The Meaning of “Reversed Electron Flow” and “High Energy Electron” in Biochemistry

Abstract

IN recent years there have been some needless controversies¹ about the use of the term “high energy bond” in biochemistry. We imagine that most biochemists are perfectly aware that, when they speak of the “high energy bonds” in ATP, they do not imply that there is something exceptional about the P-O-P linkages; they mean that ATP is a compound that reacts (enzymically) with other compounds in biochemical systems, the net result being the formation of products with a lower free energy content. Some of the free energy released may be utilized in various ways but a certain fraction (usually about 25%) is dissipated as heat, and thus the reaction is virtually irreversible, whether it is anabolic or catabolic. Such irreversibility conferred by heat loss is the basis of the steady state we call “life”. No one supposes that the P-O-P (anhydride) bonds enshrine the chemical potential in ATP, but Lipmann's term “high energy bond” remains a useful metaphor for at least two reasons. First, the phosphate group can be readily attached to all kinds of biochemical compounds and is therefore an excellent intermediary in linking one reaction with another. Secondly, the free energy of hydrolysis of the P-O-P anhydride bond at pH 7 is relatively high. Thus a reaction such as with an equilibrium lying to the left can be reversed by coupling it enzymically with ATP hydrolysis: in which the equilibrium is to the right.