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Phosphoryl Group Transfers and ATP:- Biochemical and Chemical Equations Are Not Identical

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  506

2026-05-31

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Phosphoryl Group Transfers and ATP:- Biochemical and Chemical Equations Are Not Identical

Biochemists write metabolic equations in a simplified way, and this is particularly evident for reactions in volving ATP. Phosphorylated compounds can exist in several ionization states and, as we have noted, the different species can bind Mg²⁺. For example, at pH 7 and 2mMMg²⁺, ATP exists in the forms ATP^4-, HATP^3-, H2ATP^2-, MgHATP^-, and Mg₂ATP. In thinking about the biological role of ATP, however, we are not always interested in all this detail, and so we consider ATP as an entity made up of a sum of species, and we write its hydrolysis as the biochemical equation

ATP H₂O →ADP Pi

where ATP, ADP, and Pi are sums of species. The corresponding apparent equilibrium constant, Keq [ADP][Pi]/[ATP], depends on the pH and the concentration of free Mg²⁺. Note that H⁺ and Mg²⁺ do not ap pear in the biochemical equation because they are held constant. Thus, a biochemical equation does not balance H, Mg, or charge, although it does balance all other elements involved in the reaction (C, N, O, and P in the equation above).

We can write a chemical equation that does balance for all elements and for charge. For example, when ATP is hydrolyzed at a pH above 8.5 in the absence of Mg²⁺, the chemical reaction is represented by

ATP^4-→ H₂O→ADP^3- +HPO4 ^-2+ H⁺

The corresponding equilibrium constant, Keq [ADP^3-][HPO4 ^2-][H⁺]/[ATP^4-], depends only on temperature, pressure, and ionic strength. Both ways of writing a metabolic reaction have value in biochemistry. Chemical equations are needed when we want to account for all atoms and charges in a re action, as when we are considering the mechanism of a chemical reaction. Biochemical equations are used to determine in which direction a reaction will proceed spontaneously, given a specified pH and [Mg²⁺], or to calculate the equilibrium constant of such a reaction. Throughout this book we use biochemical equations, unless the focus is on chemical mechanism, and we use values of ΔG⁰ and Keq as determined at pH 7 and 1 mM Mg²⁺.

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