(from Fundamentals of Biochemistry by Voet, 5th ed.)
In this step of glycolysis, I'm not seeing where the \ceH+ ion on the product side is coming from. It seems to me that the G3P's aldehydic H is replaced by phosphate, and that H is given to \ceNAD+ to make NADH. So where is the extra \ceH+ coming from?
Answer
Although texts such as Berg et al. tend to refer to inorganic phosphate, \cePi, as orthophosphate (\cePO43−), the term inorganic phosphate is used because in aqueous solution at pH 7.6 several phosphate species exist, the predominant one being \ceHPO42−. If this is regarded as \cePi, then it is the source of the \ceH+, and the equation balances.
Note added by David:
On checking I find, in contrast to Berg, Lehninger’s book defines \cePi as monohydrogen phosphate (\ceHPO42−), and Fersht actually writes the equation of the reaction (16-1) with \ceHPO42− rather than \cePi.
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