BC - II Unit 3 Slide 1 Flashcards
(180 cards)
1
Q
Primary site of fatty acid synthesis in yeast and vertebrate cells?
A
Cytosol
2
Q
Primary site of fatty acid synthesis in higher plant cells?
A
Plastids (stroma of chloroplasts and other plastids)
3
Q
Main source of NADPH for fatty acid synthesis in the cytosol of yeast and vertebrate cells?
A
Pentose phosphate pathway and malic enzyme
4
Q
Main source of NADPH for fatty acid synthesis in the plastids of higher plant cells?
A
Photosynthetic light reactions and plastidial pentose phosphate pathway
5
Q
Primary site of fatty acid oxidation (beta-oxidation) in vertebrate cells?
A
Mitochondria (peroxisomes also involved in very long-chain fatty acids)
6
Q
Primary site of fatty acid oxidation (beta-oxidation) in yeast cells?
A
Peroxisomes
7
Q
Sites of fatty acid oxidation (beta-oxidation) in higher plant cells?
A
Mitochondria and Peroxisomes
8
Q
Primary site of triacylglycerol (TAG) synthesis in yeast and vertebrate cells?
A
Endoplasmic Reticulum (ER)
9
Q
Storage form of TAGs in yeast and vertebrate cells?
A
Lipid Droplets (bud off from the ER)
10
Q
Primary site of triacylglycerol (TAG) synthesis in higher plant cells?
A
Endoplasmic Reticulum (ER)
11
Q
Storage form of TAGs in higher plant cells?
A
Oil Bodies (Oleosomes)
12
Q
Primary site of phospholipid and cholesterol synthesis in yeast and vertebrate cells?
A
Endoplasmic Reticulum (ER)
13
Q
Site of glycolipid synthesis in higher plant cells?
A
Plastids
14
Q
Why is the subcellular localization of fatty acid synthesis important in relation to NADPH?
A
Fatty acid synthesis is a reductive process requiring NADPH. The pathway is localized in compartments where NADPH is readily available.
15
Q
Primary location for the digestion and absorption of dietary lipids in vertebrates?
A
Small Intestine (specifically the jejunum and ileum for absorption)
16
Q
What is the role of bile salts in dietary lipid digestion in the small intestine? How do they achieve this?
A
Emulsification: Bile salts are amphipathic molecules that break down large, hydrophobic fat globules into smaller droplets (micelles). Their hydrophobic regions interact with fats, while hydrophilic regions interact with the aqueous environment, increasing the surface area for enzymatic action.
17
Q
List the major pancreatic enzymes involved in dietary lipid digestion and their primary substrates.
A
Pancreatic Lipase: Hydrolyzes triglycerides into 2-monoacylglycerol (MAG) and free fatty acids.
Cholesterol Esterase: Hydrolyzes cholesterol esters into cholesterol and free fatty acids.
Phospholipase A2: Hydrolyzes phospholipids into lysophospholipids and a free fatty acid.
18
Q
What are micelles and why are they important for lipid absorption in the small intestine?
A
Micelles are small, spherical aggregates formed by bile salts and the products of lipid digestion (MAG, free fatty acids, cholesterol, lysophospholipids). They are crucial for transporting these relatively hydrophobic molecules through the aqueous environment of the intestinal lumen to the surface of the enterocytes for absorption.
19
Q
Describe the process by which digested lipids (MAG, fatty acids, cholesterol) are taken up by the enterocytes.
A
At the brush border membrane of the enterocytes, the digested lipids diffuse out of the micelles and across the plasma membrane into the enterocytes. Bile salts are mostly left behind in the lumen to be reabsorbed later in the ileum.
20
Q
What happens to long-chain fatty acids and 2-monoacylglycerol inside the enterocytes?
A
They are re-esterified to form triglycerides. Long-chain fatty acids are first activated by the addition of Coenzyme A (CoA) to form fatty acyl-CoA, which then reacts with 2-monoacylglycerol.
21
Q
What happens to cholesterol inside the enterocytes?
A
It is re-esterified with fatty acids to form cholesterol esters.
22
Q
What are chylomicrons and what is their composition?
A
Chylomicrons are large lipoprotein particles synthesized in the enterocytes. They are composed primarily of triglycerides and cholesterol esters, along with phospholipids and apolipoprotein B-48. They serve to transport dietary lipids.
23
Q
How do chylomicrons enter the bloodstream? Why do they take this route?
A
Chylomicrons are too large to directly enter the bloodstream. They are secreted from the basolateral membrane of the enterocytes into the lymphatic system, entering lacteals. The lymphatic vessels eventually drain into the bloodstream via the thoracic duct.
24
Q
What is lipoprotein lipase (LPL) and where is it located?
A
Lipoprotein lipase (LPL) is an enzyme anchored to the endothelial surface of capillaries in various tissues, including muscle and adipose tissue.
25
How does lipoprotein lipase (LPL) facilitate the delivery of fatty acids to muscle and adipose tissues from chylomicrons?
