Lipids Flashcards
Function of Lipids
Major source of energy Provide hydrophobic barrier Serve as coenzymes, regulators Hormones Mediators of inflammation
Group of compounds related by certain physical properties: Insoluble in water, Soluble in nonpolar solvents
Lipids
Amphipathic; have both hydrophilic and hydrophobic groups; enables formation of bilayers
Phospholipids
Long chains of carboxylic acids
Fatty acids
Degree of Saturation: Contain 0 double bond
Saturated Fatty Acids
Degree of Saturation: Contain 1 double bond
Monounsaturated Fatty Acids
Degree of Saturation: Contain >1 double bond
Polyunsaturated Fatty Acids
Are associated with increased risk of cardiovascular diseases
Trans- and saturated fatty acids
Are thought to be protective
Mono- and polyunsaturated fatty acids
Geometric Isomerism of Unsaturated Fatty Acids
Cis fatty acids
Trans fatty acids
On the opposite side of the double bond
Cis fatty acids
On the same sides of double bonds
Trans fatty acids
Fluidity decreases with
Increasing chain length
Increasing saturation
Essential Fatty Acids
Linoleic Acid
Linolenic Acid
Arachidonic Acid
Precursor of arachidonic acid 20:4 (5,8,11,14) which is essential in prostaglandin synthesis
Linoleic Acid
18:3 (9,12,15)
Deficiency results in decreased vision and altered learning vision
Linolenic Acid
Becomes essential if Linoleic Acid is deficient
Arachidonic Acid
Numbering starts from the last carbon atom
Omega Fatty Acids
Are correlated with a decreased risk of cardiovascular disease; Lowers thromboxane production; Reduced tendency of platelets to aggregate
Omega-3 Fatty Acid
Omega-6 Fatty Acid
Activation of Fatty Acids
Must first be activated before being used in metabolism
Enzyme: fatty acyl-CoA synthetase
Co-factor: pantothenic acid
Energy used: 2 ATP equivalents
Formation of Palmitate (16:0)
Fatty Acid Synthesis
Fatty Acid Synthesis: Where does it occur?
In the cytosol
Major: liver and lactating mammary glands
Minor: adipose tissue
Fatty Acid Synthesis: Substrates
1 Acetyl CoA
7 Malonyl CoA
NADPH
ATP
Fatty Acid Synthesis: Product
Palmitate only
Fatty Acid Synthesis: Rate limiting step
Reaction: Acetyl CoA + ATP➡️Malonyl CoA
Enzyme: Acetyl CoA carboxylase
Necessary co-factor for fatty acid synthesis
Biotin
Fatty Acid Synthesis: Step 1
Synthesis of cytoplasmic Acetyl CoA
Fatty Acid Synthesis: Step 2
Acetyl CoA carboxylated to Malonyl CoA Rate limiting step Enzyme: Acetyl CoA carboxylase Cofactor: biotin Activators: insulin and citrate
Fatty Acid Synthesis: Step 3
Assembly of Palmitate
Enzyme: Fatty acid synthase
Where does the cell primarily get the necessary NADPH?
Hexose monophosphate Pathway or
Pentose phosphate Pathway and
NADPH-dependent malate dehydrogenase (malic enzyme)
Assembly is a sequence of steps
Condensation➡️
Reduction➡️
Dehydration➡️
Reduction
Regulation of Lipogenesis
Activated by: Citrate, Insulin
Inhibited by: Fatty acyl-CoA, Glucagon, Epinephrine
Fate of Fatty Acids
Further elongation in smooth endoplasmic reticulum and mitochondria;
Desaturation in the ER through mixed function oxidases (cytochrome b5)
Essential in the diet because they have double bonds that exceed the 9th carbon
Linoleic Acid
Linolenic Acid
Esters of the trihydric alcohol Glycerol and fatty acids; Main storage forms of fatty acids; Coalesce within adipocytes to form oily droplets that are the major energy reserve of the body
Triacylglycerols (TAGs)
Synthesis of TAGs: Where does it occur?
Liver
Adipose tissue
Synthesis of TAGs
Glycerol-3-phosphate + 3 fatty acyl CoA➡️triglyceride
Sources of glycerol-3-phosphate
DHAP from glycolysis
Phosphorylation of free glycerol
DHAP from glycolysis
Enzyme: glycerol-3-phosphate dehydrogenase
In liver and adipose tissue
Phosphorylation of free glycerol
Enzyme: glycerol kinase
In liver only
What organs synthesize fatty acids?
