The pyruvate →lactate reaction in animals is reversible, but...

Key Concepts

Reversibility of Enzymatic Reactions

Enzymatic reactions can be reversible or irreversible based on their thermodynamic and kinetic properties. A reversible reaction typically has a small free energy difference between reactants and products, allowing the enzyme to catalyze the reaction in both directions depending on cellular conditions. In contrast, an irreversible reaction has a large negative change in free energy, making the backward reaction highly unfavorable under physiological conditions.

Gibbs Free Energy and Reaction Coupling

The overall spontaneity of a biochemical reaction is governed by its Gibbs free energy change (?G). When a reaction step has a very negative ?G, it commits the substrate to that pathway, effectively serving as a regulatory irreversible step. In many fermentation processes, coupling reactions such as decarboxylation can significantly lower ?G, thereby driving the process predominantly in one direction.

Decarboxylation and Product Removal

Decarboxylation is a process in which a carboxyl group is removed from a molecule, often releasing carbon dioxide. This removal of a product shifts the equilibrium of the reaction, such that the reverse reaction is thermodynamically unfavorable. In metabolic pathways, the escape of a gaseous product like CO2 ensures that the reaction proceeds irreversibly, as the loss of this intermediate prevents backflow to the original substrate.

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