Section 4: Lipids Flashcards
Lipids - amphipathic
Mostly hydrophobic (hydrocarbon), but with a polar or charged region (carboxylate)
Lipids - solubility
Usually not water soluble
What do lipids form
Don’t form large covalent polymers
Tend to form non-covalent higher-order structures
Lipids: Formation of non-covalent higher-order structures
Sequester the hydrophobic hydrocarbon component(s) from the (polar) aqueous environment
Stabilised by vdW interactions between hydrocarbon part
Fatty acids - strength
Weak acids - deprotonated at physiological pH (carboxylate form)
Fatty acids: Alkyl chains may be…
Saturated (fully reduced)
Unsaturated (some C=C):
- monounsaturated: one double bond
- polyunsaturated: many double bonds
Fatty acids are a type of _____
Lipid
Fatty acids: Saturated hydrocarbon chains
Can rotate freely about any C-C bond
Fatty acids: Unsaturated hydrocarbon chains
Can’t rotate around the double bond
Double bond is usually cis, which makes the hydrocarbon chain bend
Fatty acids: Number of carbons
Most naturally occurring fatty acids have an even no of C atoms because fatty acid synthesis involves adding 2C units
Fatty acids: Temperature and C
As no of Cs increase, melting point increases for both saturated and unsaturated
Fatty acids: Temperature and double bonds
Double bonds greatly reduce temp of melting point
Essential fatty acids
Required for good health and must be ingested, because mammals can’t introduce double bonds in fatty acids beyond carbon 9 and 10
Fatty acids: Major physiological roles
- Source of hormones and intracellular messengers
- Building blocks of micelles and membranes
- Post-translational modification of proteins
- Fuel
Fatty acids and lipids: Micelle
Fatty acids are wedge-shaped and tend to form spherical micelles
Polar head groups tend to be larger than their single hydrocarbon chain –> forms curved structure
Fatty acids and lipids: Micelles and phospholipids - number of tails
Micelle: one tail
Phospholipids: 2 tails
Fatty acids and lipids: Phospholipids
More cylindrical and pack tgt to form a bilayer structure
Biological membranes: Hydrophobic core - length
Hydrophobic core ~30Å
Hydrophobic core + interfacial on either side = ~60Å
Biological membranes: Interfacial region
Polar
Has some lipid headgroups, but also some water molecules - not a sharp boundary
Biological membranes: Lipid tails
Never perfectly straight
Biological membranes: Lipid tails - temp
Higher temp = more mobile
Biological membranes: How does the cell modify its curvature
By putting diff kinds of lipids in the membrane
What are found in biological membranes
Proteins, channels, sugars
Provide info to cell and ways to pass signals through the membrane
Lipid bilayers: States
Gel state (below Tm) Liquid crystal state (above Tm)
Lipid bilayers: Gel state
Lower temp and more saturated fatty acids
Hydrocarbon tails are packed tgt in a highly ordered gel state
Lipid bilayers: Liquid crystal state
Higher temp an d more unsaturated fatty acids
Movement of chains become more dynamic and interior of membrane resembles a liquid hydrocarbon
Fatty acids: Saturated vs unsaturated
Unsaturated fatty acids bent –> can’t pack as well tgt
- fewer vdW can form
- more dynamic / liquid-like
Major types of membrane lipids
Glycerophospholipids (glycerol backbone)
Sphingolipids (sphingosine backbone)
Sterols
Membrane lipids: Glycerophospholipids
Built on a glycerol backbone
Has a phosphate
2 fatty acid tails (any kind) added onto an O each
Membrane lipids: Sphingolipids
Already has a long hydrocarbon chain, so only one more fatty acid needs to be added, which is added onto a N
Head group attached to C1
Membrane lipids: Glycerophospholipid - basic structure
Phosphate group Glycerol backbone R1 and R2 = fatty acids R3 = head group 2 hydrophobic (fatty acid) chains point into membrane and phosphate group points in opp direction
Membrane lipids: Glycerophospholipids - phosphatidylserine (PS)
‘Eat me’ signal
Normally located in inner leaflet of PM
Moves to outer leaflet in apoptosis and attracts phagocytes to consume cell remnants
Membrane lipids: Cardiolipin (diphosphatidylglycerol)
4-tailed glycerophospholipid
Very large head group
Glycoglycerolipids are found where
Less common in animal membranes
Common in plant and bacterial membranes
Glycoglycerolipids - structure
Has a glycerol backbone
Carbohydrate/sugar attached via a glycosidic bond
Sphingolipids - where is it found
PM of all eukaryotic cells
Highest conc in CNS cells
Sphingolipids - function
Participate in cell signaling, e.g. regulating cell differentiation, proliferation, programmed cell death
Ceramides - basic structure
Also built on sphingosine backbone
Sphingolipid vs ceramide
In a ceramide, there isn’t really a head group attached to C1, just an OH that’s already attached
Whereas sphingolipid has a head group
Ganglioside - function
Sit in cell membrane and send out a signal which is recognised by other molecules that then lead to certain functions
Ganglioside - structure
Oligosaccharide linked to terminal hydroxyl group of a ceramide via a glucose molecule
Oligosaccharide chain contains at least an acidic sugar or sialic acid
Ganglioside - diarrhea
Ganglioside recognition and binding is the first step in the development of at least 2 diarrhea conditions
Ganglioside - immune system
Crucial for binding of immune system cells to sites of injury in the inflammation response
Gangliosides - Cholera
Pathological condition characterised by severe diarrhea
Cholera toxin recognises and binds to gangliosides (GM1) to gain access to inside of cell
Gangliosides - Enterotoxigenic E. coli
Most common cause of diarrhea
Like cholera, also produces a toxin that recognises and binds to gangliosides to gain access to cell
Cholera and diarrhea
Large V of water are normally secreted into small intestinal lumen, driven by Cl- secretion
Most of this water is absorbed before reaching the large intestine
Diarrhea occurs when secretion of water into intestinal lumen exceeds absorption
Cholera and diarrhea - steps
Cholera toxins (and other bacterial toxins) bind to gangliosides and strongly activate adenylyl cyclase --> increase in intracellular conc of cAMP Cl- channels open --> uncontrolled secretion of water (and Na+, K+, HCO3-) into intestinal lumen Cholera toxin also affects enteric nervous system --> independent stimulus of secretion
Secretory diarrhea - fasting
Not resolved by fasting
Often lethal
Cholera toxin absorption stimulates / inhibits…
Stimulates cAMP production
Stimulates epithelial Cl- secretion
Inhibits Na+ absorption
Cholera - treatment
GM1-coated nanoparticles act as decoys to absorb cholera toxin before it binds to epithelial cells
Where is the highest conc of gangliosides found
In nervous system - 6% of lipids
Gangliosides - Tay-Sachs disease
