Lipids 2 Flashcards
Hormone sensitive lipase
Acts on stored TAG (in adipocytes) and is converted to FA and glycerol.
Located in the adipocytes
Inhibited by insulin (activated during diabetic ketoacidosis).
Ketone body synthesis reactions upto primary ketone body formation
- Acetoacetyl CoA (from beta oxidation) and Acetyl CoA combine with the help of HMG CoA Synthase (RDS)
- The HMG CoA formed is split by HMG CoA Lyase to acetoacetate (and Acetyl CoA)
Ketone body synthesis occurs in
Exclusively Liver mitochondria
Secondary ketone body synthesis from primary
Acetoacetate is either
a) spontaneously decarboxylated to acetone
b) converted to Beta Hydroxy Butyrate by b-OH Butyrate dehydrogenase utilising NADH
Two organs that cannot utilise ketone bodies are
Liver, RBC
Ketone body utilisation from primary ketone body
First step
Acetoacetate accepts CoA by Thiophorase (S- CoA Acetoacetate CoA transferase) from succinyl CoA to become acetoacetyl CoA (and succinate but no GTP/ATP is generated)
Ketone body utilisation from primary ketone body
Second step
Acetoacetyl CoA is converted to 2 Acetyl CoA as part of beta oxidation by thiolase
Fate of secondary ketone bodies
- Beta OH butyrate is converted to acetoacetate producing NADH+
- Acetone is volatilised and excreted through lungs (fruity smell in ketosis)
Energetics of ketone body utilisation from acetoacetate
2 Acetyl CoA are formed
TCA cycle occurs twice but in one cycle thiophorase is used instead of thiokinase
So 20-1= 19 ATP
If it is beta OH butyrate 19+2.5= 21.5 ATP
Most common ketone body in a normal person
Beta Hydroxy butyrate = acetoacetate
Most common ketone body during starvation
Beta Hydroxy Butyrate: Acetoacetate = 6:1
Neutral ketone body
Acetone
Test for ketone body
- Rothera’s test
a) Purple ring - acetoacetate and acetone
b) Beta Hydroxy Butyrate does not answer Rothera’s test - Gerhard’s test answered only by acetoacetate
- Ketostix - dipstick test
Organs where FA are synthesised
Liver, adipose tissue, brain, kidney, lungs, lactating mammary glands
Steps of FA is elucidated by
Feodor lynen
Hence FA synthesis is also called Lynen’s spiral
Steps of FA synthesis
- Transfer of Acetyl CoA from mitochondria to cytoplasm
- Acetyl CoA carboxylase
- FA synthase complex reactions requires Mn+2
Transport of Acetyl CoA into cytoplasm for FA synthesis
- First step of TCA occurs
- Citrate exits via Tricarboxylic Acid Transporter
- It is split into Acetyl CoA and OAA by ATP Citrate Lyase
Acetyl CoA carboxylase , the second step of FA synthesis
Acetyl CoA is carboxylated to Malonyl CoA using:
- bicarbonate (HCO3-)
- Acetyl CoA carboxylase
- ATP
- Biotin
FA synthase complex structure
- Homodimer
- Each monomer unit has 6 enzyme activity + 1 Acyl carrier protein (ACP)
- ACP has a pantothenic acid as 4 phosphopantotheine.
- Multifunction enzyme-single polypeptide has more than 2 enzyme activity.
- X shaped (using X-ray crystallography)
Domains of FA synthase
- Condensing unit
- Reduction unit
- Releasing unit
- Acyl Carrier Protein
Condensing unit- enzymes
- Acetyl/ Malonyl transacylase
- Ketoacyl synthase
ACP
Reduction unit-enzymes
- Ketoacyl reductase
- Dehydratase
- Enoyl reductase
Releasing unit-enzyme
Thioesterase
This unit takes place only once per FA
Cys-SH group of first monomer unit and Pan-SH group of 2nd monomer unit
By Acetyl/Malonyl transacylase
a Acetyl group combines with Cys-SH and
a Malonyl group combines with Pan-SH
Action of ketoacyl synthase, 2nd enzyme of first unit
From Malonyl group, a CO2 is removed and then the Acetyl group condenses with it
A keto compound of 4C is formed at pan-SH
Reduction unit
Ketoacyl compound is reduced using NADPH to Acyl group (Acetoacetyl initially)
Releasing unit
Acyl group combines with CoA and separated from the complex by thioesterase.
