Complex sentence: Proprotein convertase 1 is the enzyme largely responsible for the synthesis of insulin; the second prohormone convertase plays a more minor role in insulin biosynthesis, but a greater role in glucagon biosynthesis.
Split into simpler sentences: Proprotein convertase 1 is the enzyme largely responsible for the synthesis of insulin. Another prohormone convertase, proprotein convertase 2 plays a more minor role in insulin biosynthesis, but a greater role in glucagon biosynthesis.