Biochemistry 608
Unit III
Lipids
Defined:soluble in organic solvents (i.e. chloroform and ether); some are insoluble in aqueous solution
Examples: fats, waxes, oils
Functions:
1.energy storage: major energy reserve
2.construction of membranes:50% lipid/50% protein, gives characterisitic sheet-like structure
3.surfactant activity:resemble detergents (they have an hydrophobic end and hydrophillic end. Ex. bile salts, emulsify ingested fats for digestion; Ex. surfactants to lower surface tension of fluid in lungs
4.insulation against cold, protection from injuries
5.energy production
6.chemical signaling:Ex. steroid hormones (androgens, estrogens, and eicosanoids--local chemical mediators)
7.sexual attraction:female profile
Fatty acids
Overview
do not occur in high [] in body
components of more complex lipids
important intermediates in lipid metabolism
general structure formula: RCOO-; R is normally unbranched w/ 0+ double bonds
carboxyl group is ionized at physiological pH (@5)
each cell has them
most abundant have 16 or 18 carbons
odd-numbered ones are rare, higher organisms do not make them; some in diet
Nomenclature (Ex. 18:2 9,12) with delta numbers being locations of the 2 double bonds
Saturated
simplest = acetic with 2 carbons, intermediate, low []
most abundant = palmitic (16:0) and stearic (18:0)
Unsaturated
double bonds are always cis, changes direction at each bond, don't pack well, lower melting point
double bonds rarely before C9, but often occurs there
multiple double bonds usually separated by two single bonds, therefore not conjugated-draw
most abundant = oleic (18:1/9), linoleic (18:2/9,12), linolenic (18:3/9,12,15) arachidonic (20:4/5,8,11,14)
Linoleic is an essential fatty acid (must be in diet)
Arachiodonic is dif. because it has 20 carbons and two bonds before C9
helpful to classify according to position of the last double bond (Ex. linoleic is an w-6 fatty acid while linolenic is an w-3 family)
Examples: prostaglandins, thromboxanes, prostacyclins, leukotrienes
Triglycerides (triacylglycerols, neutral fats)
Overview
Defined:a molecule of glycerol (draw) + three esterified fatty acid molecules
characterized by fatty acid composition and arrangement of the molecules on the glycerol
draw typical
Systematic name = triacylglycerols
contain neither acidic nor basic groups so called neutral fats
no charged or other highly polar groups so very hydrophobic/quite insoluble in water
with unsaturated fats are liquids (corn oil); with saturated fats are solids (beef tallow)
Function
stored in adipose cells (mostly triglycerides)
principal energy reserve in higher animals (100x the caloric content of carbos in body)
confined almost entirely to adipose cells and have the same function: energy storage, insulation from the cold, padding
Phosphoglycerides
Overview
derivatives of phospatidic acid (draw)
not in high [], but is an impt. intermediate in lipid metabolism
draw general structure
R1 is usually a satruated fatty acid
R2 is usually an unsaturated fatty aci
R3 is an alcohol
Most common R3 are Ethanolamine (draw)
and Choline (draw)
Less common R3 are Serine (draw)
and Inositol
Named as derivatives of phosphatidic acid (Ex. inositol = phosphatidylinositol,etc.)
