lipid metabolism Flashcards
fatty acids are stored in what form
in the form of Triacylglycerols
how are triacylglycerols formed
Condensation reaction combines 1 Glycerol and 3 Fatty Acids
- Hydroxyl group (-OH)
- Carboxyl group (-COOH)
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 Lipase
504 amino acids in length with a weight of 55 kDa – so is a big protein
The rate limiting step of Triacylglycerol breakdown- so why its at the beginning
- Occurs on lipid droplets and in the cytosol
Produces either 2,3-diacylglycerol or 1,3-diacylglycerol
- Catalytic activity is strongly enhanced by CGI-58
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
explain Monoacylglycerol Lipase
303 amino acids in length, 33 kDa
- Cytosolic
Hydrolyses 1- and 2- diacylglycerols at the same rate
- Other monoacylglycerol lipases, such as ABHD6, do show a preference
Found in many tissues and is thought to be a ‘housekeeping’ enzyme for lipid metabolism
why can glycerol not be metabolised by adipocytes
so what happens instead
because they do not have glycerol kinase.
Instead, glycerol is transported in the blood to the liver which can then subject it to one of 3 possible fates.
All require production of glycerol 3-phosphate by glycerol kinase
what is the fate of glycerol dependent upon
upon conversion of glycerol 3-phosphate to dihydroxyacetone phosphate by glycerol 3-phosphate dehydrogenase
what are the 2 possible fates of Dihydroxyacetone phosphate
Glucose or Pyruvate
as it is a glycolytic / gluconeogenic intermediate
what can Glycerol 3-phosphate be used to produce
more triacylglycerides
The liver then stores this, but not in its raw state
- Instead, it will be stored as Very Low-Density Lipoprotein (VLDL)
give a brief description of a lipoprotein
Lipoproteins are like the fat alternative for storing glycogen, its basically s membrane and stores fat
what is the fat of free fatty acids
Free Fatty Acids (FFAs) move through the plasma membrane of the adipocyte into the bloodstream.
- When in the blood, 99% of FFAs bind to albumin
Albumin has 7 binding sites for fatty acids – so concentrated the amount of fatty acids in one place
Only free FAs can enter cells
Fatty acids are quite big
Only free FAs can enter cells Depending upon their what
and enter by what
chain-length, FFAs can enter cells by either:
- Passive diffusion (short (2-6 Cs) & medium chain (7-12 Cs)
- Specialised FA transport proteins (Long chain (13-21+ Cs)
explain how fatty acids Cross the Plasma Membrane
The precise mechanisms of fatty acid transport remain an area of active research
- small fatty acids can Cross by diffusion
- CD36 (Fatty Acid Translocase) and/or FABPpm (Fatty Acid Binding Protein) increase the local concentration of fatty acids, increasing diffusion events
- CD36 can act as a shuttle for fatty acids if bound to the cytosolic form of FABP
- Fatty Acid Transport Protein (FATP) transport Fatty acids (10-15 carbons in length) through the membrane, where they activate Acyl-CoA synthetase (ACS1) to form Acyl-CoA (not acetyl-CoA) esters
- Very Long Chain Fatty Acids (22+ Cs) are transported by FATPs (fatty acid transport proteins) and directly converted to Acy-CoA esters
explain Fate of the Fatty Acids: Activation
Free Fatty Acids are toxic to cells – as amphipathic
- The COO- makes them amphipathic (hydrophilic and hydrophobic) and thus they would act like detergents if not neutralised
- CH3- (CH2)n- COO-
-This is achieved by Acyl CoA Synthase (also called Thiokinase – as sulfur of the coenzyme group)
The 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:
- Re-esterification into Triacylglycerols
- Metabolised more by Β-oxidation to the Citric Acid Cycle and Ketone Bodies
explain the beta oxidation of fatty caids
The major oxidation pathway for fatty acids catabolism
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.
explain the β-oxidation of Fatty Acids: Transport into the mitochondria
This is the rate limiting step of FA oxidation as fatty acyl CoA molecules cannot diffuse into the mitochondrial
The passage of fatty acyl CoA from the cytosol into the mitochondrial matrix is mediated by a special transport system – The Carnitine Shuttle
- Carnitine replaces CoA in the fatty acyl CoA by carnitine palmitoyltransferase I (CPT I) to form an acyl carnitine.
