Lecture 33: Cholesterol and Steroid Biosynthesis/Metabolism Pathwats

What is the overview of cholesterol biosynthesis?

1. acetate condensation to form mevalonate

2. mevalonate is converted to activated isoprene

3. 6 five-carbon isoprenes are condensed to form squalene

4. squalene is cyclized to form lanosterol

Describe the condensation of 3 acetyl-CoA to HMG-CoA.

1. acetyl-CoA is converted to acetoacetyl-CoA by thiolase

2. CoA-SH is given off

3. acetoacetyl-CoA is converted to HMG-CoA synthase

4. CoA-SH is given off

Describe the reduction of HMG-CoA to mevalonate.

1. HMG-CoA reductase (with the help of 2 NADPH and 2 H⁺) converts HMG-CoA to mevalonate

2. 2 NADP⁺ and CoA-SH is given off

rate-limiting step, committed step

Describe the pathway of activated isoprene formation.

1. mevalonate 5-phosphotransferas (with the help of ATP) converts mevalonate to 5-phosphomevalonate

2. phosphomevalonate kinase (with the help of ATP) converts 5-phosphomevalonate to 5-pyrophosphomevalonate

3. pyro phosphomevalonate decarboxylase (with the help of ATP) converts 5-pyrophosphomevalonate to 3-phospho-5-pyrophosphomevalonate

4. CO₂ and P_i is given off

5. 3-phospho-5-pyrophosphomevalonate is converted to delta3-isopentynyl pyrophosphate (IPP)

6. IPP is isomerized to dimethylallyl pyrophosphate (DMAP)

Describe the formation of squalene.

1. prenyl transferase converts DMAP and IPP to geranyl pyrophosphate [head-to-tail condensation]

2. PP_i is given off

3. prenyl transferase converts geranyl pyrophosphate and delta3-isopentenyl pyrophosphate (IPP) to farnesyl pyrophosphate [head-to-tail condensation]

4. PP_i is given off

5. squalene synthase (with the help of NADPH and H⁺) converts farnesyl pyrophosphate to squalene [head-to-head condensation; production of cyclopropyl intermediates]

6. NADP⁺ and 2 PP_i are given off

Describe squalene cyclization.

• substrate(s): squalene

• coactivator(s): NADPH, H⁺, and O₂

• enzyme: squalene monooxygenase

• product(s): squalene 2,3-epoxide, H₂O, and NADP⁺

Describe the cyclization of squalene 2,3-epoxide to cholesterol.

1. cyclase converts squalene 2,3-epoxide to lanosterol

2. multistep pathway to form cholesterol from lanosterol

Describe the mechanism for how LDL cholesterol enters the cell.

1. LDL receptor is synthesized in the rough ER

2. LDL receptor moves to the PM via the golgi

3. LDL receptor binds apo B-100 on LDL—initiating endocytosis

4. LDL is internalized in the endosome

5. LDL receptor is segregated into vesicles and recycled to the surface

6. endosome with LDL fuses with the lysosome

7. lytic enzymes in lysosome degrade apo B-100 and cholesteryl esters—releasing amino acids, fatty acids, and cholesterol