polar lipids; hydrophobic tail of two fatty acids + hydrophilic tail w/ "-" phosphate group and a very hydrophilic, usually "+" R3 group
Functions
principal membrane lipids; form a bilayer in aqueous solutions
precursors to molecules having a signaling function
phospatidylcholine (w/ two palmitate chains) is a lung surfactant, prevent alveoli collaps
Clinical--newborn respiratory distress syndrome = hyaline membrane disease
specialized pulmonary phosphoglycerides begins five weeks prior to birth
if more than five weeks premature there is insufficient surfactant, alveoli collapse, lungs tissues injured, poor gas exchange
treatment involves introducing the missing lipid into the lungs
Sphingolipids
Overview
derivatives of sphingosine (draw)
attaching a fatty acid to the amino group gives a ceramide (draw)
ceramides not in high [], but are intermediates to sphingolipids
synthesize by substituting the terminal hydroxyl of a ceramide with a polar residue; the nature of the substitution determines the class
phosphoylcholine = sphingomyelin
glucose or galactose = cerebroside
oligosaccharide = ganglioside
have an hydrophilic head and two-stranded hydrophobic tail (one is hydrocarbon chain of fatty acid, other is hydrocarbon chain of sphingosine)
cerebrosides are classified as glucocerebrosides or galactocerebrosides based on monosaccharide residue; attachment via glycosidic linkage
dozen different gangliosides each w/ dif. oligosaccharide head; many common features
all begin w/ glucose residue; glucocerebrosides are the starting materials for the synthesis of gangliosides
all are short, usually between 3 and 6 monosaccharide residues
larger oligosaccharides are branched
contain between 1 and 3 residues of sialic acid (acidic sugar derivative); makes head of oligosaccharide acidic
Function
high [] in nervous tissues; present in other tissue as well
resemble phosphoglycerides; also components of membranes
Clinical
catabolized in lysozomes; have hydrolases to cleave each specific substituent group
deficiency in any one hydrolase and the body cannot degrade a specific sphingolipid; accumulates; condition known as lipid storage disease (a dozen dif. kinds); most are serious, all are rare
Steroids
Overview
derivatives of the polycyclic hydrocarbon having the following structure: (draw)
rings in parent compound are completely reduced (no = bonds)
in many steroids, double bonds have been introduced into the ring system
nucleus is fairly planer; alpha groups extend below (dashed line), beta groups extend above (solid line)
Cholesterol
steroids with 8-10 carbon side chains at 17 + hydroxyl group at 3 are "sterols"
most abundant = cholesterol (draw)
has methyl groups (#18 & 19) at 10 and 13; termed "angular methyl groups", present in most naturally occurring steroids
all attached groups are beta
solitary double bond between 5 and 6
The hydroxyl is frequently esterified to an unsaturated fatty acid; 2/3 of cholesterol in blood is esterified.
cholesterol oleate (draw) is a typical cholesterol ester
Cholesterol is the biosynthetic precursor to steroid hormones, bile salts, and Vitamin D
structural component of membranes; lots in myelin sheaths and plasma membranes, little in endoplasmic reticulum
Bile acids and bile salts
bile acids synthesized from cholesterol by the liver
most common = cholic acid (draw as typical of group)
side chain at 17 is the same in all bile acids
differ from each other only in # and position of OH groups
COO- and OH groups lie on one side of the planar molecule, making it amphipathic; thus, they are detergents
bile salts are derivatives of bile acids where the carboxyl group has been couple to a molecule of glycine or, less commonly, taurine via an amide linkage
most bile acids produced in the liver are converted to salts before leaving the organ
pass from liver into bile and eptined into S.I.
powerful detergents which emulsify lipids for digestion
higly soluble in water
Steroid hormones
endocrinology
Triglyceride synthesis
Introduction
caloric content: carbos (4 kcal/g), protein (9 kcal/g...6?), lipid (9 kcal/g)
hyrdophillic molecules (proteins, carbos) are highly hydrated; hydrophobic molecules (lipids) are not.
