Lipid Metabolism Flashcards
1
Q
What are fatty acids stored as?
A
Fatty acids stored in the form of triaclglycerols
2
Q
What does a condensation reaction combine?
A
1 glycerol and 3 fatty acids
3
Q
What do triacylglycerols act as?
A
Act as the bodies major energy reserves
4
Q
What does the release of fatty acids from triaclyglycerols require?
A
Requires hydrolysis
5
Q
What is hydrolysis?
A
- 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
6
Q
how are triacylglycerols formed
A
Condensation reaction combines 1 Glycerol and 3 Fatty Acids
- Hydroxyl group (-OH)
- Carboxyl group (-COOH)
7
Q
explain hormone sensitive lipase
A
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
8
Q
triacylglycerols act as the body’s what
A
major energy reserve
When hydrolysed, fatty acids released can yield 9kcal/g
Carbohydrates and amino acids only yield 4kcal/g
9
Q
how are Fatty Acids released from Triacylglycerols
A
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
10
Q
Explain adipose triglyceride
A
- 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
11
Q
Explain hormone sensitive lipase
A
- 786 amino acids
- weight= 85.5 kDa
- it is cytosolic
- Hydrolysis occurs in 2 steps
12
Q
What are the 2 steps of hydrolysis in hormone sensitive lipase?
A
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
13
Q
Explain monoacylglycerol lipase
A
- 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
14
Q
Why can glycerol not be metabolised by adipocytes? And what happens instead
A
- 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
15
Q
What is the fate of glycerol dependent on?
A
- the conversion of glycerol 3-phosphate to dihydroxyacetone phosphate
- done by glycerol 3-phosphate dehydrogenase
16
Q
What are the 2 possible fates of dihydroxyacetone phosphate?
A
- Glucose
- Pyruvate
As it is a gycolytic/gluconeogenic intermediate
17
Q
What can glycerol 3-phosphate be used to produce
A
- more triaclyglycerides
18
Q
Where and how are the triacylglycerides produced from glycerol 3-phosphate stored?
A
- Stored in liver
- not in its raw state
- will be stored as very low density lipoprotien (VLDL)
19
Q
Give a brief description of a lipoprotien
A
- like the fat alternative for storing glycogen
- basically a membrane that stores fat
20
Q
Where do free fatty acids move through?
A
- Free Fatty Acids (FFAs) move into the bloodstream
-do this through the plasma membrane of the adipocyte
21
Q
When free fatty acids are in the blood what percentage binds to where?
A
- When in the blood:
99% of FFAs bind to albumin
22
Q
How many binding sites does albumin have for fatty acids? and why?
A
- Albumin has 7 binding sites for fatty acids
- concentrates the amount of fatty acids in one place
23
Q
What are 2 characteristics of free fatty acids?
A
- Only free fatty acids can enter cells
- Fatty acids are quite big
24
Q
A
25
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 +)
26
Explain how fatty acids cross the plasma membrane.
1. Small fatty acids can cross by diffusion
2. CD36 (fatty acid translocase) or FABPpm (fatty acid binding protein) increase the local concentration of fatty acids, increasing diffusion events
3. When bound to the cytosolic form of FABP, CD36 acts as a shuttle for fatty acids
4. Fatty acid transport proteins transport fatty acids ( 10-15C in length). Where they activate Acyl-CoA synthetase to form Acyl-CoA esters
5. Very long chain fatty acids (>22C) are transported by FATPs and directly converted to Acyl-CoA esters)
27
What do all 3 fates of glycerol require?
All require the production of glycerol 3-phosphate by glycerol kinase
28
What are the two fates of fatty acids?
1. Crossing the Plasma Membrane
2. Activation
29
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)
30
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
31
Once the Acyl-CoA has been formed it can follow one of two paths: What are they?
1. Re-esterification into Triacylglycerols
2. Β-oxidation to the Citric Acid Cycle and Ketone Bodies
32
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.