- Acyl carnitine is transported through the inner mitochondrial membrane by a translocase.
- In the mitochondrial matrix, carnitine palmitoyltransferase II (CPT II) converts the acyl carnitine to fatty acyl CoA
- Fatty acyl CoA now undergoes β-Oxidation
- Carnitine is transported back to the inner membrane space by the translocase for further use.
to summarise: CoA enzyme is broken off and then eventually added back on after enters mitochondrial matrix
β-oxidation of Fatty Acids comprises of how many reactions ? and whats the net result
4
Net result is that 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
acyl CoA dehydrogenase:
it is specific to chain length. Members of this enzyme family include long-chain, medium-chain, and short-chain acyl CoA dehydrogenases (LCAD), (MCAD), and (SCAD), respectively.
These enzymes catalyze the formation of a double bond between the alpha and beta carbons on acyl CoA molecules by removing two electrons to produce one molecule of FADH2, which 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, such as a carbonyl. The second carbon atom is called the β-carbon
explain step 2 of β-oxidation of Fatty Acids
Enoyl CoA hydratase,
performs a hydration step of the double bond between the alpha and beta carbons; this results in the addition of a hydroxyl (OH-) group to the beta carbon and a proton (H+) to the alpha carbon.
There is no energy production associated with this step
(basically u add water)
explain step 3 of β-oxidation of Fatty Acids
3(β)-hydroxyl acyl CoA dehydrogenase; as the name implies, electrons and two protons are removed from the hydroxyl group, and the attached 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
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. (5 carbons in length)
explain β-oxidation of unsaturated Fatty Acids
Unsaturated fatty acids, such as oleate (18:1) and linoleate (18:2), contain double bonds that must be isomerized (enoyl CoA isomerase) or reduced at the expense of an NADPH molecule (2,4-dienoyl CoA reductase)
explain the β-oxidation of 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.
explain β-oxidation of Fatty Acids: High ATP yield
Produces more ATP so is why fats are important for energy
what do ketone bodies include
Acetoacetate
3-hydroxybutyrate
Acetone (not metabolisable)
where are ketone bodies synthesised
Synthesised in the mitochondria of liver cells, transported in the blood to peripheral tissues
when are ketone bodies produced
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 [Acetyl CoA] ([…] = concentration
what do words in brackets […] mean
concentration of word in brackets
eg; [Acetyl CoA]
Acetyl CoAs can be joined together to form what
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.
explain the process of ketone bodies
Fatty acids are brought into the mitochondria via CPT-I and then broken down into acetyl CoA via beta-oxidation.
Two acetyl-CoA molecules are converted into acetoacetyl-CoA via the enzyme thiolase; this is also known as acetyl coenzyme A acetyltransferase (ACAT).
Acetoacetyl-CoA is converted to HMG-CoA via the enzyme HMG-CoA synthase. HMG-CoA lyase then converts HMG-CoA to acetoacetate.
- HMG = β-Hydroxy β-methylglutaryl-
Acetoacetate can be converted to either acetone through non-enzymatic decarboxylation or to 3(β)-hydroxybutyrate via 3(β)-hydroxybutyrate dehydrogenase.
Liver cannot reconvert acetoacetate into what
acetoacetyl CoA and hence cannot use ketone bodies as a fuel.
all othe cells in body with mitochondria apart from liver can oxidise what
acetoacetate and 3-hydroxybutyrate
3-hydroxybutyrate is oxidised to acetoacetate by 3-hydroxybutyrate dehydrogenase
Acetoacetate is converted back to acetyl-CoA via the enzyme beta-ketoacyl-CoA transferase and thiolase.
in ketone bodies Acetone does not convert back to acetyl-CoA so what happens to it
it is either excreted through urine or exhaled.