Gangliosides in nervous system usually degraded in lysosomes by sequential removal of their terminal sugars
In Tay-Sachs disease, one removal enzyme is missing or deficient –> neurons become swollen with lipid-filled lysosomes
Gangliosides - Tay-Sachs disease - symptoms
Severe
Weakness and retarded psycho-motor skills before age 1
Demented and blind by age 2
Usually dead before age 3
Lipid composition
Varies between cell types and leaflets
Allows fine-tuning of membrane properties
Lipid membranes - asymmetric
Biological membranes are made of two layers / leaflets
Each leaflet faces a diff environment –> asymmetric
High conc of a lipid on one side = low conc of lipid on other side
Lipid membranes: Organelles vs cell membrane
Organelles:
Inner leaflet faces organelle interior
Outer leaflet faces cytoplasm
Cell membrane:
Inner leaflet faces cytoplasm
Outer leaflet faces environment
PM typically comprises __ diff lipid types
~60
Lipidation of proteins
Allows anchoring to membrane
Hydrophobic part slots into cell membrane
Type of lipid determines…
Protein location
Glycolipids are built on…
Either a glycerol backbone or sphingosine backbone
Fatty acids: How is fuel/fat stored
Stored as triacylglycerols
Excess from diet will also be stored here
Triacylglycerols AKA…
TAG
Neutral fats
Triglycerides
What are triacylglycerols formed from
Ester bonds between carboxyl groups of (same or diff) fatty acids (includes the C=O) and hydroxyl groups of glycerol
Fatty acids: Fuel - oxidation
Oxidation of fatty acids released from triacylglycerols produce energy for cellular process
Why are triacylglycerols/fats an efficient energy store
They are highly concentrated stores of metabolic energy;
- highly reduced (lots of ability to be oxidised)
- non-polar and so anhydrous
Can carry lots of energy into a small space and weight
Define anhydrous
Not much water
Fat yields ___x more energy than carbohydrates/proteins
~6.5x
What is the major energy storage form in most organisms
Fat
3 sources of fatty acids
- Digestion
- Adipose tissue
- Synthesis
Sources of fatty acids: Digestion - small intestine
Small intestine contains hydrolytic enzymes from pancreas which can be absorbed into the bloodstream
Sources of fatty acids: Digestion - chylomicrons
Transport triacylglycerols through lymph and bloodstream
What are bile salts
Emulsifiers
Amphipathic molecules synthesised from cholesterol in the liver and secreted from gall bladder
Transport of fatty acids and other lipids: Lipoprotein particles - function
Emulsify lipids for transport in the blood
Transport of fatty acids and other lipids: Lipoprotein particles - structure
Consists of a core of hydrophobic lipids (oil droplet) surrounded by a shell of more polar lipids and proteins
Transport of fatty acids and other lipids: Lipoprotein particles - apolipoproteins
Solubilise hydrophobic lipids
Contain cell-targeting signals; helps make sure the fats being transported end up in the right cell
Have a hydrophilic part that points outward
Lipoprotein particles - families
Chylomicron (Chylomicron remnant) VLDL (very low-density lipoprotein) IDL (intermediate density lipoprotein) LDL (low-density lipoprotein) HDL (high-density lipoprotein)
Lipoprotein density
Increases with increasing protein content (decreasing lipid content) because lipids are less dense than protein
Fats - solubility
Not soluble in water, therefore aren’t soluble in blood
How are fatty acids transported in blood
By lipoprotein particles
Density of water
Just under 1
Density of lipids vs water/protein
Lipids are generally less dense than protein and water
Lipids and proteins - density
More lipids = less dense
More proteins = more dense
Chylomicron remnants
What’s left over once the chylomicron has dropped off all its lipids
Lipoproteins: Chylomicron
Delivers dietary triacylglycerides to target tissues
Lipoproteins: Chylomicron remnant - function
Delivers dietary cholesterol esters left from chylomicron to liver
Lipoproteins: VLDL
Transports endogenous triacylglycerides from liver to periphery
Lipoproteins: IDL
Remnants of VLDL
Lipoproteins: LDL
Major transporter of cholesterol to periphery
Lipoproteins: HDL
Picks up cholesterol that’s no longer needed from circulation
Lipoproteins: Good vs bad cholesterol
HDL = good cholesterol LDL = bad cholesterol
Most lipids are ingested in the form of _____
Triacylglycerols, so must be degraded to fatty acids for absorption across the intestinal epithelium
How are triacylglyceroles in intestinal lumen solubilised
By bile salts
Digestion of dietary lipids - steps
- Cholic acid ionises to give its bile salt
- Hydrophobic surface of bile salt molecule associates with triacylglycerol, several of which aggregate to form a micelle
- Hydrophilic surface of bile salts face outward, allowing micelle to associate with pancreatic lipase/colipase
- Hydrolytic action of lipase frees fatty acids to associate in a much smaller micelle that is absorbed through the intestinal mucosa
What is cholic acid
A typical bile acid
COOH loses H to become COO- (ionised)
Triacylglycerides are hydrolysed by…
Pancreatic lipases
Digestion of dietary lipids: Pancreatic lipases
Secreted from pancreas
Catalyse hydrolysis of ester bonds between fatty acyl group and glycerol of triacylglycerols
- first hydrolyses off one of the outer fatty acids –> diacylglycerol
- then acts on other outer fatty acid –> monoacylglycerol
Releases 2 free fatty acids
Structure of bile salts
Triacylglycerols point in the same direction - this side is the hydrophilic face –> becomes the outer part of micelle
Other side is hydrophobic
Monoacylglycerol
Glycerol backbone with one acyl chain attached
Where do chylomicrons bind to membrane-bound lipases
At adipose and muscle cells
Fatty acids from adipose tissue: Stages of processing
- Mobilisation
- Activation and transport
- Degradation
Fatty acids from adipose tissue: Mobilisation
Triacylglycerols are degraded to free fatty acids and glycerol
hydrolysed by hormone-stimulated lipases in adipose tissue
Free fatty acids and glycerol are released from adipose tissue and transported to energy-requiring tissues
Fatty acids from adipose tissue: Mobilisation - enzymes
ATGL: adipose triglyceride lipase
triacylglycerol –> diacylglycerol
HSL: hormone-sensitive lipase
diacylglycerol –> monoacylglycerol
MGL: monoacylglycerol lipase
monoacylglycerol –> glycerol
Fatty acids from adipose tissue: Mobilisation - what can glycerol be used for
Glycolysis
Gluconeogenesis
Fatty acids from adipose tissue: Mobilisation - what happens to the free fatty acids produced
Transported into the blood plasma and undergoes fatty acid oxidation –> acetyl CoA –CAC–> CO2 + H2O
How are fatty acids transported
Since not soluble, they are transported bound to protein serum albumin
Serum albumin - function
Bind molecules that are insoluble in water and deliver them to tissues via blood, e.g.