LPL recognizes and binds to apolipoprotein C-II on the surface of chylomicrons. It then hydrolyzes the triglycerides within the chylomicrons into glycerol and free fatty acids.
26
What happens to the free fatty acids released by LPL in muscle tissue?
They are taken up by muscle cells and can be oxidized for energy production through beta-oxidation.
27
What happens to the free fatty acids released by LPL in adipose tissue?
They are taken up by adipocytes and re-esterified with glycerol (derived from glucose metabolism) to form triglycerides, which are stored as fat droplets.
28
What happens to the glycerol released by LPL?
It is transported to the liver and kidneys, where it can be used in glycolysis or gluconeogenesis.
29
What are chylomicron remnants and what is their fate?
Chylomicron remnants are the particles remaining after most of the triglycerides have been removed by LPL. They are relatively enriched in cholesterol and are taken up by the liver via receptor-mediated endocytosis.
30
What is the overall structure of a chylomicron? What is its main function?
A lipoprotein particle with a surface layer of phospholipids and apolipoproteins surrounding a core primarily composed of triacylglycerols. Its main function is to transport dietary fats from the intestines to other tissues.
31
Describe the surface layer of a chylomicron. What type of lipid forms this layer and how is it oriented?
A monolayer of phospholipids. The hydrophilic head groups face the aqueous phase (blood/lymph), while the hydrophobic tails point inwards, interacting with the lipid core.
32
What is the primary component of the chylomicron's interior? Approximately what percentage of its mass does it constitute?
Triacylglycerols (TAGs). They make up more than 80% of the chylomicron's mass.
33
What are apolipoproteins? Where are they located on a chylomicron?
Proteins that are embedded within the phospholipid surface layer and protrude outwards into the aqueous environment.
34
Name three important apolipoproteins found on chylomicrons.
Apolipoprotein B-48, Apolipoprotein C-II, and Apolipoprotein C-III.
35
What is the role of Apolipoprotein B-48 on chylomicrons?
Essential for the secretion of chylomicrons from the enterocytes. It also serves as a structural component.
36
What is the role of Apolipoprotein C-II on chylomicrons?
It acts as an activator of lipoprotein lipase (LPL), which hydrolyzes triglycerides in capillaries.
37
What is the role of Apolipoprotein C-III on chylomicrons?
It acts as an inhibitor of lipoprotein lipase and plays a role in the hepatic uptake of chylomicron remnants.
38
What is the typical diameter range of chylomicrons? How does this relate to their composition?
Approximately 100 to 500 nm. Their large size is due to their high triglyceride content.
39
40
What is the overall structure of a chylomicron? What is its main function?
A lipoprotein particle with a surface layer of phospholipids and apolipoproteins surrounding a core primarily composed of triacylglycerols. Its main function is to transport dietary fats from the intestines to other tissues.
41
Describe the surface layer of a chylomicron. What type of lipid forms this layer and how is it oriented?
A monolayer of phospholipids. The hydrophilic head groups face the aqueous phase (blood/lymph), while the hydrophobic tails point inwards, interacting with the lipid core.
42
What is the primary component of the chylomicron's interior? Approximately what percentage of its mass does it constitute?
Triacylglycerols (TAGs). They make up more than 80% of the chylomicron's mass.
43
What are apolipoproteins? Where are they located on a chylomicron?
Proteins that are embedded within the phospholipid surface layer and protrude outwards into the aqueous environment.
44
Name three important apolipoproteins found on chylomicrons.
Apolipoprotein B-48, Apolipoprotein C-II, and Apolipoprotein C-III.
45
What is the role of Apolipoprotein B-48 on chylomicrons?
Essential for the secretion of chylomicrons from the enterocytes. It also serves as a structural component.
46
What is the role of Apolipoprotein C-II on chylomicrons?
It acts as an activator of lipoprotein lipase (LPL), which hydrolyzes triglycerides in capillaries.
47
What is the role of Apolipoprotein C-III on chylomicrons?
It acts as an inhibitor of lipoprotein lipase and plays a role in the hepatic uptake of chylomicron remnants.
48
What is the typical diameter range of chylomicrons? How does this relate to their composition?
Approximately 100 to 500 nm. Their large size is due to their high triglyceride content.
49
Explain the overall purpose of the chylomicron's molecular structure in the context of lipid transport.
The phospholipid surface allows transport in the aqueous bloodstream. The triacylglycerol core carries the hydrophobic dietary fats. The apolipoproteins act as signals for uptake and metabolism by different tissues and enzymes.
50
What hormone, triggered by low blood glucose, initiates the mobilization of triacylglycerols in adipose tissue as shown in Figure 17-3?
Glucagon
51
According to Figure 17-3, where does the hormone bind on the adipocyte to initiate lipolysis?
To a specific Receptor on the adipocyte membrane.
52
Following hormone binding, what enzyme is activated in the adipocyte membrane via a G protein (Gs)? What second messenger is produced? (Figure 17-3, step 2)
Adenylyl cyclase is activated, leading to the production of cAMP.
