3 - Fatty Acid Nomenclature, Modification and Metabolism in the Fed/Fasting State Flashcards
(137 cards)
1
Q
How tightly controlled is entry into fatty acid synthesis?
A
Very unregulated.
2
Q
What causes entry into fatty acid synthesis?
A
An abundance of pyruvate.
3
Q
What is the fate of most pyruvate?
A
Conversion to Acetyl CoA in the mitochondrial matrix by pyruvate dehydrogenase.
4
Q
Describe the action of pyruvate dehydrogenase.
A
Pyruvate + CoASH + NADH
->
Acetyl CoA + CO2 + NAD
5
Q
Why is pyruvate dehydrogenase so tightly regulated?
A
Because it catalyses the decarboxylation of a 3C pyruvate to a 2C Acetyl CoA. In mammals 2C compounds cannot be used in gluconeogenesis.
6
Q
What regulatory strategies are employed on pyruvate dehydrogenase (PDH)?
A
Substrate activation & product inhibition. Energy charge sensitivity. Redox state sensitivity.
+ Pyruvate - Acetyl CoA
+ NAD+ - NADH
+ ADP/AMP - ATP
7
Q
How does pyruvate travel to and from the mitochondrial matrix?
A
Freely.
8
Q
How does Acetyl CoA travel to and from the mitochondrial matrix?
A
Via the citrate-malate shuttle, which is active only when the CAC is saturated.
9
Q
When is Acetyl CoA found in the cytosol?
A
When the aconitase enzyme CAC is saturated, as this allows it substrate, citrate, to diffuse into the matrix and so allow the citrate-malate shuttle to work.
10
Q
What two enzymes could pyruvate be the substrate for in the mitochondrial matrix?
A
Pyruvate dehydrogenase to form Acetyl CoA (Link Reaction).
Pyruvate carboxylase to form oxaloacetate which feeds into the CAC.
11
Q
What enzyme is totally dependent upon Acetyl CoA?
A
Pyruvate carboxylase.
12
Q
Describe the function of pyruvate carboxylase.
A
Pyruvate -> Oxaloacetate
When the citric acid cycle is saturated by Acetyl CoA in the fed state the excess Acetyl CoA activates pyruvate carboxylase to use CO2 and ATP to convert it to oxaloacetate.
13
Q
What does the citrate-malate shuttle transport from the matrix to the cytosol?
A
Acetyl CoA
NADPH
CO2
14
Q
What is the cost of the citrate-malate shuttle per cycle?
A
2x ATP
1x NADH
The latter is arguable offset by production of 1x NADPH
15
Q
What two molecules are responsible for enabling the citrate-malate shuttle by being able to freely move between the matrix and the cytosol?
A
Pyruvate and citrate.
16
Q
What is the NADPH transported into the cytosol by the citrate malate shuttle used for?
A
Synthesis of the fatty acid palmitate from Acetyl CoA.
17
Q
What are the primary and backup sources of NADPH?
A
The CAC and Pentose Phosphate pathway respectively.
18
Q
What two carbon compounds are used to synthesise fatty acid chains, and in what ratio?
A
Malonyl CoA and Acetyl CoA, 7:1
19
Q
Which enzyme converts Acetyl CoA to Malonyl CoA for fatty acid synthesis?
A
Acetyl CoA Carboxylase, which uses ATP and CO2 to carboxylate ACoA.
20
Q
In what state is Acetyl CoA Carboxylase active?
A
Acetyl CoA Carboxylase is only active when it is polymerised.
21
Q
What molecules regulate Acetyl CoA Carboxylase?
A
+ Citrate (only present in cytosol when CAC saturated)
- Malonyl CoA (product inhibition)
- Fatty Acids (Feedback Inhibition)
22
Q
How does Malonyl CoA concentration affect the rate of Beta Oxidation?
A
High [Malonyl CoA] inhibits FA Ox by inhibiting CPTI, which transports the acids into the mitochondrion.
23
Q
What is the structure of the enzyme that produces fatty acids?
A
Fatty Acid Synthase is a giant multifunctional enzyme that uses seven Malonyl CoA and one Acetyl CoA to make a 16C Palmitic Acid.
There are seven catalytic domains, with the nascent chain being bound by a cysteine residue on the ACP domain.
24
Q
The number of carbons in a fatty acid chain will always be…
A
Even, as a single 2C Malonyl CoA is added by each elongation or synthesis event.
25
What is a 'Methylene Interruption'?
The two sigma bonded carbons that must separate each unsaturated double in the fatty acid chain.
26
What three systems are used to name fatty acids?
The Delta Numbering System, the Miller System and the practically identical Omega system.