- fatty acids
- hydrophobic hormones
- drugs
- metal ions
Uptake of fatty acids - passive?
Originally thought to occur largely by passive diffusion
Now thought to be mostly facilitated and regulated by proteins
Fatty acids from adipose tissue: Activation and transport
Fatty acids arrive in cytosol, but fatty acid degradation occurs in mitochondria so must be activated and transported
Fatty acids from adipose tissue: Activation and transport - steps
- Activation via adenylylation (requires ATP)
- Transfer to carnitine (replace CoA with carnitine molecule)
Coenzyme A recycled and goes back to activate next fatty acid - Transport through mitochondrial inner membrane
- Reconjugation with CoA
Fatty acyl CoA goes to degradation pathway
Fatty acids from adipose tissue: Activation and transport - activated by?
Activated by formation of a thioester linkage to coenzyme A
Fatty acids from adipose tissue: Activation and transport - where does this take place
Outer mitochondrial membrane
Fatty acids from adipose tissue: Activation and transport - for fats to cross the membrane…
They must be conjugated to carnitine to enter the mitochondrial matrix
Fatty acids from adipose tissue: Activation and transport - carnitine acyltransferase I (CPT I)
Bound to outer mitochondrial membrane
Catalyses transfer of acyl group from coenzyme A to carnitine –> acyl carnitine
Fatty acids from adipose tissue: Activation and transport - translocase
Shuttles acyl carnitine across membrane
Fatty acids from adipose tissue: Activation and transport - carnitine acyltransferase II (CPT II)
Transfers acyl group back to coenzyme A
Is fatty acid degradation anabolic or catabolic
Catabolic - produces e- for oxidative phosphorylation
Is fatty acid synthesis anabolic or catabolic
Anabolic
What is fatty acid degradation
Degradation of a saturated acyl chain with an even no of C atoms attached to coenzyme A
Fatty acid degradation - double bonds / odd no of C atoms
Oxidation of an acyl chain containing double bonds or an odd no of C atoms requires additional steps
Fatty acid degradation: Pathway name
β-oxidation pathway
Fatty acid degradation: β-oxidation pathway - steps
Recurring sequence of 4 reactions:
- Oxidation of single bond to double bond. FAD –> FADH2
- Hydration - addition of water across double bond –> single bond. Forms an alcohol group
- Oxidation of alcohol to ketone. NAD+ –> NADH + H+
- Thiolysis - cleavage at β-C to release acetyl-CoA. Adds CoA to activate remainder of acyl chain for further rounds of β-oxidation
Fatty acid degradation: In each round of the β-oxidation pathway…
An acyl chain is shortened by 2Cs (in the form of acetyl-CoA)
Acetyl-CoA, NADH and FADH2 are generated
Fatty acid degradation: Where does the β-oxidation pathway occur
All reactions happen between α and β carbon of acyl-CoA molecule
α-C is next to carboxyl group, and next one is the β-C
Often the β-C is cleaved
Fatty acid degradation: β-oxidation pathway continues until…
There are no Cs left to remove
e.g. 7 rounds for C16
Fatty acid degradation: Fates of acetyl-CoA
Enters CAC or forms ketone bodies
Fatty acid degradation: Fates of acetyl-CoA - CAC
If fat and carbohydrate degradation are balanced, acetyl-CoA enters CAC
CAC: What does availability of oxaloacetate depend on
On the carbohydrate supply - formed from pyruvate
Fatty acid degradation: Fates of acetyl-CoA - Ketone bodies
When more fat than carbohydrate degradation (glycolysis)
Oxaloacetate is consumed to form glucose via gluconeogenesis
Examples of ketone bodies
Acetoacetate
β/D-3-hydroxybutyrate
Acetone
When are ketone bodies present in individuals
High levels of ketone bodies often present in blood of untreated diabetics
Also occurs when fasting or on low-carb diets
What are ketone bodies
Major fuel source for heart and kidney
Need to know what a ketone body looks like!
