Lipid Metabolism Flashcards
What are fatty acids stored as?
Fatty acids stored in the form of triaclglycerols
What does a condensation reaction combine?
1 glycerol and 3 fatty acids
What do triacylglycerols act as?
Act as the bodies major energy reserves
What does the release of fatty acids from triaclyglycerols require?
Requires hydrolysis
What is hydrolysis?
- A process initiated by the adipose triglyceride lipase and hormone sensitive lipase.
- Includes the removal of fatty acids from Carbons 1 and 3 of the glycerol backbone
- remaining fatty acid at carbon 2 is removed by monoacylglcerol lipase
how are triacylglycerols formed
Condensation reaction combines 1 Glycerol and 3 Fatty Acids
- Hydroxyl group (-OH)
- Carboxyl group (-COOH)
explain hormone sensitive lipase
786 amino acids with a molecular weight of 85.5 kDa
- Cytosolic
Hydrolysis occurs in 2 steps;
- Cleaves the covalent ester bond between glycerol and the fatty acid
- Water displaces the covalent intermediate
Cleaves the 1- and 3- ester bonds 3-4-fold faster than the 2- ester bonds
triacylglycerols act as the body’s what
major energy reserve
When hydrolysed, fatty acids released can yield 9kcal/g
Carbohydrates and amino acids only yield 4kcal/g
how are Fatty Acids released from Triacylglycerols
Requires hydrolysis
- Process is initiated by Adipose triglyceride lipase and hormone sensitive lipase, which remove fatty acids from C1 and C3 of the glycerol backbone.
- The remaining fatty acid at C2 is removed by monoacylglycerol lipase
Explain adipose triglyceride
- 504 amino acids. Weight=55kDa (large protien)
- occurs on lipid droplets and in the cytosol
- produces either:
1. 2,3-diaclglycerol
2. 1,3-diacylglycerol - catalytic activity is strongly enhanced by CGI-58
Explain hormone sensitive lipase
- 786 amino acids
- weight= 85.5 kDa
- it is cytosolic
- Hydrolysis occurs in 2 steps
What are the 2 steps of hydrolysis in hormone sensitive lipase?
Step 1
- Cleaves the covalent ester bond between glycerol and the fatty acid
- water displaces the covalent intermediate
Step 2
- cleaves the 1 and 3 ester bonds
- ester bonds 3-4 fold faster than the 2 ester bond
Explain monoacylglycerol lipase
- 303 amino acids
- weight= 33 kDa
- cytosolic
- hydrolyses 1 and 2 diaclygylcerols at the same rate
- found in many tissues and thought to be a housekeeping enzyme for lipid metabolism
Why can glycerol not be metabolised by adipocytes? And what happens instead
- They do not have enough glycerol kinase
Instead:
- glycerol is transported in the blood to the liver which can then subject it to one of the 3 possible fates
What is the fate of glycerol dependent on?
- the conversion of glycerol 3-phosphate to dihydroxyacetone phosphate
- done by glycerol 3-phosphate dehydrogenase
What are the 2 possible fates of dihydroxyacetone phosphate?
- Glucose
- Pyruvate
As it is a gycolytic/gluconeogenic intermediate
What can glycerol 3-phosphate be used to produce
- more triaclyglycerides
Where and how are the triacylglycerides produced from glycerol 3-phosphate stored?
- Stored in liver
- not in its raw state
- will be stored as very low density lipoprotien (VLDL)
Give a brief description of a lipoprotien
- like the fat alternative for storing glycogen
- basically a membrane that stores fat
Where do free fatty acids move through?
- Free Fatty Acids (FFAs) move into the bloodstream
-do this through the plasma membrane of the adipocyte
When free fatty acids are in the blood what percentage binds to where?
- When in the blood:
99% of FFAs bind to albumin
How many binding sites does albumin have for fatty acids? and why?
- Albumin has 7 binding sites for fatty acids
- concentrates the amount of fatty acids in one place
What are 2 characteristics of free fatty acids?
- Only free fatty acids can enter cells
- Fatty acids are quite big
Only free fatty acids can enter cells. What does this depend on and what processes are used for them to enter cells?
