lipid and fatty acid metabolism Flashcards
describe digestion and absorption
- Dietary fats consumed through diet
- Emulsified by bile salts into fat droplets
- Triacylglycerols degraded by lipases producing FREE fatty acids
- Free Fatty Acids are absorbed in epithelial cells triacylglycerols then synthesised
- Triacyclglycerols are insoluble in blood and lymph therefore transported via chylomicrons (LIPOPROTEIN)
- Transported to MUSCLE for ENERGY (or ADIPOSE TISSUE for STORAGE)
what happens in troglycerol breakdown
triclyglycerol is converted to glycerol
tricylglycerol = diacylglycerol (=lipase is removed)
diacylglycerol= monoacylglycerol (=lipase removed)
monoaacylglycerol = glycerol (=lipase removed)
what happens in glycerol breakdown in the liver
glycerol is converted to dihydroxyacetone phosphate
glycerol = glycerol-3-phosphate = dihydroxyacetone phosphate
1st = glycerol kinase (ATP = ADP)
2nd = glycerol-3-phosphate dehydrogenase (NADH+ = NADH+ H+)
What is dihydroxyacetone phosphate
an intermediate in glycolysis used for energy production
what happens if blood glucose is low in triacylglycerol breakdown
energy is still needed and therefore breakdown of triacylglycerol
what happens if blood glucose is high in triacylglycerol breakdown
insulin responds to high glucose levels and inactivates lipase and stops lipolysis
what is fatty acid breakdown
called beta oxidation and occurs in the mitochondria (somoetimes the peroxisomes) and is a reoccurring cycle that has 4 steps and it produces FADH2 AND NADH
What happens in the activation step
FA is activated by a coenzyme A resulting in an acyl-CoA. This allows FA and acyl CoA to diffuse into the mitochondria.
what is the first step
Oxidation
Removes a pair of hydrogen atoms from the FA
Reduction of FAD to FADH2 (2ATP)
Outcome = forms a C=C bond on the fatty acyl chain
what is the second step
hydration
Hydration of the double bond
Beta-carbon is hydroxylated
what is the third step
Oxidation reaction
Hydroxyl group of the beta-carbon is now hydrogenated
NAD+ is reduced to NADH –
what is the fourth step
Final reaction
Catalysed by thiolase
Cleavage and release of Acetyl CoA
The thiol is inserted between C-2 (alpha) and C-3 (beta)
An acetyl CoA molecule, and an acyl CoA molecule that is two carbons shorter
what are peroxisomes
small organelles with single membrane and found in cytoplasm.
Longer chain FAs are initially broken down into shorter units in peroxisomes
Partly degraded FAs move to mitochondria where process is complete
what happens in the first step of fatty acid breakdown in peroxisomes
Step 1 – enzyme converts electrons to H20 producing Hydrogen peroxide
Catalase converts hydrogen peroxide back to water and oxygen species
Associated with variety of toxic compounds such as phenols and alcohol
what are the main points for tricylglycerol breakdown
broken down by lipases (releasing FAs to be further degraded)
Glycerol further degradation
Fatty acid breakdown (beta-oxidation):
Occurs in mitochondria or peroxisomes
Activation step Fatty acids converted to Acyl CoA derivatives
Carnitine used for longer and/or unsaturated FA
4 key steps
Outcome to yield ENERGY
Glycolysis and Beta-oxidation occur at the same rate
Steady supply of pyruvate oxaloacetate
An adequate supply of oxaloacetate is needed for acetyl CoA (from beta-oxidation) to efficiently enter TCA/Kerb cycle
Low supply of carbohydrates energy comes from FA degradation
Ketone bodies are produced from Acetyl CoA
what are three examples of ketone bodies
Acetone
Acetoacetic acid
Beta-hydroxybutyric acid
what are ketones functions
Ketone bodies can be used by the heart and brain for energy
how are fatty acids synthesised from acetyl-CoA and malonyl CoA
through fatty acid synthase FAS enzymes
where does fatty acid synthase occurs
Occurs in the cytosol, when carbohydrate and energy supplies are plentiful
what is fatty acid synthase not
a simple reverse of beta oxidation
what cannot be synthesised and has to be obtained from the diet
Essential PUFA (alpha-linolenic and linoleic acid)
what is the first stage of fatty acid biosynthesis
Elongation/Condensation of acetyl-ACP to malonyl-CoA to form acetoacetyl-ACP, a ketoacid, releasing CO2
what is the second stage of fatty acid biosynthesis
Reduction of acetoacetyl-ACP to form β-hydroxy acid derivative, using NADPH
what is the third stage of fatty acid biosynthesis
Dehydration to make a trans-double bond
what is the fourth stage of fatty acid biosynthesis
Reduction by NADPH to form the saturated fatty acid butyryl-ACP (4 C)
what number can humans synthesise
Humans can only synthesise up to 16-carbons long fatty acids
C18 fatty acids (i.e., ESSENTIAL) only obtained in DIET!
what 2 acids are obtained from dietary sources and are deemed essential
Linoleic acid (LA) (C18 n-6)
Alpha-linolenic acid (ALA) (C18 n-3)
what acids obtained from the diet are not termed essential
Arachidonic acid (AA) (C20 n-6)
Eicosapentaenoic acid (EPA) (C20 n-3)
Docosahexaenoic acid (DHA) (C22 n-3)
what are the key differences between beta oxidation and fatty acid biosynthesis
beta= occurs in mitochondria, has 4 non associated enzymes, 2Cs removed as acetyl CoA, starts at the carboxyl end, occurs in muscle and liver, nutritional status is starvation and has a low insulin/glucagon ratio.
bio= occurs in cytosol, a multi enzyme complex, 2Cs added using malonyl CoA, starts at methyl end, occurs in liver, nutritional status is well-fed and has a high insulin/glucagon ratio
what occurs in Triacylglycerol synthesis/storage
Adipose tissue STORAGE
Muscle cells ENERGY
liver
Major role in regulation of blood glucose concentrations
role in lipid metabolism as Excess fuel
liver converts FA to ketones
brain
Normal conditions – brain uses glucose as source of energy
Starvation depletes glycogen stores
ketone bodies (acetoacetate and beta-hydroxybutyrate) are used as alternative energy source
muscle tissue
Energy demands of resting muscle is usually via beta-oxidation
Working muscle – energy obtained from degradation of own supply of glycogen
Heart muscle prefers ketone bodies over glucose
adipose tissue
Adipose tissue is a major storage depot of fatty acids
Triglycerides released from liver are carried by VLDL and absorbed by adipose tissue