Which structures prefer ketone bodies
Heart muscle and renal cortex
Brain prefers glucose, but under prolonged starvation can adapt to get 75% of its energy from ketone bodies
Energy sources under starvation
After several days,
- slight increase in fatty acids
- decrease in glucose
- large increase in ketone bodies in plasma
Palmitoyl-CoA (C16) needs _ rounds of β-oxidation
7
in 7th cycle, C4 is cleaved into 2 molecules of acetyl CoA
Fatty acid degradation - equation
Palmitoyl-CoA + 7FAD + 7NAD+ + 7CoA + 7H2O –>
8 acetyl-CoA + 7 FADH2 + 7NADH + 7H+
Molecules of ATP produced from fatty acid degradation
~2.5 ATP per NADH
~1.5 ATP per FADH2
~12 ATP per acetyl-CoA
~2 used to activate palmitate (-2)
Where does fatty acid synthesis occur
Cytoplasm
Fatty acid synthesis: What is acetyl-CoA formed from
From pyruvate in the mitochondria (glycolysis)
Mitochondria aren’t naturally permeable to…
Acetyl-CoA
So citrate carries acetyl groups through the inner mitochondrial membrane to cytoplasm for fatty acid synthesis
Fatty acid synthesis: Committed step
Starts with carboxylation of acetyl-CoA (2C) to form malonyl-CoA (3C)
Burns 1 ATP - irreversible
Catalysed by acetyl-CoA carboxylase
Fatty acid synthesis: Acetyl-CoA Carboxylase 1 and 2
Cytoplasmic enzymes
Regulates fatty acid synthesis and degradation
Fatty acid synthesis: -KS
Ketoacyl synthase
Fatty acid synthesis: ACP - structure
Acyl carrier protein
A single polypeptide chain of 77 amino acids
Like a giant version of CoA
Fatty acid synthesis: ACP - function
Used to carry around acyl chains
ACP and coenzyme A - similarities
Both have a phosphopantethine group - long end of coenzyme A
Both end in a sulfur that can be joined onto molecules
ACP and coenzyme A - differences
Coenzyme A:
Fatty acid degradation
Adenosine phosphate attached
ACP:
Fatty acid synthesis
Protein attached
Synthesis pipeline: Fatty acid synthase (FAS)
Giant, multifunctional enzyme complex where all steps in fatty acid synthesis take place
One continuous polypeptide chain folded into domains
Contains enzymes for fatty acid synthesis
Acts as a dimer
Synthesis pipeline: Where are fatty acid synthases found
In higher organisms only
Fatty acid synthesis: Steps
Fatty acids are elongated by repetition:
- Condensation of malonyl-ACP and acetyl-KS
- Reduction of carboxyl group to OH (uses NADPH)
- Dehydration - forms double bond
- Reduction (uses NADPH) - double bond becomes single bond
- Translocation - shuffle growing chain onto KS protein –> ACP is free and cycle repeats
- C16-acyl-ACP is hydrolysed by a thioesterase (TE) to yield palmitate and ACP
Fatty acid synthesis: When does it stop
Elongation cycle is repeated until C16-acyl-ACP is formed
Fatty acid synthesis: Thioesterase (TE)
Acts as a ruler to determine fatty acid chain length
Most commonly observed fatty acids - no of Cs
C16 and C18
Bonds - fully reduced state
All single bonds
Fatty acid synthesis: Condensation - cycles
Cycle 1: acetyl-KS + malonyl-ACP
Cycles 2-6: growing acyl chain-KS + malonyl-ACP
To carry out fatty acid synthesis, you need…
2 molecules of NADPH
Fatty acid synthesis: Where does NADPH come from
1 mole of NADPH per molecule of acetyl-CoA released from citrate
Additional NADPH required comes from pentose phosphate pathway
The accumulation of precursors for fatty acid synthesis involve…
The coordinated use of multiple biochemical pathways
Fatty acid synthesis - overall equation
8 acetyl-CoA + 7ATP + 14NADPH + 6H+ –>
Palmitate + 14NADP+ + 8CoA + 6H2O + 7ADP + 7Pi
Major product of fatty acid synthase
Palmitate
Elongation and unsaturation: ER
Chain lengthening
Introduction of double bonds into long-chain acyl-CoAs
Longer fatty acids - eukaryotes
Formed by elongation reactions catalysed mainly by enzymes on the cytosolic face of the ER membrane
Fatty acid degradation vs synthesis - location
Degradation - mitochondria
Synthesis - cytosol
Fatty acid degradation vs synthesis - carrier of acyl chain
Degradation - coenzyme A
Synthesis - ACP
Both attach the acetyl to a sulfhydryl group
Fatty acid degradation vs synthesis - adds/removes _____
Degradation - removes acetyl-CoA
Synthesis - adds malonyl-CoA
Fatty acid degradation vs synthesis - oxidant/reductant
Degradation - oxidant is NAD+/FAD
Synthesis: reductant is NADPH
Fatty acid degradation vs synthesis - starts from
Degradation - saturated fatty acid, any length
Synthesis - acetyl-CoA
Fatty acid degradation vs synthesis - ends with
Degradation - acetyl-CoA
Synthesis - 16C saturated fatty acid
Fatty acid degradation vs synthesis - processing pipeline
Degradation - no processing pipeline
Synthesis - FAS processing pipeline
Fatty acid synthesis: ACC
Acetyl-CoA carboxylase
Helps regulate fatty acid synthesis
Fatty acid synthesis AKA…
Fatty acid metabolism
Fatty acid synthesis: Regulation of ACC
Subject to 2 types of allosteric regulation
Also regulated by a variety of hormones
Fatty acid synthesis: ACC - allosteric regulation
Allosteric stimulation by citrate
Allosteric inhibition by palmitoyl-CoA
Fatty acid synthesis: ACC - allosteric regulation by citrate
High citrate when both acetyl-CoA and ATP are abundant - signals raw material and energy are available for fatty acid synthesis
Leads to increased fatty acid synthesis
Fatty acid synthesis: ACC - allosteric regulation by palmitoyl-CoA
Abundant when there’s an excess of fatty acids
Leads to decreased fatty acid synthesis
Fatty acid synthesis: ACC - hormones
Inhibited by glucagon and epinephrine
Stimulated by insulin
Fatty acid synthesis: ACC - glucagon and epinephrine
Stimulates its phosphorylation by AMP-activated protein –> inhibits ACC
Fatty acid synthesis: ACC - insulin
Activates PDH phosphatase –> removes phosphate to activate PDH
Activates citrate lyase to create acetyl-CoA
Stimulates glucose uptake
End product of fatty acid synthesis
Palmitoyl CoA
Cholesterol - Janus-faced molecule
Described as a Janus-faced molecule because it has both hydrophobic and hydrophilic parts
Cholesterol - solubility
Absolute insoluble in water - makes it useful in membranes but also potentially lethal if too much accumulates in one place
Cholesterol - structure
Built on a saturated tetracyclic hydrocarbon
Fused cyclohexane rings all in chair conformation - makes it bulky and rigid compared with other types of lipids
Cholesterol tends to _______ lipid membrane structure
Disrupt
Cholesterol - amphipathic?
