Week 3A: Fatty acid metabolism and Membrane lipids Flashcards
-HC -WC -Chapter 22 -Chapter 27
Functions fatty acids
-Building blocks of phospholipids in cell membranes
-Targeting: bind proteins, directing them to designated locations in membranes
-Messengers: FA derivatives serve as hormones or messenger molecules like diacylglycerol (DAG)
-Energy: storage as triacylglycerols (TAGs)
Which bonds links FAs and glycerol in TAGs?
ester bonds
> condensation of hydroxylic and carboxylic group
Why are TAGs called neutral fats
Neutral charge
> free fatty acids are charged
High concentrations of DAG are correlated to..
defect in lipid degradation
How is the FA precursor of the hormones prostaglandins called?
Arachidonic acid
Type of chains found in fatty acids
acyl chains
Why do FAs yield much energy compared to proteins and carbohydrates?
They are strongly reduced
What is the natural polarity and hydro state of FAs?
Nonpolar and hydrophobic, forms hydrophobic interactions
Where are TAGs stored in the cell (which special organelle)
Fat droplets: dense in energy
Fats are stored anhydrous, what does this mean?
Water free
Why are fats dense in energy
They do not bind water and therefore weigh less per energy output
FAs do not have hydrogen acceptors. What are hydrogen bond acceptors? What about TAGs?
Free O group.
> The ester bonds of TAGs contain hydrogen bond acceptors and bind a bit of water
How does the unilocular lipid droplet in adipocytes develop?
Little droplets fuse to one with hydrophobic interactions
Adipocytes store …, when the lipid droplet grows, there is ….
TAGs, hypertrophy of the adipocyte
How is the function of the pancreas to secrete lipases into the GI tract called?
Exogeneous function
Uptake TAGs from food
-TAGs + H2O -> FAs + MAGs (by pancreatic lipase)
- Import into mucosal cell / enterocyte
- Make TAGs from FAs and MAGs
- Form chylomicrons (lipoproteins)
- To lymphatics
- To blood
-To adipose tissue and muscle using LPL (lipoprotein lipase, digests chylomicron to 3 FAs and glycerol, uptake FAs by target cell)
Pancreatic lipase has a lot of activity but limited operation rate, explain
It removes the outer fatty acids of TAG but cannot reach the middle one, products are 2 FAs and 1 MAG.
Why are proteins added to the lipids to make chylomicrons?
To make them water soluble for transport in blood
Where does LPL operate
LPL is secreted by organs which need the FAs, like muscle cells and adipocyes
What happens to fats stored in adipocytes in the fasted state?
They are taken for mobilization of energy
Three global steps of burning fat: lipolysis and beta-oxidation
- Mobilization
-Breakdown TAGs in adipocytes into glycerol and FAs > transport to other tissues using carrier - Activation and transport
-Activation FAs (stay inside cell, charge, and to get more energy, destabilization) > transport into mitochondrial matrix for degradation - Degradation: FAs are broken down into acetyl-CoA in step-by-step manner > acetyl-CoA is oxidized in TCA cycle to CO2 and H2), generating high energy electrons on carriers for respiratory chain and ATP synthesis
Which enzyme of the fasted state promotes TAG breakdown?
Glucagon
Where is fat stored?
In adipocytes and muscle
Which central enzyme has a role in TAG breakdown in adipocytes and muscle cells? How is it activated?
PKA
Activation:
> Glucagon/epinephrine binds to 7TM receptor
> Activation Gas –> activation adenylate cyclase
> Conversion ATP to cAMP
> cAMP binds regulatory subunits PKA: activation
How does PKA induce TAG breakdown / lipolysis
-PKA activates perilipin and hormone sensitive lipase (HS lipase)
-Perilipin-P adds CA to ATGL > ATGL catalyzes TAG to DAG with FA
-HS lipase-P catalyzes DAG to MAG and FA
-MAG lipase catalyzes MAG to glycerol and FA
How is glycerol used when TAGs are broken down?
Glycerol is a precursor of gluconeogenesis for the liver in the fasted state
> or glycolysis to generate pyruvate
Released FAs after lipolysis are used for…
energy > transport to blood
Main human FA
Palmitate (C16)
> also product of FA synthesis
Unsaturated FAs contain…
double bonds
Fats in human mainly consist … double bonds
cis
Oleate is a C18, unsaturated, cis omega-9 FA. How is this determined?
From the omega carbon (last counted from carboxylic end), count backwards until double bond.
Numbered carbon counts in FAs
Count from carboxylic end, and include the carbolic acid carbon
Greek letters carbons counting in FAs
Count from carboxylic end but do not include the carboxylic acid carbon
Which carbon is oxidated in FA breakdown?
Beta carbon > beta-oxidation
Which greek letter indicates a double bond in numbered nomenclature
the delta triangle
> use cis or trans
Trans unsaturated fats are uncommon and usually…
not so good
Which abundant C20 FA do we have?
