FA degradation and ketone body metabolism week 2 Flashcards
TGs must be enzymatically hydrolyzed to release the FA and glycerol. What 2 lipases are involved? (just list)
- lipoprotein lipase
- hormone sensitive lipase
Where is lipoprotein lipase found?
What is the source of TGs that is hydrolyzes?
Where do the resultant free FA go? (2 options)
Where do monoacylglycerol products go? (2 option)
What hormone inhibits it? What protein activates it?
- Lipoprotein lipase is located on the surface of endothelial cells of capillaries and possibly of adjoining tissue cells. It hydrolyzes fatty acids from the 1 or 3 position of tri- and diacylglycerols present in VLDL or chylomicrons.
- Resultant fatty acids are either bound to serum albumin or taken up by the tissue.
- Monoacylglycerol products may either pass into the cells or be hydrolyzed by serum monoacylglycerol hydrolase.
- Insulin inhibits lipoprotein lipase. Apo CII (present on lipoproteins) activates it.
Where is hormone sensitive lipase found?
What position on TGs does it hydrolyze FAs? What happens to the resultant mono and diacylglycerols?
What is hormone senstive lipase controlled by? (2nd messenger system, hormones that stimulate and inhibit)
Where do the free FA go? Where does the glycerol go?
- A completely distinct type of lipase controls mobilization of fatty acids from triacylglycerols stored in adipose tissue (cytosol of adipocytes). The lipase that removes the first fatty acid is a controlled enzyme, sensitive to hormones, called hormone-sensitive lipase. The first lipase is controlled by cAMP and removes fatty acids from 1 or 3 position. Two additional lipases complete hydrolysis of mono and diacylglycerols.
- Epinephrine, glucagon and ACTH stimulate hormone sensitive lipase, while insulin inhibits it.
- Fatty acids and glycerol produced by adipose tissue lipases are released to circulating blood, where fatty acids are bound by serum albumin and transported to tissues for use.
- Glycerol returns to liver, where it is converted to DHAP and enters glycolysis or gluconeogenesis.
The degree of utilization of FA for energy varies considerably from tissue to tissue and depends on what 2 factors?
What 2 tissues depend heavily on FA for energy? What tissue cannot utilize FA for energy and why not?
What energy sources are used by most tissues during prolonged fasting?
- The degree of utilization of fatty acids for energy varies considerable form tissue to tissue and depends on metabolic status of the body (and the presence of mitochondria in the cell).
- For example, both cardiac and skeletal muscles depend heavily on fatty acids for energy while nervous tissue does not (blood-brain barrier does not let FA go through). Hydrolyzation of FA produces more energy than usage of glucose. Also, tissues that use FA for energy save glucose for the brain and other tissues that can only utilize glucose.
- However, during prolonged fasting most tissues use fatty acids or ketone bodies for energy. Brain can also use ketone bodies. Will still take up glucose but amount of glucose uptake will be less.
What is the cellular localization of B-oxidation of FA?
What are the general steps of this process?
What is produced? (reducing factors, molecules, energy)
How many ATP are produced per palmitate?
Where do these products go?
- Mitochondrial process
- A step-by-step process
- Steps are similar to the reversal of palmitate synthesis.
- AcCoA (C2) are removed sequentially from the carboxyl end of the FA
- dehydrogenation, hydration, oxidation to form a beta keto acid which is then split by thiolysis.
- FADH2 and NADH are produced along with every AcCoA
- ACCoA –> TCA;
- FADH2, NADH –> terminal oxidation
- 129 ATP / palmitate
What is the first step of beta-oxidation of FA?
What enzyme is involved? Where is this enzyme located within the cell?
How many ATP are used?
Fatty acids are first activated to fatty acylCoA by fatty acylCoA synthase located on the outer mitochondrial membrane. This requires 1 molecule of ATP.
What transporter is required to get FA across the mitochondrial membrane? FA with what number carbons need this transport system? What FA can cross the mitochondrial membrane without this system?
How do get this transporter? (is it ingested through food, synthesized?)
Carnitine is the carrier of acyl groups across the membrane. This is needed for C12-C18 chains.
Entry of shorter chains (medium- and short-chain fatty acids) is independent of carnitine and are activated to acyl-CoA in the mitochondrium. However, when in access, medium chain fatty acids can bind to carnitine.
Carnitine needs to be taken up by food. Some can be synthesized from Lysine in the liver.
Explain how FA get into the mitochondria with the carnitine transport system.
How many ATP are required for this process? Which enzymes utilize ATP?
- On the outer mitochondrial membrane the acyl group is transferred to carnitine catalyzed by carnitine pamitoyltransferase I (CPT I). (needs 1 ATP)
- Acyl carnitine then exchanges across the inner mitochondrial membrane with free carnitine by a carnitine-acylcarnitine antiporter translocase.
- Finally, the fatty acyl group is transferred to CoA by carnitine palmitoyltansferase II (CPT II) located on the matrix side of the inner membrane. (needs 1 ATP)
Once the acyl groups have been transferred back to CoA at the inner surface of the inner mitochondrial membrane, what enzyme acts on them? Where is this enzyme located within the mitochondria?
What is required for the fxn of this enzyme? (cofactor)?
What are the 4 types of this enzyme?
- Once the acyl groups have been transferred back to CoA at the inner surface of the inner mitochondria membrane, they can be oxidized by a group of acyl-CoA dehydrogenases on the inner membrane that work with FAD coenzyme.
