If you look at the greek prefix it can kind of give you a clue as to what it means. Apo- is a common prefix you'll see in science. In the greek it means "apart from" "away from" or "out of" (think of "symplastic" vs "apoplastic" in plants)
An apoenzyme is an enzyme that is not functional until it has a specific cofactor bound to it. So you could think of it being "apart from" the cofactor. Once the cofactor has bound, the resulting complex is called a "holoenzyme" The prefix "Holo-" meaning "whole" or "complete" (its where we get the term "Holistic")
As for apoprotein. Well all enzymes are proteins, so what do you think an apoenzyme could also be called?
(a bit overload I know, but I took Greek as my language in college so I love putting it use 😛 )
