Harden-Yong Equation

HARDEN-YONG EQUATION (Harden, Young). Harden and Young (1909) showed that the fermentation of sugar requires the presence of inorganic phosphate and is accompanied by the formation of a complex ester of one molecule of sugar (hexose) with two molecules of phosphoric acid. The formation of this compound, which they named "zymophosphate," Harden and Young consider as a reaction coupled with sugar fermentation, and express both processes by the equation: 2 C₆H₁₂O₆ + 2 PO₄HMe₂ = 2 CO₂ + 2 C₂H₅OH + C₆H₁₀O₅(PO₄Me₂) + 2 H₂O.

(i). In the second phase of fermentation, zymophosphate is split by a special enzyme, hexose-diphosphatase, according to the equation: C₆H₁₀O₅(PO₄Me₂) + 2 H₂O = C₆H₁₂O₆ + 2 PO₄HMe₂ (2), after which the separated sugar and phosphate again enter into reaction according to equation (1). Meyerhof, however, proved in more recent works that sugar during fermentation is first converted into an unstable hexose-monophosphate, from two molecules of which one is fermented and the other passes into a stable hexose-diphosphate (Harden's zymophosphate): 2C₆H₁₂O₆ + 2 PO₄HMe₂ = 2C₆H₁₁O₅(PO₄Me₂)+2H₂O (1a) active form. C₆H₁₀O₅(PO₄Me₂) = 2 CO₂ + 2 C₂H₅OH + C₆H₁₀O₅(PO₄Me₂), stabilized form (2a). Equations (1a) and (2a) express the first phase of fermentation and correspond to the H.-Y. equation (1). The second phase, according to Meyerhof, consists in the direct fermentation of zymophosphate without preliminary cleavage of phosphoric acid. A similar scheme is also followed in the breakdown of carbohydrate into lactic acid in muscle (see Glycolysis, as well as Fermentation, Hexosephosphoric acids).