Download 13synthesis

* Lipid Biosynthesis
- These are endergonic and reductive reactions, use ATP as source of
energy and reduced electron carrier usually NADPH as reductant.
* Fatty acid synthesis:
- F.A synthesis is not the reversal of the degradative pathway,
different sets of enzymes.
using
1-synthesis in cytosol, degradation in mitochondria (mitochondrial matrix)
2-intermediates of F.A synthesis are covalently linked to -SH group of
ACP
3-the growing F.A is elongated by sequential addition of 2-carbon units.
4-the reductant in fatty acid synthesis is NADPH,
5-elongation of F.A is stopped at C16 and further elongation or insertion of
double bonds are carried by other enzyme systems.
* Large proportion of F.A used in the body is supplied by diet excess CHO and
protein are converted into F.A.
- F.A are synthesized mainly in liver and lacting mammary gland and to lesser
extent in adipose tissue and kidney.
* Formation of Malonyl CoA from acetyl CoA and HCO3*This step is mediated by Acetyl CoA carboxylase that consist of three domains.
Biotin
Prosthetic
group
2nd unit of
Acetyl CoA
carboxylase
3rd domain
first part of
acetyl CoA
carboxylase
NET : Acetyl CoA + ATP + HCO3-
Malonyl CoA + ADP + Pi + H+
Acetyl CoA carboxylase enzyme
•The F.A grows by 2 carbon units donated
by Malonyl CoA
• for palmitate synthesis 16 C
7 steps
Single polypeptide 77 a.a
Linked to Ser & ACP
* intermediates are attached to acyl carrier protein (ACP)
•The F.A synthase complex consist of 7 separated polypeptides.
•These portions act together to catalyze the formation of F.A from
acetyl CoA and Malonyl CoA
Acetyl CoA is attached to KS by -SH group
thioester linkage.
Malonyl CoA is attached to ACP through -SH group forming thioester linkage.
* The -SH of KS is
charged to acetyl CoA by
Acetyl - CoA - ACP
transacetylase (AT)
KS : - ketoacyl-ACP
synthase
* The -SH group of ACP
is bonded to malonyl
CoA by enzyme
Malonyl - CoA - ACP
transferase (MT)
Of phosphopantetheine
group
* CO2 is released is the same
HCO3- that condensed into
acetyl CoA to form malonyl
CoA by acetyl CoA
carboxylase.Why ?
Good nucleophile
•Coupling the
decarboxylation
and condensation
* HCO3- that reacts
with Acetyl CoA to
form malonyl CoA is
released in form CO2.
The cleavage of acetyl
group from F.A is highly
exergonic. & condensation
of acetyl CoA endergonic.
So introduction of CO2
make the condensation more
thermodynamically favored.
exergonic
Is the first
step in F.A
synthesis


- ketoacyl - ACP
synthase
* electron donor
for this
reduction
NADPH
Enzyme :
- ketoacyl ACP reductase
KR
DD isomer
-D-hydroxyacyl
dehydratase
enzyme
NADPH is the
electron donor
Enoyl ACP
reductase enzyme
This process is
catalyzed by acetyl CoA
ACP transacetylase
(AT)
*Then the butyryl group is
transferred from
phosphopantetheine -SH of ACP
to the Cys-SH of - ketoacyl
ACP-synthase (KS)
By
MT
By
KS
* The biosynthesis of F.A requires energy
Needs ATP and reducing power of NADPH
* For synthesis of 16C F.A (palmitic acid)
7 cycles of condensation, 7 reduction, 7 dehydration and 7 reduction.
7 acetyl CoA + 7 CO2 + 7 ATP
7 malonyl CoA + 7 ADP + 7 Pi
Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+
+ 8 CoA + 14 NADP+ + 6 H2O
8 acetyl CoA + 7 ATP + 14 NADPH + 14 H+
H2O + 7 ADP + 7 Pi + 14 NADP+
palmitate + 7 CO2
palmitate + 8 CoA + 6
* Fatty acid synthase of different organisms
F.A synthase from :
- bacteria, plants : 7 activities in 7 separated polypeptides
- Yeast : 7 activities in two separated polypeptides.
