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Lipid and Ketone Metabolism Week 12
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Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

Jan 11, 2016

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Page 1: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

Lipid and Ketone Metabolism

Week 12

Page 2: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipids

• Lipids: structure & function• Transport of lipids: albumin-binding & lipoprotein• Storage & release of lipids – adipose tissue• Catabolism: energy release from fatty acid• Activation of FA & carnitine shuttle• oxidation of palmitic acid• Ketone bodies

Page 3: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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What are lipids?

• Not soluble in water – man (or animal) is ~80% water

• Soluble in organic solvents – acetone, ether• Saponifiable (can be split by hyrolysis)

• Triglyceride (fats), waxes

• Phospholipids, sphingolipids, glycolipids, lipoproteins

• Non-saponifiable• Carotenoids, cholesterol, fat soluble vitamins (D, K, E, A)

Page 4: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Functions of lipids• Fuel for energy metabolism• Membrane structure

• Phospholipid, cholesterol

• Hormone action• Steroids, prostaglandins

• Electron transfer• Ubiquinone

• Antioxidant• Vitamin E

Page 5: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Importance of Lipid Metabolism

• In health– Provision of energy in fasting/starvation

– Provision of energy in pregnancy & for lactation

• In diseases– Endocrine disease

• Diabetes mellitus, Cushing’s syndrome

– Production/metabolic diseases• Ketosis, pregnancy toxaemia, peri-parturient syndrome, fatty

liver disease, hyperlipidaemia

Page 6: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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• Linoleic, linolenic and arachidonic are essential fatty acids with >1 double bond

• Important for membrane function

• Cis double bond not trans

Page 7: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipid of di-acyl glycerol provides the hydrophobic nature of phospholipid membrane

Page 8: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Transport of lipid

• Problem of solubility

• From intestine – lipoprotein (chylomicrons)

• From liver – very low density lipoprotein

• From tissues to liver – high density lipoprotein

• From adipose tissue – fatty acid binds to albumin

Page 9: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipoproteins

• Chylomicrons: • from intestine to adipose & other tissues

• Very low density lipoprotein (VLDL)• TG, cholesterol ester & phospholipid to tissues

• Low density lipoprotein (LDL)• Remnant of VLDL after release of lipid

• High density lipoprotein (HDL)• Collects lipid, especially cholesterol for return to

liver

Page 10: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipoproteins:

Central hydrophobic core of triglyceride, cholesterol ester

Monolayer of phospholipid in outer surface, hydrophilic head to outer-face

Apolipoprotein in phospholipid monolayer

Page 11: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipoprotein composition (%)

Chylo VLDL LDL HDL

Protein 1 10 25 50

Triglyceride 90 60 10 3

Cholesterol ester & cholesterol

5 15 45 20

Phopholipid 4 15 20 27

Size (m) 1000 50 20 10

Page 12: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Apolipoproteins

• Apolipoprotein:– Have many

hydrophobic amino acids

– Hydrophilic amino acid on surface

– Provide structure

– Some have activity in lipid metabolism

• Apo A: predominantly in HDL, Cofactor for lecithin cholestrol acyl transferase (LCAT)

• Apo B :Predominantly in HDL & VLDL

• Apo C:Predominantly in HDL VLDL; Activates lipoprotein lipase in transfer of TG to adipose tissue

• Apo D:Predominantly in HDL

• Apo E: Predominantly in HDL & VLDL

Page 13: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Fats in diet carried by chylomicron to tissues eg adipose

Liver exports TG, CE PL in VLDL to tissues

VLDL release lipids and become LDL

LDL remnants to tissues and liver

HDL is collecting lipid from tissues & from LDL to return to liver (reverse transport)

Page 14: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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HDL released from liver

Collects excess cholesterol from tissues

Produces cholesterol ester and lyso-lecithin from cholesterol and lecithin (phosphotidyl choline)

Cholesterol ester to VLDL and TG from VLDL

HDL absorbed by liver

Page 15: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Transport from Adipose tissue –When energy needed

• Hormones (adrenaline, glucagon, cortisol) activate hormone sensitive lipase in Adipose Tissue

• Triglyceride 3 Fatty acid + Glycerol • Released from adipose to blood• Fatty acid disrupts membranes• Fatty acid bound to albumin – protects membranes• Fatty acid goes to tissues (eg muscle & liver) for

energy• Glycerol goes to liver for gluconeogenesis

Page 16: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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In cell (eg liver, muscle), FA linked to coenzyme A by high energy bond

Required ATP, releases AMP and PPi

2 x high energy ~phos bonds used

Page 17: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Carnitine Shuttle

CoA can not enter mitochondria

Outer membrane: After activation FA-CoA transfers FA to carnitine to produce fatty acyl-carnitine- enzyme inhibited by malonyl CoA

