Synthesis of FA Flashcards
How is FA synthesis initiated? how is this done?
transport of acetyl coA out of the mitochondria
1)citrate shuttle system converts Acetly coA + OAA to citrate (using citrate synthase)
2) citrate first crosses inner MM through tricarboxylate acid transporter, then crosses porins across the outer MM into the cytosol
3) citrate is split by citrate lyase into OAA + acetyl coA which USES 1 ATP
where does FA synthesis occur?
LIVER
-adipose, mammarly tissue and intestine
what is the first reaction in the synthesis of FA? what cofactor is involved? what is produced? what is important about this rxn?
acetyl coA carboxylase rxn (ACC)
-ATP and HCO3- use biotin as a co-factor to generate malonyl coA from acetyl coA
-this is an irreversible commited step in FAS (tightly regulated rxn)
explain the structure of the acetyl coA carboxylase. where is this found?
similar to PDC
3 domains:
1) adds CO2 to biotin
2) binds biotin
3) transfers CO2 from biotin to Acetyl coA
-found in the cytoplasm
what is the active form of the ACC? what does phosphorylation do to this?
it is active in the polymerized form
-when phosphorylated it will be inactive (glucagon, will phosphorylate in the fasted state, storage moleucles would not be generated if this were the case)
when ACC dephosphorylated, what state of polymerozation is it in?
it is polymerized when dephosphorylated
-active state to push metabolism to FA synthesis
what is the effect of citosolic citrate on the ACC?
It will activate the ACC
-citrate allows us to produce Acetyl coA in the cytosol which is a substrate for the ACC
what is the effect of LCFA on the ACC?
inhibitors of ACC, FA linked to coA in the cytosol are destined for FA oxidation
what effect does AMP dependent protein kinase have on ACC?
inactivates it
-phosphorylates ACC, depolymerizing it, deactivating it
AMPK turns on pathways that make ATP and turns off pathways that use ATP
- this is a more important regulator than glucagon
what is ACP? what does it do? where is it located? what part of the structure is essential for the attachment of FA?
Acyl carrier protien
-anchors the growing FA chain to the FA synthase complex
-located inside the fatty acid synthase complex
-SH group
what reactions doe the fatty acid synthase complex allow us to carry out? what do these reactions have in common with beta oxidation?
-allows us to carry out reduction, condensation, and dehydration reactions
-opposite reactions of beta oxidation
what is KS in the FAS complex? what does it contain in its structure that is important?
keto synthase
-allows us to join incoming FA units
-contains SH group
how many acetyl coA does the FAS complex recieve? what happens to this acetyl coA?What enzyme catalyzes this?
it only receives ONE acetyl coA, the rest come in as malonyl coA
-this Acetyl coA has its acetyl group transferred to ACP and coA is released
-Malonyl/acetylcoA-ACP transacetylase catalyzes this (MAT)
how many C is malonyl coA? what happens to malonyl coA coming into the ACC? What enzyme catalyzes this?
3C structure
-malonyl group transferred to ACP, coA released
-Malonyl/acetylcoA-ACP transacetylase catalyzes this (MAT)
what is the relation of the reactions in beta oxidation vs FA synthesis? where do each of these pathways occur?
FA synthesis is the opposite reactions in the reverse order
condensation, reduction,dehydration,reduction (CRDR)
FA oxidation: mitochondria
FA synthesis: cytosol
explain the condensation reaction in FA synthesis. what is coming in? what enzymes are involed? what is produced?
1) One acetyl coA comes in and becomes the last C on the FA chain
2) MAT transfers acetyl group from Acetyl coA to ACP, produing Acetyl-ACP (releasing coA)
3) KS transfers acetyl from Acetyl-ACP to SH group on KS to free up the ACP
1) malonyl coA comes in and malonyl is transferred to ACP (releasing coA) using MAT
2) removal of CO2 from malonyl coA allows the malonyl-ACP group to join with the KS-Acetyl group, using a condensing enzyme
-generates Acetoacetyl-ACP (C4 molecule)
how does the structure of malonyl coA allow it to join carbons from acetate to growing FA chain?
malonyl coA has 3 C, the extra CO2 provides energy to join C from acetate to growing FA chain
explain how the carboxylation of acetyl coA to malonyl coA is an indirect transfer of energy?
