Complex sentence: Vitamin K 2 (menaquinone) includes several subtypes; two subtypes most studied are menaquinone-4 (menatetrenone, MK4) and menaquinone-7 (MK7).
Split into simpler sentences: Vitamin K 2 (menaquinone) includes several subtypes. The two subtypes of vitamin K 2 that have been most studied are menaquinone-4 (menatetrenone, MK4) and menaquinone-7 (MK7).