Seminar 8 - Warszawski Uniwersytet Medyczny · Amide linkage Fatty acid phosphocholine Ester bond ....
Transcript of Seminar 8 - Warszawski Uniwersytet Medyczny · Amide linkage Fatty acid phosphocholine Ester bond ....
Seminar 8
Fatty acid metabolism
Fatty acid metabolism
• Why are fatty acids important to cells?
• fuel molecules
• stored as triacylglycerols
• building blocks
• phospholipids
• glycolipids
• precursors of hormones and other messengers
• used to target proteins to membrane sites
Fatty acid metabolism
• Why do triacylglycerols store large amounts of energy?
• fatty acid portion is highly reduced
• nonpolar molecules are stored in anhydrous form
• Where are triacylglycerols stored?
• adipocytes
Fatty acid metabolism
• What is needed for triacylglycerol breakdown?
• bile salts
• made in liver, stored in gall bladder
• glycocholate
• lipases
• from pancreas
• hydrolyze ester bond
Triglyceride hydrolysis
• fatty acids and monoacylglcerols are absorbed across plasma membrane of intestinal epithelial cells
Lipid transport system
• chylomicrons are particles consisting of triacylglycerols and proteins (apolipoproteins)
Fatty acids as an energy source
• How are fatty acids made available to peripheral tissues as an energy source?
• hormones trigger lipolysis in adipose tissue
• epinephrine, glucagon, ACTH
• insulin inhibits lipolysis
• released fatty acids are insoluble in plasma
• must be attached to serum albumin for transport
Fatty acids as an energy source
• What happens to the glycerol released?
• It is converted to glyceraldehyde-3-phosphate
• glycolysis
• gluconeogenesis
Fatty acid degradation
Beta-oxidation of fatty acids
• the first step is an activation of fatty acids
Beta-oxidation of fatty acids
• carnitine transports activated fatty acids into mitochondria matrix in fatty acid oxidation
Carnitine shuttle
Beta-oxidation of fatty acids
• what is the reaction sequence for the oxidation of fatty acids?
1. first step is an oxidation (acyl CoA dehydrogenase)
Beta-oxidation of fatty acids
2. second step is a hydration (enoyl CoA hydratase) this enzyme is stereospecific only L isomer is formed
Beta-oxidation of fatty acids
3. third step is a second oxidation (L-3-hydroxyacyl CoA dehydrogenase)
Bata-oxidation of fatty acids
4. last step is cleavage of 3-ketoacyl CoA by thiol group of CoA acyl CoA shortened by 2 carbons acetyl CoA formed
Beta-oxidation of fatty acids
• what are the products of fatty acid degradation?
for a C16 fatty acid:
• 8 acetyl CoA
• 7 FADH2
• 7 NADH + 7 H+
how much energy does this generate?
8 x 10 ATP = 80
7 x 1.5 ATP = 10.5
7 x 2.5 ATP = 17.5
Total = 108 ATP – 2 ATP (activation)
= 106 ATP
Beta-oxidation of unsaturated fatty acids
• unsaturated fatty acids require additional steps for degradation
isomerization shifts position and configuration of a double bond
reduction needed to remove double bond in wrong position
Beta-oxidation of odd-chain fatty acids
• how is the oxidation of odd-chain fatty acids different from even-chain ones?
• in final round of degradation products are acetyl CoA and proprionyl CoA (3C)
• proprionyl CoA is converted to succinyl CoA
Beta-oxidation of odd-chain fatty acids
• how is the oxidation of odd-chain fatty acids different from even-chain ones?
• in final round of degradation products are acetyl CoA and proprionyl CoA (3C)
• proprionyl CoA is converted to succinyl CoA
Step 1 Step 2 Step 3
Beta-oxidation of odd-chain fatty acids
Step 1
• proprionyl CoA is carboxylated to give D-methylmalonyl CoA (proprionyl CoA carboxylase) it uses biotin
Beta-oxidation of odd-chain fatty acids
Step 2
• D-methylmalonyl CoA is racemized to L form (methylmalonyl CoA mutase) it uses a derivative of vitamin B12
Beta-oxidation of odd-chain fatty acids
Step 3
• 5-deoxyadenosyl free radical removes a H atom to aid in rearrangement of L-methylmalonyl CoA to succinyl CoA
Beta-oxidation of fatty acids
• where, in addition to the mitochondria does fatty acid oxidation take place?
• peroxisomes
• how is this different from oxidation?
• in first step electrons are transferred to O2
Ketone bodies
• what are ketone bodies?
• acetoacetate
• -hydroxybutyrate
• acetone
• under what conditions are they formed?
