What does it mean by "PC" in this article?
Answer from @user438383, @matteo-ferla and @m, converted from comments [anyone, feel free to edit this answer to improve it]:
According to the intro it means 'pseudocenter'. All measurements are relative to an assigned point because a protein doesn't have a 'centre' they assigned it as a pseudocentre.
Note that PC normally means principal component, but that does not seem to be the case here.