- depends on chain length
Enters through:
1. Passive diffusion ( short 2-6C and medium 7-12C chain)
2. Specialised fatty acid transport proteins (long chain 13-21C +)
Explain how fatty acids cross the plasma membrane.
- Small fatty acids can cross by diffusion
- CD36 (fatty acid translocase) or FABPpm (fatty acid binding protein) increase the local concentration of fatty acids, increasing diffusion events
- When bound to the cytosolic form of FABP, CD36 acts as a shuttle for fatty acids
- Fatty acid transport proteins transport fatty acids ( 10-15C in length). Where they activate Acyl-CoA synthetase to form Acyl-CoA esters
- Very long chain fatty acids (>22C) are transported by FATPs and directly converted to Acyl-CoA esters)
What do all 3 fates of glycerol require?
All require the production of glycerol 3-phosphate by glycerol kinase
What are the two fates of fatty acids?
- Crossing the Plasma Membrane
- Activation
free fatty acids are toxic to what?
- toxic to cells
- The COO- makes them amphipathic (hydrophilic and hydrophobic)
- so they would act like detergents if not neutralised
- achieved by Acyl CoA Synthase (Thiokinase)
explain what happens in the following reaction:
R-COO- + ATP + CoA-SH <—> R-CO-S-CoA + AMP + PPi
-reaction is driven forward by a pyrophosphatase
- Seen before in Glycogen production
- Hydrolyses PPi into 2Pi
Once the Acyl-CoA has been formed it can follow one of two paths: What are they?
- Re-esterification into Triacylglycerols
- Β-oxidation to the Citric Acid Cycle and Ketone Bodies
What is the major oxidation pathway for fatty acids catabolism?
β-oxidation of Fatty Acids
β-Oxidation takes place in the mitochondrial matrix
Essentially, 2C fragments are removed, sequentially from the fatty acyl CoA producing acetyl CoA which can be oxidised in the citrate cycle.
Where does β-Oxidation take place?
- takes place in the mitochondrial matrix
What happens in the β-Oxidation of fatty acids?
- 2C fragments are removed, sequentially from the fatty acyl CoA
- this produces acetyl CoA which can be oxidised in the citrate cycle
What is the rate limiting step of fatty acid oxidation?
β-oxidation of Fatty Acids: Transport into the mitochondria
Why is the following considered the rate limiting step of fatty acid oxidation?
- because fatty acyl CoA molecules cannot diffuse into the mitochondrial
What does the The Carnitine Shuttle do?
- it mediates the passage of fatty acyl CoA from the cytosol into the mitochondrial matrix
How is acyl carnitine formed?
- Carnitine replaces CoA in the fatty acyl CoA
- replaces by carnitine palmitoyltransferase I (CPT I)
Acyl carnitine is transported through the inner mitochondrial membrane by what?
a translocase
In the mitochondrial matrix, what converts the acyl carnitine to fatty acyl CoA
carnitine palmitoyltransferase II (CPT II)
What happens when fatty acyl CoA undergoes β-Oxidation?
- Carnitine is transported back to the inner membrane space
-transported by the translocase for further use
How many reaction does the β-oxidation of Fatty Acids comprise of?
- Comprises 4 reactions
what is the net result of β-oxidation of Fatty Acids
- The fatty acid is shortened by two carbon atoms
Each β-Oxidation cycle produces what?
