All most all of the reactions that occurs in metabolic pathways are enzymatically catalyzed organic reactions. Various mechanisms enzymes have at that their disposal for catalyzing reactions: Acid-Base catalysis, covalent catalysis, metal ion catalysis, electrostatic catalysis, proximity and orientation effects, and transition state binding. Yet few enzymes alter the chemical mechanisms of these reactions, so much can be learned about enzymatic mechanisms from the study of nonenzymatic model reactions.
Christopher Walsh has classified biochemical reactions into four categories:
1-Group Transfer Reactions,
2-Oxidations and Reductions
3-Eliminations, isomerizations and rearrangements
4-Reactions that make or break carbon-carbon bonds.
A-Chemical Logic:
A covalent bond consists of an electron pair shared between two atoms. In breaking such a bond, the electron pair can either remain with one of the atoms-Heterolytic bond cleavage or separate such that one electron accompanies each of the atoms- Homolytic bond cleavage.
Homolytic bond cleavage usually produces unstable radicals, occurs mostly in oxidation-reduction reactions.
Heterolytic C-H bond cleavage involves either cabanion and protein H+ formation or carbocation (carbonium ion) and hydride ion H- formation. Since hydride ions are highly reactive species and carbon atoms are slightly more electonegative than hydrogen atoms, bond cleavage in which the electron pair remains with the carbon atom is the predominant mode of C-H bond breaking in biochemical systems. Hydride ion abstraction occurs only if the hydride is transferred directly to an acceptor such as NAD+ or NADP+.
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