53
What enzyme is activated by the increase in cAMP levels within the adipocyte? (Figure 17-3, step 3)
Protein Kinase A (PKA)
54
What two key proteins are phosphorylated by PKA in the vicinity of the lipid droplet? (Figure 17-3, step 4)
Hormone-Sensitive Lipase (HSL) and Perilipin.
55
How does the phosphorylation of perilipin contribute to lipolysis? (Figure 17-3, step 4 & 5)
Phosphorylation of perilipin alters its structure, allowing hormone-sensitive lipase (HSL) to gain access to the stored triacylglycerols on the surface of the lipid droplet.
56
What is the primary enzymatic activity of activated hormone-sensitive lipase (HSL)? What are the products? (Figure 17-3, step 5)
HSL hydrolyzes triacylglycerols stored in the lipid droplet, releasing fatty acids and glycerol (glycerol not explicitly shown exiting in this figure).
57
How are the released fatty acids transported in the bloodstream away from the adipocyte? (Figure 17-3, step 6)
They bind to serum albumin, a major protein in the blood plasma, which acts as a carrier.
58
How do fatty acids enter a muscle cell (myocyte) from the bloodstream? (Figure 17-3, step 7)
Via specific fatty acid transporters embedded in the myocyte plasma membrane.
59
What metabolic process do fatty acids undergo in the myocyte to generate energy? What are the key products? (Figure 17-3, step 8)
β-oxidation in the mitochondria, producing acetyl-CoA, NADH, and FADH2. Acetyl-CoA then enters the citric acid cycle, and the reducing equivalents fuel the respiratory chain to produce ATP. CO2 is a byproduct.
60
Where in the animal cell does fatty acid oxidation (beta-oxidation) occur?
The mitochondrial matrix.
61
How do fatty acids with chain lengths of 12 or fewer carbons enter the mitochondria?
They can cross the mitochondrial membranes without the help of membrane transporters.
62
What mechanism is required for fatty acids with 14 or more carbons to enter the mitochondrial matrix?
The carnitine shuttle.
63
What is the first step of the carnitine shuttle and where does it take place? What enzyme catalyzes this reaction?
Activation of the fatty acid by the attachment of Coenzyme A (CoA) to form fatty acyl-CoA. This occurs in the outer mitochondrial membrane (cytosolic side) and is catalyzed by acyl-CoA synthetases.
64
What are the two potential fates of fatty acyl-CoA formed on the cytosolic side of the outer mitochondrial membrane?
1. It can be used in the cytosol to synthesize membrane lipids.
2. It can be transported into the mitochondrion for oxidation via the carnitine shuttle.
65
What is the second step of the carnitine shuttle? What enzyme catalyzes this reaction and where is it located?
Transesterification of the acyl group from fatty acyl-CoA to carnitine, forming fatty acyl-carnitine. This is catalyzed by carnitine acyltransferase I (CAT I), located in the outer mitochondrial membrane.
66
How does fatty acyl-carnitine move from the cytosolic side to the intermembrane space?
Either the acyl-CoA passes through the outer membrane and is converted to carnitine ester in the intermembrane space, or the carnitine ester is formed on the cytosolic face and moves through pores formed by porin in the outer membrane.
67
How does fatty acyl-carnitine enter the mitochondrial matrix from the intermembrane space? What transporter is involved?
By facilitated diffusion through the acyl-carnitine/carnitine transporter (CACT) located in the inner mitochondrial membrane. This transporter exchanges fatty acyl-carnitine for free carnitine.
68
What is the third and final step of the carnitine shuttle? What enzyme catalyzes this reaction and where is it located?
The fatty acyl group is enzymatically transferred from carnitine back to intramitochondrial Coenzyme A, regenerating fatty acyl-CoA in the matrix. This is catalyzed by carnitine acyltransferase II (CAT II), located on the inner face of the inner mitochondrial membrane.
69
What are the primary functions of Coenzyme A in the mitochondrial matrix versus the cytosol?
Mitochondrial CoA: Used in the oxidative degradation of pyruvate, fatty acids, and some amino acids.
Cytosolic CoA: Used in the biosynthesis of fatty acids.
70
What is the rate-limiting step for the oxidation of long-chain fatty acids in mitochondria? What enzyme is responsible for this regulation point?
The carnitine-mediated entry process, specifically the activity of carnitine acyltransferase I (CAT I).
71
What molecule is a potent inhibitor of carnitine acyltransferase I (CAT I), and what is its metabolic significance?
Malonyl-CoA. It is the first committed intermediate in fatty acid synthesis. Its inhibition of CAT I prevents the simultaneous synthesis and breakdown of fatty acids, ensuring metabolic efficiency.
72
What is the role of the acyl-carnitine/carnitine transporter (CACT) in the inner mitochondrial membrane?
It facilitates the exchange of fatty acyl-carnitine from the intermembrane space into the mitochondrial matrix for free carnitine moving from the matrix back to the intermembrane space. This exchange is crucial for the continuous operation of the carnitine shuttle.
