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