Arachidonate, unsaturated, made in our body to make prostaglandins after conversion to arachidonic acid (arachidonate is the base/alkaline form)
FAs are insoluble, how do they travel through blood?
Bound to serum albumin (abundant plasma protein)
The reaction of FA binding to albumin is a … reaction
equilibrium
How does the FA bind to serum albumin
to the hydrophobic pocket
How are FAs taken up by target cells?
The cells express FA uptake transporters
Do all FAs strictly need transporters for import?
No, very small FAs can wiggle and diffuse through the PM, long ones need help
Which membrane transporters transport long-chain FAs over the PM?
FATP and CD36
> FATP: Fatty acid transport protein.
Activation step inside target cell to retain FAs
Add a CoA with thioester bond (high energy)
> make acyl-CoA
CoA is a carrier of …
carbons
Net reaction and ATP costs FA activation
FA-COO- + CoA-HS + ATP <=> AMP + PPi + acyl-CoA (acyl-C=O-S-CoA)
> it costs 2 ATP to regenerate ATP from AMP
> acyl-CoA synthetase
Intermediate made in FA activation
acyl-adenylate
-addition of AMP to fatty acid carboxyl end using ATP> AMP
-energy used to switch AMP to S-CoA (thioester bond) + AMP
When is fatty acid breakdown (beta-oxidation) committed?
When the FA is transported to the mitochondrial matrix (past the inner mitochondrial membrane)
Can FAs pass the outer mitochondrial membrane without energy use?
Yes, through a gradient.
Which FAs require a special transport mechanism to pass the inner membrane of mitochondria?
Long-chain FAs (12-20 C)
To which molecule is the acyl group transferred to tranfer across the inner membrane. Which enzyme is involved in the committed step?
To carnitine, enzyme: carnitine palmitoyl transferase 1 (CPT-1)
How is acyl-carnitine flipped over the inner membrane and what happens next?
A translocase transfers the molecule. In the matrix CPT-2 transfers the acyl group back to a CoA to form acyl-CoA
CPT-1 is the committed step in ..
beta-oxidation
Which process will acyl-CoA enter in the matrix?
The beta-oxidation cycle
Reaction sequence beta-oxidation cycle
Oxidation (generate FADH2)
Hydration
Oxidation (generate NADH+H)
Thiolysis
End products one beta-oxidation cycle
acetyl-CoA (C2) + acyl-CoA (Cn-2)
FAD is the electron acceptor in the first oxidation step of the beta-oxidation cycle. What happens to it?
Transfer to Coenzyme Q in inner membrane which transfers the electrons to complex 3 of the respiratory chain
First oxidation step b-oxidation cycle
Acyl-CoA + FAD > trans-delta2-enoyl-CoA + FADH2 (double bond at C2-C3, release 2 H)
Enzyme: acyl-CoA dehydrogenase
Isozymes acyl-CoA dehydrogenase
-LCAD: long chain
-MCAD: medium chain
-SCAD: short chain
Hydration in b-oxi cycle
Trans-delta2-enoyl-CoA + H2O > L-3-hydroxyacyl-CoA (addition OH group at C3 and extra H group at C2, counting from carboxylic end connected to CoA)
-Stereospecific reaction
-Enzyme: enoyl-CoA hydratase
Oxidation 2 in b-oxi cycle
NAD+ as electron acceptor
-Enzyme specific for L-isomer: L-3-hydroxyacyl-CoA dehydrogeanase
-L-3-hydroxyacyl-CoA + NAD+ > 3-ketoacyl-CoA + NADH + H+
Thiolysis in b-oxi cycle
-3-Ketoacyl-CoA + HS-CoA > Acyl-CoA + Acetyl-CoA
-Enzyme: Thiolase ( beta-ketothiolase)
Range of fatty acids with site of catabolism and membrane transport
-Short chain (C2-4), Mitochondrion, Diffusion
-Medium (C4-12), Mitochondrion, Diffusion
-Long (C12-20), Mitochondrion, Carnitine cycle
-Very-long (C20+), Peroxisome, Carnitine cycle (export)
Palmitate is a … (range) FA
Long-chain (C16 is within 12-20)
Peroxisomes, unlinke mitochondria, lack the …
electron transport chain
How does the peroxisome take care of released electrons in oxidation of very-long FAs?
Oxygen is used as electron acceptor
> Acyl-CoA dehydrogenase has FADH2 bound in reducted state, this is oxidized by reducing O2 to H2O2 (hydrogen peroxide)
-H2O2 is converted to H2O and 0.5 O2 by enzyme Catalase.
What happens to the remainder of the reactions beside the first oxidation in peroxisomes?
They are the same as in the mitochondria but performed in the peroxisomes.