33
Where does β-Oxidation take place?
- takes place in the mitochondrial matrix
34
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
35
What is the rate limiting step of fatty acid oxidation?
β-oxidation of Fatty Acids: Transport into the mitochondria
36
Why is the following considered the rate limiting step of fatty acid oxidation?
- because fatty acyl CoA molecules cannot diffuse into the mitochondrial
37
What does the The Carnitine Shuttle do?
- it mediates the passage of fatty acyl CoA from the cytosol into the mitochondrial matrix
38
How is acyl carnitine formed?
- Carnitine replaces CoA in the fatty acyl CoA
- replaces by carnitine palmitoyltransferase I (CPT I)
39
Acyl carnitine is transported through the inner mitochondrial membrane by what?
a translocase
40
In the mitochondrial matrix, what converts the acyl carnitine to fatty acyl CoA
carnitine palmitoyltransferase II (CPT II)
41
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
42
How many reaction does the β-oxidation of Fatty Acids comprise of?
- Comprises 4 reactions
43
what is the net result of β-oxidation of Fatty Acids
- The fatty acid is shortened by two carbon atoms
44
Each β-Oxidation cycle produces what?
1 acetyl CoA
1 Fatty Acyl CoA (minus 2C)
1 NADH
1 FADH2
45
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**
46
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+)
47
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
48
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
49
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)
50
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.
51
What are ketone bodies used as?
a fuel of starvation
52
What do ketone bodies include?
- Acetoacetate
- 3-hydroxybutyrate
- Acetone (not metabolisable)
53
Where are Ketone bodies synthesised?
- in the mitochondria of liver cells, transported in the blood to peripheral tissues
54
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
55
Acetyl CoAs can be joined together to form what?
Ketone bodies
56
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.
57
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 ______
1. CPT-I
2. Acetyl CoA
3. via beta-oxidation.
58
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
59
What is the enzyme Thiolase also known as?
acetyl coenzyme A acetyltransferase (ACAT).
60
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.
61
Ketone bodies: Fuel of starvation
Step 4
1. 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
62
What does HMG in full terms?
HMG = β-Hydroxy β-methylglutaryl-
63
The liver cannot use ketone bodies as fuel, explain why.
As the liver cannot reconvert acetoacetate into acetoacetyl CoA
64
All other cells that have mitochondria can oxidise what 2 things?
1. acetoacetate
2. 3-hydroxybutyrate
65
3-hydroxybutyrate is oxidised to acetoacetate by what?
3 -hydroxybutyrate dehydrogenase
66
Acetoacetate is converted back to what? via what enzymes?
1. Acetyl-CoA
2. Beta-ketoacyl-CoA transferase and thiolase
67
Where is acetone excreted due to it not converting back to Acetyl-CoA?
excreted through urine or exhaled
68
A large proportion of fatty acids used by the body are supplied by what?
The diet
69
What 2 things obtained in excess amounts through the diet can be converted into fatty acids?
In turn what can this be incorporated into?
1. Excess amounts of -CHO and protein obtained from the diet
2.Incorporated into Dietary triacylglycerols (TAGs.)
70
In humans, where does fatty acid synthesis primarily occur?
In the liver and lactating mammary glands
71
The fatty acid synthesis process incorporates carbons from where? into what chain?
-Carbons come from the acetyl CoA
- Incorporates carbons into the growing fatty acid chain
72
The fatty acid synthesis process utilises what 2 things?
1. ATP
2.Reduced nicotinamide adenine dinucleotide phosphate
(NADPH)
73
What pathway is NADPH produced in?
Pentose Phosphate Pathway
74
What is produced in step 1 of fatty Acid Synthesis
Step 1= Production of Cytosolic Acetyl-CoA
75
What is step 1 of FA synthesis?
- Transfer of acetyl units from mitochondrial acetyl CoA to the cytosol
- Forms cytosolic acetyl CoA
76
CoA moiety cannot cross what? And in contrast what can?
