Fatty acid catabolism Flashcards
What is the end product of beta oxidation?
Acetyl-CoA
What are lipids stored as?
As triglycerides
How do lipids yield energy?
Through beta oxidation of fatty acids
Liver and heart derive __% energy from FA oxidation!
Liver and heart derive 80% energy from FA oxidation!
β-Oxidation: a __-step enzyme catalyzed process of oxidative removal of __ units from FA to form __
β-Oxidation: a four-step enzyme catalyzed process of oxidative removal of 2-carbon units from FA to form acetyl-CoA
Why are triglycerides the best storage fuels?
v FA chains are highly reduced compounds
v Yield >2-fold energy than CHO and protein
v Insoluble in water – do not increase osmolarity
v Relatively inert – no risk of undesirable reactions
Bile salts are made in __, stored in __ and emptied into __
What is their role?
Bile salts are made in liver, stored in gallbladder and emptied into duodenum
They emulsify fat- increase SA for enzymes to attack
What are the degradation stages of lipids?
TG-> Diacylglycerol-> Monoacylglycerol-> Free FA-> Glycerol
Which form of FA is absorbed after digestion? Where? What happens next?
FA and glycerol are absorbed in mucosal cells where they are then reassembled into triglycerides
They are then packaged into chylomicrons
Surface of chylomicron has __
Surface of chylomicron has cholesterol
After absorption TG are incorporated, with __ and __ into __
After absorption TG are incorporated, with cholesterol and apolipoproteins into chylomicrons
How can lipids travel around the body?
While they travel through blood stream they can go as free FA or chylomicrons
Lymphatic or circulatory system,
How and in which forms can lipids be absorbed in circulatory
When they go as free FA they can be absorbed
When they travel as chylomicrons they can be absorbed only by as specific mechanism - efficient in liver, but not everywhere else
What are lipid droplets?
TG are stored in lipid droplets in adipocytes - similar to chylomicrons but no lipoproteins
Spherical in structure
What do lipid droplets have on their surface?
Have surface proteins- CGI (comparative gene identification) protein and perilipins
Describe lipid mobilization from the storage
- When low levels of glucose in the blood trigger the release of glucagon, the hormone binds its receptor in the adipocyte membrane
- Adenylyl cyclase is stimulated via a G protein, to produce cAMP. This activates PKA
- PKA phosphorylates the hormone-sensitive lipase enzyme (HSL) and perilipin molecules on the surface of the lipid droplet.
- Phosphorylation of perilipin causes dissociation of the protein CGI from perilipin. CGI then associates with the enzyme adipose triacylglycerol lipase (ATGL), activating it.
- Active ATGL converts triacylglycerols to diacylglycerols.
- The phosphorylated perilipin associates with phosphorylated HSL, allowing it access to the surface of the lipid droplet, where it converts diacylglycerols to monoacylglycerols.
- A third lipase, monoacylglycerol lipase (MGL) hydrolyzes monoacylglycerols -> 3 fatty acids + glycerol
- Fatty acids leave the adipocyte, bind serum albumin in the blood, and are carried in the blood
Are there free FA in the circulation?
No, they are always bound to albumin
Albumin can bind up to _ FA
Albumin can bind up to 10 FA
What are NEFA?
non-esterified fatty acids (non-esterified to glycerol)
FA bound to albumin
How do FA in circulation enter the cells that need them?
they enter the cell through fatty acid transporters
What happens to glycerol released after TF are converted to free fatty acids? What is the end product used for?
The glycerol released by lipase action is phosphorylated by glycerol kinase and the resulting glycerol 3-phosphate is oxidized to dihydroxyacetone
phosphate. NADH is produced
The glycolytic enzyme triose phosphate isomerase converts this compound to glyceraldehyde 3-phosphate, which is oxidized via glycolysis- can yield energy
Glycerol contributes only _% of the energy from TG
Glycerol contributes only 5% of the energy from TG
What need to occur before GA can be transported to mitochondria for beta-oxidation?