Weakly amphipathic
Very large hydrophobic part relative to hydrophilic part (polar bit = 1 OH)
Roles of cholesterol: Membrane fluidity
Lowers the temp at which the membrane transitions from the gel to liquid crystal phase
Roles of cholesterol: Lipid rafts
Cholesterol, sphingolipids and GPI-anchored proteins tend to associate in the membrane to form lipid rafts
Multiple lipid rafts can associate to form ‘platforms’ where certain proteins will preferentially interact - helps organise membranes
GPI
Glycophosphatidylinositol
Cholesterol derivatives - examples
Vitamin D
Bile salts
Steroid hormones
Cholesterol derivatives: Vitamin D
Group of fat-soluble secosteroids
Most important forms in humans are vitamin D3 and D2
Cholesterol derivatives: Vitamin D - function
Responsible for increasing intestinal absorption of calcium, magnesium and phosphate
Has many other biological effects
Cholesterol derivatives: Vitamin D - source
Major natural source is synthesis of cholecalciferol in lower layers of skin epidermis, which is dependent on sun radiation
Only a few foods contain significant amounts of vitamin D
Cholesterol derivatives: Vitamin D - not technically a vitamin?
Not essential as can be synthesised in adequate amounts by most mammals if exposed to sufficient sunlight, so not technically a vitamin
Cholesterol derivatives: Vitamin D - activation
Activated by 2 hydroxylation steps, the first in the liver and the second in the kidney
What are secosteroids
Steroids where part of the ring structure is broken open
Cholesterol derivatives: Bile salts
Polar derivatives of cholesterol
Highly effect detergents
Cholesterol derivatives: Steroid hormones
Precursor of potent signalling molecules, including the 5 major classes of steroid hormones
e.g. oestrogen, androgen, glucocorticoids, minerolocorticoids
Cholesterol is essential for…
Animal life
Cholesterol synthesis - de novo
Can be synthesised de novo (as new);
Principle sterol synthesised by all animals
Cholesterol - plants and eukaryotes
Very little made by plants - make phytosterol instead
Absent in most prokaryotes
Major site of cholesterol synthesis
In mammals, the liver (hepatic cells)
Cholesterol synthesis: Where are the C atoms derived from
All 27 C atoms of cholesterol are derived from acetyl-CoA
Occurs in 3 stages
Cholesterol synthesis: Stages
- Synthesis of isopentenyl pyrophosphate (cytoplasm)
- Condensation of 6 molecules of isopentenyl pyrophosphate to form squalene (ER)
- Cyclisation of squalene in an ‘astounding reaction’ and subsequent 18-step conversion of the tetracyclic product into cholesterol (ER)
Cholesterol: Isopentenyl pyrophosphate (IPP)
An inactivated isoprene that is the key building block of cholesterol
Cholesterol synthesis - committed step
Part of stage 1
Reduction of HMG-CoA to mevalonate
HMG-CoA reductase is an important control site in cholesterol biosynthesis
Cholesterol synthesis: Stage 1 - regulation of HMG CoA reductase
Regulated by controlling:
- rate of synthesis of reductase mRNA (activated by SREBP)
- rate of translation of reductase mRNA
- rate of degradation of reductase
- phosphorylation of reductase (decreases activity)
Regulation at levels of transcription, translation and degradation can later the amount of enzyme in the cell more than 200x
Cholesterol: SREBP
Sterol regulatory element-binding protein
Intracellular sensor that detects low cholesterol levels
Cholesterol synthesis: Stage 1 - conversion of mevalonate to isopentenyl
3 consecutive reactions requiring ATP
Cholesterol synthesis: Stage 1 - isopentenyl pyrophosphate and dimethylallyl pyrophosphate
Can readily interconvert
Cholesterol synthesis: Stage 2
3 molecules of isopentenyl pyrophosphate condense to form farnesyl pyrophosphate
2 molecules of farnesyl pyrophosphate condense to form squalene
Cholesterol synthesis: Stage 3
Squalene folds up from an open/linear structure into a ring-like structure
Squalene –> oxidosqualene –3 steps–> lanosterol –19 steps–> cholesterol
Cholesterol: Heart disease
Fatty yellow-ish material on arterial walls of patients
What is the essential control point of the cholesterol biosynthetic pathway
HMG-CoA reductase
Cholesterol synthesis: Managing heart disease
Statin drugs use to treat heart disease target HMG-CoA reductase
Lovastatin and related compounds are potent competitive inhibitors (Ki = 1 nM) of HMG-CoA reductase
Control of cholesterol uptake to manage heart disease: Loss of bile salts…
Reduces total cholesterol content body
Control of cholesterol uptake to manage heart disease: Bile acid sequestrants
Binders that inhibit the intestinal reabsorption of bile salts
Orally administered +vely charged polymers
Bind -vely charged bile salts
Aren’t absorbed themselves
Control of cholesterol uptake to manage heart disease: Statins
Resemble mevalonate - structurally and chemically similar
Mimic both the substrate and product of HMG-CoA reductase
Cholesterol: Mevalonate in an enzyme
Folded up into a ring-like structure
Cholesterol is carried by __________ as __________
Lipoprotein particles
Cholesterol esters
Cholesterol metabolism: LDL
Cholesterol esters in LDLs are too hydrophobic to pass through the cell membrane, so the LDLs enter the cell through receptor-mediated endocytosis
Cholesterol metabolism: LDL - what is engulfed
The entire LDL-receptor complex is engulfed and taken into the cell
Cholesterol metabolism: LDL - steps
When LDL gets to cell, it’s recognised by LDL receptors in PM
Results in pips where LDL is coated with LDL receptor - the entire thing fuses around the LDL –> endocytic vesicle
Endocytic vesicle fuses with others –> endosome with multiple LDL particles in it - fuses with a lysosome –> lowers pH and breaks up the LDL particles
Releases amino acids, cholesterol, and cholesterol esters
After vesicle releases its contents, it goes back to the PM and waits for the next LDL to arrive