73
Why is the compartmentalization of CoA and fatty acyl-CoA pools (cytosolic vs. mitochondrial) important for cellular metabolism?
It allows for the independent regulation of distinct metabolic pathways. Cytosolic CoA is primarily dedicated to biosynthesis (e.g., fatty acid synthesis), while mitochondrial CoA is dedicated to energy production through oxidative degradation. This prevents futile cycles and allows for coordinated metabolic control.
74
Briefly describe the overall three-step process of the carnitine shuttle and its net effect.
1. Esterification of a long-chain fatty acid to CoA in the cytosol (outer mitochondrial membrane).
2. Transesterification of the fatty acyl group to carnitine, allowing transport across the inner mitochondrial membrane.
3. Transesterification back to CoA in the mitochondrial matrix.
Net Effect: It allows the transport of long-chain fatty acyl groups across the impermeable inner mitochondrial membrane, linking cytosolic fatty acid activation to mitochondrial beta-oxidation.
75
What happens to the free carnitine that is released in the mitochondrial matrix by carnitine acyltransferase II (CAT II)?
It is transported back to the intermembrane space via the acyl-carnitine/carnitine transporter (CACT) to participate in another round of fatty acid import.
76
How does the carnitine shuttle contribute to the regulation of energy production from fats?
The rate-limiting step at CAT I allows for control based on cellular energy needs and the availability of other fuels. Inhibition by malonyl-CoA links fatty acid oxidation to the status of fatty acid synthesis and glucose metabolism.
77
Where does the beta-oxidation of long and very long-chain fatty acids occur in animal cells?
The mitochondrial matrix.
78
What is the initial step for fatty acids entering the mitochondrial matrix for beta-oxidation? Where does this occur?
Activation of fatty acids to Acyl-CoA by Acyl-CoA Synthetase on the outer mitochondrial membrane.
79
What is the role of Carnitine Acyltransferase I (CAT I) in fatty acid transport? Where is it located?
CAT I, located on the outer mitochondrial membrane, exchanges CoA on Acyl-CoA for carnitine, forming Acylcarnitine.
80
How does Acylcarnitine cross the inner mitochondrial membrane? What transporter is involved?
Via the Translocase (Acylcarnitine/Carnitine Transporter) in the inner mitochondrial membrane, which exchanges Acylcarnitine for Carnitine.
81
What is the role of Carnitine Acyltransferase II (CAT II) in fatty acid transport? Where is it located?
CAT II, located on the inner mitochondrial membrane (matrix side), converts Acylcarnitine back to Acyl-CoA by transferring the acyl group to CoA-SH in the matrix.
82
What are the four enzymatic steps of beta-oxidation that occur in the mitochondrial matrix?
1. Acyl-CoA Dehydrogenase
2. Enoyl-CoA Hydratase
3. 3-L-hydroxyacyl-CoA Dehydrogenase
4. β-Ketothiolase (Thiolase)
83
What are the products of each cycle of beta-oxidation?
1. One molecule of Acetyl-CoA
2. One molecule of FADH₂ (from Acyl-CoA Dehydrogenase)
3. One molecule of NADH (from 3-L-hydroxyacyl-CoA Dehydrogenase)
4. A Fatty acyl-CoA that is two carbons shorter.
84
What is the fate of the Acetyl-CoA produced during beta-oxidation?
It enters the citric acid cycle for further oxidation and ATP production.
85
What is the role of FADH₂ and NADH produced during beta-oxidation?
They donate electrons to the electron transport chain (respiratory chain) to generate ATP through oxidative phosphorylation.
86
What are some potential consequences of deficiencies in VLCAD, MCAD, and LCHAD?
Impaired ability to oxidize very long-chain (VLCAD), medium-chain (MCAD), or long-chain (LCHAD) fatty acids, potentially leading to issues like hypoglycemia (especially MCAD deficiency) and other metabolic disturbances.
87
What is the role of CoA-SH in both the activation and the final step of beta-oxidation?
CoA-SH is required for the initial activation of fatty acids by Acyl-CoA Synthetase and is also used by β-Ketothiolase to cleave β-ketoacyl-CoA, releasing Acetyl-CoA and a shorter fatty acyl-CoA.
88
What are the three main stages of mitochondrial fatty acid oxidation?
1. β-oxidation: Oxidative removal of two-carbon units as acetyl-CoA.
2. Citric Acid Cycle: Oxidation of acetyl groups to CO₂.
3.Electron Transport Chain & Oxidative Phosphorylation: Generation of ATP from reduced electron carriers.
89
How many passes of β-oxidation are required to completely oxidize a 16-carbon fatty acid (palmitate)? How many acetyl-CoA molecules are produced?
Seven passes, producing eight acetyl-CoA molecules.
90
What is the net output of each cycle of β-oxidation (excluding the final acetyl-CoA)?
One molecule of acetyl-CoA, one molecule of FADH₂, and one molecule of NADH. The fatty acyl chain is also shortened by two carbons.
