Download Lipid Metabolism

BIOCHEMISTRY
SFA 2073
Lipid Metabolism
NIK NORMA NIK MAHMOOD-Ph.D
FACULTY OF SCIENCE AND
TECHNOLOGY
ISLAMIC SCIENCE UNIVERSITY OF
MALAYSIA
Digestion & Absorption
• Lipids that gets into the digestive system are
dietary lipids, normally free fatty acids,
cholesterols and triglycerides (TAG) and many
minor components.
• Digestion starts in the duodenum portion of small
intestine:
- is mix with bile that contains HCO3ˉ ions and
bile salts to solubilize fat. This process is called
emulsification. The big lipids droplets are broken
down into smaller droplets.
(bile is made in liver and stored in gall bladder
between meals. When there is food, bile is
delivered to the intestine from gall bladder via
bile duct)
- TAG is acted by lipase secreted by pancreas
liberating monoglyceride and two fatty acids.
Monoglyceride, cholesterol and f.fas and bile salts
form amphipathic micelles. These micelles keep the
insoluble lipid components in soluble aggregates
from which small amounts are released and
absorbed by epithelial cells via diffusion.
- Free fatty acids and monoglycerides then
recombine into triacylglycerols at the smooth ER
and together with cholesterols moves on to Golgi to
be converted to chylymicrons. It enters interstitial
fluid, then taken up by the lacteals in the intestinal
wall and delivered to liver via hepatic portal vein
for processing.
• exposure to a large
aggregate of
triglyceride, the
hydrophobic portions of
bile acids intercalate
into the lipid, with the
hydrophilic domains
remaining at the surface.
Such coating with bile
acids aids in breakdown
of large aggregates or
droplets into smaller
and smaller droplets.
• Pancreatic lipases hydrolyse triglyceride
into monoglyceride and free fatty acids.
The activity of this enzyme is clipping the
fatty acids at positions 1 and 3 of the
triglyceride, leaving two free fatty acids
and a 2-monoglyceride.
**Lipase is a water-soluble enzyme
• Lipids, and products of their digestion are
transported through aqueous compartments
within the cell as well as in the blood and tissue
spaces in the forms LIPOPROTEINS
Why in the form of LIPOPROTEINS?
• Large portion of the lipids’ structures comprise of
C-C &C-H rendering lipids hydrophobic in nature
i.e lipids are insoluble in aqueous environment
thus creates problem to its transport within bodymedium which is aqueous in nature.
Dietary triacylglycerols (Tag) & cholesterol and
in-vivo Tag and cholesterol (synthesized in liver),
must be converted to the soluble form to
overcome the problem. This is achieved by
forming LIPOPROTEINS
Lipolysis
• Is the breakdown of fat (Tag) stored in fat cells
into free fatty acids + glycerol + mono &
diglycerides which is catalysed by enzyme lipase
• Induced by hormone epinephrine ,
norepinephrine, glucagon and adreno
corticotropic hormone
• the lipolytic products are then released into the
blood
• The free fatty acids bind to serum albumin and
transport to tissues that require energy. The
energy is generated by catabolic β-oxidation
pathway (a 4 steps pathway/cycle)
• How the hormones induce lipolysis ?
The
hormones trigger 7TM receptors, which activate
adenylate cyclase. This results in increased
production of cAMP, which activates protein
kinase A, which subsequently activate lipases
found in adipose tissue.
• β–oxidation of free fatty acid
Fatty acid degradation and synthesis are
relatively simple processes and essentially the
reverse of each other.
• f.f.a first are activated to acyl-CoA
catalyse by Acyl-CoA synthase
(ACosyn)prior to transport into
mitochondria.
• acylCoA not permeable to inner
mitochondria membrane, hence
carried across by carnitine carrier
system into mitochondrion matrix. It
is by conjugation to carnitine.
AcylCoA + Carnitine → Acyl-carnitine + Co-ASH
• carnitine carrier system consisted of
2 enzyme-units CAT I & CAT II
Carnitine Acyl
Transferase
Carnitine (a quaternary ammonium
compound) is hydrophilic amino acid
derivative, produced endogenously in
the kidneys and liver from lysine and
methionine of diet’s meat and dairy
products. Carnitine binds acyl
residues conjugated with coenzyme
A.