1 acetyl CoA
1 Fatty Acyl CoA (minus 2C)
1 NADH
1 FADH2
Explain step 1 of β-oxidation of Fatty Acids in depth
Acyl CoA dehydrogenase:
- it is specific to chain length
- Members of this enzyme family include long, medium, and short chain acyl CoA dehydrogenases
- These enzymes catalyse the formation of a double bond between the alpha and beta carbons on acyl CoA molecules
- this is done by removing two electrons to produce one molecule of FADH2
- This eventually accounts for ~2 ATP molecules produced in the electron transport chain
The α-carbon refers to the first carbon atom that attaches to a functional group (eg. a carbonyl) The second carbon atom is called the β-carbon
Explain step 2 of β-oxidation of Fatty Acids in depth
- Enoyl CoA hydratase performs a hydration step of the double bond between the alpha and beta carbons
- There is no energy production associated with this step
Results in:
Beta carbon= addition of a hydroxyl (OH-) group
Alpha carbon= addition of a proton (H+)
Explain step 3 of β-oxidation of Fatty Acids in depth
- 3(β)-hydroxyl acyl CoA dehydrogenase
- electrons and two protons are removed from the hydroxyl group
- then go on to attach to the beta carbon
- to oxidize the beta carbon and produce a molecule of NADH
- Each molecule of NADH will result in the production of ~3 ATP molecules from the ETC
Explain step 4 of β-oxidation of Fatty Acids in depth
- 3 (β)-keto thiolase
- Cleavage of the bond between the alpha and beta carbon by CoA
- The reaction produces one molecule of acetyl CoA and a fatty acyl CoA that is two carbons shorter
- The process may repeat until the even chain fatty acid has completely converted into acetyl CoA
how is β-oxidation of Fatty Acids effected by unsaturated fatty acids?
- Unsaturated fatty acids such as oleate and linoleate contain double bonds that must be isomerized (enoyl CoA isomerase)
- Or reduced at the expense of an NADPH molecule (2,4-dienoyl CoA reductase)
How is β-oxidation of Fatty Acids effected by Odd-chain fatty acids?
- Odd-chain fatty acids undergo beta-oxidation in the same manner as even chain fatty acids
- Once a five-carbon chain remains, the final spiral of beta-oxidation will yield one molecule of acetyl CoA and one molecule of propionyl CoA.
- This three-carbon molecule can be enzymatically converted to succinyl CoA, forming a bridge between the TCA cycle and fatty acid oxidation.
What are ketone bodies used as?
a fuel of starvation
What do ketone bodies include?
- Acetoacetate
- 3-hydroxybutyrate
- Acetone (not metabolisable)
Where are Ketone bodies synthesised?
- in the mitochondria of liver cells, transported in the blood to peripheral tissues
When are ketone bodies produced?
- During periods when the amount of acetyl CoA produced EXCEEDS the oxidative capacity of the liver
- E.g. During starvation:
- Increased FAs from adipose tissue
- Oxidation in liver leads to an increased concentration of Acetyl CoA
Acetyl CoAs can be joined together to form what?
Ketone bodies
What is the function of ketone bodies?
- Fuel by skeletal muscle, cardiac muscle, kidney and about 70% of the brain (if pushed)
- Act as a ‘substitute’ for glucose for those tissues that can adapt themselves during starvation.
Ketone bodies: fuel of starvation
Step 1: Fill in the blanks of the following
Fatty acids are brought into the mitochondria via _____ and then broken down into ______ via ______
- CPT-I
- Acetyl CoA
- via beta-oxidation.
Ketone bodies: fuel of starvation
Step 2:
In step 2… How many acetyle-CoA molecules are converted into acetoacetyl-CoA via what enzyme?
- Two acetyl-CoA molecules are converted into acetoacetyl-CoA
-Via the enzyme thiolase
What is the enzyme Thiolase also known as?
acetyl coenzyme A acetyltransferase (ACAT).
Ketone bodies: fuel of starvation
Describe step 3
- Acetoacetyl-CoA converted to HMG-CoA
-Via the enzyme HMG-CoA synthase
- HMG-CoA lyase then converts HMG-CoA to acetoacetate.
Ketone bodies: Fuel of starvation
Step 4
- Acetoacetate can be converted into what 2 things via which processes?
- Acetoacetate can be converted to either:
1. Acetone
2. 3(β)-hydroxybutyrate
Processes:
1. Acetone= through non-enzymatic decarboxylation
2.3(β)-hydroxybutyrate= via 3(β)-hydroxybutyrate dehydrogenase
What does HMG in full terms?
HMG = β-Hydroxy β-methylglutaryl-
The liver cannot use ketone bodies as fuel, explain why.
As the liver cannot reconvert acetoacetate into acetoacetyl CoA
All other cells that have mitochondria can oxidise what 2 things?
- acetoacetate
- 3-hydroxybutyrate
3-hydroxybutyrate is oxidised to acetoacetate by what?
3 -hydroxybutyrate dehydrogenase
Acetoacetate is converted back to what? via what enzymes?