Inner membrane: FA transferred back to CoA, carnitine recycles

Page 18: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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-Oxidation of saturated fatty acid

a) Dehydrogenase (oxidation) at -carbon with FAD reduced and trans == formed

b) Hydratase, hydrates double bond

c) Dehydrogenase with NAD reduced

d) Thiolase cleavage with release of acetyl CoA and CoA on shortened fatty acid

Page 19: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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-Oxidation of saturated fatty acid

a) Dehydrogenase (oxidation) at b-carbon with FAD reduced and trans == formed

b) Hydratase, hydrates double bond

c) Dehydrogenase with NAD reduced

d) Thiolase cleavage with release of acetyl CoA and CoA on shortened fatty acid

e) Note similarity to steps of TCA cycle

Page 20: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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-oxidation of palmitic acid

16 carbon fatty acid

7 cycles yields

8 acetyl CoA

7 FADH2

7 NADH + H+

Which gives 131 ATP

Equivalent of 2 ATP used in activation of FA:- net 129 ATP

Page 21: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Palmitic acid yields >2x ATP per gram compared to glucose

Fatty acids have no water of hydration

Page 22: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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If FA has odd number of carbons, -oxidation proceeds till C3 fatty acyl CoA (propionyl CoA)

Carboxylase requiring biotin

Racemase (isomerase) converts between sterioisomers

Mutase – internal transfer of carboxyl group- required Vit B12

Succinyl CoA to TCA cycle and gluconeogenesis

THIS IS VERY IMPORTANT FOR RUMINANTS

Page 23: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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-Oxidation of poly unsaturated fatty acid

-oxidation operates for saturated FA (no double bonds)

When unsaturated other enzymes are involved for oxidation of double bonds

Reaction requires 2 x isomerases and NADPH dependent reductase as well as the usual enzymes

Altered reaction costs 5 ATP per double bond

Page 24: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Catabolism of lipid

Stimulated by hormone, (glucagon, adrenalin)

Adipose tissue releases FA & glycerol

In liver & muscle FA is activated & enters mitochondria

Oxidised to acetyl CoA & enters TCA cycle

In liver, excess acetyl CoA forms ketone bodies, secreted to blood & used in other tissues (not brain)

Page 25: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Control of lipid catabolism

• Whole animal level– Hormone sensitive lipase

• Triglyceride fatty acid + glycerol

– Activated by glucagon, adrenaline

• Cellular regulation– Carnitine shuttle inhibited by malonyl CoA– Malonyl CoA is a reactant in FA synthesis– Malonyl CoA elevated when cell is energy rich

Page 26: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Formation of Ketone Bodies

In Liver (only organ with enzymes):

3 x acetyl CoA form 3-OH, 3-methyl glutaryl CoA

1 acetyl CoA released

Acetoacetate* formed

Reduction forms -hydroxybutyrate*

Decarboxylation forms acetone*

Exported to blood

Ketone bodies are more soluble that fatty acid & help to make energy available for tissues in fasting & starvation

* These are the ketone bodies

Page 27: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Ketosis of acetyl Co-A

Page 28: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Use of Ketone Bodies

In tissues (not brain in most species):

OH butyrate & acetoacetate to acetyl CoA

Enters TCA cycle for ATP production

Excess production of ketone bodies occurs in metabolic diseases – diabetes mellitus, bovine ketosis, ovine toxaemia

Page 29: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Ketones in Ruminants

• Carbohydrate in diet of ruminants forms ‘volatile fatty acids’ in rumen

• These are acetate C2, propionate C3 and butyrate C4 fatty acids

• Butyrate converted to -OH butyrate (ie ketone body) in rumen wall before entry to blood

• Much energy for ruminant tissue metabolism come from this ketone body

Page 30: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipid Anabolism – FA Synthesis• When body/cell is energy rich• Liver, adipose tissue & mammary gland• In cytoplasm, separate from -oxidation• Substrate is acetyl CoA (2 carbons)

• From glycolysis or amino acid catabolism

• Exported from mitochondria by citrate shuttle

• Primary product – palmitoyl CoA, (16 carbons)• Then modified - elongated, desaturated, conjugated

Page 31: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Lipid Anabolism- FA Synthesis

• Co-factors: NADPH + H+, CoA

• Multi-enzyme complex• Fatty Acid Synthase complex

• Control point - Acetyl CoA carboxylase• Activated by citrate; inhibited by palmitoyl CoA

• Hormone activation (insulin); inhibition (glucagon)

• Nutrition: cabrohydrate activates; lipid inhibits

Page 32: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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FA Synthesis in Ruminants

• Fatty acid synthesis primarily in adipose tissue and mammary gland

• Little synthesis in liver• Substrate:

– Acetyl CoA from acetate in blood

– Acetate in blood is rumen product ‘volatile fatty acid’

