Glossary of Biosynthesis of Sterols
- 2 sources of cholesterol:
- 1. Diet
2. De novo synthesis in all cells
- 4 predominant tissues of Sterol biosynthesis:
- 1. Liver
2. Adrenal cortex
3. Intestine
4. Reproductive tissues
- Main sites where cholesterol functions: (2)
- 1. Cell membrane component
2. CNS/brain myelinated structures
- 2 Direct Derivatives of Cholesterol
- 1. Steroid hormones
2. Bile acids
- 8 Derivatives of Cholesterol INTERMEDIATES:
- -Vitamens D/E/A/K
-Carotenoids
-Rubber
-Plant hormones
-Phytol chain of chlorophyll
-Dolichols
-Ubiquinone/Plastiquinone
-Isoprene
- Plant hormones (2):
- -Abscisic Acid
-Gibberellic acid
- 3 Most important derivatives of cholesterol to remember:
- -Steroid hormones
-Bile acids
-Vitamin D
- In what form is cholesterol in membranes?
- Unesterified - FREE
- What distinguishes free unesterified cholesterol from esterified?
- An -OH instead of Acyl
- What enzyme makes free cholesterol esterified?
- ACAT - Acyl-CoA Cholesterol Acyl Transferase
- What reaction does ACAT catalyze?
- Incorporates Fatty AcylCoA into Cholesterol w/ subsequent release of CoASH.
- When does ACAT work? Why?
- When membranes have too much free cholesterol ACAT esterifies it for intracellular storage or lipoprotein transfer.
- What is the function of cholesterol in membranes?
- Regulates fluidity and lateral mobility of proteins in the lipid bilayer.
- What is the storage form of cholesterol called?
- Cholesteryl Ester
- What type of molecule is cholesterol?
- Amphipathic
Nonpolar = 4 hydrocarbon rings
Polar head = OH
- How many carbons are in cholesterol?
- 27
- What allows cholesterol to circulate in the blood?
- Apolipoproteins
- What is the carbon source for cholesterol synthesis de novo?
- Acetyl CoA
- 3 Sources of AcCoA:
- 1. FA beta-oxidation (mitochon)
2. Ketogenic amino acid oxidation (leucine/isoleucine)
3. PDH reaction
- PDH Cofactors:
- -TPP
-Lipoamide
-FAD
- Reaction of PDH:
- Pyruvate -> AcCoA
- What enzyme catalyzes the COMMITTED STEP of Cholesterol biosynthesis?
- HMG-CoA Reductase
- What are the 4 major stages of cholesterol biosynthesis?
- 0. Acetate
1. Mevalonate
2. Activated Isoprenes
3. Squalene
4. Cholesterol
- What happens in Cholesterol biosynthesis Stage 1? Enzymes?
- AcCoA -> Mevalonate
-3 reactions
-Thiolase, HMG-CoA Synthase, HMG-CoA Reductase
- What is important re: first 2 reactions in Mevalonate synth?
- They are shared with Ketogenesis
- What is different about Ketogenesis vs. Mevalonate?
- Ketogenesis = mitochondria
Cholest Synth = Cytosol
Hence dif. pools of enzymes
- What are the enzymes shared by Ketogenesis & Mevalonate synth?
- -Thiolase
-HMG-CoA Synthase
- In what tissue are the cytosolic and mitosolic pools found?
- Liver Parenchymal cells
- Why is Mevalonate synthesis so very important?
- Its 3rd reaction is the Commitment step of cholesterol biosynthesis
- Where do HMG-CoA reductase and subsequent reactions occur?
- Probably in peroxisomes.
- What occurs in the 1st reaction of Mevalonate synthesis? Enzyme?
- 2 AcCoA condensation - releases one CoASH
-Via Thiolase
- What is the product of Thiolase?
- Acetoacetyl-CoA
- What occurs in the 2nd reaction of Mevalonate synth? Enzyme?
