There may be two separate questions here. One, the Michaelis constant Km is sometimes approximately equal to the dissociation constant of the substrate, but it can be less than, equal to, or greater than the dissociation constant in general. Two, I sometimes use the word "apparent" to mean only for a certain, specified set of conditions. Let us take creatine kinase as an example. It has two substrates, creatine and MgATP. Suppose that I fix the MgATP concentration at a value that is not saturating and vary the [creatine]. I would find the [creatine] that produces a velocity equal to Vmax/2, and I would call this the apparent Km of creatine. The true Km of creatine only refers to the situation in which all other substrates are saturating. Borek may be a good person to ask about things like apparent pKa values.