- Acetyl-CoA
- Beta-ketoacyl-CoA transferase and thiolase
Where is acetone excreted due to it not converting back to Acetyl-CoA?
excreted through urine or exhaled
A large proportion of fatty acids used by the body are supplied by what?
The diet
What 2 things obtained in excess amounts through the diet can be converted into fatty acids?
In turn what can this be incorporated into?
- Excess amounts of -CHO and protein obtained from the diet
2.Incorporated into Dietary triacylglycerols (TAGs.)
In humans, where does fatty acid synthesis primarily occur?
In the liver and lactatingmammary glands
The fatty acid synthesis process incorporates carbons from where? into what chain?
-Carbons come from the acetyl CoA
- Incorporates carbons into thegrowing fatty acid chain
The fatty acid synthesis process utilises what 2 things?
- ATP
2.Reducednicotinamideadenine dinucleotide phosphate
(NADPH)
What pathway is NADPH produced in?
PentosePhosphate Pathway
What is produced in step 1 of fatty Acid Synthesis
Step 1= Production of Cytosolic Acetyl-CoA
What is step 1 of FA synthesis?
- Transfer of acetyl units frommitochondrial acetyl CoA to the cytosol
- Forms cytosolicacetyl CoA
CoA moiety cannot cross what? And in contrast what can?
- CoA moiety cannot cross the mitochondrial membrane
- Acetyl can
Transfer of acetyl units frommitochondrial acetyl CoA to the cytosol is achieved by what?
-Condensation of oxaloacetate andacetyl CoA
-Forms citrate (TCA cycle)
-and when mitochondrialconcentration citrate is high
Citrate spills out from the mitochondrialmatrix into where? via what?
- Cytosol
- Via a transporter
the mitochondrial citrate transporter (MCT)
What transporter is used for citrate to go from the mitochondrial matrix to the cytosol?
the mitochondrial citrate transporter (MCT)
Cytosolic citrate is then cleaved by what to produce what?
- Cleaved by ATP citratelyase
- produces acetyl CoA
What is the main thing that occurs in step 2 of Fatty Acid Synthesis?
Carboxylation of Acetyl-CoA to form Malonyl-CoA
The energy for the carbon-to-carbon condensations in FAsynthesis is supplied by what processes?
Carboxylation +decarboxylation of acetyl groups in the cytosol
What does the carboxylation of acetyl CoA form?
malonylCoA
The carboxylation of acetyl CoA forms malonylCoA. What catalyses this reaction and what else does the reaction require?
- Catalysed by acetyl CoA carboxylase
- Reaction requires ATP
The rest of the steps in fatty acid synthesis are catalysed by what?
Amultienzymecomplex:fatty acid synthase (FAS)
The remaining steps of FA synthesis are catalysed by the multienzyme complex: fatty acid synthase.
What is the reaction initiated by?
- Initiated by the transfer of the acetyl moiety to the acyl carrier protein (ACP)
- Which is catalysed by the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT)
- acetyl moiety is from the starter substrate acetyl-CoA
The β-ketoacyl synthase (KS) catalyses what?
- Catalyses the decarboxylative condensation of the acyl intermediate with malonyl-ACP
-forms into b-ketoacyl-ACP intermediate
The β-carbon is processed by what?
-Nicotinamide adenine dinucleotide phosphate (NADPH)
–dependent reduction through β-ketoacyl reductase (KR)
Β-hydroxyacyl-ACP is dehydrated by a dehydratase (DH) , to form what?
-A b-enoyl intermediate
The b-enoyl intermediate formed by the dehydration of Β-hydroxyacyl-ACP is reduced by what? and why is it reduced?
What:
Reduced by the NADPH-dependent: β-enoyl reductase (ER)
Why:
-to yield a four-carbon acyl substrate
-allows for further cyclic elongation
-with two-carbon units derived from malonyl-CoA
-until a substrate length of C16 to C18 is reached
The product of fatty acid synthesis in the last step is released from where by what part?
The product is released from the ACP
By the thioesterase (TE)
Mono-,Di-and Triacylglycerolsconsist of one,two or three molecules of fatty acids that are….
