Physiological pH vs. Isoelectric Point

[LadyPikachuu]

This is probably a really basic question and I am just confusing myself, but...

The isoelectric point or pI of an amino acid is the pH at which an amino acid has a net charge of zero. At physiological pH (pH~7.4), amino acids with unionizable side chains will exist with a net charge of zero. So how can it be that that isoelectric points and physiological pH be different values?

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[aldol16]

Well, remember the Henderson-Hasselbalch equation. At various pH values, there will be varying population of the HA and A- states. So at a given pH, say, 60% of a molecule could be uncharged whereas 40% could still contain a net negative charge. The isoelectric point is defined as the point exactly halfway between the pKa values corresponding to the molecule losing a negative charge and gaining a positive charge.