> Shortened until medium chain or long chain FAs and then transported to mitochondria for further degradation.
What happens to the formed acetyl-CoA and NADH in peroxisomes?
Peroxisomes do not have a TCA cycle, the acetyl-CoA and NADH is transported to mitochondria and degraded to CO2 and H2O there.
How many beta-oxidation cycles for palmitate (C16)?
7 cycles
16/2 = 8 but the last cycle yields two acetyl-CoA (C2)
Degradation odd numbered carbons (FAs)
Make propionyl-CoA (C3) and acetyl-CoA (C2) out of 3-ketopentanoyl-CoA (C5) in the final thiolysis step.
> with the C3 you can make glucose, with the C2 you cannot
> C3: intermediate TCA cycle made: anaplerosis. Succinyl CoA which can be converterted to oxaloacetate
Propionyl-CoA conversion to make glucose
Propionyl-CoA > Succinyl-CoA (intermediate TCA cycle) > Oxaloacetate > Gluconeogenesis in liver
Which fats can be used to make glucose?
Odd numbered, done in the mitochondria.
Additional enzymes for breakdown unsaturated FAs
-For degradation double bond on odd-carbon: isomerase (enoyl-CoA isomerase) needed to shift position double bond
-Degradation double bond on even-carbon: both isomerase and reductase (dienoyl-CoA reductase) required.
> Only these two needed additionally for complete degration of (poly)unsaturated fatty acids
Ketone body synthesis substrate
Acetyl-CoA
Ketone body synthesis organ
The liver, in mitochondrial matrix
When ketone body synthesis
When fat breakdown predominates: long fasting
Committed step enzyme in ketogenesis
HMG-CoA synthase
Reactions ketogenesis
2 acetyl-CoA (from beta-oxidation) <=> acetoacetyl-CoA (C4) + CoA
Acetoacetyl-CoA + Acetyl-CoA (C2) <=> HMG-CoA (C6) (HMG-CoA synthase, committed step)
HMG-CoA > acetoacetate (C4) + Acetyl-CoA (C2)
(Acetoacetate + NADH + H+ <=> D-3 hydroxybutyrate (C4) + NAD+)
(Spontaneous: Acetoacetate > Acetone (C3) + CO2)
The two ketone bodies
-Acetoacetate
-D-3-hydroxybutyrate: more reduced, more energy carried, NADH needed for synthesis from acetoacetate.
Cell is very reduced > Lots of NADH > more of this ketone body:
d-3-hydroxybutyrate
Acetoacetate is instable: explain
It can spontaneously be converted to acetone which releases CO2
> decarboxylation
> fruity scent: smelled when diabetes: acetone is a gas and is released in breath
Ketone body breakdown (for which organs)
Fuel for heart, muscle and brain in starvation
Acetoacetate breakdown (ketone body)
Acetoacetate + succinyl-CoA (intermediate TCA cycle, cataplerosis) > acetoacetyl-CoA + succinate (by enzyme CoA transferase)
Acetoacetyl-CoA + CoA > 2 acetyl-CoA (by enzyme thiolase)
How is excess acetyl-CoA shipped to the brain and other organs during starvation?
Through ketone bodies made by the liver
> liver does not have the transferase and cannot use ketone bodies to make acetyl-CoA (it has enough)
> brain can do without glucose for longer by using ketone bodies (1-2 weeks)
Liver provides ketone bodies to the peripheral tissue during two states:
Fasted and starvation state
HC16: Fatty acid synthesis cycle
Condesation
Reduction
Dehydration
Reduction
> reverse of beta-oxidation cycle
Where in the cell does FA synthesis occur?
In the cytosol
In FA degradation, the acyl carrier is the SH group of CoA. In FA synthesis, this is the … group of …
SH group of acyl-carrier protein (ACP)
> thioester bond
Activation of acetyl-CoA for FA synthesis
Acetyl-CoA (C2) > Malonyl-CoA (C3) (the activated C2 donor, used to extent FA chains)
(adding a carbon: carboxylation, using HCO3-)
Which electron carrier is required in FA synthesis. And which carriers are made in the breakdown?
NADPH used in synthesis
NADH and FADH2 made in breakdown
Making Palmitate from 8 Acetyl-CoA costs …
7 ATP and 14 NADPH
How are acetyl groups transferred to the cytosol?
It is transferred to citrate (C6) in TCA cycle and citrate in the cytosol can be converted to acetyl-CoA
Committed step FA synthesis with enzyme
Acetyl-CoA + ATP + HCO3- > Malonyl-CoA + ADP + Pi + H+
(enzyme: acetyl-CoA carboxylase)
> the rate limiting step
Prosthetic group of Acetyl-CoA carboxylase
Biotin (vitamin B7)
Which enzyme also has biotin as prosthetic group?
Pyruvate carboxylase
What is the biotin able to bind? What does acetyl-CoA carboxylase facilitate?
