tisdag 1 februari 2011

Kertausta: STEROLIEN biosynteesin otsikoita

Glossary of Biosynthesis of Sterols

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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
-Plant hormones
-Phytol chain of chlorophyll
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?
Nonpolar = 4 hydrocarbon rings
Polar head = OH
How many carbons are in cholesterol?
What allows cholesterol to circulate in the blood?
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:
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?
-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?
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:
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:
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?
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?
-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
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
What enzyme cyclizes Squalene 2,3-Epoxide?
Oxidosqualene cyclase
What is the product of oxidosqualene cyclase action?
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?
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?
-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.

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