A large proportion of the FAs used by the body are suppliedby what
the diet
Excess amounts of -CHO and protein obtained from the dietcan be converted to what and why
to fatty acids which in turn can beincorporated into TAGs.
In humans FA synthesis occurs primarily in what
liver and
lactatingmammary glands.
fatty acid synthesis process incorporates what
incorporates carbons from acetyl CoA into thegrowing fatty acid chain, utilising ATP and reducednicotinamideadenine dinucleotide phosphate (NADPH).
NADPH is produced in a pathway called the PentosePhosphate Pathway.
explain Fatty Acid Synthesis: Production of Cytosolic Acetyl-CoA
First step of FA synthesis is the transfer of acetyl units frommitochondrial acetyl CoA to the cytosol, forming cytosolicacetyl CoA
CoA moiety cannot cross the mitochondrial membrane, but acetyl can
This is achieved by the condensation of oxaloacetate andacetyl CoA to form citrate (TCA cycle) and when mitochondrial[citrate] is high
Citrate spills out from the mitochondrialmatrix and into the cytosol via a transporter, the mitochondrial citrate transporter (MCT)
Cytosolic citrate is then cleaved byATP citratelyaseto produce acetyl CoA.
explain the Fatty Acid Synthesis: Carboxylation of Acetyl-CoA to form Malonyl-CoA
The energy for the carbon-to-carbon condensations in FAsynthesis is supplied by the process of carboxylation and thendecarboxylation of acetyl groups in the cytosol.
The carboxylation of acetyl CoA to formmalonylCoA iscatalysed by acetyl CoA carboxylase and requires ATP.
the remaining series of reactions of FA synthesis is catalysed by what
amultienzymecomplex,fatty acid synthase (FAS)
explain the series of reactions catalysed by fatty acid synthase
Initiated by the transfer of the acetyl moiety of the starter substrate acetyl-CoA to the acyl carrier protein (ACP) catalyzed by the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT).
The β-ketoacyl synthase (KS) catalyzes the decarboxylative condensation of the acyl intermediate with malonyl-ACP to a b-ketoacyl-ACP intermediate
acetoacetyl-ACP in the first cycle
The β-carbon is processed by nicotinamide adenine dinucleotide phosphate (NADPH)–dependent reduction through β-ketoacyl reductase (KR).
Β-hydroxyacyl-ACP is dehydrated by a dehydratase (DH) to a b-enoyl intermediate, which is reduced by the NADPH-dependent β-enoyl reductase (ER) to yield a four-carbon acyl substrate for further cyclic elongation with two-carbon units derived from malonyl-CoA until a substrate length of C16 to C18 is reached.
The product is released from the ACP by the thioesterase (TE)
Mono-, Di- andTriacylglycerols
consist of what
one,two or three molecules of FA esterified to amolecule of glycerol.
FAs are esterified through what
their carboxylgroups, resulting in a loss of negative chargeand formation of “neutral fat”.
stored fatty acids on the glycerol molecule are usually …..the same type
not the same type
Carbon 1 - Normally saturated
Carbon 2 - Unsaturated (double bonds)
Carbon 3 - Either
fatty acids are stored as what
components of triacylglycerols
how are triacylglycerols stored
VLDLs
Inside the cell
Because they are only ‘slightly soluble in waterand cannot form stable micelles themselves,they coalesce within adipocytes to form oilydroplets
Glycerol phosphate is the initial acceptor of FAs during thesynthesis of what
of TAG
explain the Synthesis of Glycerol-3-phosphate
Route of synthesis is different in liver and adipose tissue
- No glycerol kinase in adipose cells thus,inlyone route toglycerol phosphatesynthesis.
- Adipose cells can ONLY take up glucose in the presence ofinsulin
explain Synthesis TAG fromglycerol phosphate andfatty acyl CoA
Pathway involves:
4reactions
These include thesequential addition of 2fatty acids from fatty acylCoA, the removal ofphosphate and the additionof a third fatty acid.