Activated CO2 (carboxyphosphate)
> acetyl-CoA carboxylase transfers CO2 group from carboxybiotin to acetyl-CoA, resulting in malonyl-CoA
> irreversible, committed, rate-limiting
Function acetyl-CoA carboxylase is similar to…
carboxylation of pyruvate to form oxaloacetate by pyruvate carboxylase in the gluconeogenesis
The thiol (SH) group of ACP is a part of the … group
Phosphopantetheine group (end in =O)
Which enzyme complex catalyzes the FA synthesis (elongation)
Fatty acid synthase complex
> individual proteins fused to one polypeptide in eukaryotes
Starting reactions of elongation phase FA synthesis
Formation acetyl-ACP and malonyl-ACP
-Acetyl/Malonyl-CoA + ACP > A/M-ACP + CoA
Enzymes: acetyl/malonyl transacylase
First step FA synthesis (remember, cycle opposites)
Condensation:
Acetyl-ACP (C2) + Malonyl-ACP (C3) > Acetoacetyl-ACP (C4) + CO2 (carbon from activation!) + ACP
Why is the condensation reaction of FA synthesis in a favorable equilibrium? Why is the reaction driven by ATP nevertheless?
Because the decarboxylation of malonyl-ACP contributes to the reduction of Gibbs free energy (negative).
The reaction is driven by ATP which was used to carboxylate acetyl-CoA to malonyl-CoA.
Elongation phase FA synthesis after condensation
Reduction>Dehydration>Reduction
-reduction keto group (C=O) of C3 carbon to methylene (CH2) group (count from carboxyl group linked to ACP)
-NADPH supplies reduction equivalents (electrons)
-Acetoacetyl-ACP + NADPH > d-3-hydroxbutyryl-ACP + NADP+
> Crotonyl-ACP + H2O
Crotonyl-ACP + NADPH > Butyryl-ACP + NADP+
Second round of elongation phase
Butyryl-ACP is extended by two carbon atoms derived from Malonyl-ACP (during condensation, one C is lost into CO2)
When does the chain extension stop for FA synthesis?
At C16: palmitate
Ruler enzyme of the chain length
Thioesterase
> hydrolyzes the thioester bond between ACP and the FA.
The human fatty acid synthase complex is one polypeptide chain with one active site, of:
Malonyl-acetyl transacylase
> this catalyzes the addition of ACP to both the groups of acetyl-CoA and malonyl-CoA.
For the human fatty acid synthase to work, … is essential
dimerization
How many acetyl-CoA make palmitate
8 (C2) make 1 (C16)
When are FAs made?
In fed state > insulin, enough energy, good times
Complete sum reaction acetyl-CoA to palmitate
8 acetyl-CoA + 7 ATP + 14 NADPH + 6 H+ >
palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ATP + 7 Pi
In which state does the FA synthesis occur?
When insulin rules, in the fed, energy-rich state
How is the acetyl-CoA transported to the cytosol, where FA synthesis takes place?
Using citrate.
In mitochondrion:
-Acetyl-CoA condenses with oxaloacetate to citrate (citrate synthase)
-Citrate moves through transporters or channels to the cytosol.
-There: citrate splits into acetyl-CoA and oxaloacetate (ATP-citrate lyase)
Redirecting oxaloacetate from cytosol to mitochondrion after acetyl-CoA shuttling
in cytosol
-Oxaloacetate + NADH > malate + NAD+ (malate dehydrogenase)
-Malate + NADP+ > Pyruvate + NADPH (malic enzyme)
-transport to mitochondrion of pyruvate
-Pyruvate > oxaloacetate (pyruvate carboxylase)
For each acetyl-CoA transferred to the cytosol … NADPH is generated
1
Oxidative phase of the PPP (glucose-6-P>ribulose-5-P) yields ….
2 NADPH and releases CO2
The two fates of pyruvate in mitochondrion
-To acetyl-CoA (in fed state, by pyruvate dehydrogenase)
-To oxaloacetate (in fasted state, gluconeogenesis for example, by pyruvate carboxylase)
End product FA synthesis: palmitate (C16). How are long-chain and very-long chain fatty acids with >C16 made?
Elongation of palmitate with C2 units (activated C2 donor: malonyl-CoA/ACP)
> these reactions take place on the cytoplasmic face of the ER and are catalyzed by fatty-acid elongase
How are unsaturated fatty acids made?
Takes place on the ER cytoplasmic side, catalyzed by fatty-acid desaturase
> requirement of O2 and NADH(/NADPH)
Which omega unsaturated fatty acids can be made in the human cell?
Omega-9
> NOT omega-3/6
Which important omega fatty acids cannot be synthesized by humans?