What is familial hypercholesterolaemia (FH)
Absence or deficiency of functional receptors for LDL
Familial hypercholesterolaemia (FH) - what happens
Cholesterol is deposited in various tissues because of the high conc of LDL cholesterol in the plasma
Familial hypercholesterolaemia (FH): Homozygotes
Rare
No functional LDL receptors
Most die of severe coronary heart disease in childhood
Familial hypercholesterolaemia (FH): Heterozygote
Common
~Half the normal number of LDL receptors
Pre-mature cardiovascular disease in 30s and 40s
Familial hypercholesterolaemia (FH): Mutation
One class of mutations that results in FH generates receptors that are reluctant to give up the LDL cargo
What is thermogenesis
Heat generation
What is obesity
A medical condition where excess body fat (adipose tissue) has accumulated to an extent that it may have a -ve effect on health
What does obesity increase the likelihood of
Various diseases and conditions, particularly: Cardiovascular diseases Type 2 diabetes Obstructive sleep apnea Certain types of cancer Osteoarthritis Depression
What is obesity caused by
Generally caused by a combination of excessive food intake, lack of physical activity and genetic susceptibility
Can also be caused by endocrine and mental disorders, or certain medications
Obesity - BMI
Often defined as having a BMI > 30 kg/m^2
Intended for statistical measurement of pops; not a measure of individual body fat, build, or health
Only detects ~50% of cases of obesity
Obesity ‘pandemic’
Most common nutritional disease in developed countries
Leading preventable cause of death worldwide
Adipose tissue AKA…
Fat
Body fat
Adipose tissue - structure
Loose CT composed mostly of adipocytes
Adipose tissue - function
Main role is to store energy in the form of lipids
Also cushions and insulates the body
Types of adipose tissue
White adipose tissue (WAT) - stores energy
Brown adipose tissue (BAT) - generates body heat
Adipocytes AKA…
Lipocytes
Fat cells
What are adipocytes
The primary constituent of adipose tissue
Adipocytes are specialised in…
Storing energy as fat
Types of adipocytes
White adipocytes - store energy as a single large lipid droplet and have important endocrine functions
Brown adipocytes - store energy in multiple small lipid droplets for use as fuel to generate body heat (thermogenesis)
White vs brown adipocytes
White adipocyte: Found in obesity Low mitochondria density One large lipid droplet Store energy Endocrine functions
Brown adipocyte: Anti-obesity High mitochondria density Numerous small lipid droplets Produce heat and dissipate energy through thermogenesis Endocrine functions
How much does white adipose tissue contribute to body weight
In healthy, non-overweight humans, white adipose tissue comprises ~20% of body weight in men and ~25% in women
White adipose tissue (WAT) - function
Energy storage
Acts as a thermal insulator - helps maintain body temp
Buffer impact
Structural roles
Where is white adipose tissue found
Found all over the body
White adipocytes - structure
Contain a single large fat droplet, which forces the nucleus to be squeezed into a thin rim at the periphery
White adipocytes - contents
Have receptors for insulin, sex hormones, NE, glucocorticoids
Endrocinologically active
Where is brown adipose tissue (BAT) found
Especially abundant in new-born humans and hibernating mammals
Also present and metabolically active in adult humans, but prevalence decreases with age
Mostly found around vasculature and organs, and shoulders and upper back
Brown adipose tissue (BAT) - main function
Thermoregulation - generates heat by non-shivering thermogenesis
Brown adipose tissue (BAT) - capillaries
Contains more capillaries that white fat, which supply the tissue with oxygen and nutrients, and remove/distribute the produced heat throughout the body
Brown adipose tissue (BAT) - structure
Contain numerous small droplets of fat
Contain a much higher no of mitochondria
What gives brown adipose tissue their colour
Their large no of (iron-containing) mitochondria
Neonates
New-born humans
Types of thermogenesis
Shivering thermogenesis
Non-shivering thermogenesis
Shivering thermogenesis
Involuntary contraction of skeletal muscle
Shivering thermogenesis - what does it occur in
All mammals exposed to cold will initially shiver to elevate heat production
Shivering thermogenesis - hibernating animals
Process by which body temp of hibernating mammals is raised as they emerge from hibernation
Shivering thermogenesis - oxygen consumption
In adult humans, it can reach intensities equivalent to 40% of maximum oxygen consumption
What does shivering thermogenesis involve
Oxidation of mainly carbohydrates and lipids
Shivering thermogenesis - how much can it increase heat production and core temp in humans
Can increase heat production by 3-4x and core temp by ~0.5°C in humans
Shivering thermogenesis - efficiency
Inefficient method of heating
- increases convective transfer of body heat away from core by increasing muscle blood flow
- increases convective heat loss to the environment via gross bodily movement (wind chill)
Shivering thermogenesis - spontaneity
When cold, it is spontaneous
What is non-shivering thermogenesis
Heat production without shivering
What is non-shivering thermogenesis carried out by
Brown adipose tissue (BAT)
Discovery of non-shivering thermogenesis
Mice in cold (-10°C) food storage rooms
- initially shivered constantly
- later stopped shivering and appeared to thrive
- found to have increased metabolic rate
Non-shivering thermogenesis - BAT
Increase in blood flow to BAT in cold conditions
Non-shivering thermogenesis - neonates
Brown fat plays an important role in helping neonates avoid hypothermia (a major death risk, especially when pre-mature)
Why are infants more susceptible to cold than adults
Higher ratio of body SA to body volume
Higher proportional SA of the head
Little musculature and inability to shiver