ATP is used to carboxylate Acetyl coA to malonyl coA, and that energy is used from the removal of CO2 to join malonyl-ACP and ketoacyl-KS
explain the reduction reaction in FA synthesis. what is coming in? what type of enzyme is involed? what is produced? what is needed as a cofactor?
acetoacetyl-ACP is converted into beta-hydroxybutyryl-ACP by a ketoreductase
-reduces ketone to hydroxyl
-uses NADPH
explain the dehydration reaction in FA synthesis. what is coming in? what type of enzyme is involed? what is produced?
hydroxyl structure has H2O removed to produce an enoyl-type structure
-uses a dehydrase
for each reaction in FA synthesis, how many C does each structure increase by?
each reaction will increase by 2 C after each round
explain the last reduction reaction in FA synthesis. what is coming in? what type of enzyme is involed? what is produced? what is needed as a cofactor?
an enoyl structure is reduced to an acyl-ACP (FA linked to ACP) using an enoyl reductase
-NADPH is used
what happens to the acyl-ACP group after each round of FA synthesis? why is this important?
the acyl-ACP is transferred from ACP to the cys-SH of KS
-this frees ACP to be used for the next malonyl coA to join to ACP
why does it make metabolic sense that only 1 Acetyl coA is used in FA synthesis?
because the energy from the malonyl coA release of CO2 is needed for every round fo condensation
after the first round of FA synthesis, how many C is the FA? when does the FAS complex stop working? what happens then?
4C
-when FA get to 16C (after 7 rxn cycles)
-The 16C FA is then released using H2O (thioesterase), freeing ACP to allow the FAS to start again
how many Acetyl coA, malonyl coA and ATP are used to make a 16C FA?
7 malonyl coA and 7ATP, 8 Acetyl coA
what is the shuttle system to transfer acetyl groups across the mitochondrial membrane?
1) Acetyl coA converted to citrate by citrate synthase in mitochondria
2) citrate crosses tricarboxylate transporter, then porins to reach cytosol
3) in the cytosol Citrate lyase uses ATP to split citrate into Acetyl coA and OAA (Acetyl coA can then be used in FAS)
4) OAA gets converted into malate by cytosolic malate dehydrogenase (using NADH)
5) malate gets converted into pyruvate by malic enzyme (generates NADPH and CO2)
what are the 2 ways OAA can be transported across the IMM? When would you expect to see each of these transport pathways?
1) OAA -> citrate -> crosses IMM through citrate transporter and OMM through porins -> citrate converted to OAA in cytosol -> malate
->pyruvate
-this allows us to GENERATE NADPH in the cytosol for anabolic metabolism
2) OAA -> citrate -> crosses IMM through citrate transporter and OMM through porins -> citrate converted to OAA in cytosol -> malate ->crosses OMM through porins and IMM through malate alpha-KG transporter -> OAA ->
enters the CAC to GENERATE ENERGY
- taken in directly in catabolic conditions
what is needed to convert citrate to acetyl coA? what else is produced?
ATP and coA-SH
-OAA is also produced
what does the malic enzyme do? what does this produce?
converts malate to pyruvate
-produces NADPH and CO2
how does beta oxidation and FAS differ? where do they occur? what rxns happen?
beta oxidation:
-occurs in mitochondria
-NAD+ is electron group acceptor
-generates NADH,FADH2, Acetyl coA and Fatty acyl coA 2C shorter each time
FA synthesis:
-occurs in cytoplasm
-NADPH is electron donor
-generates CO2, H2O, NADP+, and a fatty acyl 2C longer each time until 16C
how many ATP does bOX vs FAS use? why?
bOX: 2ATP used
-1ATP to activate FA (Fatty acyl coA synthetase), 1ATP in ADK rxn
FAS: 2 ATP used
-1 ATP in ACC, 1 ATP used in citrate lyase
explain the regulation of ACC? what is the most important regulator?
ACC is regulated by cellular needs and [blood glucose]
-inhibited by AMP and cAMP-dependent phosphorylation
-activated by insulin-dependent dephosphorylation
energy levels in the liver is a more important regulator (AMP)
what 2 big switches turn us from synthesizing FA to oxidizing them
low insulin and high AMP (low energy)
how does xylulose 5-P impact fat metabolism in the liver?
it activates enzymes that dephosphorylate PFK-2
-activates kinase domain, increasing F,2-6BP, which activates glycolysis
-this increases Acetyl coA levels for FA synthesis
what type of enzyme are desaturases? how do they function? where can we not add double bonds in mammalian cells?
mixed-function oxidases
-they oxidize FA and NADH and the electrons are transferred to O2 to generate H2O (4e- generate 1 H2O)
-cannot add db after C9
what 2 ways is glycerol-3-phosphate used in TAG and glycerophospholipid synthesis generated? where do you not expect both of these to occur?
