Figure 1.

Sterol reductases in cholesterol biosynthesis. Four sterol reductases, DHCR24, DHCR7, DHCR14, and LBR, catalyze three distinct reactions in cholesterol synthesis. In the Bloch pathway, LBR or DHCR14 first reduces the C(14–15) double bond. Then DHCR7 and DHCR24 act consecutively to form cholesterol. In the Kandutsch–Russell pathway, DHCR24 acts first, followed by LBR or DHCR14 and DHCR7 as the terminal enzyme. In addition to these two defined pathways, DHCR24 can shunt intermediates from the Bloch pathway into the Kandutsch–Russell pathway at any point. Dashed arrows represent multiple enzymatic steps. The purple box indicates the C(14–15) bond on FF-MAS and T-MAS, the reaction catalyzed by DHCR14 and LBR. The green box indicates the C(24–25) bond on sterol intermediates, the reaction catalyzed by DHCR24 at numerous steps in the pathway. The red box indicates the C(7–8) bond on 7DHC and cholesterol, the reaction catalyzed by DHCR7.