen is considered a reduction.      The actual addition or removal of electrons changes the molecule's electrical charge. This occurs during oxidative phosphorylation. The following equation illustrates oxidation when it goes forward: H2 ¾® ¬ 2H+ + 2e- and reduction when it goes in reverse: H2 ® ¬¾ 2H+ + 2e-