one gram of carbo occupies @ 3x the volume of one gram of nonpolar lipid
nonpolar lipids provide the most efficient storage form for energy
stored fat in adipose cells (depot fat) is almost exclusively triglycerides
each of three major groups can be efficiently transformed into triacylglycerol
triclycerides produced by adipose tissue, the liver, and the lactating mammary gland
one molecule = synthesis and joining together of 3 fatty acid molecules + one glycerol
Fatty acid synthesis
synthesized from acetly CoA;
several steps to initiate the fatty acid chain; elongated by repetitive cycles of a sequence of four reactions; elongated by two carbons each cycle
Acetyl CoA is active acetate but needs further activation before it can be used; the enzyme is acetyl CoA carboxylase and the product of activation is malonyl CoA (draw)
Acetyl CoA Carboxylase mechanism of action
three subunits
Biotin carboxyl carrier protein (BCCP)- biotin subunit which serves as a carrier for an activated molecule of carbon dioxide
Biotin carboxylase - forms the activated biotin-CO2 intermediate
Transcarboxylase - catalyzes the transfer of the activated CO2 group
See reaction in notes (13)
Regulation, synthesis of malonyl CoA is the committed step in fatty acid synthesis; acetyl CoA carboylase is regulated both allosterically and through phosphorylation-dephosphorylation
citrate and isocitrate are allosteric effectors which increase the Vmax of the enzyme. In the absence of these compounds, the enzyme exists as an inactive monomer; in their presence, the monomers aggregate to form long chains which are enzymatically active
The CoA derivatives of long-chain fatty acids inhibit the enzyme
Glucagon and epinephrine cause a decrease in the activity of acetyl CoA carboxylase; insulin, in contrast, activates the enzyme
glucagon and epinephrine inactivation works by an increase in phosphorylation and a decrease in polymerization of the enzyme
activation by insulin is accompanied by dephosphorylation and an increase in polymerization
fatty acid synthetase complex--the enzymes which catalyze the synthesis from malonyl CoA units are organized into this large structure
The bacterial version has eight types of polypeptide chains, each with its own enzymatic activity; higher organisms have only one or two chains but the same activities and function
higher organisms carry out synthesis in the cytosol
Acyl carrier protein--part of the fatty acid synthetase complex; protein to which the growing fatty acid chain is attached; in higher organisms, it is part of a large polypeptide chain that has additional activities
contains a covalently-bound phosphopantetheine group, identical in structure to the functional end of a CoA molecule
SH group supplied by phosphopantetheine is designated the central SH group
the growing fatty acid chain is attached via a thioester linkage to this central SH group
Phosphopantetheine is long, flexible; appears to pass the growing chain from one enzyme of the complex to the next
Peripheral SH group--an SH group contributed by a cysteine residue of one of the enzymes of the synthetase complex
Steps in synthesis. See figure 2 in notes (16)
a. acetyl group is transferred form acetyl CoA to the SH group of ACP (central SH); reaction occurs only once, provides the first two carbon atoms of the fatty aid (palmitate 15 and 16); all subsequent carbons derive from malonyl CoA
b. A malonyl group is transferred from malonyl CoA to the central SH, displacing the acetyl group to the peripheral SH
c. A condensing enzyme transfers the acetyl group to the malonyl group with liberation of CO2 and formation of _-ketoacyl ACP
d. The _-ketoacyl ACP is reduced to a D-_-hydroxyacyl ACP by a reductase which uses NADPH as the reductant
e. The D-_-hydroxylacyl ACP is converted too a transenoyl ACP by a dehydratase
f. The transenoyl ACP is converted to an acyl ACP by a reductase
g. steps b-f are repeated again and again until a fatty acid chain sixteen carbons long has been produced; not further elongated, but hydrolyzed from the synthetase complex
Formation of fatty acy CoA
before further elongation beyond 16 C's, desaturation, or incorporation into triglycerides, fatty acids must be activated by conversion to the CoA derivatives
this reaction occurs in the cytosol--catalyzed by ACYL CoA SYNTHETASE
reaction driven by inorganic pyrophosphatase, which removes the inorganic pyrophosphatase as it is formed
Synthesis of fatty acids with more that 16 carbons
Upon release at 16 carbons, mitochondria and microsomes are capable of elongating the CoA derivatives of preformed fatty acids by successive additionof two-carbon units to the carboxyl-terminal end of the fatty acyl CoA molecule.