91
Where does the second stage (oxidation of acetyl groups) of fatty acid oxidation occur? What pathway is involved?
The mitochondrial matrix, via the citric acid cycle (Krebs cycle).
92
What is the fate of the NADH and FADH₂ produced during the first two stages of fatty acid oxidation? Where does this occur?
They donate electrons to the electron transport chain in the inner mitochondrial membrane, leading to ATP production through oxidative phosphorylation.
93
How are fatty acids taken up by proximal tubule cells in the kidney? What transporter is involved?
Via the fatty acid transporter CD36.
94
Where are fatty acids activated to acyl-CoA? What enzyme catalyzes this?
In the cytosol, by long-chain acyl-CoA synthetase (ACSL).
95
Why is the carnitine shuttle necessary for the oxidation of long-chain fatty acids?
The inner mitochondrial membrane is impermeable to long-chain acyl-CoA.
96
What enzyme on the outer mitochondrial membrane initiates the carnitine shuttle? What does it do?
Carnitine Palmitoyltransferase 1 (CPT1) (or CAT I) converts cytosolic acyl-CoA to acylcarnitine.
97
How does acylcarnitine cross the inner mitochondrial membrane? What transporter is involved?
Via the carnitine acylcarnitine carrier (CAC) (or SLC25A20), which mediates an antiport exchange with free carnitine.
98
What enzyme on the inner mitochondrial membrane regenerates acyl-CoA in the matrix? What does it do?
Carnitine Palmitoyltransferase 2 (CPT2) (or CAT II) transfers the acyl group from acylcarnitine back to mitochondrial CoA.
99
What are the four types of Acyl-CoA Dehydrogenase (ACAD) and what is their cofactor?
SCAD (Short-chain), MCAD (Medium-chain), LCAD (Long-chain), VLCAD (Very long-chain). Their cofactor is FAD.
100
What is the role of Electron-Transferring Factor (ETF) and ETF dehydrogenase in fatty acid oxidation?
ETF accepts electrons from FADH₂ produced by Acyl-CoA Dehydrogenase and transfers them to ETF dehydrogenase, which is linked to the electron transport chain via Coenzyme Q.
101
What are the two distinct forms of 3-hydroxyacyl-CoA dehydrogenase and what cofactor do they use?
Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) and short/medium-chain hydroxyacyl-CoA dehydrogenase (HAD). They use NAD⁺ as a cofactor.
102
What enzyme catalyzes the final cleavage step in each round of β-oxidation? What are the products?
Long-chain 3-ketoacyl-CoA thiolase (HADHB) (also known as β-ketothiolase or thiolase), producing acetyl-CoA and a two-carbon shortened acyl-CoA.
103
Why is carnitine availability crucial for fatty acid oxidation?
It is essential for the transport of long-chain fatty acyl groups into the mitochondrial matrix, where β-oxidation occurs.
104
How does the body obtain carnitine? What is the major transporter involved in its absorption and distribution?
Endogenous synthesis and absorption from the diet (about half). The major transporter is Organic Cation Transporter Novel 2 (OCTN2 – SLC22A5).
105
What role does the kidney play in carnitine homeostasis? What happens to carnitine homeostasis in kidney diseases?
The kidney plays a major role in the renal tubular reabsorption of carnitine, preventing excessive loss. Kidney diseases are often associated with derangements in carnitine homeostasis.
106
What is the primary outcome of fatty acid β-oxidation?
The breakdown of a long-chain acyl-CoA molecule to acetyl-CoA molecules.
107
List the four main enzymes involved in each cycle of fatty acid β-oxidation, in order.
1. Acyl-CoA dehydrogenase (ACAD)
2. Enoyl-CoA hydratase (ECH)
3. Hydroxyacyl-CoA dehydrogenase (HADH)
4. Ketoacyl-CoA thiolase (KAT) (β-ketothiolase)
108
What are the two main energy-carrying molecules produced during each cycle of β-oxidation?
FADH₂ (by acyl-CoA dehydrogenase) and NADH (by hydroxyacyl-CoA dehydrogenase).
109
What is the fate of the FADH₂ and NADH produced during β-oxidation?
They are used by the electron transport chain to produce ATP through oxidative phosphorylation.
110
What is the role of different isoforms of β-oxidation enzymes? Give examples.
They have different affinities for fatty acids of different chain lengths, allowing efficient oxidation of various fats. Examples: VLCAD, LCAD, MCAD, SCAD.
111
What three long-chain specific β-oxidation enzymes form a complex on the inner mitochondrial membrane? What is this complex called?
Long-chain enoyl-CoA hydratase (LCEH), long-chain (S)-3-hydroxyacyl CoA dehydrogenase (LCHAD), and long-chain 3-ketoacyl-CoA thiolase (LCKAT). This complex is called the mitochondrial trifunctional protein (MTP).
112
Under what conditions can CPT2 and CACT operate in reverse? What does this facilitate?
When intramitochondrial acyl-CoA levels rise. This facilitates the export of acyl groups as acylcarnitines out of the mitochondria.