Lack of thermal insulation
Inability to move away from cold areas and keep warm
Nervous system not fully developed - doesn’t respond quickly/properly to cold
What is heat loss and heat production proportional to
Heat loss proportional to body SA
Heat production proportional to body V
Brown adipose tissue (BAT) - neonates
Especially abundant in neonates, especially those born without fur, and hibernating animals
How does non-shivering thermogenesis by BAT maintain body temp during cold exposure
Warms blood in surrounding blood vessels before its distribution to the periphery
Ensures an optimal temp for biochemical processes in adjacent organs
Mechanism of non-shivering thermogenesis
Energy is instead released as heat by allowing protons to flow down their gradient without producing ATP (proton leak) - ATP synthase is blocked
Uses UCP1
UCP1
Uncoupling protein 1
Allows protons to leak across the inner membrane of the mitochondria
Releases stored energy as heat
Brown adipocytes - efficiency
Energy inefficient for ATP production
Energy efficient for heat production
Endotherms
Organisms that generate their own heat
What cells can uncouple proton transport from ATP production
All cells of endotherms
How is BAT specialised for non-shivering thermogenesis
Each cell has more mitochondria than usual
These mitochondria have a higher-than-normal conc of UCP1 in the inner membrane
Brown adipose tissue: NA
Noradrenaline
Brown adipose tissue: SNS
Sympathetic nervous system
Brown adipose tissue: β1,2,3
β-adrenoceptors
Sit in outer membrane of brown adipocytes
Brown adipocytes mainly have β3, other cells will have other β-adrenoceptors
Brown adipose tissue: AC
Adenylyl cyclase
Brown adipose tissue: PKA
Protein kianse A
Brown adipose tissue: CREB
cAMP regulatory element binding protein
Brown adipose tissue: UCP1
Uncoupling protein 1
Brown adipose tissue: HSL
Hormone sensitive lipase
Brown adipose tissue: TG
Triacylglycerol
Brown adipose tissue: FA
Fatty acids
Brown adipose tissue: CM
Chylomicrons
Brown adipose tissue: LPL
Lipoprotein lipase
Activation of brown adipose tissue - steps
- NA released by SNS acts on β-adrenoceptors, primarily β3
- This stimulates generation of cAMP by adenylyl cyclase, which activates PKA
- PKA catalyses phosphorylation of CREB –> increased ucp1 gene expression
- PKA also catalyses phosphorylation of HSL and perilipin –> activates HSL and dissociates perilipin from lipid droplets –> activates lipolysis of TG stores
- Released FA stimulate UCP1 and are channeled to the mitochodnria where they enter the β-oxidation pathway and CAC –> ETC –> proton gradient
- UCP1 dissipates the proton gradient generated by the respiratory chain –> release of energy as heat (thermogenesis)
Adipocytes: Perilipin
The protein that covers the intracellular lipid droplets
DNP - safety
Not safe - can be deadly
DNP - what does it act as
Acts as a proton ionophore to shuttle H+ across cell membranes
Similar to UCP
What are kinases
Proteins that phosphorylate other proteins
DNP - steps
Dissipates the proton gradient across the mitochondrial membrane
Instead of producing ATP, the energy of the proton gradient is lost as heat
More DNP –>
Less efficiency energy production –> metabolic rate increases (and more fat is burned) to compensate
BAT: Humans - children
Presence of BAT in newborns and children is well established
What was BAT believed to become? What was confirmed recently
Was believed to become more like white adipose tissue in adult humans, but presence and role in thermogenesis only recently confirmed
BAT: Modern imaging technology
Detection of BAT was enabled by modern imaging tech:
- PET - metabolic info
- CT - structural info
Combing these overlays functional and anatomical data
BAT: Modern imaging technology - PET
Positron emission tomography
Can identify diff types of metabolic activity
BAT: Modern imaging technology - CT
Computed tomography
Structural info
How are types of BAT categorised
Categorised based on cell morphology and location
Both types of BAT have…
Small lipid droplets and numerous mitochondria
Types of adult BAT
Classic or Constitutive
Beige or Brite (brown-in-white) or recruitable
Adult BAT: Classic / Constitutive - where is it found
Found in highly vascularised deposits, typically between the shoulder blades, surrounding the kidneys, neck, and supraclavicular area, and along the spinal cord
Adult BAT: Classic / Constitutive - lipid droplets
Smaller of the two types with numerous small lipid droplets
Adult BAT: Beige / Brite / Recruitable - where is it found
Interspersed with white adipocytes in WAT
Develops from white adipocytes after stimulation by SNS (noradrenalin)
Adult BAT: Beige / Brite / Recruitable - lipid droplets
Greater variability in lipid droplet size and a greater proportion of lipid droplets to mitochondria than BAT –> beige appearance
Adult BAT: Beige / Brite / Recruitable - when is it found
Recruited when you need it to generate heat
Adult BAT: Classic / Constitutive - when is it found
Always occurring
Browning of WAT to form BAT - reversibility
Adaptive and reversible response to environmental challenges
Adult BAT: Beige / Brite / Recruitable - what is it rich in
Rich in UCP1 and mitochondria
What factors cause browning / whitening of adipocytes
White adipocytes –browning–> beige adipocytes:
- cold
- β3-agonism
Beige adipocytes –whitening–> white adipocytes:
- thermoneutrality
- high-fat diet (HFD)
Almost everything used to combat obesity is ________
Reversible
Browning of WAT: Evidence in rodents - newborn lacking BAT
Newborn (neonate) mice lacking BAT (knockout, K) have reduced body temp
Browning of WAT: Evidence in rodents - adult mice lacking BAT
Have normal body temp at 22°C
Adult slowly to prolonged cold temp
Exhibit increased browning of some types of WAT for thermoregulation
Is conversion of WAT to BAT in rodents a short or long term process