27
Which nomenclature system is the most common and useful?
The Miller System (or v. similar omega system).
28
What properties of fatty acids do the Miller/Omega systems rely upon?
The areas of unsaturated bonds occurring with methylene interruptions only - no larger gaps and no lack of them.
29
Describe the notation of the Miller System.
(No. of carbons in chain) : (number of double bonds)n - (carbon on which the double bond region begins).
30
From which end of the fatty acid do the Miller and Omega systems number the carbon atoms? Why is this useful?
C1 at the ALIPHATIC end, highest numbered carbon at the acid end.
Because the chains are elongated from the acid end, with these systems only the number of carbons in the chain increases with elongation. The others are constant as the double bond position remains the same W.R.T. the aliphatic end.
31
Describe the notation of the Omega System.
(No. of carbons in chain) : (number of double bonds)w - (carbon on which the double bond region begins).
32
From which end does the Delta numbering system number the carbon atoms?
From the acid end, so the relative position of the unsaturated bonds changes with elongation.
33
Describe the Delta Numbering System notation.
(No. of carbons in chain) : (number of double bonds) ^ D (position of each double bond)
34
What is the Delta System useful for?
Fatty acids with unorthodox double bond positions.
35
What is an essential fatty acid?
One that cannot be synthesised by the body.
36
What are the two main categories of essential fatty acid?
N-3 and N-6.
37
How are phospholipids modified to suit their purpose?
Many double bonds added to phospholipids to increase their fluidity.
38
Where are n-6 fatty acids obtained from?
Terrestrial plants.
39
What is the Miller designation for Linoleic Acid?
18:2n-6
40
What is the Miller designation for Arachidonic Acid?
20:4n-6
41
What is the most common n-6 essential fatty acid?
Linoleic Acid
42
Which is the most important n-6 fatty acid for the body, and what does it do?
Arachidonic acid is used to produce inositol phosphoglyceride signalling molecules and can be cleaved to produce eicosanoid signalling molecules.
43
What stimulates Arachidonic acid to be cleaved to form eicosanoids?
Cytokines.
44
What are two possible products created from the cleavage of Arachdonic acid and what enzyme produces them?
Cyclooxygenase produces A-Prostaglandin, which is involved in pain signalling.
5-Lipoxygenase produces A-Leukotriene.
45
What is the mechanism of the analgesic aspirin?
Inhibition of cyclooxygenase prevents the production of the pain signalling molecule A-prostaglandin from the products of arachidonic acid cleavage.
46
How do n-3 acids enter the human diet?
They are produced by marine plants, are eaten and modified by fish which we eat.
47
What are the two main classes of n-3 acids and how do they arise?
DHA and EPA. They are a result of the modifications made by the fish that obtain them.
48
What is the Miller designation of DHAs?
22:6n-3
49
What is the Miller designation of EPAs?
20:5n-3
50
What are DHAs used for in humans?
They are phospholipids needed for brain and retinal function. They are thought to be involved in the function of membrane embedded proteins.
51
What effect do EPAs have in humans, and by what mechanism?
Anti-thrombotic and anti-inflammatory, by competing with Arachidonic acids for incorporation into phospholipids.
52
What are PUFAs?
Poly-Unsaturated Fatty Acids, an essential acid group.
53
What major process are PUFAs involved in and how?
Gene regulation by binding to nuclear receptors that in turn bind to response elements on the DNA.
54
What metabolic activity do PUFAs inhibit?
Lipogenesis, by repressing the relevant genes.
55
What is an example of the action of a nuclear receptor stimulated by a PUFA?
PPAR. When stimulated it binds a retinoid X receptor (RXR) which then bind to DNA sequences calles Peroxisome Proliferator Hormone Response Elements (PPREs).
56
Where does Fatty Acid Modification take place?
On the smooth endoplasmic reticulum.
57
What are the two main mechanisms of fatty acid modification?
Elongation and desaturation.
58
What enzymes catalyse fatty acid elongation?
Fatty acid elongases: ELOVL 1 to 5.
59
What coenzyme is needed for fatty acid elongation?
NADPH, for the same reduction mechanism as in fatty acid synthesis.
60
What substrates is added to the chain to elongate fatty acids?
Malonyl CoA
61
What are the side products of elongation?
CoASH
CO2
Water
NADP+
62
What two types of fatty acid are produced by elongation for their health benefits?
PUFAs and Arachidonic acids.
63
What are linolenic acids (18:2) extended to form?
Arachidonic acids.
64
Other than synthesis of useful acids, what is fatty acid elongation used for? Give an example of this.