1. CoA moiety cannot cross the mitochondrial membrane
2. Acetyl can
77
Transfer of acetyl units from mitochondrial acetyl CoA to the cytosol is achieved by what?
-Condensation of oxaloacetate and acetyl CoA
-Forms citrate (TCA cycle)
-and when mitochondrial concentration citrate is high
78
Citrate spills out from the mitochondrial matrix into where? via what?
1. Cytosol
2. Via a transporter
the mitochondrial citrate transporter (MCT)
79
What transporter is used for citrate to go from the mitochondrial matrix to the cytosol?
the mitochondrial citrate transporter (MCT)
80
Cytosolic citrate is then cleaved by what to produce what?
1. Cleaved by ATP citrate lyase
2. produces acetyl CoA
81
What is the main thing that occurs in step 2 of Fatty Acid Synthesis?
Carboxylation of Acetyl-CoA to form Malonyl-CoA
82
The energy for the carbon-to-carbon condensations in FA synthesis is supplied by what processes?
Carboxylation + decarboxylation of acetyl groups in the cytosol
83
What does the carboxylation of acetyl CoA form?
malonyl CoA
84
The carboxylation of acetyl CoA forms malonyl CoA. What catalyses this reaction and what else does the reaction require?
- Catalysed by acetyl CoA carboxylase
- Reaction requires ATP
85
The rest of the steps in fatty acid synthesis are catalysed by what?
A multienzyme complex: fatty acid synthase (FAS)
86
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
87
The β-ketoacyl synthase (KS) catalyses what?
- Catalyses the decarboxylative condensation of the acyl intermediate with malonyl-ACP
-forms into b-ketoacyl-ACP intermediate
88
The β-carbon is processed by what?
-Nicotinamide adenine dinucleotide phosphate (NADPH)
–dependent reduction through β-ketoacyl reductase (KR)
89
Β-hydroxyacyl-ACP is dehydrated by a dehydratase (DH) , to form what?
-A b-enoyl intermediate
90
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
91
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)
92
Mono-,Di- and Triacylglycerols consist of one, two or three molecules of fatty acids that are....
Esterified to a molecule of glycerol
93
fatty acids are esterified through their carboxyl groups, resulting in what?
A loss of negative charge and formation of “neutral fat”
94
Are fatty acids on one glycerol molecule usually the same type?
Yes or no?
NO: The fatty acids on one glycerol molecule are usually not of the same type
95
provide what types of fatty acids the 3 carbons are on one glycerol molecule
Carbon 1 - Normally saturated
Carbon 2 - Unsaturated
Carbon 3 - Either
96
Where is triaclyglycerol stored?
- Stored in very low density lipoprotein (VLDLs)
- Inside the cell
97
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 oily droplets
98
In the synthesis of Glycerol-3-phosphate... What is the initial acceptor of fatty acids during the synthysis of TAG
Glycerol phosphate
99
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
100
Why is there only one route to glycerol phosphate synthesis?
As there is no glycerol kinase in adipose cells
101
Adipose cells can only take up glucose in the presence of what?
Insulin
102
Synthesis TAG from glycerol phosphate and fatty acyl CoA:
The pathway involves how many reactions?
Pathway involves 4 reactions
103
What 4 reactions are involved in the pathway for the synthesis TAG from glycerol phosphate and fatty acyl CoA?
- Addition of 2x fatty acids from fatty acyl CoA
- The addition of a third fatty acid
- The removal of phosphate
104
1. 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.
105
2. 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
106
3. 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
107
5. 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
1. Hydrolysis of triacylglycerols
2. Activation of the fatty acid by joining to CoA
3. Conversion of fatty acyl-CoA to a fatty acyl carnitine
4. Acyl-CoA in mitochondrion
5. FAD-linked oxidation
6. NAD+-linked oxidation
7. Thiolysis
8. Acetyl CoA enters the citrate cycle
9. Electron transport and oxidative phosphorylation
108