113
What is the role of FAD in the acyl-CoA dehydrogenase reaction? What happens to the resulting FADH₂?
FAD acts as an electron acceptor, oxidizing acyl-CoA. The enzyme-bound FADH₂ is then reoxidized by the electron transfer flavoprotein (ETF).
114
How are electrons from FADH₂ transferred to the respiratory chain? What enzymes are involved?
Electrons are transferred from FADH₂ on acyl-CoA dehydrogenase to electron transfer flavoprotein (ETF), and then to ETF dehydrogenase (ETFDH), which donates them to coenzyme Q (Q) in the respiratory chain.
115
What is the role of NAD⁺ in the hydroxyacyl-CoA dehydrogenase reaction? What is the fate of the produced NADH?
NAD⁺ acts as an electron acceptor, oxidizing β-hydroxyacyl-CoA to β-ketoacyl-CoA and producing NADH. The NADH is reoxidized by oxidative phosphorylation (OXPHOS) to generate ATP.
116
What are the two products of the ketoacyl-CoA thiolase reaction?
Acetyl-CoA and an acyl-CoA shortened by two carbon atoms.
117
What are the two main metabolic fates of the acetyl-CoA produced by β-oxidation?
It can enter the TCA cycle for further oxidation and ATP production, or it can be used for ketogenesis.
118
What happens to the shortened acyl-CoA molecule after each β-oxidation cycle?
It re-enters the β-oxidation cycle for further breakdown.
119
Define the β-carbon and the ω-carbon in a fatty acid chain.
The β-carbon is the third carbon atom from the carboxyl group. The ω-carbon is the last carbon atom in the chain.
120
What type of fatty acids require auxiliary enzymes for complete β-oxidation?
Mono- and polyunsaturated fatty acids (those containing double bonds).
121
Name the three obligatory auxiliary enzymes involved in the β-oxidation of unsaturated fatty acids.
1. 2,4-Dienoyl-CoA Reductase (DECR)
2. 3,5-2,4-Dienoyl-CoA Isomerase
3. 3,2-Enoyl-CoA Isomerase (ECI) (Δ³-Δ²-enoyl-CoA isomerase)
122
What is the function of 2,4-Dienoyl-CoA Reductase (DECR)? What cofactor does it use?
It reduces 2,4-dienoyl-CoA (an intermediate from polyunsaturated fatty acid oxidation) to trans-Δ³-enoyl-CoA. It uses NADPH as a cofactor.
123
What is the function of 3,5-2,4-Dienoyl-CoA Isomerase?
It acts on a 2,4-dienoyl-CoA intermediate (from polyunsaturated fatty acid oxidation) and can isomerize it to a cis-Δ³-enoyl-CoA.
124
What is the function of 3,2-Enoyl-CoA Isomerase (ECI)? What type of double bond configurations does it act on?
It isomerizes cis- or trans-Δ³-enoyl-CoA intermediates (from both mono- and polyunsaturated fatty acids) to the trans-Δ²-enoyl-CoA configuration.
125
Why is the trans-Δ²-enoyl-CoA configuration important in β-oxidation?
It is a normal intermediate in the standard β-oxidation pathway and a substrate for enoyl-CoA hydratase.
126
What are the two main types of reactions catalyzed by the auxiliary enzymes to facilitate the β-oxidation of unsaturated fatty acids?
Reduction (by DECR) and isomerization (by the isomerases).
127
What is the overall goal of the auxiliary enzymes in the context of unsaturated fatty acid metabolism?
To position the double bonds in unsaturated fatty acid intermediates into the trans-Δ² configuration, allowing them to re-enter the standard β-oxidation cycle for complete degradation to acetyl-CoA.
128
What are the two main levels at which mitochondrial fatty acid β-oxidation (FAO) is regulated?
Transcriptional regulation (controlling gene expression) and post-transcriptional regulation (controlling enzyme activity).
129
Name three ligand-activated nuclear receptors that play a key role in the transcriptional regulation of fatty acid metabolism, including FAO.
Peroxisome Proliferator-Activated Receptor α (PPARα), PPARβ/δ, and PPARγ.
130
What are PPARs activated by? What is their general mechanism of action?
Activated by fatty acids (and their derivatives). They act as transcription factors, binding to DNA to regulate the expression of genes involved in FAO and other metabolic processes.
131
What are some key roles of PPARα in fatty acid metabolism, particularly in the liver during starvation?
Controls hepatic expression of many FAO genes, induces microsomal ω-oxidation, peroxisomal dicarboxylic acid metabolism, and ketogenesis.
132
What transcriptional coactivator interacts with PPARs and ERRs to regulate FAO and other metabolic processes?
PPARγ Coactivator-1α (PGC-1α).
133
What is the key post-transcriptional regulatory step in mitochondrial fatty acid β-oxidation? What enzyme is inhibited?
The inhibition of Carnitine Palmitoyltransferase 1 (CPT1) by malonyl-CoA.
134
Name the two important isoforms of CPT1 and their tissue-specific expression.