– Activated by acetyl CoA synthase• Acetate + CoASH + ATP Acetyl CoA + AMP + PPi

Page 33: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Acetyl CoA +

OxaloacetateADP, Pi

Citrate + CoA

Citrate + CoA+ ATP

Oxaloacetate+

Acetyl CoA +

ADP + Pi

MalatePyruvate

Pyruvate

Mitochondria

Cytoplasm

COO- , ATP

carboxylaseCitrate synthase (TCA cycle)

FATTY ACID SYNTHESIS

Citrate lyase

NAD+ NADH+ + H+

Malate DH

Malate enzyme

NADPH+ + H+ NADP+

CITRATE SHUTTLE

(Malate Shuttle also operates)

Page 34: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Acetyl CoA carboxylase, activated by citrate, inhibited by palmitoyl CoA (product)

Activated by insulin, inhibited by glucagon

Upregulated by hi carbon/low fat diet

Downregulated by hi fat/low carbon diet

Control site for FA synthesis

Page 35: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Page 36: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Fatty acid synthase complex: 7 enzymes and acyl carrier protein (ACP) in dimer formation

ACP has active –SH and binds growing FA chain

Cys with –SH also present

Each enzyme required for FA synthesis

Page 37: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Transacylase: acetyl CoA to cys-SH, malonyl CoA to ACP-SH

Condensation by ketoacyl synthase: acetyl replaces carboxyl grp of malonyl, becomes C4

Reductase: C3 keto group becomes hydroxyl (NADPH)

Dehydratase: hydroxyl grp lost, double bond in place

Reductase: double bond is hydrogenated (NADPH)

Acetyl Transacylase: C4 FA transferred to cys-SH, new malonyl to ACP-SH

Reactions catalysed by fatty acid synthase

Page 38: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Page 39: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Fatty Acid Synthesis

• Continues for 6 cycles

• C16 palmitoyl Co A is released by thioesterase

• Released palmitoyl CoA further modified– Elongated by addition of acetyl groups– Desaturated to give unsaturated FA– Conjugated to triglyceride and phospholipid

Page 40: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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In endoplasmic reticulum

C2 groups from malonyl CoA with loss of CO2

Multi-enzyme complex needed – FA elongase

Very long chain FA (22-24 carbons) needed for central nervous system

In Mitochnodria

Elongation by reversal of -oxidation

Page 41: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Example: palmitoyl CoA (C16) Palmitoleioyl CoA C16 C16, cis 9

In endoplasmic reticulum, NADH & cytochrome b5 required

Page 42: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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In endoplasmic reticulum

In Liver: glycerol-3-phos from glycerol, product (TG) goes to VLDL synthesis

In adipose:glycerol-3 phos from glucose via glycolysis (no glycerol kinase), therefore only when animal has high glucose

* Note error in fig: Pi released from phosphatidic acid not taken up

*

Page 43: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Control of FA synthesis

• In fed state with excess of energy– Increased insulin from pancreas

• Activates acetyl CoA carboxylase

• Activates lipoprotein lipase in adipose tissue, increasing TG breakdown in VLDL & transfer of Fatty acid to adipocyte

– Decreased glucagon – (insulin antagonist)

– High glucose • Enters adipose tissue, provides glycerol, essential for TG re-

synthesis

Page 44: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Synthesis of Phospholipid

• In endoplasmic reticulum• A) diacyl glycerol (as in TG formation) reacts

with CDP-choline giving phosphatidyl choline and CMP

• Same with CDP-serine & CDP-ethanolamine

• B) phosphatidic acid (TG formation), react with CTP to form CDP-diacyl glycerol then with inositol to form phosphatidyl inositol

• From endoplasmic reticulum to membranes

Page 45: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Synthesis of cholesterol

Liver predominant site

Acetyl CoA to mevalonate

3 x acetyl CoA forms HMG CoA (as in ketone formation)

Reduction to mevalonate (C6)

Converted to isopentenyl pyro phosphate (C5; diphosphate)

5 x isopent pyrophos condensed to give squalene

Page 46: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Natural conformation of squalene encourages ring formation by cyclase

Page 47: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Cholesterol

• Forms cholesterol ester for transport in VLDL (enzyme is LCAT)

• Forms bile acids with glycine, taurine for digestion

• Modifications lead to hormones

• No degradation in body (ie no enzyme to break ring structure)

Page 48: Lipid and Ketone Metabolism Week 12. 2 Lipids Lipids: structure & function Transport of lipids: albumin-binding & lipoprotein Storage & release of lipids.

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Regulation of cholesterol synthesis

• Committing enzyme:- 3-hydroxy 3-methyl glutaryl reductase (formation of mevalonic acid)

• Control by cholesterol concentration in cell– Hi chol inhibits: low chol activates

• Control by hormones– Insulin & thyroid hormone activates: glucagon & cortisol inhibit

• Control by diet – Enzyme synthesis increases in fasting– Enzyme synthesis decreases when dietary cholesterol is high

• Target for ‘statin’ drugs