Carnitine + Acyl-CoA
Inner membrane
+ CoASH
matrix
CAT II
Acyl-carnitine
Acyl-CoA
Intermembrane space
ACosyn
cytosol
CAT I
+ CoASH
Outer membrane
Fatty acid
CAT: carnitine Acyl
transferase
Simplified mitochondrial layout
ACosyn: acyl-CoA
synthetase
• Followed by (4 steps )
• - oxidation by FAD
• - hydration
• - oxidation by NAD+
• - thiolysis
• The cycle then repeats on the larger
fragment while acetyl-CoA fragment
channeled to Krebs Cycle
Step 1
• oxidation by FAD/Acyl-CoA DH :
The activated fatty acid is
oxidized to introduce a double
bond.
Step 2
• Hydration/Enoyl-CoA Hydratase:
to introduce an oxygen via
formation of alcohol
Step 3
• oxidation by NAD+/Hydroxy-CoADH: the alcohol is oxidized to a
ketone.
Step 4
• Thiolysis- Thiolase/CoA-SH : cleaving
of the acylCoA into two fragments,
acetyl CoA and an acylCoA of fatty
acid chain two carbons shorter.
Steps: 1 and 2
3 and 4
• β-oxidation of unsaturated fatty
acids poses a problem.
Unsaturated f.a are the cis type.
This prevents the formation of
the required bond orientation,
trans-δ2 bond, in the enoyl
intermediate. These situations
are handled by an additional of
two enzymes.
Anabolism Of Fatty Acid
in Human
• Process occurs in cytoplasm of liver (major)
and adipose tissue cells.
• Fatty acids are formed by the following 3
rxn-stages:
i- acetyl-CoA Carboxylase rxn
ii- fatty acid synthase rxn
iii- desaturase rxn
• This process is the de novo synthesis of F.A
• The initiator substrate acetyl-CoA is the
product of β-oxidation catabolic pathway
Cytoplasm
Mitochondria
Acetyl CoA
Acetyl CoA
synthesis
β-oxidation
citrate
citrate
Fatty Acid
oxaloacetate
Fatty Acid
NADH
oxaloacetate
Citrate
synthase
NADH
malate
malate
Malate DH
ATP-citrate
lyase
NAD+
NADP+
NAD+
Malic enzyme
NADP+
NADPH
pyruvate
NADPH
pyruvate
transporter
Malate-oxaloacetate shuttle: Transfer OF Acetyl CoA from Mitochondria to
cytoplasm
Glucose
Oxidation
Pyruvate
Fatty Acid
&
Cholesterol
Fatty Acids
ACETYL-CoA
Steroid hormones
Ketogenic
Ketone bodies
Amino Acids
SOURCES AND UTILIZATION OF ACETYL-CoA
Acetyl-CoA Carboxylase rxn
• Initiator to fatty acid synthesis is acetyl-CoA
• Acetyl-CoA carboxylase catalyses carboxylation of acetyl-CoA to
malonyl-CoA via 2-steps reaction.
• The enzyme is biotin bound. In mammals acetyl-CoA carboxylase is
a large enzyme controlled by conversion inactive ══> active
(inactive: protomers (4 subunits; one biotin); active: 1 unit)
conversion promoted by citrate, but inhibited by fatty acyl CoA. Also
by hormonal controlled: in liver by glucagon – PO4rylation to inactive
form; in adipose tissue by adrenalin (epinephrin) – PO4rylation to
inactive form
Additional note
• Acetyl-CoA originated from pyruvate in mitochondria and
transported to cytosol as citrate by condensing with oxaloacetate
• In cytosol citrate is broken down to yield acetyl-CoA and
oxaloacetate by ATP-citrate lyase.
• Acetyl-CoA undergoes carboxylation by Acetyl-CoA carboxylase to
malonyl-CoA
Figure at right is expansion of reaction in figure on left
reaction at site 2
• ATP-dependent carboxylation of the biotin,
carried out at one active site (1)
• transfer of the carboxyl group to acetyl-CoA at a
second active site (2).
• Reaction is spontaneous,
HCO3- + ATP + acetyl-CoA → ADP + Pi
+ malonyl-CoA
Fatty acid synthase rxn
• The reaction is a multi-steps .
• The enzyme(in mammal) is a very large
polypeptide of many domains that includes an
acyl carrier protein domain.
• Has a number of prosthetic grps.
• Individual domain catalyses a single step .