Liver
Adipose tissue
Hydrolyzes TAGs to yielding free fatty acids and glycerol; Can only release fatty acids from carbon 1 and carbon 3 of the TAG in stored fat
Hormone-sensitive Lipase
Bound to Albumin in blood for beta-oxidation
Free fatty acids
Carbon backbone for gluconeogenesis
Glycerol
Increase glucagon
Increase cAMP➡️phosphorylation
Active Hormone-sensitive lipase
Increase Insulin
Decrease cAMP➡️dephosphorylation
Inactive Hormone-sensitive lipase
Removal of Acetyl CoA fragments from ends of Fatty acids; Acetyl CoA can enter the citric acid cycle; generates NADH and FADH2 that can enter the ETC
Beta-oxidation of Fatty Acids
Beta-oxidation of Fatty Acids: Where does it occur?
In the mitochondria of almost all cells but fatty acid activation occurs in the cytosol;
Exceptions are: neurons, RBC, testis, kidney medulla
Beta-oxidation of Fatty Acids: Substrate
Palmitate
NAD+ + FAD
ATP
Beta-oxidation of Fatty Acids: Products
8 Acetyl CoA
7 FADH2
7 NADH
Beta-oxidation of Fatty Acids: Rate limiting step
Reaction: fatty acyl CoA + Carnitine➡️fatty acyl carnitine + CoA
Enzyme: carnithine acyltransferase
Beta-oxidation of Fatty Acids reverses the process of fatty acid synthesis by
Oxidizing and releasing units of acetyl-CoA
Oxidation of a fatty acid with an odd number of carbon atoms will yield
Acetyl CoA
Propionyl-CoA
Propionyl-CoA is converted to a TCA intermediate
Succinyl-CoA
Propionyl-CoA carboxylase requires
Biotin
Methylmalonyl-CoA mutase requires
Vit. B12
Oxidize very long chains of fatty acids (C20, C22)
Peroxisomes
Oxidation of unsaturated FAs require an additional enzyme
3,2 enoyl-CoA isomerase
Energy Yield of Beta-oxidation
129 ATP
Regulation of Beta-oxidation
Activated by: Glucagon
Inhibited by: Malonyl-CoA, Insulin
Alcohol leads to fat accumulation in the liver, called steatosis, which ultimately leads to cirrhosis; Alcohol dehydrogenase eats up NAD+ to reduce beta-oxidation in the liver
Fatty liver
Can occur in newborn and manifest as hypoglycemia from impaired FA oxidation and muscle weakness from lipid accumulation
Carnitine Deficiency
Affects only the liver resulting in reduced FA oxidation and ketogenesis with hypoglycemia
CPT I Deficiency
Affects skeletal muscle and when severe the liver
CPT II Deficiency
Decreased FA oxidation; During fasting, hypoglycemia can become profound due to lack of ATP to support gluconeogenesis; can manifest as Sudden Infant Death Syndrome
Medium-chain Fatty Acyl-CoA Dehydrogenase Deficiency (MCAD)
Caused by eating unripe fruit of the akee tree , which contains hypoglycin, a toxin that inactivates medium and short-chain Acyl CoA dehydrogenase and leads to hypoglycemia
Jamaican Vomiting Sickness
Rare neurologic disorder due to a defect that causes accumulation of Phytanic Acid which is found in plant foodstuff and blocks Beta-oxidation; Causes neurologic symptoms due to improper myelinization
Refsum’s Disease
Cerebrohepatorenal syndrome, which occurs in individuals with rare inherited absence of peroxisomes in all tissues; Characterized by liver dysfunction with jaundice, marked mental retardation, weakness, hypotonia, and craniofacial dysmorphism
Zellweger’s Syndrome
Defect in peroxisomal activation of VLCFA leads to accumulation of VLCFA in blood and tissues; Initial abnormalities are apathy and behavioral change; Visual loss, spasticity and ataxia follow
X-linked Adrenoleukodystrophy
Converts acetyl CoA to ketone bodies
Ketogenesis
Ketogenesis: Where does it occur?
In liver mitochondria
Ketogenesis: Substrate
Acetyl CoA
Ketogenesis: Products
Ketone Bodies
Ketogenesis: Rate limiting step
Reaction: Acetoacetyl CoA + Acetyl CoA➡️HMG-CoA
Enzyme: HMG CoA synthase
6-Hydroxybutyrate➡️Acetoacetate➡️Acetyl-CoA
Ketogenolysis
Can serve as fuel for extrahepatic tissues especially during fasting
Ketone Bodies
In prolonged starvation and diabetic ketoacidosis, oxaloacetate is depleted for gluconeogenesis; In alcoholism, excess NADH shunts oxaloacetate to malate; Rate of Ketone Body Formation > Rate of Ketone Body Use
Ketoacidosis
A steroid alcohol; Very hydrophobic compound; has a single hydroxyl group
Cholesterol
Adrenal hormone not derived from cholesterol
Epinephrine
De novo synthesis of Cholesterol
Cholesterol Synthesis
Cholesterol Synthesis: Where does it occur?