1) through glycolysis
2) glycerol kinase converts glycerol to glycerol-3-phosphate using ATP
-adipocytes lack this enzyme so 100% of glycerol is released to other tissues
how does the glcyerol phosphate shuttle system differ from the glycerol kinase reaction?
the glycerol phosphate shuttle occurs in muscle / brain
-it does the opposing reaction in the IMM, this shuttle system is important there because those tissues do not make TAGs
what must first happen to FA being linked to form TAGs? where does this occur? how many ATP are needed?
they are activated
-occurs in the ER
-requires 2 ATP (1 to link FA to ATP and 1 to regenerate AMP to ATP)
what is phosphatidate? how are phospatidates formed? what is the structure of the intermediate formed? what is the structure of a phosphatidate?
Phosphatidate is an intermediate in the synthesis of TAGs and glycerophospholipids
1) glycerol-3-phosphate is converted into lysophoshatidate by an acyl transferase
2) lysophosphatidate is converted into a phoshatidate by another acyl transferase
-lysophosphatidate has a glycerol backbone with a FA at C1, nothing at C2, and a phosphate at C3
-phosphatidate has 2 FA, at C1/C2, and a phosphate at C3
what must occur to phosphatidate to generate a TAG?
1) phosphatate removed using water and a phosphatase to generate a DAG
2) TAG formed from an acyl transferase
how is TAG biosynthesis indirectly regulated by hormones? how does this happen?
insulin stimulates FAS -> more FAS -> more TAG synthesis
Insulin:
-stimulates glycolysis (increases PFK-2-> increase F-2,6BP)
-activates PDC (acivated pyruvate DH phosphatase -> depohsphorylates it-> activates it -> increases acetyl coA)
-activates ACC by dephosphorylating ACC, polymerizing it
-inhibits HSL which inhibits TAG breakdown
how do adipocytes generate glycerol-3-phosphate?
1) glycolysis
2) glyceroneogenesis (synthesis of glycerol-3-phosphate without using glycerol or glucose)
-pyruvate coming from lactate /AA is converted into OAA -> PEP -> DHAP -> glycerol 3-phosphate dehydrogenase -> glycerol 3-phospahte (goes up similar pathway of gluconeogenesis)
explain the TAG cycle:
occurs within a tissue or between tissues
1) TAGs broken down in adipocytes releasing glycerol (100% shipped out) and FFA
2) FFA released into blood and taken to tissues for energy
3) FFA not used for energy are taken to adipocytes and converted back into TAGs
how can glycerol-3-phosphate be made in the liver? how does this differ from adipocytes?
1) glycolysis
2) glyceroneogenesis
3) glyerol kinase
adipocytes lack glycerol kinase and will only do the first 2
what percent of FFA are directly converted back into TAGs in adipocytes? how does this ratio differ during periods of fasting? why?
40% converted back into TAGs
-during starvation the amount of FFA converted back to TAGs increases to 60%
-when in starving conditions gluconeogenesis will increas the number of FFA in the blood so that they can be taken to tissues for energy, therefore, the ratio of FFA is decreased to not overwhelm the body with the larger flux of FFA
-high levels of FA in the blood stream harm pancreas and decrease insulin sensitivity
How is the flux through the TAG cycle controlled by PEP carboxlylase? how does this differ in adipose tissue vs liver?
glucocortocoids: released during stress/anxiety
IN ADIPOSE:
-decrease PEPCK -> decreases glyceroneogenesis -> decreases glycerol 3-phosphate levels (used as backbone for TAG synthesis) -> increasing FFA release for energy
IN LIVER:
-increase PEPCK -> increases glyceroneogenesis and gluconeogenesis -> increases glycerol 3-phosphate levels (used as backbone for TAG synthesis) -> reducing FFA release and increased TAG formation for export to other tissues
glucagon (same affect in both tissues): increases PEPCK expression, stimulating gluconeogenesis and glyceroneogenesis in the liver / adipose
-stimulate HSL which breaks down TAGs during starvation
insulin (same affect in both tissues): Suppresses PEPCK expression, decreasing glyceroneogenesis -> decreases glycerol 3-phosphate levels (used as backbone for TAG synthesis) -> favoring FFA release
-PEPCK activity determines the balance between lipolysis (TAG breakdown) and re-esterification (TAG synthesis) in adipose tissue