- Acetoacetyl-CoA is condensed with another AcCoA
-Via HMG-CoA synthase
- What is the product of HMG-CoA Synthase?
- HMG - b-hydroxy-b-methylgutaryl-CoA
- What occurs in the 3rd reaction of Mevalonate synth? Enzyme?
- CoASH released from far end of HMG-CoA; C=O reduced to CH2-OH; the protons donated by 2NADPH;
-Via HMG-CoA Reductase
- Where is HMG-CoA reductase found, and how is it situated?
- -Integral - in the cell membrane
-Active site on cytosolic side
- Product of HMG-CoA reductase is:
- Mevalonate
- What does Mevalonate get converted to? How?
- 5-Carbon Activated isoprenes by Decarboxylation
- How many reactions and enzymes are needed for stage 2 of cholesterol biosynthesis?
- 4 Reactions/3 enzymes
- Enzymes in 5C Activated Isoprene synthesis:
- 1. Mevalonate 5-phosphotransferase
2. Phosphomevalonate kinase
3. Pyrophosphomevalonate Decarboxylase
- Key thing to remember about stage 2 of cholest. biosynth:
- IT REQUIRES 3 total ATP
- What happens in the 1st reaction of stage 2?
- PO4 added to terminal carbon of Mevalonate - replaces the -OH created by HMGCoA reductase.
- What happens in the 2nd reaction of stage 2?
- Add another PO4 right onto the one added in reaction 1.
- What happens in the 3rd reaction of stage 2?
- Add another PO4 onto the beta carbon of pyrophosphomevalonate.
- What enzymes catalyze reactions 1 and 2?
- 1. Mevalnt 5-phosphotransferase
2. Phosphomevalonate kinase
- What enzyme catalyzes reactions 3 and 4 of stage 2?
- Pyrophosphomevalonate decarboxylase - same enzyme for both reactions.
- What occurs in reaction 4 of stage 2?
- Decarboxylation of carbon 1 and loss of PO4 from carbon 3
- What results from decarboxylating 3-Phospho-5-pyrophosphomevalonate?
- 2 isomers:
-d3-isopentenyl pyrophosphate -Dimethylallyl pyrophosphate
- What happens in Stage 3 of cholesterol biosynthesis? (in broad terms)
- Condensation of 6 activated 5-C isoprene units to make Squalene
- What substrate is used in stage 3?
- 1 NADPH
- So substrates for:
-Stage 1
-Stage 2
-Stage 3 - 1 = 2 NADPH
2 = 3 ATP
3 = 1 NADPH
- How many steps are entailed in Stage 3 of cholesterol synth?
- 3 steps: C5 -> C10 -> C15 -> C30
- What enzymes are used in Stage 3?
- -Prenyl transferase (Rxns 1/2)
-Squalene synthase (Rxn 3)
- What terms describe the nature of the 3 condensation reactions?
- Rxns 1/2 = head-to-tail
Rxn 3 = head-to-head
- What are the intermediates in Squalene synthesis? How many Cs?
- 1. Geranyl PPi (10 C)
2. Farnesyl PPi (15 C)
- What is important about Farnesyl PPi?
- Used in post-translational protein modification
- Where does Farnesylation occur on proteins?
- C-terminal Cysteine residue
- What is unique about the final condensation of 2 Farnesyl PPi?
- -Head-to-Head
-Requires NADPH
- What enzyme catalyzes the final step of stage 3?
- Squalene synthase
- How many carbons are in squalene? How many in cholesterol?
- Squalene = 30
Cholesterol = 27
- What needs to happen to squalene to make cholesterol?
- Cyclization to close rings
- In Squalene-Cholest conversion:
-How many reactions?
-How many enzymes?
-What substrates? - -13 reactions
-11 enzymes
-1 NADPH
- How does Squalene cyclization get started?
- By activating it to Squalene Epoxide
- What enzyme makes Squalene 2,3-Epoxide?