Virtually all cells, in the cytosol and smooth endoplasmic reticulum
Majority: in the liver and intestines
Cholesterol Synthesis: Substrate
Acetyl CoA
NADPH
ATP
Cholesterol Synthesis: Product
Lanosterol➡️Cholesterol
Cholesterol Synthesis: Rate limiting step
Reaction: HMG CoA➡️mevalonate
Enzyme: HMG CoA reductase
Drugs used for the treatment of hypercholesterolemia, to reduce the risk for cardiovascular diseases
Statins
Cholesterol Synthesis: Step 1
Biosynthesis of mevalonate
Rate limiting step
Cholesterol Synthesis: Step 2
Formation of isoprenoid units
Isopentenyl diphosphate
Cholesterol Synthesis: Step 3
Six isoprenoid units form isoprene
Cholesterol Synthesis: Step 4
Formation of Lanosterol
Cholesterol Synthesis: Step 5
Formation of Cholesterol
An intermediate in the pathway
Farnesyl pyrophosphate
How does the Acetyl CoA reach the cytosol for cholesterol biosynthesis?
Citrate shuttle
High cholesterol limits the expression of HMG-CoA reductase gene by transcription factor (SREBP)
Production inhibition
Insulin dephosphorylates and activates; Glucagon phosphorylates and inactivates
Enzyme phosphorylation
Elimination through conversion to bile salts then secretion into the bile
Cholesterol degradation
Synthesized in the liver from cholesterol
Bile Acids
Rate limiting enzyme of Bile Acids
Cholesterol-7-alpha-hydroxylase
Regulation of Bile Acids
Activated by: Cholesterol
Inhibited by: Bile Acids
Bile acid conjugated with either glycine or taurine; Primary means of excreting cholesterol; Emulsify lipids in the intestines
Bile salts
Excreted bile is reabsorbed in?
Terminal ileum
95% reabsorbed
5% excreted in feces = amount that liver must make
Steroid Hormone Synthesis: What is it for?
Precursor of ALL steroid hormones
Glucocorticoids (Cortisol)
Mineralocorticoids (Aldosterone)
Sex Hormones (Testosterone and Estradiol)
Steroid Hormone Synthesis: Location
In the smooth endoplasmic reticulum of the adrenal cortex, ovaries, testes, placenta
Steroid Hormone Synthesis: Substrate
Cholesterol
Pregnenolone - “mother hormone” from which all other hormones are derived
Steroid Hormone Synthesis: Rate limiting step
Reaction: Cholesterol➡️Pregnenolone
Enzyme: Desmolase
Blocker: Aminogluthetimide
Lipid digestion begins in the
Stomach
Reach the capillaries of skeletal muscle and adipose tissue; Triglycerides broken to FA and glycerol via lipoprotein lipase
Chylomicrons
Directly enter adjacent muscle cells or adipocytes, or may be transported in blood bound to albumin
Free fatty acids
Converted to DHAP then enters glycolysis or gluconeogenesis
Glycerol
Manifests as steatorrhea (greasy stools); results in deficiency in fat-soluble vitamins and essential fatty acids
Lipid Malabsorption
Causes of Lipid Malabsorption
Liver Disease Pancreatic Disease Cholelithiasis Shortened bowel Intestinal mucosa defects
Spherical macromolecule complexes composed of neutral lipid core surrounded by shell of amphipathic apoptoteins, phospholipid and nonesterified cholesterol
Plasma Lipoproteins
Functions of Lipid Transport
1) Keep their component lipids soluble as they transport them in plasma
2) Provides an efficient mechanism for transporting their lipid contents to and from the tissues
Represent the protein moiety of lipoproteins; Some are integral while others are free to transfer to other lipoproteins
Apolipoproteins or APO Proteins
Transport dietary triglyceride and cholesterol from intestine to tissues
Chylomicrons
Apoproteins: Chylomicrons assembly + secretion; Secreted by epithelial cells
Apo B-48
Apoproteins: Chylomicron, VLDL; Cofactor for lipoprotein lipase; Shuttled by HDLs
Apo C-II
Apoproteins: Chylomicron, VLDL; Mediates uptake of chylomicron remnant
Apo E
Transport triglyceride from liver to tissues
VLDL
Picks up Cholesterol from HDL to become LDL; Picked up by the liver
IDL
Delivers cholesterol into cells
LDL
Picks up cholesterol accumulating in blood vessels; Delivers cholesterol to liver and steroidogenic tissues via scavenger receptor; Shuttles apo C-II and apo E in blood
HDL
Apoproteins: HDL; Activates LCAT
Apo A-I