- There are 4 kinds of acyl-CoA dehydrogenases (ADs) that will react with acylCoAs depending on their length:
VLCAD – for very-long chain FAs (C20-24)– works in peroxisomes only
LCAD – for long chain FAs (C12-18)
MCAD – for medium chain FAs (C6-10)
SCAD – for short chain FAs (C4-6)
Genetic defects of acyl-CoA dehydrogenases are known. What is the result of these genetic defects? What is one way (other than genetic testing) that they can be detected?
Genetic defects of these dehydrogenases are known, resulting in accumulation of fatty acids in liver, causing hepatic mitochondrial damage and impaired liver function (liver failure, jaundice), thus metabolic derangements. Mitochondrial damage=metabolic derangements. This is incompatible with life because mitochondria are so important for energy generation and other cellular processes.
Fatty acids not processed by liver can bind to carnitine and released to the blood then to the urine, where they can be detected.
Explain the sequence of 4 rxns of beta oxidation.
What is produced?
What is a main difference btwn beta oxidation and synthesis of FA? (hint: activity of certain enzyme)
- dehydrogenase acts: FADH2 is produced
- hydration
- dehydrogenase acts: NADH is produced
- thiolase ACoA cleaved.
Thiolase enzyme works after every step to release ACoA. In synthesis, this enzyme cleaves CoA at the end of FA synthesis.
attached is slide 16 of notes
How is beta-oxidation regulated?
What is the rate limiting step?
How is CPT I regulated?
- Control is exerted by availability of substrates and cofactors and by the rate of processing acetyl CoA in the TCA cycle.
- Transport of FAs to the mitochondria by the carnitine shuttle system is the rate limiting step.
- Malonyl CoA (product of Ac CoA carboxylase for FA synthesis) inhibits CPT I.
How many ACoA, FADH, and NADH are produced in a single round of beta oxidation?
How many are produced from a single palmitate?
How many total ATP are produced?
- In one round, one acetyl CoA is produced, and one FADH2 and one NADH are generated.
- Overall, 8 AcCoA, 7 FADH and 7 NADH will be produced from a palmitate.
- Palmitate: [C-C C-C C-C C-C C-C C-C C-C C-C] beta-oxidation yields
- 7 FADH2 and 7 NADH. This results in 14+21=35 ATP production in terminal oxidation.
- Additionally, the 8 acetyl CoA molecules can go to TCA and provide 12 ATPs each or 96 ATP total.
each or 96 ATP total.
However, 2 ATP were used to activate palmitate, so the net production is 131-2=129
ATPs by one molecule of palmitate.
How is the catabolism of FA kept separate from synthesis? Why is this done?
- Catabolism of fatty acids is kept separate (i.e., in separate intracellular compartments) from synthesis since both pathways have similarities and this would allow cross control.
- B-oxidation is in mitochondria, while palmitate synthesis is in the cytosol.
- NADPH is used in synthesis, FAD and NAD+ in oxidation.
What is the final product of oxidation of odd chain FA?
What is this converted to? Where does this molecule go?
What 2 cofactors are needed for this conversion?
The final product of oxidation of an odd chain fatty acid is propionyl CoA, which is then carboxylated to methylmalonyl CoA (enzyme is biotin dependent), rearranged and then converted to succinyl CoA (enzyme is a mutase, which is vitamin B12 dependent). Succinyl CoA can then enter TCA. A similar breakdown of propionyl CoA occurs from some amino acids.
Where must very long chain FA (VLCFA) go for beta-oxidation? Why is this?
What is produced in this organelle from the beta-oxidation of FA? What is it used for? What enzyme is involved?
What is Zellweger syndrome? What is Adrenoleukodystrophy? What do they both lead to?
Very-long chain fatty acids (C20 or longer) must go to the peroxisomes for preliminary beta-oxidation for chain shortening (once down to palmitate, can go the mitochondria for further beta-oxidation). The initial dehydrogenase produces FADH2, which is oxidized by molecular oxygen to hydrogen peroxide. Catalase then reduces this to water.
Genetic diseases associated with VLCFA oxidation:
Zellweger syndrome – a defect in peroxisomal assembly in all tissues
Adrenoleukodystrophy – X-linked, defect in peroxisomal activation of VLCFA
Both lead to accumulation of VLCFA in the blood and tissues.
What must acetoacetate and B-hydroxybutyrate be converted to in cells to be used for energy generation?
What enzyme is required for this? What tissues is it found in? What is the cellular location of this enzyme?
After action by this enzyme, where does its product go?
Why does the liver make ketone bodies? (hint: 2 reasons. one is in neonates)
Acetoacetate and B-hydroxybutyrate made by liver serve as excellent fuels for many nonhepatic tissues, such as cardiac and skeletal muscle, especially when glucose is in short supply (starvation) or when inefficiently used (insulin deficiency).
Why does liver make ketone bodies?
o During prolonged starvation, the ketone bodies replace glucose as the major fuel for the CNS, which cannot get energy from fatty acid oxidation.
o Also, during neonatal development, acetoacetate and B-hydroxybutyrate serve as precursors for cerebral lipid synthesis.
Use of ketone bodies requires that acetoacetate first be reactivated to its CoA derivative (acetoacetyl CoA). This is done by a mitochondrial enzyme, acetoacetate-succinyl CoA transferase, present in most non-hepatic tissues but absent from liver.
Through action of B-ketothiolase, acetoacetyl CoA is then converted into acetyl CoA, which in turn enters the TCA cycle.
What are the 3 possible fates of Acetyl CoA in the liver?
What are the differences btwn the fates of ACoA under insulin and glucagon stimulation?
In the liver, ACoA can go through the TCA cycle for energy production, go to the cytosol for cholesterol synthesis (HMG CoA) or be used for FA synthesis.
Insulin: glycolysis –> AcCoA –> energy + FA + cholesterol
Glucagon: AAs and FAs –> energy + ketone body
