FA catabolism and anabolism both use this as an intermediate:
Acetyl-CoA [Beta oxidation (FA degradation, losing C2 unit), whereas in FA synthesis, involves addition of C2 unit onto fatty acyl chain]
Malonyl-CoA
A C3 intermediate used to add acetyl-CoA to growing fatty acyl chain; occurs via condensation rxn followed by decarboxylation to remove C1);
What is the overall reaction for FA synthesis
Cn + C3 = Cn+2 + C1
FA synthesis is the reversal of
Beta Oxidation
What is the main difference b/w FA synthesis and Beta Oxidation?
ENZYMATIC MACHINERY: FA synthesis proceeds through a humongous multi-domain, multifunction enzyme complex (a homodimer) that catalyzes up to seven consecutive reactions
B Oxidation occurs in the ____, whereas FA synthesis occurs in the ____
mitochondria, cytosol
Citrate
Intermediate formed so that Acetyl-CoA can be transported from the mitochondrial matrix to the cytosol; Acetyl-CoA in the mitochondrial matrix is condensed with OXALOACETATE to form citrate, catalyzed by CITRATE SYNTHASE
TRICARBOXYLATE TRANSPORTER SYSTEM:
Facilitates directional transport of citrate from the mitochondria to the cytosol.
How is Acetyl-CoA reformed in the cytosol?
Citrate is transported into the cytosolic side where it is cleaved back into acetyl-CoA and oxaloacetate, catalyzed by CITRATE LYASE. ATP mediated step
Citrate Synthase
The enzyme that catalyzes the fomation of citrate within the mitochondrial matrix
ATP-Citrate Lyase
The enzyme that catalyzes the reverse reaction in the cytosol to release acetyl-CoA
What are the starting substrates for FA synthesis
Acetyl-CoA (precursor -C2 atoms) and malonyl-CoA (C2 donor- C3 unit)
ACC (Acetyl-CoA carboxylase):
catalyzes malonyl-CoA synthesis; has a biotin prosthetic group involved in carboxyl group transfer from CO2 to substrates
ACC is also a multi-domain, multi-function enzyme that catalyzes (i) ___ and (ii) ___ reaction
biotin carboxylation (using biotin carboxylase), carboxyl group transfer (using carboxyltransferase)
What are the functional domains of ACC
Biotin Carboxylating (BC) domain; Biotin Binding (BB) Domain; and Carbozyltransferase (CT) domain
The immediate substrates for FA synthesis are attached to ___
ACP [ acetyl-CoA and malonyl-CoA are further converted to acetyl-ACP (starter) and malonyl-ACP (extender) before proceeding into the subsequent condensation]
ACC catalyzed malonyl-CoA synthesis are catalyzed by this one enzyme
FAS (fatty acid synthase) : multi-domain, multi-function, megasythethase
What are the 6 rxns catalyzed by FAS?
(ia) acetyl-ACP formation and (ib) malonyl-ACP formation which are catalyzed using same enzyme, and the other intermediates catalyzed via enzymatic modified FAs: (ii) C-C bond condensation, (iii-v) Beta carbon modification (reduction), and (vi) cleavage o
___ is a homodimer
mammilian FAs (mFAS)
What is ACP?
ACP= Acyl-Carrier Protein; It is the prosthetic group involved in all steps of the enzymatic reactions
formation of acetyl-ACP and maloneyl-ACP is catalyzed by ___
MAT; Malonyl/acetyl-CoA-ACP transferase
acetyl-ACP is anchored onto ___ domain of FAS
KS [loading of acetyl group onto KS and subsequent condensation with malonyl-ACP]
What are the two major components of FAS catalyzed reactions?
(i) C-C bond condensation, and (ii) Beta carbon modification-->Acetyl-ACP releases acetyl grp which binds to Thiol group on KS, this catalyzes the condensation reation with malonyl-ACP and one carbon is released. Then another malonly coA is accepted and t
Beta Carbon Modification is catalyzed by?
KR (ketoacyl-ACP reductase), DH (hydroxyacyl ACP dehydrogenase), and ER (enoyl-ACP reductase).
Condensation reaction is catalyized by ...
MAT (malonyl-/acetyl-CoA-ACP transferase) and KS (ketoacyl synthase)
In the last step of FA Synthesis, ___ cleaves palmitoyl-ACP
TE (thioesterase) ; this is a hydrodrogenation
Why do we care about FAS?
De novo fatty acid synthesis is not active in humans consuming western diet rich in fat. However it has been shown that cancer cells exhibited high FAS activity. Selective inhibition of FAS activity may suppress tumorigenesis and abnormal cell growth.