• when fats are rapidly broken down
Synthesis of ketone bodies
1. formation of acetoacetyl CoA (thiolase)
Synthesis of ketone bodies
2. third molecule of acetyl CoA condenses with the acetoacetyl CoA, forming 3-hydroxy-3-methylglutaryl CoA (HMG CoA synthase) present only in the liver
Synthesis of ketone bodies
3. HMG CoA is cleaved to yield acetoacetate and one molecule of acetyl CoA (HMG CoA lyase) present only in the liver
Synthesis of ketone bodies
4. Acetoacetate can be reduced to beta-hydroxybutyrate (-hydroxybutyrate dehydrogenase)
4. Acetoacetate spontaneously decarboxylates to yield acetone (acetoacetate decarboxylase) (it cannot be metabolised any further and excreted through lungs)
Ketone bodies
• how can ketone bodies be used?
• major fuel source for heart muscle and kidney cortex
• during starvation or diabetes may be used by brain
• high levels of acetoacetate decreases lipolysis
Ketone body synthesis in the liver and use in peripheral tissues
Ketone bodies
• what is one important difference between plants and animals with respect to fatty acid metabolism?
• animals cannot use fatty acids to make glucose
• acetyl CoA cannot be converted to oxaloacetate („bypass 3” in gluconeogenesis)
Fatty acid synthesis
Transport of acetyl CoA to the cytosol
To synthesise palmitate you need: • 8 acetyl CoA • 14 NADPH • 7 ATP
Sources of NADPH for fatty acid synthesis
• Oxaloacetate must re-enter mitochondria
What is a main source of NADPH for lipogenesis?
Fatty acid synthesis
• what is the first committed step in fatty acid synthesis?
• formation of malonyl CoA (acetyl CoA carboxylase) it uses biotin
Fatty acid synthesis
• intermediates in fatty acid synthesis are linked to an acyl carrier protein (ACP)
Syntaza kwasów tłuszczowych
Stage 2 – formation of acetyl ACP and malonyl ACP
Stage 3 – condensation of acetyl ACP and malonyl ACP (+ decarboxylation)
Stage 4 – reduction of ketone group (C3) to form a methylene grup
Cycles 0. C2 + HCO3
- = C3
1. C2 + C3 = C4 – CO2
2. C4 + C3 = C6 – CO2
3. C6 + C3 = C8 – CO2
…
7. C14 + C3 = C16 – CO2
Stechiometry
Acetyl CoA + 7 malonyl CoA + 14 NADPH + 20 H+ →
Palmitate + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O
Control of fatty acid metabolism
• Acetyl CoA carboxylase (ACC) regulates this metabolism
• ACC is regulated by:
– glucagon i adrenaline (-)
– insulin (+)
– AMP (AMP-activated protein kinase) = phosphorylated ACC is inactive
– regulation of phosphatase
– concenration of citrate (allosterc control) = restores partially the activity of phosphorylated (inactive) ACC
Long-term control
• Adaptative control
• Based on a change of the rate of synthesis and degradation of enzymes in:
– -oxidation
– Synthesis of fatty acids
– Lipolysis (lipases)
Elongation and the synthesis of
unsturated fatty acids
Elongation and the synthesis of unsaturated bonds
• enzymes are on the cytosolic surface of ER
• process can investigated in microsomal systems (fragmentaded ER)
• to make unsaturated bonts additional enzymes are needed:
– cytochrome b5 reductase – cytochrome b5
– desaturase
e-
FAD
reductasa cyt b5 cytochrom b5 desaturase
Fe3+→Fe2+
hemowe Fe3+→Fe2+
niehemowe
e- e-
stearoil CoA NADH+H+
O2
oleil CoA NAD+
2H2O
• in mammals unsaturted fatty acids come from: – palmitoleic acid (16:1)
– oleic acid (18:1)
– linoleic acid (18:2)
– linolenic acid (18:3)
lack of enzymes to make double bonds in position futher than C9
Phospholipids
• Lecithin, was discovered in 1870 by the German
biochemist Ernst Hoppe- Selyer
• Strecker characterized choline in 1861
• In 1884, Thudichum JLW described sphingosine,
sphingomyelin, cerebrosides, cephalin and
lecithin in brain tissue
Phospholipids
• These are derivatives of phosphatidic acid, which is the
simplest phospholipid.
• Phosphatidic acid is made up of one glycerol to which two
fatty acid residues are esterified to carbon atoms 1 and 2
The 3rd hydroxyl group is esterified to a phosphoric acid
L-Phosphatidic acid
Phospholipids
• Phospholipids in general are amphipathic,
particularly Lecithin
• They have both hydrophobic and hydrophilic
portion in their molecule
Phospholipids
Phospholipids form the bilayer
Amphipathic nature
Phospholipids form micelles and liposomes
Amphipathic nature
• The glycerol along with the phosphoric acid and
choline constitute the polar „head” of a
phospholipid molecule, whereas the hydrocarbon
chains of the fatty acids represent the nonpolar
„tail”
Amphipathic nature
Micellar Formation
• When phospholipids are distributed in water, their
hydrophobic parts keep away from water, forming
molecular aggregates called micelle.