CPT1A (Liver CPT1): Ubiquitously expressed, high in the liver.
CPT1B: Muscle- and heart-specific.
135
What is the effect of malonyl-CoA on CPT1? What is the physiological significance of this regulation?
Malonyl-CoA is an allosteric inhibitor of CPT1. This regulation is important because CPT1 is the rate-limiting step for fatty acid entry into mitochondria for β-oxidation.
136
What two enzymes regulate cellular levels of malonyl-CoA?
Acetyl-CoA carboxylase (ACC) (produces malonyl-CoA) and malonyl-CoA decarboxylase (MCD) (degrades malonyl-CoA).
137
How does PPAR activation influence malonyl-CoA levels and subsequently FAO?
PPAR activation induces transcription of malonyl-CoA decarboxylase (MCD), increasing its activity. This leads to lower malonyl-CoA levels, relieving inhibition of CPT1 and stimulating FAO.
138
How does AMP-activated protein kinase (AMPK) influence malonyl-CoA levels and subsequently FAO? What activates AMPK?
AMPK phosphorylates and inactivates acetyl-CoA carboxylase (ACC), reducing malonyl-CoA production. Lower malonyl-CoA levels relieve inhibition of CPT1 and stimulate FAO. AMPK is activated by AMP, which rises when ATP levels are low.
139
What is the overall effect of activated AMPK on cellular energy homeostasis in relation to FAO?
Activated AMPK, sensing low ATP, initiates a signaling cascade to restore ATP levels by inhibiting ATP-consuming pathways (like fatty acid synthesis by inactivating ACC) and stimulating ATP-producing pathways (like FAO by lowering malonyl-CoA and activating CPT1).
140
Name two main membrane-bound transporters responsible for the uptake of long-chain fatty acids into the cell.
CD36 and Fatty Acid Transport Protein (FATP).
141
What cytosolic proteins facilitate the intracellular transport of fatty acids?
Fatty Acid Binding Proteins (FABPpm, membrane-associated; FABP, cytosolic).
142
Where in the cell are fatty acids stored?
In lipid droplets in the cytosol.
143
What molecule is the precursor for fatty acid synthesis in the cytosol? What enzyme catalyzes its formation?
Malonyl-CoA, formed from acetyl-CoA by Acetyl-CoA Carboxylase 1 (ACC1).
144
What enzyme is responsible for synthesizing new fatty acids in the cytosol? What are its substrates?
Fatty Acid Synthase (FAS), using acetyl-CoA and malonyl-CoA as substrates.
145
What is the rate-limiting step in fatty acid synthesis? What enzyme catalyzes it?
The conversion of acetyl-CoA to malonyl-CoA, catalyzed by Acetyl-CoA Carboxylase 1 (ACC1).
146
What process is required for long-chain fatty acyl-CoA to enter the mitochondrial matrix for β-oxidation? Name the key enzymes involved.
The carnitine shuttle, involving Carnitine Palmitoyltransferase 1 (CPT1), Carnitine Acylcarnitine Translocase (CACT), and Carnitine Palmitoyltransferase 2 (CPT2).
147
Where does β-oxidation of fatty acyl-CoA occur? What are the main products?
In the mitochondrial matrix, producing acetyl-CoA, FADH₂, and NADH.
148
What is the fate of the acetyl-CoA produced from β-oxidation in the mitochondria?
It enters the tricarboxylic acid cycle (TCA cycle) for further oxidation.
149
What is the role of FADH₂ and NADH produced by β-oxidation and the TCA cycle?
They donate electrons to the electron transport chain (ETC) in the inner mitochondrial membrane to produce ATP.
150
How does malonyl-CoA regulate mitochondrial fatty acid β-oxidation? What enzyme does it inhibit?
Malonyl-CoA inhibits Carnitine Palmitoyltransferase 1 (CPT1), preventing the entry of fatty acyl-CoA into the mitochondria for β-oxidation.
151
What is the role of citrate in linking mitochondrial metabolism to cytosolic fatty acid synthesis?
Citrate produced in the TCA cycle can be transported to the cytosol, where it is cleaved to generate acetyl-CoA, a precursor for fatty acid synthesis.
152
What potential role does CPT1 activation have in relation to apoptosis?
Activation of CPT1 may act as a survival signal by inhibiting the oligomerization of pro-apoptotic proteins Bak and Bax.
153
What are the main transcription factors involved in the transcriptional regulation of fatty acid synthesis in response to high glucose?
Carbohydrate-Responsive Element-Binding Protein (ChREBP) and Sterol Regulatory Element-Binding Protein (SREBP).
154
How is ChREBP activated and translocated to the nucleus in response to high glucose?
High glucose promotes its translocation from the cytoplasm to the nucleus.
155
How is SREBP activated and translocated to the nucleus? What inhibits this process?
SREBP is cleaved in the Golgi apparatus, releasing the active form that translocates to the nucleus. This process is inhibited by high levels of cholesterol.