• the precursor of fatty acid synthesis is malonylCoA
• the initial action is binding of acetyl-CoA (2C) and
malonyl-CoA (3C) to specific domain of FAS
leading to formation of acyl-ACP intermediates 5C (steps 1&2)
• Followed by formation of βketoacyl-ACP (4C) with
evolution of CO2 (step 3)
• β-ketoacyl is reduced to an
alcohol, by electron transfer
from NADPH (step 4).
• Dehydration yields a trans
double bond (step 5).
• Reduction at the double bond
by NADPH yields a saturated
Acyl-ACP chain- 4C (step 6).
This is 1 cycle.
• Acyl-ACP and malonyl ACP then
repeat step 3 and reaction
proceeds to step 7.
• Acyl chain lengthens by 2C /
cycle.
• Elongation process stops when
acyl 16C is formed. Hydrolysis
of the ester bond takes place
with liberation of palmitate.
FAS
**The active enzyme is a dimer of identical subunits.
All of the reactions of fatty acid synthesis are carried out by the
multiple enzymatic activities of fatty acid synthase FAS
REGULATION of f.f a synthesis
• The major site of fatty acid synthesis
regulation is at reaction catalysed by
acetyl-CoA carboxylase (ACC). ACC
requires a biotin co-factor
Activity of ACC is associated
with conformational change
of the enzyme, and conc. of
citrate and palmitoyl-CoA.
When [citrate] is high,
monomeric form associates to
the multimeric form. Active
conformation is the
multimeric form. When
[palmitoyl] is high,
multimeric form dissociates
into monomeric,it becomes
inactive.
citrate
n monomeric –PO4
inactive
(multimeric)n + Pi
active
palmitoyl
Catabolism Of
Triglycerides (TG)
• Initial rxn is in small intestine
where TG is mixed with bile salt.
• Bile salt are steroids with
detergent properties. 2 most
abundant componants are
cholate and deoxycholate, and
they are normally conjugated
with either glycine or taurine
Glycocholic acid (Cholic
acid+ glycin). It occurs as
a sodium salt in the bile
of mammals
Taurocholic acid. In mammal it
exists as Na+ salt. In medical
use, it is administered as
cholagogue and choleretic.
• Starts by break-up of the
glyceride (TG) into fatty acids
and monoacylglycerol by
pancreatic lipases. This step
takes place because TG cannot
be transported across the plasma
membrane of the intestinal wall
cells (enterocytes) due to its
size.
• The 2 products are transported
into the cell. Once in the cell,
recombination occurs and
triacylglyerols TG1 are reformed.
• TG1 is combined with dietary
cholesterol, newly synthesized
phospholipids and protein into
compound chylomicrons (a large,
low-density lipoproteins).
• Lipoprotein lipase (synthesize by
a number of sources) acts on
TG1 portion in the chylomicron
liberating F.F.A and glycerol.
F.F.A is metabolised by either:
- converted to new TG
- catabolic pathway(β-oxidation)
- used in membrane synthesis
• The glycerol is transported to and
absorbed by the liver or kidney
where it is converted to glycerol-3phosphate by the enzyme glycerol
kinase,GK. Glycerol 3-phosphate
(especially from hepatic) converted
into dihydroxyacetonephosphate
(DHAP) then glyceraldehyde-3phosephate(G3P) to join glycolysis
and gluconeogenesis pathway.
IN INTESTINE
TG1 in chylomicron
Lipoprotein lipase
F.F.A
Glycerol
Glycerol kinase
TG
Membrane
synthesis
Catabolism
β-oxidation
Glycerol-3-PO4
TG
phospholipids
Glucose
Anabolism Of Triglyceride
• Precursor is L-glycero-3phosphate
• Proceed by condensation with
acyl-CoA to form lysophosphatidic acid (l.p.a), catalyse by
enzyme E1
PEP
1
L-glycerol-3-PO4
2
Glycerol
*1 is glycerol-3-PO4 DH ; 2 is glycerol kinase
*E1 is glycerol-3-PO4 acyltranferase
Further reactions on l.p.a till formation of TG
Cholesterols
CHOLESTEROL
• Is a soft, fat-like, waxy substance
found in the bloodstream and
membrane of cells (especially of the
liver, spinal cord), and myelin
sheaths and some hormones.
• require by cells as a precursor to bile
acids.