Phospholipids form micelles and liposomes
Liposomes
• A lipid bilayer will close on itself under appropriate conditions
to form liposomes. Unilamellar or multilamellar liposomes
may be formed. They may be prepared by sonication of
mixtures of phospholipids and cholesterol.
Liposomes
• Liposomes are microscopic spherical vesicles
• When mixed in water under special conditions,
the phospholipids arrange themselves to form a
bilayer membrane which encloses some of the
water in a phospholipid sphere
Liposomes
• Drugs, proteins, enzymes, genes, etc. may be
encapsulated by the liposomes which could act as
carriers for these substances to target organs
• Liposome-entrapped drugs exhibit superior
pharmacological properties than those observed
with conventional formulations
Liposomes
• Liposomes have important applications in:
• cancer chemotherapy
• antimicrobial therapy
• gene therapy
• vaccines and diagnostic imaging
Aquasomes
• They are one of the most recently developed delivery systems that are making a niche as the peptide/protein carriers
• These are nano particulate carrier systems with three layered self-assembled structures
Aquasomes
• They comprise the central solid nanocrystalline core coated
with polyhydroxy oligomers onto which biochemically
active molecules are adsorbed
• The solid core provides the structural stability
• The carbohydrate coating stabilizes the biochemically
active molecules
• As the conformational integrity of bioactive molecules is
maintained, aquasomes are being proposed as a carrier
system for delivery of peptide based pharmaceuticals
Membrane lipids
Phospholipids
• Derivatives of glycerol OR sphingosine
glycerol sphingosine
Fosfoglicerydy
• phosphatides (diacylglycerol 3-phosphate): – Hydroxylic groups C1 i C2 are estriried with fatty acid
– Hydroxylic groups C1 i C2 are estriried with phosphoric acid
• Not common in membranes
-
Fosfoglycerides in membranes
• Derivatives of phosphatides (phosphate is eterificated)
serine etanolamine choline
glycerol inozytol
Sphingolipids
• Phospholipids which are derivatives of sphingosine
• Amine group is connected to fatty acid (amide linkage)
• Hydroxyl group is esrificated with phosphocholine
sphingosine
sphingomyeline
sphingosine
Fatty acid Amide linkage
phosphocholine
Ester bond
Glycolipids
• Aminal glycolipods are derivatives of sphingosine
• Amine group is connected to fatty acid (amide)
• Hydroxyl group is connected to sugar
sphingosine
Fatty acid
sugar
ceramide
Cerebroside
• The simpliest glycolipid
sphingosine
Fatty acid
Amide linkage Galactose (or glucose)
Ester bond
Gangliosides
• Cantain branched chain up to 7 sugar residues
N-acetylneuraminic acid
Biomembranes • The molecules align themselves to form
monolayers with the polar heads pointing in one
direction and the nonpolar tails in the opposite
direction.
Phospholipids form the bilayer
Phosphatidylcholine or Lecithin
• This is a nitrogen containing phospholipid
• The word lecithin is derived from the Greek word,
lekithos = egg yolk
• It contains glycerol
Phosphatidylcholine or Lecithin
• The alpha and beta positions are esterified with
fatty acids
• Usually, the fatty acid attached to the
betacarbon, is a PUFA molecule
Phosphatidylcholine or Lecithin
Lecithin R1 and R2 are fatty acids Red rectangle – glycerol group The blue rectangle is choline which shows polar or hydrophilic property
Phosphatidylcholine or Lecithin
• The phosphoric acid is added to the third position, to form
hosphatidic acid. The phosphate group is esterified to the
quaternary nitrogen containing group – Choline
Action of Phospholipases
• phospholipases are enzymes that hydrolyze
phospholipids
• different phospholipases are involved in the
hydrolysis of specific bonds in lecithin
Action of Phospholipases
• Phospholipase A2 acts on an intact lecithin molecule
hydrolyzing the fatty acid esterified to the beta (second)
carbon atom
• The products are Lysolecithin and fatty acid
• Lysolecithin is a detergent and hemolytic agent
• The enzyme is present in the venom of viper snakes
• The hemolysis and consequent renal failure seen in viper
poisoning could be thus explained
Action of Phospholipases
• The products formed in each case may be summarized
as follows:
Phospholipase A2
Lecithin Lysolecithin + fatty acid
Phospholipase A1
Lecithin 1 Acyl glycerophosphorylcholine + fatty acid
Phospholipase C
Lecithin 1,2 diacylglycerol + Phosphoryl choline
Phospholipase D
Lecithin Phosphatidic acid + choline
Lung Surfactants
• Normal lung function depends on a constant
supply of lung surfactants
• It is produced by epithelial cells
• It decreases surface tension of the aqueous