Different types of regulation of FA synthesis
- Short term
a) allosteric
b) covalent
c) compartmentalisation - Long term
Increased Acyl CoA decreases the expression of enzymes that synthesise FA
Acetyl CoA carboxylase regulation
Inactive state- dimeric
Active- polymeric
Activator-citrate (also activates TCA transporter)
Inhibitor-LCFA (also inhibits TCA transporter)
Compartmentalisation of FA acid synthesis
Beta oxidation occurs in the mitochondria while FA synthesis occurs in the cytoplasm
Elongation of FA
- Major
In SER by microsomal FA elongase system - Minor
By mitochondrial FA elongase
For myelination of brain
Synthesis of unsaturated FA
Involves the enzyme in ER :
- Desaturase
- Elongase
Humans cannot insert a double bond between C10 and terminal methyl
Cholesterol concepts
Regulated by insulin Cannot generate energy Purely animal sterol 27C 50% excreted
Cholesterol is synthesised in
all nucleated cells especially in liver, adipose tissue, adrenal cortex, gonads,intestine
It is synthesised in SER and cytoplasm
Stages of synthesis of cholesterol
- Synthesis of HMG CoA (6C)
- Synthesis of Mevalonate (6C)
- Synthesis of isoprenoid unit(5C)
- Isoprenoid units join to form 30C squalene
- Trimmed to Cholesterol
HMG CoA is involved in
- Synthesis of Cholesterol (cytoplasmic)
- Synthesis of Ketone body (mitochondrial)
- Leucine metabolism
Synthesis of Mevalonate
HMG CoA is converted to Mevalonate by HMG CoA reductase (RDS)
Statins is a competitive inhibitor of this enzymes
Occurs in SER
Synthesis of isoprenoid units
Mevalonate is decarboxylated and phosphorylated to isoprenoid unit (5C)
Squalene formation
2 Isoprenoid unit = Geranyl PPi 10C
Combines with isoprenoid unit (5C) to form Farnesyl PPi
2 Farnesyl PPi combine to form Squalene (30C)
Trimming to cholesterol
Squalene Lanosterol (first cyclical compound) Zymosterol Desmosterol Cholesterol
SLZDC
Regulation of cholesterol synthesis
- Feedback regulation
- Feedback inhibition
- Hormonal regulation
Long-term feedback regulation of Cholesterol synthesis
Dietary cholesterol decreases binding of SREBP at genes which in turn reduces expression of HMG CoA reductase (RDS)
Feedback inhibition of cholesterol synthesis
Mevalonate inhibits HMG CoA reductase
Formation of primary bile acids in liver
From cholesterol by 7-alpha Hydroxylase (Cytochrome P7A1 or CYP7A1) using 1. NADPH 2. vitamin C 3. O2 To get 7-Hydroxy cholesterol
This after multiple steps forms primary bile acids cholic acid and chenodeoxycholic acids requiring NADPH and producing propionyl CoA
Formation of secondary bile acids
After deconjugation and dehydroxylation of primary bile acids:
- cholic acid to deoxycholic acid
- chenodeoxycholic acid to lithocholic acid
98-99% of secondary bile acids undergo enterohepatic circulation
Least enterohepatic circulation is for
Lithicolic acid
Regulation of bile acid synthesis
Farnesoid X receptor (FXR)
RDE is 7-alpha hydroxylase (CYP7A1)
Increased bile acid (Chenodeoxycholic acid) will decrease the binding of this receptor to the gene
Thus decreasing expression of this enzyme
Layers in lipoprotein
- Hydrophobic lipids (TAG,cholesterol ester)
- Amphipathic lipids (cholesterol, phospholipids)
- Proteins (integral, peripheral)
Maximum lipid content (TAG) and least protein content is in
Chylomicron
Apolipoproteins in chylomicron and remnant chylomicron
Unique :
app B 48
Major :
Apo C2
Apo E
VLDL is assembles in
Liver
and carries endogenous TAG from liver to peripheral organs
VLDL contains the apo lipoproteins
Apo B100
From HDL:
Apo C2
Apo E
Lipoprotein cascade pathway
VLDL to IDL to LDL
Maximum cholesterol and cholesterol ester content is present in
LDL or beta lipoprotein
Apolipoprotein present in LDL
Apo B100 only
Maximum apoprotein and phospholipid content is in
HDL
Formed from liver and intestine
Participates in reverse cholesterol transport
Repository for apo E and Apo C2
HDL or alpha lipoprotein
Lipoprotein of HDL
Enzyme activity of HDL
Apo A1
Enzyme activity:
- LCAT (Lecithin Cholesterol Acyl Transferase) activated by apo A1