Storage of an excess of useful acids in an elongated form, which can be shortened later when they are needed.
Arachidonic acids (20:4) are often stored as adrenic acids (22:4)
65
What two enzymes are involved in desaturation modification?
Cytochrome b5 Reductase
Desaturase
66
What kind of process is desaturation?
Oxidative.
67
What coenzyme is found with cytochrome b5 reductase?
FAD/FADH2
68
Describe the mechanism of desaturation.
Cytochrome b5 reductase collects 2e- from reduction of NADH and passes them to the b5 electron carrier. b5 releases these electrons to Fe3+ groups on desaturase which uses them and two electrons from the target carbons to reduce an O2 to H2O.
69
What is the acid end of an FA noted as?
The alpha end.
70
What are the three forms of human desaturases named for?
Which number carbon (from the alpha end) the double bond they produce begins on.
71
What are the three human desaturases?
Delta 9, delta 6 and delta 5.
72
What two processes remove two carbons from the fatty acid chain?
Beta oxidation and retroconversion.
73
Where does retroconversion occur?
The peroxisome.
74
Where does beta oxidation occur?
The mitochondrial matrix.
75
What is the primary difference between beta oxidation and retroconversion?
In beta oxidation the reduced FAD is used to fuel oxidative phosphorylation, in retoconversion they are used to reduce water.
76
What are the products of retroconversion per two carbons?
1x FADH2
1x NADH
1x Acetyl CoA
77
What happens to the NADH and Acetyl CoA produced in retroconversion?
They are exported from the peroxisome.
78
What happens to the FADH2 produced in retroconversion?
Immediate reoxidation by water to produce hydrogen peroxide, which itself is immediately removed by catalase.
79
What substrates are used in the process of retroconversion?
1x H2O
1x CoASH
2 carbons from the fatty acid chain.
80
What end of the fatty acid chain are carbons removed from?
The alpha acid end.
81
What two ways can the liver obtain fatty acids?
Uptake from the blood or de novo synthesis.
82
What is the primary site of TAG synthesis in the fed state?
The small intestine.
83
What two glycolytic products can be added to fatty acids to produce TAG?
Glycerol 3-phosphate (directly) and Dihydroxyacetone phosphate (after reduction).
84
By what mechanism is glycerol converted to glycerol 3-phosphate?
Glycerol Kinase
| 1x ATP used.
85
By what mechanism is DHAP converted to glycerol 3-P?
Glycerol 3-P Dehydrogenase
| 1x NADH used
86
What pathway synthesised TAG?
The glycerol phosphate pathway.
87
What is the most common fate of hepatic produced TAG?
Secretion into the blood in VLDLs.
88
What is often used to produce VLDLs?
Diacylglycerols
89
What non-glycolytic product can be used to produce glycerol 3-phosphate?
Glycerol!
90
What are the products of TAG digestion?
2x Fatty acids
| 1x Monoacyl glycerol
91
What process produces TAG in the small intestine?
The Monoacylglycerol Pathway
92
What modification must be made to digested fatty acids before they can enter the monoacylglycerol pathway?
They must have CoASH added to their alpha end by double hydrolysis of an ATP to AMP.
93
What are the substrates used to produce TAG by the Monoacylglycerol pathway?
2x fatty acyl CoA
| 1x 2-monoacylglycerol
94
What are the roles of the two enzymes involved in the monoacylglycerol pathway?
Monoacylglycerol Acyltransferase - adds the first fatty acyl CoA to MAG make DAG.
Diacylglycerol Acyltransferase - adds the second fatty acyl CoA to DAG to make TAG.
95
What is the fate of TAG synthesized in the small intestine?
Packages into chylomicrons and released into the lymphatic system via the lacteal villi.
96
Where does the lymphatic system drain into the blood?
The thoracic duct.
97
In the fed state, what is the concentration of FFAs in the blood?
High
98
How does adipose tissue respond to insulin?
Increased FA uptake by LPL expression.
Increased FA synthesis.
Increased TAG synthesis/storage.
Increased glucose uptake through increased GLUT4 expression.
99
How does adipose tissue regulate glycolysis?
In the same manner as the liver.
100
What are the consequences of diabetes on adipose tissue in the fed state?
GLUT4 not expressed properly, so little glucose taken up. Even more FAs released into the blood.
101
What does the adipose tissue use to take up fatty acids (free or lipoprotein based)?
Lipoprotein lipase.
102
What pathway is used by the adipose tissue to synthesise TAG? Where else is this pathway used?
The glycerol phosphate pathway, as found in the liver.
103
What is the fate of adipose tissue synthesised TAG?