- Squalene monooxygenase
- What does Squalene monooxygenase require?
- 1 molecule of O2
1 NADPH
- What enzyme cyclizes Squalene 2,3-Epoxide?
- Oxidosqualene cyclase
- What is the product of oxidosqualene cyclase action?
- Lanosterol
- What needs to happen to Lanosterol to make cholesterol?
(3 things) - -Demethylation of 3 Carbons
-Reduce a double bond
-Migrate another double bond
- Substrates used in
-Stage 1 (HMG-CoA reductase)
-Stage 2
-Stage 3 (squalene synthase)
-Stage 4 (squalene monoxygnase - -Stage 1: 2 NADPH
-Stage 2: 3 ATP
-Stage 3: 1 NADPH
-Stage 4: 1 NADPH
- 3 Inherited disorders of Cholesterol Biosynthesis:
- 1. Chondrodysplasia punctata
2. Latherosterolosis
3. Smith-lemli-Opitz syndrome
- What are the cholest biosynth inherited disorders associated with?
- Developmental malformities
- What is the problem in these disorders?
- Low cholesterol levels - lack of Hedgehog morphogens b/c they are made by post-transl. attachment of cholesterol.
- What is the main regulatory target in cholesterol biosynth?
- HMG-CoA Reductase
- 3 levels of Regulating HMG-CoA reductase:
- 1. Gene Transcription
2. Proteolysis
3. Phosphorylation
- How is gene transcription of HMG-CoA reductase regulated?
- By SREBPs
Sterol Regulatory Element Binding Proteins
- Where are SREBPs normally located? When is this the case?
- In the ER - when cholesterol is high (biosynthesis unnecessary)
- What happens to SREBPs when serum cholesterol gets low?
- SCAP (on ER membrane) senses low levels; it travels to Golgi w/ SREBP, then cleaves SREBP
- What part of SREBP gets cleaved; where does it go?
- N-terminus -> goes to the nucleus to bind the SRE for HMG-CoA reductase.
- What is SRE?
- Sterol Regulatory Element
- Result of SREBP binding to SRE?
- Activated transcription of HMG-CoA reductase hence increased cholesterol biosynth.
- What causes proteolysis of HMG-CoA reducatase?
- metabolites of cholesterol
- What 2 things do Cholesterol metabolites inhibit?
What do they ACTIVATE? - -HMGCoA reductase (proteolysis)
-Extracellular uptake from LDL via receptor mediated endocytos.
-Activates ACAT for esterifictn
- Phosphorylated HMG-CoA is ____
Dephosphorylated HMG-CoA is ____ - Phosph = inactive
Dephosph = active
- What stimulates HMG-CoA phosphorylation/dephosphoryltn?
- Glucagon -> phosphorylate
Insulin -> dephosphorylate
- How does glucagon stimulate phosphorylation of HMG-CoA red?
- Via AMP-activated protein kinase
- How does insulin stimulate dephosphorylation of HMG-CoA rd?
- Via HMG-CoA Reductase Phosphatase
- What are Statins?
- Competitive Inhibitors of HMG-CoA reductase
- What are the 4 Statins?
- -Compactin
-Simvastatin (Zocor)
-Pravastatin (Pravachol)
-Lovastatin (Mevacor)
- What are Statins used for?
- To treat familial hypercholesterolemia
- What pleiotropic effect is exhibited by statins?
- Improved endothelial function via increased ENOS activity -
is how Viagra was discovered.
- End product of cholesterol is:
- Bile acids
- Where are bile acids made? From what?
- In liver from cholic acid
- What is cholic acid?
- Derivative of Cholesterol that is more soluble - 24 Carbons and 3 OH
- What is the function of bile acids?
- To emulsify fats in prep for pancreatic lipase
- What happens to bile acids after release from gallbladder to intestine?
- Reabsorbed - synthesis is not enough to meet physiolog demands.