Have to be activated before they can be transported
Describe use of ATP by Synthases vs Synthetases
Synthases do not use ATP
Synthetases use ATP
How are FA activated?
Fatty acyl-CoA synthetase firstly uses ATP to charge a FA
This charged ATP- fatty acyl-adenylate is attached to thiol group of coenzyme A by fatty acyl-CoA synthetase forming thioester fatty acyl-CoA
What can fatty acyl-CoA be used for?
To synthesize longer membrane lipids
Beta-oxidation
Where are the enzymes for beta-oxidation located?
Mitochondria
Fate of small vs long FA in transport into mitochondria
- Small (< 12 carbons) FA diffuse freely across mitochondrial
membranes - Longer fatty acids are transported
Where are fatty acyl-CoA formed?
Fatty acyl–CoA esters formed at the cytosolic side of the outer mitochondrial membrane
What is used for transport of fatty acyl-CoAs into mitochondria?
carnitine transport system
How does carnitine transport work?
When fatty acyl-CoAs are destined to be transported into mitochondria, they are attached to hydroxyl group of carnitine to form fatty acyl–carnitine
This transesterification is catalyzed by carnitine acyltransferase I (I as it in the outer membrane)
This enzyme removes Acetyl CoA and attaches carnitine
Fatty acyl-carnitine passes into intermembrane space through large pores in the outer membrane (moves from cytosol to the intermembrane space)
The fatty acyl–carnitine ester then enters the matrix by facilitated diffusion through the acyl-carnitine/carnitine transporter of the inner mitochondrial membrane (moves from intermembrane space to matrix)
Carnitine acyltransferase II (in the matrix) then transfers fatty acyl group from carnitine to intramitochondrial coenzyme A
Carnitine acyltransferase II regenerates fatty acyl–CoA and releases it, along with free carnitine, into the
matrix
Fatty acyl-CoA is now available for b-oxidation
Carnitine reenters the intermembrane space via the acylcarnitine/carnitine transporter.
What links links two separate pools of coenzyme A and of fatty acyl–CoA and where are they located?
three-step process for transferring fatty acids into the mitochondrion—esterification to CoA, transesterification to carnitine followed by transport, and transesterification back to CoA—links two separate pools of coenzyme A and of fatty acyl–CoA, one in the
cytosol, the other in mitochondria.
What is the function of coenzyme A pool in the mitochondrial matrix vs cytosolic coenzyme A?
Coenzyme A in the mitochondrial matrix is largely used in oxidative degradation of pyruvate, fatty acids, and some amino acids, whereas cytosolic coenzyme A is used in the biosynthesis of fatty acids
What commits the fatty acyl to the oxidative fate?
Conversion to the carnitine ester commits the fatty acyl to the oxidative fate
Why is carnitine used as a diagnostic tool?
more carnitine, more FA can be transported
In b-oxidation there are _ reactions and _ enzymes
In b-oxidation there are 4 reactions and 4 enzymes
At each cycle of b-oxidation there is a product of __
At each cycle of b-oxidation there is a product of Acetyl CoA
Describe b-oxidation
1) Dehydrogenation of fatty acyl–CoA produces a double bond between the alphaand beta carbon atoms (C-2 and C-3), yielding a trans-Δ2-enoyl-CoA- acyl-CoA dehydrogenase. The electrons removed from the fatty acyl–CoA are transferred to FAD-> FADH
2) In the second step of the β-oxidation cycle water is added to the double bond of the trans-Δ2-enoyl-CoA to form the L-stereoisomer of β-hydroxyacyl-CoA (3-hydroxyacyl-CoA)- enoyl-CoA hydratase
3) In the third step, L- β-hydroxyacyl-CoA is dehydrogenated to form β-ketoacyl-CoA, by the action of β-hydroxyacyl-CoA dehydrogenase. NAD+ is the electron acceptor. The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to O2. The reaction catalyzed by β-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle-
4) The fourth and last step of the β-oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of β-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA. The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms
β-oxidation occurs at __ end
β-oxidation occurs at carboxyl end
β-oxidation of 16 carbon FA yields: _Acetyl-CoA, _FADH2, _NADH, _H+
How much ATP?