LA (omega-6)
ALA (omega-3)
EPA (omega-3)
DHA (omega-3)
omega-3/6 fatty acids are called
essential fatty acids > uptake by diet
Concern essential fatty acids for vegans
ALA is from plants, EPA and DHA from algae eating fatty fish
> conversion ALA to EPA and DHA is slow
LA (linoleic acid) and a-linolenic acid (ALA) serve as precursors for a large number of unsaturated fatty acids, among which the substrate for prostaglandin H2 synthase: ??
arachidonic acid
Why is the slow conversion of ALA to other essential omega-3 FAs a problem?
Needed in the brain
when the pKa is lower than the pH, then the ..
base/alkaline is made more
In humans, more arachidonic acid or arachidonate?
More arachidonate: pKa=4.8
Arachidonic acid is an important precursor for synthesis of ….
eicosanoids (C20)
> hormones which act locally
Arachidonate can be converted to the eicosanoid prostaglandin H2, which can be converted to other prostaglandins. Name all enzymes, and inhibitors
Prostaglandin H2 synthase makes prostaglandin H2 (inhibited by aspirin and ibuprofen: broad action: all prostaglandins inhibited)
> Prostaglandin H2 can be converted to prostacyclin or thromboxanes or others by their synthases
Effects prostaglandins
-Stimulate inflammation
-Regulate blood flow to organs
-Modulate ion transport across membranes
How is the enzyme (which) in the committed step of FA synthesis regulated?
Acetyl-CoA carboxylase (ACC, for acetyl-CoA> malonyl-CoA) is regulated through hormones (kinases) and allosteric regulators
Phosphorylation regulation of ACC
-ACC is inactivated by phosphorylatoin by AMPK, AMP-dependent kinase, a sensor of energy status (low energy activated) > the phosphorylation costs ATP as well!
-ACC is activated by dephosphorylation by protein phosphatase 2A (PP2A)
Allosteric regulation ACC
-Positive: citrate (enough energy, TCA cycle halted, accumulation, fed state)
-Negative: palmitoyl-CoA (product inhibition, only in FA breakdown: reciprocal regulation)
How are citrate concentrations high when the energy status of the cell is high?
High concentration ATP and NADH because Electron transport chain is halted.
> conversion isocitrate to a-ketoglutarate (by ATP and NADH, pos by ADP) and that to succinyl-CoA are inhibited (ATP, succinyl-CoA and NADH)
Without activation, the conformation of ACC is?
In dimers, but the ACC is only active when activated by citrate, which promotes the polymerization enzyme for ACC. ACC is only active as a polymer. (ACC filaments)
Hormonal regulation of ACC
Glucagon and adrenaline promote AMPK.
Insulin promotes PP2A
Reciprocal regulation fatty acid metabolism
-Palmitoyl (acyl)-CoA (degradation intermediate) inhibits ACC (synthesis committed step)
-Malonyl-CoA (committed intermediate synthesis) inhibits CPT-1 (commited step enzyme beta-oxidation/FA breakdown)
MCAD deficiency
Deficiency Medium-chain acyl-CoA dehydrogenase (first step beta-oxidation cycle)
> not enough glucose made
> acetyl-CoA not formed anymore when C10 FAs are formed after few beta-oxidation cycles of palimitoyl(C16)-CoA.
> accumulation Medium chain acyl-CoA esters
> Acetyl-CoA cannot be made, all CoA occupied, no activation of pyruvate carboxylase by acetyl-CoA, no glucogenesis
> no substrate ketogenesis (important source of energy for brains of small children
HC17: For membranes to function, membrane lipids need to move freely, so what is essential
The polar hydrophilic side
Types of membrane lipids: structure phospholipids
Glycerol with 2 ester bound FAs and a polar head group
Why are TAGs not found in membranes?
No polar head group
> same synthesis pathway as phospholipids
What is the main intermediate in the phospholipid/TAG synthesis
Phosphatidate/ diacylglycerol-3-P (DAG-3-P)
Synthesis TAGs and phospholipids pathway
Glucose is converted to glycerol-3-phosphate through DHAP (intermediate)
or: TAGs from adipocytes or diet is split into glycerol and FFAs and the glycerol gets a Pi added to become glycerol-3-P.
>In the ER: Conversion glycerol-3-P to Phosphatidate (DAG-3-P) adding 2 FAs
>Phosphatedate/DAG-3-P can be converted to TAG in the liver by adding FFAs
> Phosphatidase/DAG-3-P can be converted to phospholipids by condensing with an alcohol, by first activating one of the two with a NTP like ATP.
Three important phospholipids
-Phosphatidyl choline (PC)
-Phosphatidyl ethanolamine (PE)
-Phosphatidyl serine (PS)
Phospholipids differ in function by different …
head groups
Synthesis PE and PC
Both through a pathway from ethanolaine/choline using CTP-activated intermediates and diacylglycerol (DAG) addition.