the two systems differ from each other and from the fatty acid synthesis complex found in the cytosol
Synthesis of unsaturated fatty acids
CoA derivatives of saturated fatty acids are used as substrates for the synthesis of unsaturated fatty acids
Several different microsomal enzyme systems identified
Mammals lack the enzymes necessary to synthesize fatty acids with double bonds beyond carbon 9. So, can't make linoleic (18:2/9,12) etc. These become essential but only in small amounts
Stoichiometry of fatty acid synthesis
overall reaction for synthesis of a molecule of palmitate (draw)
reaction including malonlyl CoA formation (draw)
Synthesis of glycerol 3-phosphate
immediate precursors of triglycerides are fatty acyl CoA's and glycerol phosphate
glycerol 3-phosphate can be produced by phosphorylation of glycerol or by reduction of dihydroxyacetone phoosphate
(draw)
the second reaction, a component of the glycerol phosphate shuttle, has been discussed
Adipose tissue lacks glycerokinase and cannot incorporate free glycerol into triglycerides
Joining the fatty acid and glycerol moieties
triglyceride synthesized by condensing two molecules of fatty acyl CoA with glycerol 3-phosphate to yield a phosphatidic acid
cleave the phosphate group to give a diglyceride
react the latter with a third fatty acyl CoA molecule to yield a triglyceride
(draw)
Breakdown of triglycerides
Mobilization of depot fat
little oxidation of stored triglycerides takes place in the adipose tissue itself.
depot fats are hydrolyzed to glycerol and free fatty acids
transported to other tissues (muscle/liver) for oxidation
hydrolysis of stored triglycerides, catalyzed by LIPASES, remove the fatty acids from a triglyceride molecule sequentially to yield a dyglyceride, a monoglyceride, and finally, free gycerol
(draw)
breakdown is regulated at the first step (rate-limiting)
TRIGLYCERIDE LIPASE is stimulated by adrenaline, noradrenaline, and glucagon and depressed by insulin
The first three hormones stimulate the synthesis of cyclic AMP
cAMP, in turn, activates a protein kinase which converts hormone-sensitive lipase from an inactive to an active form.
Insulin increases the level of the inactive, dephospharylated form of hormone-sensitive lipase (unknown mechanism)
(draw)
glycrol is soluble and needs no transport help
some long-chain fatty acids are sparingly soluble and also cause red cell lysis
transported as soluble, nonhemolytic complex with serum albumin
Oxidation of glycerol
glycerol can be converted to dihydroxyacetone phosphate in two steps as previously mentioned (II.C.2)
dihydroxyacetone phosphate can be used for the formation of glucose (gluconeogenesis) or can be oxidized via the glycolytic pathway and citric acid cycle
Fatty acid oxidation
formation of fatty acyl CoA
fatty acis are not oxidized as such
first converted to CoA derivatives in the cytoplasm
(see II.B.9)
To circumvent the permeability problem, fatty acid is transferred at the inner surface of the outer mitochondrial membrane, to a carrier molecule, CARNITINE
(draw) the reaction is catalyzed by CARNITINE ACYL TRANSFERASE I
the O-acyl carnitine diffuses to the inner mitochondrial membrane
transported across the membrane, mediated by TRANSLOCASE, an integral protein in inner mitochondrial membrane
at inner surface, fatty acid is transferred to a molecule of CoA by reversing the above reaction; catalyzed by CARNITINE ACYLTRANSFERASE II
carnitine acyltransferase I is inhibited by malonyl CoA, so...fatty acid synthesis and fatty acid oxidation do not occur simultaneously, thereby preventing a futile cycle
When fatty synthesis is active, the malonyl CoA [] is high, carnitine acyltransferase I is inhibited, and fatty acids cannot enter the mitochondrial matrix to be oxidized
vice versa
Oxidation of fatty acyl CoA (beta-oxidation)
involves a sequence of four reactions which convert a molecule of fatty acyl CoA to one molecule of acetyl CoA and a fatty acyl CoA which is two carbons shorter than the original molecule; repeated until the original molecule of fatty acyl coA has been completely converted to acetyl CoA; termed BETA-OXIDATION since each cycle of reactions results in the oxidation of the beta carbon atom of the fatty acyl CoA molecule
uses the same four reactions as those involved in fatty acid synthesis, but operating in the reverse direction
(draw)
the two processes are distinct--catalyzed by completely different enzyme systems
overall reaction given: (draw)