β-oxidation of 16 carbon FA yields: 8Acetyl-CoA, 7FADH2, 7NADH, 7H+
1.5 ATP/FADH2 and 2.5 ATP/NADH->
8Acetyl-CoA, 28ATP, 7H2O
What is the formula for calculating number of AcetylCoA, FADH, NADH, H, ATP produced in the b-oxidation?
# of AcetylCoA= #C/2 # of FADH, NADH, H+= #C/2-1 ATP=(FADH+NADH)*4
What happens if the FA undergoing b-oxidation is unsaturated?
B-oxidation occurs normally until double bond is encountered between carbons 3 and 4
They require additional enzyme 3,2-enol-CoA isomerase which transfers double bond to the carbons 2,3 from 3,4-
The double bond is available- B-oxidation occurs smoothly
Where should the double bond be for b-oxidation to occur normally?
Between carbon 2 and 3
What does 3,2-enol-CoA isomerase do and is it’s reaction reversible?
enzyme 3,2-enol-CoA isomerase transfers double bond to the carbons 2,3 from 3,4
Reversible reaction
What is the downside of 3,2-enol-CoA isomerase which allows b-oxidation to use unsaturated FA?
One FADH2 less is produced as double bond didn’t have to be created between carbons 2 and 3 - was already there -1st step of b-oxidation was skipped
What is the function of 2,4-Dienoyl-CoA reductase?
2,4-dienoyl-CoA uses NADH to change cis conformation (not suitable for b-oxidation) to trans
Describe β-Oxidation of FA with odd number carbons
3 carbon compound- propionyl CoA is formed in the last step of b-oxidation
It goes through reactions forming succinyl-CoA in the end (TCA)
Vitamin B12 is important in situations when cells encounter odd number FA
This doesn’t occur often
Enz_ and _ are the ones that creates energy molecules in the form of FADH and NADH
In bactria, Enz1 and 3 are the ones that creates energy molecules in the form of FADH and NADH
Where are short chain and long chain specific enzymes found?
Short chain specific enzymes float around
Long chain specific enzymes are imbedded in cell membrane
How are b-oxidation enzymes enz1-enz4 are regulated?
Regulated by both transcription factors (e.g. PPARg) and kinases (e.g. AMPK)
Fatty acyl-CoA synthesized in the cytoplasm is directed to __ or __
Fatty acyl-CoA synthesized in the cytoplasm is directed to β-Oxidation or TG synthesis
__ is the point of commitment to β-Oxidation
Carnitine Shuttle is the point of commitment to β-Oxidation
___, the first intermediate of FA synthesis, inhibits ___
Malonyl-CoA, the first intermediate of FA synthesis, inhibits Carnitine Acetyltransferase I
Malonyl-CoA is necessary for FA synthesis-> inhibits beta-oxidation process
__ stimulates AMPK leading to activation of Carnitine shuttle
Low ATP (low ATP/AMP ratio) stimulates AMPK leading to activation of Carnitine shuttle
__ ratio inhibits Acyl-CoA dehydrogenase
High NADH/NAD+ ratio inhibits Acyl-CoA dehydrogenase
What does AMPK senses?
AMP kinase senses the amount of AMP
What is the function of Acyl-CoA dehydrogenase?
First enzyme of b-oxidation which creates an energy molecule - FADH2
WHat does high NADH/NAD+ ratio mean ?
HIGH ENERGY LEVELS
Describe ω-oxidation
Oxidation of ω carbon of FAthe carbon most distant from the carboxyl group
Happens in endoplasmic reticulum in liver and kidney
A rare event
Inβ-oxidation enzymes
act at the __ end of a fatty acid
Inβ-oxidation enzymes
act at the carboxyl end of a fatty acid
Where are enzymes of ω-oxidation located?