From PE to PC or PS
PE > PC by PE-methyltransferase
PE/PC > PS
In which leaflet of lipid bilayer is PS present?
Intracellular laeflet
What happens if PS accidentally appears in the outer leaflet of the PM?
Flippase flips it back to the inner leaflet
What if flippase is blocked?
PS appears on extracellular leaflet: induces apoptosis
Where are PE and PC naturally found in the membrane?
PE inner leaflet (intracellular) and PC outer leaflet.
Which phospholipid has an important role in signal transduction
Phosphatidyl inositol
> Phosphorylated to PIP, PIP2 or PIP3.
Which enzyme cleaves PIP2? What are the signal transduction products?
Phospholipase C
> creates membrane bound DAG and free IP3
Outside the cell, … bind the transmembrane proteins
sugar chains
Structure of cardiolipin
Polar heads attached to each other (2)
What process is promoted by cardiolipin (CL)?
The curvature of the inner mitochondrial membrane > proper oxidative phosphorylation
Defect cardiolipin effect
No curvature inner mitochondrial membrane
> complexes of electron transport chain do not get together > no proper oxidative phosphorylation and ATP synthesis
Sphingolipids structure
Backbone of ceramide with a OH group of his backbone as polar head.
> The ceramide already contains a FA bound to its backbone with a peptide bond (amino acid with a FA bound to N terminus and OH as R group)
> can bind 1 FA at carboxyl end
Synthesis ceramide
Palmitoyl-CoA (FA) + serine (amino acid) > ceramide.
Synthesis sphingomyelin and function and pathology
Ceramide + PC > Sphingomyelin + DAG
(addition PC to the OH group
- important for nerve function and insulation (impulse transduction)
- Defect: MS, Niemann-Pick
Ceramide is also a substrate to make cerebroside, how?
Ceramide + UDP-glucose > cerebroside + UDP (glucose added to OH R-group of the ceramide)
Synthesis ganglioside
Addition of activated sugars to glycosphingolipids like cerebroside
> sphingolipids with oligosaccharide chain
Many lysosomal storage disordered are through…
defect of enzyme in sphingolipid metabolism
Functions cholesterol
-Membrane flexibility
-Bile acids
-Hormones
Cholesterol synthesis
-Acetyl-CoA + Acetoacetyl-CoA + H2O > HMG-CoA + CoA (HMG-CoA synthase)
-HMG-CoA + 2 NADPH + 2H+ > mevalonate + 2 NADP+ + CoA (HMG-CoA reductase)
Committed step cholesterol synthesis
By HMG-CoA reductase
(HMG-CoA > Mevalonate)
> costs a lot of energy: 2 NADPH
Which molecules inhibit HMG-CoA reductase and how
Statins like lovastatin competitively bind for the HMG group in HMG-CoA reductase. (resemble the structure)
Besides cholesterol, HMG-CoA has another fate. Which?
In mitochondria:
HMG-CoA > acetyl-CoA and acetoacetate (ketone body: for energy)
Mevalonate > Cholesterol
Mevalonate (C6) > Isopentenyl pyrophosphate (C5)
3xC5 > C15
2xC15 > C30 (squalene, by squalene synthase)
Squalene > lanosterol (ring formation)
Lanosterol > Cholesterol (C27, three methyl groups removed, two methyl group together on outer ring and the ring directed methyl group in the third outer ring)
Which transcription factor is activated when there is much cholesterol in the cell? What are the effects?
LXR
> Less uptake cholesterol
> Promote cholesterol efflux
> Inhibit cholesterol synthesis
Which transcription factor is active when there is little cholesterol in the cell and what are the effects?
SREBP
> More uptake cholesterol
> Inhibit cholesterol efflux
> Promote cholesterol synthesis
How is LXR activated
LXR binds to target DNA
> cholesterol levels increase
> Oxy-sterols bind to LXR > go into nucleus > find target genes and inhibit pathways for cholesterol synthesis
Which gene is activated by LXR?
ABCA1, for cholesterol efflux
Transcriptional regulation SREBP
SREBP is bound to SCAP in membrane ER and held by Insig
> cholesterol levels fall, release Insig, move Scap+SREBP to Golgi membrane
> Activation, serine proteases cleave and release SREBP to move to nucleus and bind SRE (sterol regulatory element)
Protease activities in activation SREBP
Serine protease cleaves bond between regulatory domain and transcription factor domain with DNA binding domain. A metalloprotease with Zn2+ cleaves the TF from the transmembrane domain to release it to move to SRE.
Binding of SREBP to SRE promotes the expression of these two proteins (and more)
-HMG-CoA reductase
-LDLR
How is SREBP held into ER when there is cholesterol?
SCAP has a sterol binding domain which keeps it (with SREBP throgh regulatory domain) bound to Insig in ER membrane. When there are low sterol levels, SCAP changes conformation and releases Insig and can travel to Golgi.