- Cholesterol ester transfer protein (CETP)
LCAT
Lecithin + Cholesterol
cholesterol ester + Lysolecithin
CETP
- Transfers cholesterol ester from HDL to other lipoproteins like LDL,…
- in turn transfers TAG from other lipoproteins to HDL
Inhibited by apo C1
LP(a)
Lipoprotein that contains apo(a) and apo B100 linked by a disulphide bond
apo(a) is a structural analog of plasminogen and hence inhibits clot lysis
Lipoprotein X
Lipoprotein produced during cholestasis from unexcreted cholesterol and phospholipids
Indicator of cholestasis
Pre beta lipoprotein
VLDL
Broad beta lipoprotein
IDL
Order of lipoproteins from cathode to anode
Chylomicron LDL VLDL IDL/remnant VLDL HDL
Protein content~ electrophoretic mobility
Nascent chylomicron contains the lipoproteins
B48
Helps in the assembly of chylomicron in intestine
LPL HP(Lipoprotein Lipase)
Anchored to the capillaries surrounding the peripheral organs
Hydrolyses the TAG
Activated by apo C2
Involved in both chylomicron and VLDL metabolism
Function of apo E
Ligand for hepatic receptors for internalisation of remnant chylomicron and IDL
Receptor mediated endocytosis
Apo proteins of nascent VLDL
B100
Helps in the assembly of CLDL
Ligand for LDL
Fates of IDL
- Receptor mediated Endocytosis into the liver via ligand apo E
- Loses some TAG (via endothelial and hepatic lipase)
Apo E and Apo C2 are removed and then converted into cholesterol ester rich LDL (having only Apo B100)
Thus is the lipoprotein cascade pathway
Fates of LDL
- 70% of LDL is taken by the liver via LDL receptors
- 30% LDL is taken by extra-hepatic tissues via LDL receptors
Ligand for LDL receptors is Apo B100 (receptor mediated endocytosis)
Oxidation may occur which is ingested by macrophages leading to atheroma
Transporters present in HDL
ABCA1
ABCG1
SRB1 (Scavenger Receptor B1)
ATP Binding Casette
Transports cholesterol and its esters from peripheral organs to HDL3 (spherical)
Why newly formed HDL is disc shaped
Cholesterol and phospholipid which it contains are both amphipathic compounds (which exists in a structure similar to cell membrane)
Change of shape from discoidal to spherical HDL3
Due to the formation of hydrophobic cholesterol ester by LCAT from cholesterol
Formation of spherical HDL2 and the conversion back to HDL3
After receiving cholesterol from peripheral cells, spherical HDL3 is converted to spherical HDL2
which later donates the cholesterol to liver to mostly form HDL3 again
Pre beta HDL
While HDL2 is donating cholesterol, certain lipase liberate apo A1.
Apo A1 accepts cholesterol, phospholipid,… to form poorly lipidated HDL, i.e, pre beta HDL
The most potent HDL
Function of apo C3
Function of apo A2
Inhibits lipoprotein lipase
Function of apo A5
Facilitates the binding of chylomicron and VLDL to lipoprotein lipase
Apo D
Associated with human neurodegenerative diseases like Parkinson’s disease
Special features of apo E
Arginine rich
Apo E4 is associated with late onset of Alzheimer’s disease
Classification of hyperlipoproteinemia is done by
Fredrickson and Levy
Primary hyperlipoproteinemia are classified into (according to Harrison’s)
- Primary hyperlipoproteinemia with hypertriglyceridemia
- with hypercholesterolemia
- with both hypertriglyceridemia and hypercholesterolemia
Primary hyperlipoproteinemia with hypertriglyceridemia (Fredrickson’s classification)
Type 1 :
Familial chylomicronemia syndrome
Type 4 :
Familial hypertriglyceridemia apo A-V defect
Type 5 :
Familial hypertriglyceridemia apo A-V defect and GPIHBP-1 defect
Primary hyperlipoproteinemia with hypercholesterolemia (Fredrickson’s classification)
Type 2
Primary hyperlipoproteinemia with both hypertriglyceridemia and hypercholesterolemia
Type 3 or Familial Dysbetalipoproteinemia (FDBL)
Type 1 hypercholesterolemia
Familial chylomicronemia syndrome
- Apo C2
- Lipoprotein lipase
Chylomicron and VLDL increased
TAG accumulates
Clinical features of type 1 hyperlipoproteinemia
- Milky white plasma
- Eruptive xanthoma
- On fundoscopy, lipemia retinalis
- TAG>1000gm leads to pancreatitis and then abdominal pain
