Complex sentence: Kallikreins (tissue and plasma kallikrein) are a subgroup of the serine protease family that liberate kinins (BK and KD) from the kininogens.
Split into simpler sentences: Kallikreins (tissue and plasma kallikrein) are a subgroup of the serine protease family. They liberate kinins (BK and KD) from the kininogens.