156
What are some key genes involved in fatty acid synthesis whose expression is increased by active ChREBP and SREBP in the nucleus?
Citrate lyase, Acetyl-CoA carboxylase 1 (ACC1), and Fatty acid synthase (FAS).
157
What are the main energy sensors that are activated in response to low cellular energy levels and play a role in regulating fatty acid oxidation transcription?
AMP-activated protein kinase (AMPK) and Sirtuin 1 (SIRT1).
158
How does AMPK activate PGC-1α? How does SIRT1 activate PGC-1α?
AMPK activates PGC-1α by phosphorylation, promoting its translocation to the nucleus. SIRT1 activates PGC-1α by deacetylation, enhancing its activity.
159
What is PGC-1α and what is its general role in transcriptional regulation?
Peroxisome Proliferator-Activated Receptor gamma Coactivator 1-alpha. It is a transcriptional coactivator that enhances the activity of various transcription factors.
160
Name three transcription factors that PGC-1α interacts with in the nucleus to upregulate fatty acid oxidation genes.
Peroxisome Proliferator-Activated Receptors (PPARs), Forkhead box protein O1 (FoxO1), and Nuclear respiratory factor 1/2 (NRF1/2).
161
What are some examples of genes involved in fatty acid oxidation whose expression is upregulated by PGC-1α and its partners?
Fatty acid transporters (e.g., CD36), and rate-limiting enzymes of β-oxidation (e.g., Acyl-CoA oxidase).
162
Besides upregulating fatty acid oxidation genes, what other major process does PGC-1α promote in the mitochondria?
Mitochondrial biogenesis (increasing the number and function of mitochondria).
163
What are the two key enzymes involved in the coordinated regulation of fatty acid synthesis and breakdown?
Acetyl-CoA carboxylase (ACC) and carnitine acyltransferase I (CPT1).
164
What hormonal change occurs in response to a high-carbohydrate meal? What is the effect on ACC?
Insulin release is triggered. Insulin activates a phosphatase that dephosphorylates and activates ACC.
165
What is the product of active ACC? What is the role of this product in fatty acid metabolism?
Malonyl-CoA. It is the first committed intermediate in fatty acid synthesis and an inhibitor of CPT1.
166
How does malonyl-CoA regulate fatty acid β-oxidation? What enzyme does it inhibit?
Malonyl-CoA inhibits carnitine acyltransferase I (CPT1), preventing fatty acid entry into the mitochondrial matrix.
167
What hormonal change occurs when blood glucose levels drop? What is the effect on ACC?
Glucagon release is triggered. Glucagon activates PKA, which phosphorylates and inactivates ACC.
168
What happens to the concentration of malonyl-CoA when ACC is inactivated? How does this affect fatty acid entry into the mitochondria?
The concentration of malonyl-CoA falls. This relieves the inhibition of CPT1, allowing fatty acids to enter the mitochondria.
169
When carbohydrate availability is low, what becomes the major fuel source for the cell?
Fatty acids, which are broken down by β-oxidation in the mitochondria.
170
What other effect does glucagon have on fatty acid metabolism besides regulating ACC?
Glucagon triggers the mobilization of fatty acids from adipose tissue, increasing their availability in the blood.
171
What is the primary consequence of deficiencies in mitochondrial β-oxidation enzymes like MCAD, LCHAD/MTP, and VLCAD?
The accumulation of specific fatty acid intermediates (MCFA, LCHFA, LCFA) within the mitochondrial matrix.
172
How does the accumulation of fatty acid metabolites affect the respiratory chain complexes?
It can inhibit the protein complexes of the respiratory chain, impairing electron flow.
173
What is a consequence of impaired electron flow in the respiratory chain due to accumulated fatty acid metabolites?
Increased production of reactive oxygen species (ROS).
174
How does the blockage of β-oxidation and the inhibition of the respiratory chain affect ATP production?
It leads to a decrease in overall ATP production, causing an energy deficit.
175
What is the mPT pore, and what factors associated with β-oxidation defects can promote its opening?
The mitochondrial permeability transition pore in the inner mitochondrial membrane. Its opening can be promoted by accumulated metabolites, increased ROS, and calcium overload.
176
What are some consequences of prolonged mPT pore opening?
Mitochondrial swelling, release of cytochrome c, and potentially apoptosis.
177
How can β-oxidation defects affect the levels of NAD(P)H in the mitochondrial matrix?
They can lead to a decrease in NAD(P)H content due to altered metabolism and potential inhibition of dehydrogenase enzymes.
178
How is mitochondrial calcium handling affected by the accumulation of fatty acid metabolites?
The mitochondrial Ca²⁺ retention capacity can be decreased, leading to calcium overload in the cytosol.
179
What is the link between mitochondrial dysfunction caused by β-oxidation defects and apoptosis?
mPT pore opening can lead to the release of cytochrome c, which triggers the apoptotic cascade.
180
Name three key tissues that are particularly vulnerable to mitochondrial dysfunction caused by β-oxidation defects.
Heart, liver, and skeletal muscle.