Regulation of fatty acid synthesis is controlled at the activity of ____
ACC
ACC is activated by
citrate, dephosphorylation, and polymerization
ACC is inactivated by
high levels of (fatty acid) Palmitoyl-CoA, and phosphorylation [high FA concentration suppresses FA synthesis by blocking malonyl-CoA formation]
Malonyl-CoA is an important intermediate b/c
It is an important intermediate for fatty acid synthesis but also exerts a potent inhibitory effect on the activity of CPT1,thus, when the fatty acid synthesis process is active, the Beta oxidation process is suppressed.
How can fatty acid elongation (beyond 16C) be achieved?
You cannot synthesize chain lengths longer than C16, however, mammilian cells have an enzyme to catalyze reactions using palmitic acid as the starting material. ACETYL-CoA is used as an extender. The Elongation rxn occurs in the ER
The most common desaturation reaction in mammalian cells? in humans?
most common conversions in mammalian cells 18:0 (stearic acid) to 18:1 (oleic acid). In humans desaturation reactions do not occur beyond position 9 (the carbon numbers are counted starting from the COOH function group)
The fatty acids with double bonds at position 12 or 15 are commonly termed ___
omega-3 or omega-6 fatty acids (these fatty acids are acquired mainly from diet)
What are essential fatty acids?
omega-3 and omega-6 fatty acids, considered essential because humans are unable to make them
1st comparison of fatty acid beta oxidation and fatty acid synthesis
(1)B-Oxidation generates Energy in the form of FADH2, NADH, and acetyl-CoA, whereas fatty acid synthesis requires energy in the form of NADPH; In terms of energy balance, B- oxidation occurs under conditions where carbohydrate supply is inadequate (e.g. s
2nd comparison of fatty acid beta oxidation and fatty acid synthesis
the two metabolic pathways are compartmentalized; B-oxidation occurs in the mitochondrial matrix and fatty acid synthesis in cytosol.The carnitine (CPT1) and citrate (tricarboxylate) shuttle systems are responsible for the substrate transport. [the two pa
3rd comparison of fatty acid beta oxidation and fatty acid synthesis
C2 units utilized for fatty acid degradation and synthesis are different. In oxidation, C2 unit is consecutively removed from the fatty acyl chain in the form of acetyl-CoA, whereas in fatty acid synthesis the C2 unit is introduced to the elongating fatt
differences between b oxidation and biosynthesis of fatty acids
B-oxidation: location-mitochondrion, acyl acrrier-HSCoA, coenzyme-FAD and NAD+, C2 uint-acetyl-CoA (product)
FA synthesis: location-cytosol, acyl-carrier-ACP, coenzyme-NADPH, C2 unit-malonyl-CoA (donor)
Explain the structure of triacylglycerol (TAG)
Triacylglycerol contains three fatty acyl chains condensed with a glycerol molecule through ester linkages
the key intermediate for TAG synthesis is
diacylglycerol (DAG) [DAG-->TAG (enzyme DGAT)]
Formation of TAG in the cells is catalyzed by
DGAT (DiacylGlycerol AcylTransferase) through condensation of DAG and acyl-CoA.
formation of DAG in the intestine is via
MAG (monoacylglycerol) acylation--> MAG condenses with acyl-CoA forming DAG
formation of diacylglycerol (DAG) in the liver
The enzyme termed PAP (Phosphatidic Acid Phosphohydrolase) catalyzes the de-phosphorylation of PA (phosphatidic acid) to form DAG
PAP1
PAP1 translocates from cytosol to endoplasmic reticulum (ER) membrane and becomes activated (PAP1 is involved in production of DAG that is ultimately utilized for TAG synthesis
); PAP1 is encoded by the lipin1 gene
PAP2
PAP2 is a membrane protein located on the cell surface (i.e. plasma membrane); PAP2 generates DAG for signal transduction (on the plasma membrane)
How many PAPs are there? Where are the PAPs located
2 PAPs (PAP1 and PAP2); located in the Liver; both generat DAG fro PA
Phosphatidic acid (PA) is synthesized from two consecutive acylation reactions catalyzed by
GPAT (glycerol 3-phosphate acyltransferase) and AGPAT (acylglycerol 3-phosphate acyltransferase)
enzymes involved in TAG synthesis are located on the...
ER [For instance Acyl-CoA synthase, GPAT, AGPAT, and DGAT are all integral membrane proteins. Once exception is PAP1 (lipin1), which is a cytosolic protein that becomes activated only after it translocates onto the cytosolic surface of the ER. ]
What are the two fates of TAG
TAG can go to the cytosol for stroage as lipid droplets (not preferable) or it can go into the ER lumen-serves as the precursor for VLDL
lipodystrophy
Lipodystrophy is characterized by abnormal loss or absent of fat tissues in the body
lipodystrophy is linked to these two mutations
AGPAT deficiency - Berardinelli-Seip congenital lipodystrophy in humans
Lipin1 deficiency - fatty liver destrophy in mice