Definition: The apparent equilibrium constant, K', and associated values. Usage: Concentrations in the equilibrium constant equation refer to the total concentrations of all forms of particular biochemical reactants. For example, in the equilibrium constant equation for the biochemical reaction in which ATP is hydrolyzed to ADP and inorganic phosphate: K' = [ADP][Pi]/[ATP], The concentration of ATP refers to the total concentration of all of the following species: [ATP] = [ATP4-] + [HATP3-] + [H2ATP2-] + [MgATP2-] + [MgHATP-] + [Mg2ATP]. The apparent equilibrium constant is formally dimensionless, and can be kept so by inclusion of as many of the terms (1 mol/dm3) in the numerator or denominator as necessary. It is a function of temperature (T), ionic strength (I), pH, and pMg (pMg = -log10[Mg2+]). Therefore, these quantities must be specified to be precise, and values for KEQ for biochemical reactions may be represented as 5-tuples of the form (K' T I pH pMg). This property may have multiple values, representing different measurements for K' obtained under the different experimental conditions listed in the 5-tuple. (This definition adapted from EcoCyc) See http://www.chem.qmul.ac.uk/iubmb/thermod/ for a thermodynamics tutorial.
Source:http://www.biopax.org/release/biopax-level3.owl#KPrime