• it is transported in the circulatory system
within lipoproteins.
• The most abundant of the steroids
**Steroids are complex derivatives of
triterpenes They are characterized by
a carbon skeleton consisting of four
fused rings.
• normal adult utilized ~1 gram of cholesterol
daily. Approximately 70% of the amount
produces by the liver. The other 30% comes
from dietary intake
• Cholesterol is the precursor for all steroids. It
is a common component of animal cell
membranes and functions to help stabilize
the membrane. Thus it is a crucial molecule
*high levels of it in the blood may contribute to
atherosclerosis.
Catabolism Of Cholestrols
• Is not the usual mode i.e brokendown to smaller molecules
• Instead it is converted to the
more soluble derivatives to
facilitate its degradation and
excretion.
• Most important mechanism is the
formation of bile acids, in liver.
• Bile Acids (BA) are mixtures of
compounds and possess
digestive function as agent for
emulsification and absorption of
dietary fats. BA are important
component of bile.
Cholic acid and deoxycholate are
2 of the components of BA.
Metabolism of Cholesterol
cholesterol
7-α-hydroxycholestrl
Many2 steps
glycocholate
Cholic acid
(-ve charge)
glycine
(-ve charge)
Oxidation
Hydroxylation
Cholic acid
Hydrogenation
Usually the BA are converted to a
more soluble form by conjugation
with glycine or taurine
glycine /NH2CH2CO2H
taurine (an a.a)/NH2CH2CH2SO3H
e.g Conjugation:
cholic acid + Glycine → glycocholate
cholic acid + taurine
→ taurocholic acid
Glycocholic acid. It occurs
as a sodium salt in the
bile of mammals
Taurocholic acid. In mammal it
exists as Na+ salt. In medical
use, it is administered as
cholagogue and choleretic.
• Regulation of cholesterols level in blood
-Absorbed dietary cholesterol increased
linearly with the increase of dietary
cholesterol intake.
- The higher the fractional and absolute
absorption of dietary cholesterol the lower
the rates of biliary secretion, fecal
elimination, and cholesterol synthesis
(regulate cholesterol elimination and
synthesis).
- high serum levels of total, LDL, and HDL
cholesterol were associated with high
cholesterol absorption
Anabolism Of Cholesterol
• Condensation of precursors, acetylCoA and acetoacetyl-CoA catalyse by
Hydroxymethyl glutaryl CoA
synthase (HMG-CoA synthase).
• Reactions proceed to formation of
Mevalonate, catalysed by HMG-CoA
Reductase . This rxn is rate-limiting
Reactions occur in cytosol
• To this structure other rings are
added to form the final product
• Enzyme HMG-CoA Reductase is
highly regulated and the target of
pharmaceutical intervention.
Regulation of Cholesterol
Synthesis:
• is not direct on cholesterol but through B.A
• B.A is removed from pool by dietary fibers
• Depletion of B.A induces synthesis of
cholesterol: activation of HMG-CoA synthase
(Hydroxymethyl glutaryl CoA formation step)
and HMG-CoA Reductase (mevalonate
formation step)
Catabolism of Phospholipids
The products of these phospholipases are called
lysophospholipids and can be substrates for acyl transferases
utilizing different acyl-CoA groups. Lysophospholipids can also
accept acyl groups from other phospholipids in an exchange
reaction catalyzed by lysolecithin:lecithin acyltransferase
(LLAT).
• phospholipase A2, lysophospholipase, and
other enzymes are involved in
phospholipid metabolism,
• Phospholipase A2 is an important enzyme,
its activity is responsible for the release of
arachidonic acid from the C-2 position of
membrane phospholipids. The released
arachidonate is then a substrate for the
synthesis of the prostaglandins and
leukotrienes.
• glycerophosphocholine (GPC) and
glycerophosphoethanolamine (GPE) are
competitive inhibitors of lysophospholipase
activity, inhibits lysophospholipid
hydrolysis
Anabolism of Phospholipids
Choline
Acetylcholine
CDP-Choline
PE
- CO2
PS
PC
+ CO2
**Phosphatidylserine (PS)
Phosphatidylcholine (PC)
CDP: cytidine-5’-diphospho
1,2-diglyceride
** CDP: cytosinediphosphate
• DPGs:diphospha
tidylglycerols.