Treatment of familial chylomicronemia syndrome
Gene therapy:
A lipogene -Tiparvovec
Adeno associated viral vector expressing LpL variant leading to myocyte expression of LpL
Type 4 hyperlipoproteinemia
Apo A5 defect which facilitate the association of chylomicron,VLDL with LpL
Familial hypertriglyceridemia
Type 5
- Apo A5 defective or
- Defect in glycosylated phosphatidyl inositol HDL binding protein-1 (GPIHBP-1) which helps in the export of lipoprotein lipase to vascular endothelium
Increased TAG
Familial hypercholesterolemia/
ADH type 1
Fredrickson’s type 2a
Most common
Defective LDL receptor
Therefore Cholesterol and cholesterol ester is elevated
Clinical features of familial hypercholesterolemia
- Corneal arcus
- Tendon xanthoma
- Clear plasma
- Increased risk of CAD and PVD
No abdominal pain
Treating of familial homozygous hypercholesterolemia
- Lomitapide-inhibits Microsomal Triglyceride Transfer Protein (decreases VLDL leading to decreased LDL)
- Mipomersin-antisense oligonucleotide to apo B
Sitosterolemia biochemical defect
Type 2a
Primary hyperlipoproteinemia with hypercholesterolemia
Defective ABCG5 and ABCG8 (which usually actively excrete plant sterols from liver and intestinal cells)
Clinical features of sitosterolemia
Decreased plant sterols in the cells leads to decreased transcription of LDL receptors
This leads to increased LDL in blood which leads to increased cholesterol in blood
ADH type 2
Familial defective apo B(FDB)
Apo B100 defective
Autosomal dominant
ADH type 3
PCSK9-secreted protein that accelerate lysosomal degradation of LDL receptors
Gain of function mutation occurs in PCSK9 (increased activity)
Decreased LDL receptor
Autosomal recessive hypercholesterolemia
Defect in LDL receptor adaptor protein (LRAP)
Decreased LDL uptake
Type 3 Hyperlipoproteinemia/ Familial dysbetalipoproteinemia (FDBL)
Clinical features
- Tuberoeruptive xanthoma (like a bunch of grapes)
- Palmar xanthoma /Lipid deposition in palmar creases
- Slight risk of CAD
- Plasma is clear
Type 3 Hyperlipoproteinemia/ Familial dysbetalipoproteinemia (FDBL)
Biochemical defect
Apo E mutation (which acted as a ligand for the uptake of chylomicron remnant and IDL/VLDL remnant)
Hence these accumulate leading to accumulation of both TAG and cholesterol
So called as Remnant removal disease or broad beta disease
Hypolipoproteinemia
- Abetalipoproteinemia
- Tangier’s disease
- LCAT deficiency
Abetalipoproteinemia biochemical defect
Defective Microsomal Triglyceride Transfer Protein (transport of lipid to the apo protein)
So decreased lipoproteins except HDL
Clinical features of abetalipoproteinemia
- Acanthocytes
- Pigmentary retinitis
- Bleeding manifestations due to decreased fat soluble vitamins
Tangier’s disease
Biochemical defect
Defective ABCA1 (transport cholesterol from peripheral organs to HDL)
Decreased spherical HDL
Other lipoproteins are normal
Clinical features of Tangier’s disease
- Orange/yellow tonsils (cholesterol accumulation)
- Hepatosplenomegaly
- Mononeuritis multiplex
Norum’s disease
Complete LCAT deficiency
Increased lecithin and cholesterol
Decreased lysolecithin and cholesterol ester
Progresses to end stage renal disease (ESRD)
Fish eye disease
Partial LCAT deficiency
Benign
Do not progress to end stage renal disease (ESRD)
Steps in the action of hormone sensitive lipase
TAG➡️2,3-diacyl glycerol ➡️ 2-Mono Acyl glycerol
Then esterases act on the product to form glycerol and FA
Activators of hormone sensitive lipase
Glucagon
Catecholamines
ACTH,TSH
Glucocorticoids, thyroid hormones
Inhibitors of hormone sensitive lipase
Insulin
Nicotinic acid
PG E1
Lipoprotein lipase is anchored to the capillaries of the organs:
Heart, adipose tissue,spleen,renal medulla,aorta, diaphragm, lactating mammary glands
Heparin and lipoprotein lipase
If we inject heparin the lipoprotein lipase is dislodged and is free to be calculated
Hepatic lipase function
Act on chylomicron remnant
Convert HDL2 to HDL3
Endothelial lipase action
Acts on HDL3 to convert it into HDL2 and pre-beta HDL (most potent)