two hydrogen atoms removed to yield the trans- 2 enoyl CoA derivative; FAD serves as the oxidant; enzyme is ACYL CoA DEHYDROGENASE
enoyl CoA derivative is hydrated to form the L-B-hydroxyacyl derivative by ENOYL CoA HYDRATASE
hydroxyl group is converted to a keto group by L-B-HYDROXYACYL CoA DEHYDROGENASE; uses NAD+ as the oxidant
B-ketoacyl CoA derivative is split by the introduction of CoA in a thiolysis reaction which forms one molecule of acetyl CoA (bearing the original CoA molecule) and a molecule of fatty acyl CoA (bearing the new CoA moiety); catalyzed by B-DETOACYL CoA THIOLASE
each reaction cycle produces one molecule of FADH2, one molecule of NADH;, and one molecule of acetyl CoA; former are oxidized via ETC, acetyl CoA enters the citric acid cycle
oxidation of unsaturated fatty acids
problems to oxidizing unsaturated fatty acid:
although unsaturated fatty acids begin by forming a double bond, if the double bond on the saturated fatty acid begins on an odd-numbered carbon, as they are wont to do, it will be in the wrong position
normal fatty acid ='s are cis where the acyl CoA dehydrogenase ='s are trans
need two additional enzymes
ISOMERASE shifts double bonds from odd carbons to correct position while converting them to trans
RACEMASE converts D-B-hydroxylacyl CoA (from hydration of a =) to L-B-hydroxyacyl CoA
energy yield from the oxidation of fatty acids
get n-2/2 equivalents per fatty acid (i.e. palmitic,16, gives 7 energy equivalents) so one NADH, one FADH2, and one acyl CoA + 1 for the left over; each NADH yields 3 ATP, each FADH = 2, each acyl CoA = 12 (palmitic = 7x2 + 7x3 + 8x12 = 131) subtract 2 for the high-energy bonds used to convert the fatty acid to an acyl CoA derivative
each double bond in an unsaturated fatty acic gives one less FADH2; so reduces the ATP by two
differences between the pathways for fatty acid synthesis and fatty acid degradation
syntheses in the cytosol; degradation in the mitochondria
synthesis enzymes in a discrete structure (synthetase complex); breakdown enzymes in a dif. complex
intermediates in synthesis are bound to acyl carrier protein; in degradation they are bound to CoA
constructed from malonyl CoA (except intial two-carbon unit); degraded to yield acetyl CoA
reductant in synthesis is NADPH; oxidants in breakdown are FAD and NAD+
synthetase complex does not make chains greater than 16 C's; mitochondria will degrade any length fatty acids
Steroid Metabolism
Introduction
most important classes are cholesterol, cholesterol esters, bile acids and bile salts and steroid hormones
cholesterol serves as the biosynthetic precursor to the steroid hormones, bile salts and Vit D
Biosynthesis of cholesterol
cells don't make cholesterol if they can get it from their environment (serum)
serum cholesterol from liver and S.I. (eat @ .3g but produces @1.5g)
synthesized from acetyl CoA in cytosol
two molecules of acetyl CoA condese to yield acetoacetyl CoA
condenses with an acetyl CoA to give B-hydroxy-B-methylglutaryl CoA
reduced to mevalonate using NADPH as reductant
Mevalonate formation is the first committed step to cholesterol biosynthesis--point of regulation is the HYDROXYMETHYLGLUTARYL CoA REDUCTASE
high levels of serum cholesterol repress synthesis of enzyme, increase its rate of degradation and decrease its activity
fasting causes a decrease in activity of HMG-CoA reductase by an unknown mechanism
three steps from mevalonate to isopentenyl pyrophsophate
two isopentenyls plus the product of its isomerization are combined to form the carbon skeleton of cholesterol
the two C5 units (isopentenyls) give a C10 cmpd, combines with the isomer to give a C15 cmpd, two C15 units combine to form squalene, C30, an open chain
(draw)
squalene cylizes to lanosterol, differs in # and position of = and three additonal methyls, converted to cholesterol in several steps removing methyls as CO2, double bond inserted and extra ='s removed
(draw)
catabolism of cholesterol
steroids can only be excreted, via feces
most cholesterol converted to bile salts and secreted into intestine
reabsorbed and returned to liver, some escapes to feces
cholesterol itself enters S.I. by secretion across intestinal mucosa and via the bile
some is reabsorbed, some is excreted; accounts for up to 50% of fecal steroid excretion
intestinal microorganisms facilitate secretion by converting them to un-repsorbable derivatives
cholesterol and atherosclerosis
localizing thickenings on inner portion of arterial wall
interferes with blood flow, leads to angina, myocardial infarction and stroke
chief cause of death in U.S.