Function cholesterol in membrane flexibility
-Makes the membranes stiff > against the cold: antifreeze
Cholesterol as base for bile acids and steroid hormones: what happens
CYP (cytochrome P450 monooxygenases) enzymes add OH group by using O2 to the carbon chain (no ring) of cholesterol.
Where does the bile acid formation occur? To which bile acids is cholesterol converted?
In the liver:
Cholesterol to Glycocholate or Taurocholate (different side chains)
Functions bile acid
Solubilization of lipids in food through micel formaion and uptake by intestine
> and secreting cholesterol from the body
How do gallstones appear
It are cholesterol crystals due to excess cholesterol secretion via bile (in gall bladder)
Steroid hormone formation through cholesterol
Cholesterol (C27)»_space; progestagens (C21)
Progestagens fates
-Glucocorticoids like cortisol (C21)
-Mineralocorticoids like aldosteron (C21) (get minerals in place)
-Androgen (C19) (> Estrogens (C18)). (sex hormones: development)
> all essential
> ligands for nuclear receptors
Cortisol function
Fight or flight
> suppresses immune system
> enhanced gluconeogenesis and enhanced insulin
> Circadian rythm (sleep/awake)
Cortisol pathology
-Too much: syndrome of Cushing (too much fat)
-Too little: Addisons disease
How is vitamin D made?
From dehydrated cholesterol which is isomerized (one ring opened) through UV-light from the sun. and some reactions afterwards
Function VitD
Calcium and phosphorus maintenance
Too little vitD:
-Problems with bones and muscle (Rickets)
-Depressions
-Cancer
-Immune system
Too much vitD
Toxic
HC18: Problem cholesterol degradation
We cannot degrade it! excretion to get rid of it.
How do we get rid of cholesterol?
Bile acid synthesis in liver to excrete with GI tract through bile bladder
Cholesterol accumulation in the blood leads to…
atherosclerosis
What happens too adipocytes when much of TAGs are stored?
They become larger and if too much energy is taken up they will proliferate
In which cells are TAGs made?
Adipocytes and liver cells
> fuel, energy storage
Lipids are insoluble to water. How are they transported in blood?
With lipoproteins
What are lipoproteins?
Vesicles floating around in the blood which form a cell around a lipid content. (like a micelle)
Outside lipoprotein particle? And inside?
Phospholipids, unesterified cholesterol, apolipoproteins.
Inside: Cholesteryl esters and TAGs
Lipoproteins are made and secreted by which cells?
Intestine and liver cells
Why are apolipoproteins important?
These proteins are involved in docking of the lipoprotein to the right receptor of a cell which is in need of lipids.
Lipoproteins in plasma give a … look
milky
Classes of lipoproteins and characteristics
-Chylomicrons > secreted by intestine cells with dietary, taken up, lipids. These travel trough the blood via the lymph and they shrink before passing the liver because bypassing tissues suck up lipids.
-VLDL (very low density): secreted by liver.
-LDL: product of VLDL with TAGs sucked up by tissues (higher concentration cholesterol)
-HDL (high dense) > secreted by liver cells to take up cholesterol from peripheral cells and bring it to the liver
Density (ratio protein / lipid) in lipoproteins
-Chylomicrons: high percentage TAG, less protein
-HDL: high percentage protein and less lipids
-VLDL: lowest percentage protein
so on
In a serum gradient, lipids float. Different density lipoproteins can be distinguished in three layers. How?
Lower layer: HDL
Middle layer: LDL
Top layer: VLDL
How are chylomicrons called after TAGs are taken up (and before uptake by the liver)?
Chylomicron remnants
Cholesterol cannot be degraded, thus needs to be secreted. Name the pathways of cholesterol.
-Food > chylomicron remnants to liver
-Forward transport route of lipoproteins: VLDL > IDL > LDL (gradual uptake of TAGs) : LDL uptake by periphery.
-IDL and LDL can also be taken up by the liver (reverse sideroutes)
-Reverse transport route: HDL takes up cholesterol from periphery and transport to liver.
-Excretion by making bile acids and through feces.
Which kind of layer of phospholipids is found on the lipoprotein surface
Monolayer
Membrane component lipoprotein
The outside of the lipoprotein.
Lipoprotein core
Triglycerides and cholesteryl esters
Different lipoproteins are determined by the different….
apolipoproteins (determine recognition by receptors)
How is cholesterol esterified?
The small polar group (hydroxyl group at end rings) is esterified with a FA which make the molecule solely hydrophobic instead of amphipathic, directing it to the lipoprotein core
LDL carriers the Apolipoprotein? and HDL?
ApoB100
HDL”: ApoA1
Lipid uptake
-Fat uptake by intestine (bile)
-Formation chylomicrons in intestine
-Via lymphatics to blood
Half-life chylomicrons
2 hours
How is VLDL converted to IDL and LDL
Tissues express LPL which breaks down TAGs to FAs and glycerol for uptake.