layer of lung and prevents collapse of lung
alveoli
Lung Surfactants
• Constituents of surfactants are dipalmitoyl lecithin,
phosphatidyl glycerol, cholesterol and surfactant
proteins A, B and C
• During fetal life, the lung synthesizes sphingomyelin
before 28th week of gestation
• But as fetus matures, more lecithin is synthesized
Lung Surfactants
• The lecithin-sphingomyelin (LS) ratio of amniotic
fluid is an index of fetal maturity
• A ratio of 2 indicates full lung maturity
• Low surfactant level can lead to respiratory
distress syndrome (RDS), which is a common
cause of neonatal morbidity
Respiratory Distress Syndrome (RDS)
• It is due to a defect in the biosynthesis of
dipalmitoyl lecithin (DPL), the main pulmonary
surfactant
• Premature infants have a higher incidence of
RDS because the immature lungs do not
synthesize enough DPL
Phosphatidylethanolamine or Cephalin
• Cephalin differs from lecithin in that the ethanolamine is
present instead of choline
• Cephalin is also found in biomembranes and possesses
amphipathic properties
Cephalin (Phosphatidylethanolamine)
Phosphatidylinositol
• Here, phosphatidic acid is esterified to inositol
• Phosphatidyl inositol bisphosphate or PIP2 is present in
biomembranes
• This compound plays a vital role in the mediation of hormone
action on biomembranes and acts as a second messenger
Phosphatidylinositol
Plasmalogens
Plasmalogens
• these are phospholipids
• presence of a vinyl ether linkage at the 1st C position and an ester linkage at the 2-nd C position
Ethanolamine plasmalogen
Plasmalogens
• The phosphoric acid is attached to choline or
ethanolamine.
Ethanolamine plasmalogen
Phosphatidylglycerol
• It is formed by esterification of phosphatidic
acid to glycerol
• When two molecules of phosphatidic acid are
linked with a molecule of glycerol,
diphosphatidylglycerol or cardiolipin is formed
Phosphatidylglycerol
• It is the major lipid of mitochondrial membrane
(Commercially, it is extracted from myocardium)
• Decreased cardiolipin level leads to
mitochondrial dysfunction, and is accounted for
• heart failure
• hypothyroidism
• some types of myopathies
Sphingolipids
• The sphingosine containing lipids may be of 3
types
• phosphosphingosides
• glycosphingolipids
• sulfatides
Phosphosphingosides
• They contain phosphoric acid group
• A common phosphosphingoside present abundantly in
membranes, especially of the nervous system, is
sphingomyelin (it contains choline)
Sphingomyelin
Sphingomyelins
• Sphingomyelins are the only sphingolipid that
contain phosphate and have no sugar moiety
• They are found in large quantities in nervous
system
Sphingomyelins
• Different sphingomyelins may be formed
depending on the fatty acid attached.
• Common fatty acids found are
• lignoceric (24 C),
• nervonic (24 C, one double bond) and
• cervonic (22 C, 6 double bonds) acids
Sphingomyelins
• Because of its amphipathic nature sphingomyelin can act
as an emulsifying agent and detergent
• The relative proportion of lecithin and sphingomyelin is
important in biological fluids like bile, amniotic fluid, etc.
• Sphingosine combined with fatty acid is called ceramide,
which is a component of glycosphingolipids
Clinical relevance of antiphospholipid antibody
Non-phosphorylated Lipids
• Glycosphingolipids (Glycolipids)
• They are seen widely in nervous tissues. This
group of lipids do not contain phosphoric acid;
instead they contain carbohydrates and ceramide
• Ceramide + Glucose → Glucocerebroside
• Ceramide + Galactose → Galactocerebroside
Globosides (Ceramide Oligosaccharides)
• They contain two or more hexoses or
hexosamines, attached to a ceramide molecule.
• Ceramide + Galactose + Glucose → Lactosyl
ceramide
• Lactosyl ceramide is a component of erythrocyte
membrane
Gangliosides
• They are formed when ceramide oligosaccharides
have at least one molecule of NANA (N-acetyl
neuraminic acid) (sialic acid) attached to them
• Ceramide—Glucose—galactose—NANA;
• This is designated as GM3 (ganglioside M3)
Gangliosides
• Gangliosides contribute to stability of
paranodal junctions and ion channel clusters
in myelinated nerve fibers.
• Autoantibodies to GM1 disrupt lipid rafts,
paranodal or nodal structures, and ion
channel clusters in peripheral motor nerves.
Sulfolipids or Sulfatides
• These are formed when sulfate groups are
attached to ceramide oligosaccharides
• All these complex lipids are important
components of membranes of nervous tissue
Sulfolipids or Sulfatides
• Failure of degradation of these compounds
results in accumulation of these complex
lipids in CNS
• This group of inborn errors is known as lipid
storage diseases