Biologically Active Form. Menaquinone-4 (MK-4). Supports Proper Calcium Metabolism. Although Vitamin K is historically known for its role in normal blood clotting function, we now know that Vitamin K is also essential to bone, cardiovascular, and nervous system health. As a biologically active form of Vitamin K, K-2 is important for the formation of healthy, strong bone matrix. Vitamin K’s role in arterial health revolves around its ability to support proper calcium metabolism in vascular structures.