- Vertebrates : 7 activities in one large polypeptide
One large
polypeptide with
two domains with 7
activities
Fatty Acid Biosynthesis
Source of Acetyl Co for F.A synthesis
Source of Acetyl Co for F.A synthesis
- Major source of acetyl CoA is from mitochondrial oxidation of pyruvate and
from catabolism of amino acids.
- Acetyl CoA produced from - oxidation of F.A is NOT a significant source of
acetate because synthesis and oxidation of F.A are reciprocally regulated.
- Acetyl CoA is
produced in the
mitochondrial matrix
- synthesis of F.A
occurs in cytosol
- Acetate is shuttled
out of mitochondria as
citrate because
membrane is NOT
permeable for acetate.
Source of Acetyl Co for F.A synthesis
Source of NADPH for F.A synthesis
- Oxidation of malate to pyruvate will produce NADPH
Reducing power of NADH is converted into NADPH
Oxaloacetate  Malate  Pyruvate
- The other NADPH are produced pentose phosphate pathway
Regulation of F.A synthesis and degradation
- The reaction catalyzed by acetyl CoA carboxylase is the rate limiting step.
- Palmitoyl CoA : -ve feed back in the biosynthesis, it inhibits the carboxylase
enzyme.
- When mitochondrial acetyl CoA + ATP increase
increase production of
citrate which transfers out of mitochondria
increase cystolic citrate.
1- citrate is an allosteric activator for this carboxylase.
2- citrate is the precursor of cystolic acetyl CoA
high level of citrate
increase F.A synthesis
- citrate inhibit the PFK1 reducing the flow of carbon through glycolysis.
- insulin stimulate F.A synthesis by activating the carboxylase.
- Glucagon + Epinephrine decrease F.A synthesis by increasing the
phosphorylation
F.A synthesis and degradation are reciprocally regulated (not active
at the same time)
-Malonyl CoA: the 1st
intermediate in the synthesis
of F.A inhibits the carnitine
acyl transferase I, so inhibits
the  - oxidation.
-Regulation of the level of gene
expression?
-Intake of large amount of
CHO for long periods
increase expression of
carboxylase and F.A synthase
enzyme.
Long-chain saturated F.A are
synthesized from palmitate
by Fatty acid elongation system
Found in smooth endoplasmic
reticulum and mitochondria
The donor for two carbon units
= acetyl CoA
Act on the F.A in the
phospholipid,
phosphatidylcholine or linked
glycerol
Desaturation by
fatty acyl CoA
desaturase (liver)
Desaturation of F.A occurs in
the phospholipid,
phosphatidylcholine or linked
glycerol
* Biosynthesis of Triacylglycerols
- Ingested or synthesized F.A have two fates :
1- Incorporation in TG for the storage of metabolic energy.
2- Incorporation in phospholipids components of membranes.
And the activated pathway is dependent on the need of cells.
Rapid growth
for phospholipid
synthesis of membranes
- if the organism has a lot of food and fats
increase demand
storage of F.A in TG
* Few hundreds mg of glycogen stored in the body, but about 15 kg of lipids
(in the normal person) can supply the body by energy for 12 weeks.
* Excess of CHO stored as fats, While fats can’t used for glucose synthesis.
* TG and phospholipids have the same precursors
“Glycerol 3-phosphate”
and
“fatty acyl CoA”
* TG is biosynthesized primarily in liver and also in adipocytes.
Biosynthesis of TG
production of L-glycerol 3-phosphate
adipocyte
liver
In liver and kidney
mainly and adipocytes,
(cytosol)
Just in liver
* phosphatidic
acid is formed in
trace amount in
the body, but it is
the central
intermediate in
the biosynthesis
of phospholipid
and in TG
Synthesis by acyl
CoA synthetase
(diacylglycerol 3-phosphate)
TG is synthesized in the
liver, adipocytes,
mammary glands and
intestinal mucosal cells.
Transesterification
by acyl transferase
Is the parent compound
of TG and phospholipid
Glycerol 3-phosphate is synthesized in the liver and adipocyte from glucose
Glucose entrance
is not insulin
dependant
Glucose entrance
is insulin
dependant
Fates of TG in liver and adipose tissue
- TG are stored in the cytosol of the cells in anhydrous form, and ready for
mobilization
- in liver, TG stored are very little.