Esterified to amolecule of glycerol
fatty acids are esterified through their carboxylgroups, resulting in what?
A loss of negative chargeand formation of “neutral fat”
Are fatty acids on one glycerol molecule usually thesame type?
Yes or no?
NO: The fatty acids on one glycerol molecule are usually not of thesame type
provide what types of fatty acids the 3 carbons are on one glycerol molecule
Carbon 1 - Normally saturated
Carbon 2 - Unsaturated
Carbon 3 - Either
Where is triaclyglycerol stored?
- Stored in very low density lipoprotein (VLDLs)
- Inside the cell
VLDLs are slightly soluble in water. This means they cannot form what by themselves?
And what do they do instead?
- Cannot form stable micelles themselves
- They coalesce within adipocytes to form oilydroplets
In the synthesis of Glycerol-3-phosphate… What is the initial acceptor of fatty acids during the synthysis of TAG
Glycerol phosphate
True or false:
The route of synthesis is different in liver and adipose tissue for the synthesis of glycerol-3-phosphate
TRUE: The route of synthesis is different in liver and adipose tissue
Why is there only one route to glycerol phosphate synthesis?
As there is no glycerol kinase in adipose cells
Adipose cells can only take up glucose in the presence of what?
Insulin
Synthesis TAG fromglycerol phosphate andfatty acyl CoA:
The pathway involves how many reactions?
Pathway involves 4reactions
What 4 reactions are involved in the pathway for the synthesis TAG fromglycerol phosphate andfatty acyl CoA?
- Addition of 2xfatty acids from fatty acylCoA
- The additionof a third fatty acid
- The removal ofphosphate
- Which one of the following statements about utilisation of ketone bodies as a fuel by the body is INCORRECT?
a. They are soluble in aqueous solution and therefore do not require carriers in the blood.
b. They are made in response to elevated levels of fatty acids in the liver, where the amount of acetyl CoA exceeds the oxidative capacity of the liver.
c. Acetone is not utilised by the body as a fuel.
d. Unlike fatty acids, they can be oxidised by the brain.
e. When plasma ketone body levels are elevated, the liver efficiently oxidises them for energy.
e. When plasma ketone body levels are elevated, the liver efficiently oxidises them for energy.
- The mobilisation of triacylglycerol-bound fatty acids from adipose tissue and their subsequent oxidation by muscle mitochondria require all of the following compounds EXCEPT:
a. Serum albumin
b. Carnitine
c. Acetyl CoA carboxylase
d. Hormone-sensitive lipase
e. Fatty acyl CoA synthase
c. Acetyl CoA carboxylase
- Which of the following statements about the triacylgycerols stored in adipose tissue are correct?
a. They are hydrolysed to form fatty acids and dihydroxyacetone.
b. They are hydrolysed by hormone-sensitive lipase and a monoacylglycerol hydrolase
c. They release fatty acids that can be oxidised to carbon dioxide and water to provide energy in the cell
d. They can yield a precursor of glucose
e. They are mobilised during starvation and during uncontrolled diabetes
b. They are hydrolysed by hormone-sensitive lipase and a monoacylglycerol hydrolase
c. They release fatty acids that can be oxidised to carbon dioxide and water to provide energy in the cell
d. They can yield a precursor of glucose
e. They are mobilised during starvation and during uncontrolled diabetes
- Place the following incomplete list of reactions or relevant locations during -oxidation, using a flow diagram:
- Electron transport and oxidative phosphorylation
- FAD-linked oxidation
- Activation of the fatty acid by joining to CoA
- NAD+-linked oxidation
- Thiolysis
- Acyl-CoA in mitochondrion
- Acetyl CoA enters the citrate cycle
- Hydrolysis of triacylglycerols
- Conversion of fatty acyl-CoA to a fatty acyl carnitine
- Hydrolysis of triacylglycerols
- Activation of the fatty acid by joining to CoA
- Conversion of fatty acyl-CoA to a fatty acyl carnitine
- Acyl-CoA in mitochondrion
- FAD-linked oxidation
- NAD+-linked oxidation
- Thiolysis
- Acetyl CoA enters the citrate cycle
- Electron transport and oxidative phosphorylation