partial to persons with high plasma cholesterol []
especially those with homozygous familial hypercholesterolemia, used to die by 20
lowering serum [] causes lesions to regress and heart attack risk decreases
rabbits cannot metabolized cholesterol and will die in weeks
early stage is localized arterial wall thickening by cholesterol and other lipids
believe cholesterol to be the causative factor
effectiveness of dietary control is limited, @10-25% reduction in serum []
daily output is greater than intake (@5x)
dietary reduction causes increased body mobilization/synthesis
still, a safe method for achieving some reduction
amt and degree of saturation influences plasma []
more polyunsaturated lower plasma []
monounsaturated and stearic acid have no effect
saturated fatty acids (less stearic) increase []
saturated 2x as good at raising as polyunsaturated are at lowering so 2:1 does nothing
more polyunsaturated and less saturated
some polyunsaturateds may increase some cancers
drugs
cholestyramine-binds bile salts preventing resorption
lovastatin-competitive inhibitor to HMG-CoA reeductase
Digestion and transport of lipids
plasma lipoproteins
generally water-insoluble
transported as lipoprotein complexes so soluble
fatty acids travel with serum albumin
five major serum lipoproteins
as density of particl increases, % protein increases
as particle density increases, proportion of triglycerides decreases while phosphoglyceride and cholesterol portions increase
low-density lipoproteins have high proportion of cholesterol
dozen+ proteins in five classes
many apoproteins in more than one class
all lipoproteins except LDL contain more than one apoprotein species
proteins not covalently linked to each other or to lipids
each apoprotein has a function
help stabilize micellar structure of lipoprotein structure
recognition signals to enable binding to cell receptors (B and E)
cofactors or activators for lipid metabolizing enzymes (A and C)
proteins, phosphoglycerides, and unesterified cholesterol on outer shell of lipoprotein; hydrophillic portions outward
triglycerides and cholesterol esters in center with hydrophobic portions
dynamic structures; component parts always moving between lipoprotein particles
continuous state of flux, undergoing conversion
digestion and transport of dietary triglycerides and cholesterol - chylomicrons
bile salts emulsify neutral fats in intestine
PACREATIC LIPASE, a hydrolase, acts on emulsifed fats, converting triglycerides to monoglycerides and free fatty acids; slow, never completed
products absorbed by intestinal mucosa; small proportion is undegraded triglyceride
neutral fats resynthesized in intestenial mucosa cells
with cholesterol, they are coated w/ phospholipid and protein to form chylomicrons
chylomicron apoproteins include three A's, one B (B48), two C's, and E
secreted into lymph capillaries; then to thoracic duct to circulation
chylomicrons give serum a milky appearance, lipemia, after fat-rich meal
LIPOPROTEIN LIPASE, bound to the capillary endothelium of a number of tissues and activated by a specific apolipoprotein (apolipoprotein C-II), cleaves fatty acid residues from the