> LPL expressed via endothelial cells in capillaries.
> not produced by these endothelial cells
> also in chylomicrons to form chylomicron remanants
How is LDL taken up by peripheral cells?
The whole LDL particle is taken up through receptor mediated endocytosis
What happens to endosome with LDL and LDLR?
LDL transported to acid lysosome, fusion and degradation LDL, cholesterol and content into cytosol through transporters and recycling of the LDLR
What happens to the balance of SREBP/LXR when there is much cholesterol in the cell
Balance towards LXR.
Which enzyme converts cholesterol (free) to a cholesterol ester in the peripheral cell?
ACAT: Acyl-CoA cholesterol acyl transferase
Functions cholesterol esters in peripheral cells
-Storage
-Protect membrane
Which transcription factor rules in the peripheral cell when the reverse route is activated?
LXR
How is HDL produced in the liver (content)?
As an empty vesicle: only membrane and a bit of protein
> forms a bilayer disc because of no content.
Which transporter is essential for the transport of cholesterol from the PM to the HDL membrane?
ABCA1: ATP-binding cassette transporter-1
What does a defect in ABCA1 cause?
Tangiers disease, high risk for cardiovascular problems since you cannot get rid of cholesterol
What are other causes of Tangiers disease?
-Apo-A1 defect (apolipoprotein of HDL defect)
-LCAT defect
Function LCAT
Lecithine cholesterol acyl transferase (remember cholesterol-ester is esterified with a acyl)
- Cholesterol + Lecithine (PC: phosphatidylcholine) > Cholesterol-ester + Lysolecithine.
-(middle acyl group of lecithine transferred to cholesterol hydroxyl group.)
Result LCAT conversion
Increased LysoPC in the HDL membrane (product of reaction to form cholesterol ester).
> cholesterol becomes insoluble: to HDL core
Where is LCAT located?
On the HDL (bilayer disc)
Extra function ApoA1
Promotes LCAT
Reverse to transfer of CE (cholesterol ester) to forward pathway
CETP (cholesteryl ester transfer protein)
> Transfers CE from HDL to VLDL/IDL/LDL
> CETP can also transfer some TAGs and PLs (phospholipids) from LDL to HDL.
> IDL and LDL will mostly be taken up by the liver (expresses LDLR as well) and thus CE will arrive in liver for excretion also via this route, so HDL can take up more CE from the periphery before it passes the liver.
Liver decides the cholesterol fates, which ones?
To VLDL
To bile / faeces
Is reabsorption of cholesterol from intestine after excretion by bile good?
Yes
Homozygous Familial Hypercholesterolemia (FH)
No functional LDLR: no life possible
Heterozygous Familial Hypercholesterolemia
Half the number of LDLR is functional.
Problems with FH
LDL accumulaion in the arteries: inflammation and atherosclerosis risk.
Treatments FH
-Statins
-Cholestyramine/ ezetimibe
Statins function
Inibit HMG-CoA reductase as competitive inhibitor and thereby reduce cholesterol biosynthesis
Cholestyramine and Ezetimibe function
Bile abosorption inhibitors > then, more bile acids should be made, so cholesterol is used to make new bile acids, because bile absorption is inefficient.
Bile secretion and uptake
-secretion: bile salts are formed from cholesterol and transport cholesterol from liver to feces
-Uptake: bile solubilizes lipids from food and is absorbed by intestine.
LDL accumulation cause
Force into a corner which leads to inflammation
> accumulation is the outer curvatures of an S form of artery.
> low shear stress areas: uptake lipids from blood due to local flow profile.
Atherosclerosis may lead to heart …
infarction, because the coronary arteries has many curves or side chains where atherosclerosis can occur
How is inflammation induced in atherosclerosis?
Macrphages accumulate to clear the lipids
> take up the lipids
Foam cell formation
Macrophage stress: macrophages get full of lipids: secrete stress cytokines > inflammation > macrophage stress (they get immobile because there is no cholesterol degradation)
Pathway macrophage stress
Uptake LDL via scavenger receptors and NOT VIA LDLR
> no degradation cholesterol: become immobile > stress > secretion inflammation cytokines > more inflammation
> efflux via ABCA1 cannot cope with lipids,
Macrophage accumulation and effects
To prevent the macrophages from bursting, the endothelial muscle cells will grow over the macrophages.
> eventually the macrophages die
> Formation cholesterol crystals (irreversible)
(from fatty streak: macrophage accumulation (reversible) to lesion (irreversible))
What is a risk factor for infarct when atherosclerosis?
Exercise > more blood flow > plaques can burst > blood clotting > big clot closes the artery > infarct
What is SRB1 (scavenger receptor class B type 1)?
Receptor which binds HDL by peripheral cells