Apoproteins: LDL, VLDL; Binds to LDL and VLDL receptors
Apo B-100
Deposition of Cholesterol and Cholesterol esters in the artery walls especially from oxidized LDL; Oxidized LDLs can cause endothelial damage
Atherosclerosis
Lipoprotein lipase deficiency; High VLDL and Chylomicron; Low LDL and HDL; Xanthomas and pancreatitis
Type I Familial Lipoprotein Lipase
LDL receptors deficiency; High LDL; Xanthomas and Xanthelasmas with increased risk of atherosclerosis and coronary heart disease
Type II Familial Hypercholesterolemia
Apo E deficiency; High remnants of VLDL and chylomicron with increased risk of atherosclerosis and coronary heart disease
Type III Familial Dysbetalipoproteinemia
Increased VLDL production; Triad of: Coronary Artery Disease, DM type 2, Obesity
Type IV Familial Hypertriglyceridemia
Apo B-48 and 100 deficiency; No chylomicron, No VLDL/LDL; Intestinal malabsorption with accumulation of lipids in intestine and liver
Abetalipoproteinemia
Apo A1 deficiency; No HDL; Triglycerides and atherosclerosis
Familial a-lipoprotein Deficiency: Tangier’s Disease
Fisheye Disease
High HDL; Associated with benefits to health and longevity
Familial Hyperalphalipoproteinemia
High LpA; Early atherosclerosis and Thrombosis
Familial Lipoprotein A Excess
Predominant lipids of cell membranes; Degraded by phospholipases
Phospholipids
Most abundant phospholipids; Represent a large proportion of the body’s store of choline, important in nervous transmission, as acetylcholine and as a store of labile methyl groups
Phosphatidylcholine
Also found in cell membranes; plays a role in programmed cell death
Phosphatidylethanolamine (Cephalin) and Phosphatidylserine (for apoptosis)
Major lipid component of lung surfactant; Inadequate levels lead to Respiratory Distress Syndrome in the newborn
Dipalmitoylphosphatidylcholine (DPPC) or Dipalmitoyllecithin
Reservoir for arachidonic acid in the membranes; Source of 2nd messengers
Phosphatidylinositol
2 molecules of phosphatidic acid esterified through their phosphate groups to an additional molecule of glycerol; Found only in mitochondria and is essential for mitochondrial function; Antigenic
Cardiolipin
Part of the glycocalyx located on the outer layer of the cell membrane and functions in cell recognition and cell adhesion; Found in high concentrations in nervous tissues
Glycolipids
Sphingosine + Fatty Acid
Ceramide
Ceramide + Glucose or Galactose
Cerebroside
Ceramide + N-acetylneuramic acid
Ganglioside
Ceramide + Oligosaccharide
Globoside
Ceramide + Sulfated Galactose
Sulfatides
Only significant sphingophospholipid in humans where it is an important constituent of the myelin sheath of nerves
Sphingomyelin
Deficiency in phospholipids and sphingolipids from white matter resulting in increase CSF phospholipids
Demyelinating Diseases
Lipid storage diseases often manifested in childhood lipid synthesis is Normal; Lipid degradation in lysosomes is Abnormal
Sphingolipidoses
Hexosaminidase A Deficiency; Cherry red macula, MR and Hypotonia
Tay-Sach’s Disease
Alpha-galactosidase Deficiency; X-linked recessive, Rash, Renal failure
Fabry’s Disease
Ceramidase Deficiency; Triad of Skin rash, Hoarseness, Bone malformation
Farber’s Disease
Arylsulfatase A Deficiency; Psychologic disturbance in adults due to demyelination
Metachromic Leukodystrophy
Beta-Galactosidase Deficiency; Mental Retardation
Krabbe’s Disease
Beta-Glucosidase Deficiency; Hepatosplenomegaly + erosion of long bones
Gaucher’s Disease
Sphingomyelinase Deficiency; Hepatosplenomegaly
Neimann-Pick Disease
Potent compounds that elicit a wide range of physiologic and pathologic responses
Eicosanoids
3 main kinds of Eicosanoids
Prostaglandin
Thromboxane
Leukotrienes
Eicosanoids: Dietary precursor
Linoleic Acid
Eicosanoids: Immediate precursor
Arachidonic Acid
Synthesized in platelets; Cause vasoconstriction and platelet aggregation
Thromboxane (TXA2)
Produced by blood vessel walls; Inhibitors of platelet aggregation
Prostacyclin (PGI2)
Mixture of leukotrienes C4, D4, and E4; Potent bronchoconstrictors
Slow-Reacting Substances of Anaphylaxis (SRS-A)