CONSTRAINTS IN BIOCHEMICAL REACTIONS

Chemical reactions satisfy element and charge conservation equations, but generally do not satisfy additional independent conservation equations. Biochemical reactions at a specified pH satisfy conservation equations for elements in addition to hydrogen, but they often satisfy additional independent conservation equations because of the coupling of reactions through the enzymatic mechanism. The enzyme may couple together two or more biochemical reactions so that only the sum of the coupled reactions is catalyzed. The biochemical reactions that are coupled together may, or may not, share reactants, but the type of coupling discussed here is stoichiometric. Since chemical equations and biochemical equations are mathematical equations, linear algebra provides the means for determining the number and types of constraints involved in the enzymatic mechanism. Constraints in addition to element balances indicate missing reactions. The identification of conservation equations is essential for calculations of equilibrium compositions using computer programs that minimize the transformed Gibbs energy at a specified pH subject to the conservation equations that apply.