What are their preferred substrates?
in the endoplasmic reticulum of liver and kidneys
preferred substrates are fatty acids of 10 or 12 carbon atoms.
What are the end products of ω-oxidation?
FA with carboxylic group at both ends is then broken down to Succinate and Adipic acid- yielding Acetyl CoA from both ends
Are ketone bodies water soluble?
yes
Where are ketone bodies produced?
Liver
How is acetone excreted?
Breath
Name the ketone bodies. Which one is maintained at low concentrations
- Acetone- usually maintained at low concentrations. Exhaled through respiration. Acetone smell is a sign of ketosis.
- Acetoacetate
- B-hydroxybutyrate (BHB)
Which ketone bodies are used for generation of Acetyl CoA and thus, the production of energy?
Acetoacetate and BHB
What is ketogenesis and when does it occur?
Ketogenesis (ketone body synthesis) occurs when we are not eating/starvation or diabetes
Where is the site of ketogenesis?
Mitochondria within hepatocytes is the site of ketogenesis
What are ketone bodies made of?
Extra acetyl-CoA
What are the 2 fates of acetyl-CoA formed in the liver during oxidation of fatty acids?
It can either enter the citric acid cycle or undergo conversion to the “ketone bodies,”
What are the fates of Acetoacetate and D-β-
hydroxybutyrate
transported by the blood to tissues other than the liver (extrahepatic tissues), where they are converted to acetyl-CoA and oxidized in the citric acid cycle, providing much of the energy required by
tissues
What are ketone bodies made of? What type of reaction is it?
2 acetyl-CoA
It is the enzymatic condensation of two molecules of acetyl-CoA, catalyzed by thiolase
Describe thiolase
Thiolase is the last enzymes of b-oxidation and first of ketogenesis. It’s reaction is reversible
Describe steps of ketogenesis
2 Acetyl-CoA are converted into acetoacetyl-CoA by thiolase
Acetoacetyl-CoA is converted into HMG-COA (B-Hydroxy-B-methylglutaryl-CoA) by HMG-CoA synthase which adds a 3rd Acetyl CoA to acetoacetyl-CoA
HMG CoA is then lysed/cleaved to free acetoacetate (worth 2 ketone bodies) and acetyl-CoA by HMG-CoA lyase
Acetoacetate can then be converted into Acetone and β-hydroxybutyrate
Where is HMG-CoA found? Why?
HMG-COA synthase is present in mitochondria as ketone body synthesis occurs there
Its isoform is present in the cytoplasm - place a role in cholesterol synthesis - the only enzyme that is present both in cytoplasm and mitochondria
Where are the enzymes of ketogenesis located?
All are located in the mitochondria, but HMG-CoA which also has a cytoplasmic isoform (both in the cytoplasm and in the mitochondria)
How are ketone bodies catabolized?
Ketone bodies are converted to acetyl-CoA
Once ketone bodies are made -> circulation-> taken up and have to be converted back to acetyl-CoA
This starts with B-hydroxybutyrate
B-hydroxybutyrate is oxidized to acetoacetate by D-B-hydroxybutyrate dehydrogenase
The acetoacetate is activated to its coenzyme A ester by transfer of CoA from succinyl-CoA, an intermediate of the citric acid cycle in a reaction catalyzed by B-ketoacyl-CoA transferase.
The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle.
What is so special about β-ketoacyl-CoA transferase
In ketone body catabolism transfers CoA from Succinyl-CoA to Acetoacetate to make Acetoacetyl-CoA
ABSENT in the liver
If it was present - ketone bodies would never leave the liver as they would have been made and instantly catabolised
Odd-number fatty acids are oxidized by theb-oxidation pathway to yield ___
Odd-number fatty acids are oxidized by the b-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-
CoA.