- TG with cholesterol and cholesterol esters are packed in lipoprotein form and
exported to other organs via blood.
Regulation of TG Biosynthesis
CHO or protein consumed in excess of energy needs is stored in the form of TG
Biosynthesis and degradation of TG are regulated reciprocally. Depending on
the metabolic recourses and requirements
- Different hormones
affect TG metabolism
Insulin, Glucagon, adrenal
cortical hormones.
Unable to utilize glucose and to
convert it to TG
- oxidation is
continued of F.A (excess acetyl
CoA)
increase keton bodies
loss of weight
* Biosynthesis of membrane phospholipids
- synthesis of membrane lipids requires in general :
1- Synthesis of backbone molecule (glycerol or
sphingosine)
2- Attachment of F.A to the backbone by
ester or amide linkage.
3- Addition of hydrophilic head group through
phosphodiester linkage.
4- Alteration or exchange of head group to
yield final phospholipids.
- synthesis occur in smooth endoplasmic
reticulum then goes to Golgi apparatus.
- The polar head is linked to the glycerol by
phosphodiester linkage.
* All cells except RBC can synthesize
phospholipid, but TG is synthesized in the
liver, adipocyte, mammary glands and
intestinal mucosal cells.
* Two strategies for attaching head groups
In the biosynthesis one of the OH group is activated by attachment of a
nucleotide (CDP)
CDP-diacyl
group
Activation of
diacyl
glycerol
CDP-HEAD
Activation
of head
group
Strategy 1
* Synthesis of cardiolipin
and phosphatidyl inositol (PI)
Strategy 2
Two strategies Biosynthesis of membrane phospholipids
Biosynthesis of Sphingolipids
1st step Palmitoyl CoA +
serine
18 Carbons
The reducing
agent
Ceramide is parent
compound of
Sphingolipids
* Polar lipids are targeted to specific
cell membrane.
After the synthesis in the S.E.R. the
polar lipids (phospho, sphingo, glyco
lipids) they transferred to different
cell membranes by Golgi apparatus in
form of vesicles.
* Degradation of Phospholipids
enzyme responsible for degradation are present in all tissues and pancreatic juice.
- phospholipases hydrolyze the esters of phosphodiester bonds of phosphoglycerides.
* Degradation of Sphingomyelin
- Sphingolipids are degraded by sphingomyelinase
which remove phosphorylcholine leaving Ceramide.
- Ceramide is cleaved by ceramidase into sphingosine and free fatty acid.
* Degradation of phospholipids
The End
Eicosanoids : synthesized from (20 : 5,8,11,14)
* potent biological signaling molecules act as short
range messengers.
* prostaglandins regulate many physiological
processes, platelet aggregation, uterine
contraction, pain, inflammation and secretion
of mucin in GIT
* COX found in smooth endoplasmic
reticulum, act on archidonate and
convert it PGH2.
* COX also called prostaglandin H2
synthase. NSAIDS inhibits COX
* Thromboxane synthase found in
platelet
Thromboxane lead to
contraction of blood vesicles and
platelet aggregation
PGH2
Thromboxane A2
Thromboxanes
other
PGE2, PGF2, ...
By enzyme :
Thromboxane
synthase
* Different leukotrienes :
(different sites of -OOH peroxide)
* leukotrienes found in leukocyte,
heart, brain.
* NSAIDS : Aspirin , Ibuprofen
Release of Arachidonic acid from
membrane lipid (20 : 5,8,11,14)
Steroidal anti
inflammatory
Steroidal SAD
* Synthesis of phosphatadyl ethanolamine (PE) and phosphatadyl
choline (PC)
PC + PE are abundant in eukarytic cells
- choline + ethanolamine obtained from diet or from the turnover
of the body’s phospholipids.
- Also choline can be synthesized from phosphatidyl serine.
Choline
ethanolamine
CDP-choline CDPethanolamine
* The reutilization of choline is important because de novo synthesis of
choline
three methyl groups needed is derived from methionine
which is essential amino acid.
* Role of PC in lung surfactant
Dipalmitoyl phosphatidyl choline :major component in lung
surfactant (extracellular fluid layer lining the alveoli)
(Respiratory distress syndrome hyaline membrane disease)
* PC is synthesized from PS