Storage in the lipid droplet.
104
What protein encases the adipose cell lipid droplet?
Perilipin.
105
What liver-expressed enzyme is not present in the adipose tissue TAG synthesis pathway? What are the consequences of this?
Glycerol kinase. Means that all glycerol 3-P needed for TAG synthesis must be from DHAP and glycerol 3-P itself produced by glycolysis.
106
How does insulin regulate adipose tissue LPL concentration?
By stabilising the mRNA transcript.
107
What is the fate of adipose expressed LPL?
It migrates to the capillaries.
108
How long is the delay between adipose LPL expression and its proper positioning? Why is this useful?
2 hours. This is the same time taken for chylomicrons to be released into the blood by the small intestine.
109
Describe the Km of adipocytic LPL. Why is this so?
High, to allow variation in fatty acid uptake across the physiological range of chlyomicron concentration.
110
What is the main action of LPL?
1 - Extraction of TAG from chlyomicrons.
| 2 - Lipolysis of TAG to fatty acids and glycerol.
111
What is the fate of the products of LPL?
Fatty acids taken up by adipose tissue for TAG synthesis. Glycerol returned to the liver.
112
Why is the glycerol produced by LPL at the adipose tissue not used in adipose TAG synthesis?
Adipose tissue does not possess glycerol kinase so cannot convert it to Glycerol 3-phosphate.
113
When in the glycerol phosphate pathway is the phosphate on the glycerol 3-P removed?
After the first two fatty acids have been esterified.
114
What is the role of hormone sensitive lipase?
Extraction of TAG from the lipid droplet and subsequent lipolysis to monoacyl glycerol.
115
What two proteins are regulated to control lipolysis?
Hormone sensitive lipase and Perilipin.
116
When HSL is phosphorylated it is...
Active!
117
What effect does insulin have on HSL?
Dephosphorylation and subsequent inhibition.
118
What effect does adrenaline and lack of insulin have on HSL?
Phosphoactivation.
119
How does perilipin regulate lipolysis?
By either blocking or allowing access to the lipid droplet for HSL.
120
When perilipin is phosphorylated it is...
Open, allowing access to HSL and hence increasing lipolysis rates.
121
What effect does insulin have on perilipin?
Dephosphorylation and subsequent closing/blocking activity.
122
What effect does adrenaline and low insulin levels have on perilipin?
Phosphorylation and subsequent opening to HSL.
123
What happens to the two fatty acids removed from TAG by HSL?
They are released into albumin.
124
Which adipocyte enzyme breaks down Monoacyl Glycerol to glycerol and a fatty acid?
MAG Lipase.
125
What is the fate of the last fatty acid removed from TAG in the adipose tissue?
It is released into the plasma.
126
How is MAG Lipase regulated?
It isn't, as its activity is dependent upon the product of HSL which is regulated by insulin and adrenaline.
127
How much lipolysis is occuring in the fed state?
A small baseline level, but the fatty acids are quickly re-esterified.
128
Why is there little muscular beta oxidation in the fed state before any direct regulation even occurs?
There are less fatty acids available as the adipose tissue is not secreting them.
129
By what two indirect mechanisms is adipocyte lipogenesis inhibited in the fasting state?
1 - Decrease in glycerol 3-P supply due to lack of glycolysis.
2 - Decrease in fatty acid availability as without insulin Lipoprotein Lipase is not expressed.
130
What enzymes respond to different regulation in different tissues in order to specify the FA uptake of a tissue?
Lipoprotein Lipase.
131
What is the Km of muscle Lipoprotein lipase? Why is this useful?
Low, giving them priority over adipose tissue on FA uptake and making it dependent on [LPL] concentration as opposed to [lipoprotein].
132
How is fatty acid oxidation stimulated?
It isn't, its activity is dependent only on concentration of substrates and lack of Malonyl CoA inhibition of CPTI.
133
What is the function of CPTI? What inhibits this?
Transporting fatty acids into the mitochondrial matrix for beta oxidation.
Inhibited by Malonyl CoA.
134
When is beta oxidation not inhibited?
When [Malonyl CoA] is low due to lack of glycolysis in the fasting state and fatty acid inhibition of Acetyl CoA Carboxylase, meaning it is not inhibitng CPTI.
135
How is fatty acid synthesis inhibited during beta oxidation?
Fatty acids inhibit Acetyl CoA Carboxylase (by preventing its polymerisation), which produces the Malonyl CoA substrate.
136
What limits the rate of fatty acid oxidation in most tissues?
The concentration of the enzymes involved.
137
Why is the brain particularly slow at performing beta oxidation?
Very low production of thiolase.
