Complex sentence: The large and highly glycosylated KLH protein cannot be reproduced synthetically; it is available only as a purified biological product from the keyhole limpet "Megathura crenulata".
Split into simpler sentences: The large and highly glycosylated KLH protein cannot be reproduced synthetically. It is available only as a purified biological product from the Keyhole Limpet "Megathura crenulata".