Allows mice to have normal body temp at colder temps over a longer time - longer term
Browning of WAT: Evidence in rodents - UCP
Control mice with plenty of BAT don’t have any UCP in their WAT
Knockout mice have lots of UCP expressed in sWAT
Suggests WAT is being converted to BAT to generate heat
What is sWAT
Subcutaneous white adipose tissue
What does NA stimulate conversion of
WAT to BAT
Browning of WAT: Evidence in rodents - NA (NE)
Knockout mice have higher levels of NA –> suggests it is needed to turn WAT into BAT
Browning of WAT: Evidence in rodents - α-tubulin
A protein found in all cells
Control to make sure there’s some cellular protein loaded onto the gel
Browning of WAT: Evidence in rodents - No BAT gives ____ skin temp, about ___ of normal
Reduced
94%
Browning of WAT: Evidence in rodents - infrared image
Tells us how much heat was being given off the mice
Browning of WAT in rodents by ‘hot’ and cold therapy - β-AR
Extensive efforts have been made to pharmacologically activate BAT thermogenesis using synthetic β-adrenergic receptor (β-AR) agonists
Browning of WAT in rodents by ‘hot’ and cold therapy - examples
Chilli peppers (capsinoids, non-pungent capsaicin analogues) and mild cold exposure
Browning of WAT in rodents by ‘hot’ and cold therapy - experiment
Mice were fed a high fat diet supplemented with capsaicin analogues under mild cold conditions for 8 weeks
This synergistically suppressed body weight gain and increased energy expenditure on a high-fat diet
Browning of WAT in rodents by ‘hot’ and cold therapy - steps
Cold sensation registered on skin and transmitted to WAT deposits through SNS and β-AR
Capsinoids bind to capsinoid receptor in gut
Browning of WAT in rodents by ‘hot’ and cold therapy - capsinoid receptor
Transmembrane receptor
Activation of it can produce a painful burning sensation
Browning of WAT in rodents by ‘hot’ and cold therapy - BAT adipogenesis is synergistically stimulated through…
Increased β-AR expression
Stabilsiation of transcription factor PRDM16, a major transcriptional regulator of BAT development
Cachexia - symptoms
Inflammation
Body weight loss
Atrophy of adipose tissue
Skeletal muscle wasting
Where is cachexia observed
In a majority of cancer patients with advanced disease
Also at the end stage of various other morbidities, e.g. infectious diseases (AIDS) or chronic conditions (heart failure)
Cachexia is responsible for ___ of total deaths from cancer
20%
What does cachexia involve
Systemic inflammation and IL-6, both of which induce and sustain WAT browning
Cachexia: IL-6
Stimulates the adrenal gland to reduce catecholeamines –> induces NA
Cancer cachexia patients stain positive for…
UCP1 in WAT
What seems like a promising approach to ameliorate cachexia in cancer patients
Inhibition of WAT browning, as it stops the the high energy expenditure
Thermogenic capactiy
Ability of the body to produce heat
WAT - burn trauma
Browning of WAT occurs in humans following burn trauma
Browning of WAT in humans: Burn trauma - what is it
Severe and prolonged adrenergic stress
Browning of WAT in humans: Burn trauma - what does it result in
Prolonged elevation of circulating NA levels for several weeks post-injury
Browning of WAT in humans: Burn trauma - what does it increase
Resting energy expenditure
Expression of UCP1
No of mitochondria
Oxidative capacity
Browning of WAT in humans: Burn trauma - what could it reflect
Need for increased thermogenesis to maintain normal body temp following loss of insulating skin barrier
Browning of WAT in humans: Burn trauma - NA
Large 10x increase in circulating NA - persists for several weeks post-burn
Much greater than the transient increases (1.5x) in NA levels seen in patients exposed to chronic cold
This ‘catecholaminergic’ surge associated with larger burns may contribute to browning of WAT
Artificially stimulating and controlling browning in humans - experiment
Healthy young male subjects:
- some had BAT already (+), others didn’t (-)
- BAT+ and BAT- subjects randomly assigned to 2 groups
- one group exposed to cold at 19°C for 2 hours
- other group at 27°C for 2 hours
Artificially stimulating and controlling browning in humans - experiment results
Exposure to lower temp increased energy expenditure
Activation of pre-existing BAT but no browning of WAT - no new BAT
Artificially stimulating and controlling browning in humans - cold vs capsinoid exposure
Prolonged cold exposure increases browning of WAT
Prolonged capsinoid treatment increases browning of WAT
Artificially stimulating and controlling browning in humans: Cold vs capsinoid exposure - interpreting experiments
Must be careful interpreting these experiments because don’t know how much of these results is because of the experiment and how much is because of the diff people
Artificially stimulating and controlling browning in humans: Bile acids
Can activate TGR5 --> Increases cAMP conc --> Activates deiodinase enzyme D2 --> Produces active thyroid hormone --> Converts T4 into T3 --> Increased UCP1 and BAT activity
TGR5
A thyroid G-protein coupled receptor
Artificially stimulating and controlling browning in humans: Bile acids - experiment
12 healthy females treated for 2 days
Orally took a bile acid (CDCA)
Artificially stimulating and controlling browning in humans: Bile acids - experiment results
Small increase in whole body energy expenditure and BAT activity, but not as large as increase under cold conditions
No browning of WAT
Probably not very useful
Cold-induced WAT browning - glucose
Increases rate of glucose uptake by BAT - more than insulin stimulates glucose uptake by skeletal muscle
Cold-induced WAT browning - cold exposure increased … in existing BAT of mice
Both glucose and fatty acid uptake
Cold-induced WAT browning - cold and capsinoid exposure increases…
Conversion of WAT to BAT
Correlated with increased body energy expenditure