Also known as
cardiolipins are
synthesized by
the
condensation of
CDPdiacylglycerol
with
phosphatidylglyc
erols (PG).
Clinical Effect
• has not yet been fully evaluated, but
scientists have studied the role of choline
and phospholipids in age related cognitive
decline (ARCD), Alzheimer’s disease, and
Parkinson’s disease
• good dietary intake of phospholipids,
cholin lead to an improvement in learning
and memory
• The fatty acid composition of
phospholipids can deteriorate with aging
and disease.
Regulation of synthesis
• The fatty acid distribution at the C-1 and C-2 positions of
glycerol within phospholipids is continually in flux, owing
to phospholipid degradation and the continuous
phospholipid remodeling that occurs while these
molecules are in membranes.
• Phospholipid degradation results from the action of
phospholipases. There are various phospholipases that
exhibit substrate specificities for different positions in
phospholipids.
In many cases the acyl group which was initially
transferred to glycerol, by the action of the acyl
transferases, is not the same acyl group present in the
phospholipid when it resides within a membrane. The
remodeling of acyl groups in phospholipids is the result
of the action of phospholipase A1 and phospholipase
A2
LIPID PROFILE
• Is a presentation of concentration of different
lipid components in blood.
• Normally it involves determination of capillary
blood cholesterol and triglyceride of fasting and
non-fasting subject.
• Concentration of the lipid component is
determined using a specific test strips and GCT
meter
• Low and high readings are indicative to some
form of health state. ** refer manual for details
Lipid Metabolic Disorder
• Abnormalities in the enzymes in lipid
metabolism result in 2 types of disorder.
1. Lipidosis : case when there is
accumulation of specific fatty substances
due to abnormalities in the enzymes that
are involved in assimilation of the specific
fatty substances eg. Gaucher's Disease,
Tay-Sachs Disease, Niemann-Pick
Disease Fabry’s Disease; rare case:
Wolman's disease, sitosterolemia,
Refsum's disease
2. Fatty acid oxidation disorder : When body
is unable to properly convert fats into
energy due to abnormalities of enzymes in
the fatty acid oxidation pathway. Eg
medium chain acyl-CoA dehydrogenase
(MCAD) deficiency
• Gaucher's Disease, most common.
- accumulation of glucocerebrosides in liver and
spleen, most common in Ashkenazi (Eastern
European) Jews leads to enlargment of the
organs and brownish pigmentation of skin.
- Accumulations of glucocerebrosides in the eyes
cause yellow spots called pingueculae to appear
in the eye
- Accumulations in the bone marrow can cause
pain and destroy bone.
- 3 types :
i) Type 1, the chronic form, with symptom of
enlarged liver and spleen and bone abnormalities.
More common among adults
ii) Type 2, develops in infancy; infants with the
disease have enlarged spleen and severe nervous
system abnormalities and usually die within a
year.
iii) Type 3, the juvenile form, can begin at any
time during childhood. Children with the disease
have an enlarged liver and spleen, bone
abnormalities, and slowly develop progressive
nervous system abnormalities. Children who
survive to adolescence may live for many years.
Gaucher's disease can be treated with enzyme
replacement therapy
• Tay-Sachs disease : accumulate gangliosides in
tissues, most common in families of Eastern
European Jewish origin.
At early age, children with this disease become
progressively retarded and appear to have floppy
muscle tone. Spasticity develops and is followed
by paralysis, dementia, and blindness.
Patient usually die by age 3 or 4. Tay-Sachs
disease can be identified in the fetus by chorionic
villus sampling or amniocentesis. The disease
cannot be treated or cured.
• Niemann-Pick disease: accumulation of
sphingomyelin or cholesterol; has several forms,
depending on the severity of the enzyme
deficiency and thus accumulation of
sphingomyelin or cholesterol. The most severe
forms tend to occur in Jewish people. The milder
forms occur in all ethnic groups.
The most severe form (type A), children fail to
grow properly and have multiple neurologic
problems. These children usually die by age 3
Type B, disease develops fatty growths in the skin,
areas of dark pigmentation, and an enlarged
liver, spleen, and lymph nodes; may be mentally
retarded.
Type C, disease develops symptoms in childhood,
with seizures and neurologic deterioration.
Some forms of the disease can be diagnosed in the
fetus by chorionic villus sampling or
amniocentesis. After birth, the diagnosis can be
made by a liver biopsy None of the types can be
cured.