Exam 4 Flashcards
(119 cards)
1
Q
What is the central energy-producing pathway in many organisms and tissues?
A
Fatty acid oxidation
Fatty acid oxidation is crucial for energy production, especially in the liver and heart.
2
Q
Which organs obtain up to 80% of their energy from fatty acid oxidation?
A
Liver and heart
These organs primarily rely on fatty acids for energy.
3
Q
What percentage of the daily energy requirement is provided by fatty acid oxidation?
A
> 40%
Fatty acid oxidation plays a significant role in meeting daily energy needs.
4
Q
What must ingested food do to be digested by water-soluble enzymes?
A
Be emulsified
Emulsification increases the surface area for enzyme action.
5
Q
How do fatty acids travel in the blood?
A
Bound to specific proteins
Fatty acids do not travel freely in the bloodstream.
6
Q
List the sources of fatty acids.
A
- Food from diet
- Storage cells (adipocytes)
- Synthesized in organs (e.g., liver)
- Obtained from autophagy
These sources contribute to the fatty acid pool in the body.
7
Q
What are lipids characterized by?
A
Amphiphilic nature = have a hydrophilic ‘head’ and hydrophobic ‘tails’ like phospholipids
Hydrophobic nature = they don’t have a water-loving part at all → like triglycerides (fats and oils)
8
Q
What are triacylglycerols also known as?
A
Triglycerides:
- nonpolar and water-insoluble
- lower specific gravity than water (oil floats on water)
• stored in adipocytes (fat cells)
• lipases (esterases) hydrolyze esters to release fatty acids and glycerol
Triacylglycerols are the main storage form of fats.
3 fatty acids (same or different) attached to each -OH of glycerol
• 3 condensation reactions between alcohol and carboxylic acid —esterification
9
Q
What is the structure of triacylglycerols?
A
Three fatty acids attached to glycerol
They are formed by esterification reactions.
10
Q
What process hydrolyzes triacylglycerols to release fatty acids?
A
Lipases hydrolyze triacylglycerols to release free fatty acids and glycerol.
Lipases (esterases) catalyze this reaction.
11
Q
What is the role of bile salts in lipid digestion?
A
The role is to solubilize and emulsify triacylglycerols so lipases can hydrolyze them to release fatty acids (Emulsify fats)
• synthesized in the liver from cholesterol (and released in the small intestine after ingestion of a fat meal)
• surround triacylglycerols to help form micelle-like structures
Bile salts help solubilize triacylglycerols for lipase action.
12
Q
What are chylomicrons made of?
A
- Triacylglycerols - 80% of mass
- Cholesterol
- Apolipoproteins
Chylomicrons transport dietary lipids in the bloodstream.
• size from 100-500 nm
• surface is a layer of phospholipids
• cholesterol gives rigidity to surface
• embedded into structure are
apolipoproteins, proteins responsible
for transport of FA between organs
• apolipoproteins act as signals for cells to uptake and metabolize chylomicron contents
13
Q
What happens to fatty acids in myocytes?
A
They are used for oxidation
Myocytes oxidize fatty acids for energy.
14
Q
What is the fate of liberated glycerol in adipocytes?
A
Phosphorylated and oxidized to dihydroxyacetone phosphate
This it is then isomerized to D-glyceraldehyde 3-phosphate.
Next G3P becomes 1,3 bisphosphoglycerate in glycolysis R6
Note: ~95% of the biologically available energy of triacylglycerols resides in their three long-chain fatty acids
15
Q
What are the stages in fatty acid oxidation?
A
Stage 1: β fatty acid oxidation
–Long chain of fatty acids is oxidized to yield acetyl in the form of acetyl-CoA
Stage 2: oxidation of acetyl-CoA groups to CO2 in the citric acid cycle
–occurs in the mitochondrial matrix
–generates NADH, FADH2, and one
ATP/GTP
Stage 3: electron transfer chain and
oxidative phosphorylation
–generates ATP from NADH and FADH2
16
Q
What is the function of carnitine in fatty acid metabolism?
A
Transport long-chain fatty acids into mitochondria
Carnitine acyltransferases facilitate this transport.
for catabolism: carnitine acyltransferase I (CAT1) attaches carnitine to FA-CoA for transfer
• enzyme is inhibited by malonyl-CoA, first intermediate in FA synthesis
17
Q
β oxidation
A
β-hydroxyacyl-CoA dehydrogenase oxidizes the OH on the β-carbon to a ketone (defines “β-oxidation”)
This process generates acetyl-CoA and reduces equivalents.
18
Q
β fatty acid oxidation
A
Step 1• fatty acids undergo oxidative removal of successive two-carbon
units in the form of acetyl-CoA
Step 2• Starting from the carboxyl end of an FA chain, e.g., palmitoyl-CoA (16-carbon FA)
Step 3• break the bond between the α- and the β-carbon
1. Acyl-CoA dehydrogenase introduces a trans-Δ² double bond
between the α- and β-carbons
2. enoyl-CoA hydratase the double bond, putting an OH group on the β-carbon.
3. β-hydroxyacyl-CoA dehydrogenase oxidizes the OH on the β-carbon to a ketone (defines “β-oxidation”)
4. Acyl-CoA acetyltransferase uses a free CoA-SH to cleave the
bond, releasing acetyl-CoA and shortening the fatty acid by 2
carbons.
Each cycle produces 1 FADH2 and 1 NADH+ H+
• FADH2 produces 1.5 ATP in electron transport chain
• NADH produces 2.5 ATP in electron transport chain
• acetyl-CoA can also go through TCA to generate more ATP
Footnote##
• 2 carbons are removed in each cycle
• total # cycles = (CN - 2) / 2
• last cycle yields 2 acetyl-CoA molecules
7 cycles but 8 total acetyl-CoA
19
Q
What happens to odd-numbered fatty acids during oxidation?
A
They are converted to succinyl-CoA
They produce propionyl-CoA in addition to acetyl-CoA. This propionyl-CoA is then converted to succinyl-CoA, which can enter the citric acid cycle
• proceed as usual until 3 carbons left (propionyl-CoA)
• 3 auxiliary enzymes:
• Propionyl-CoA carboxylase → adds CO₂ to C2, generating 4C species (needs biotin + ATP).
• Methylmalonyl-CoA epimerase → change enantiomer (D to L)
• Methylmalonyl-CoA mutase mutase uses coenzyme B12 to swap the C1 hydrogen and C3 methyl-CoA to generate succinyl-CoA
20
Q
What effect does high glucose concentration have on fatty acid metabolism?
A
Inhibits fatty acid catabolism and stimulates fatty acid anabolism
Insulin is involved in this regulatory process.
21
Q
What enzyme is activated by insulin to promote fatty acid synthesis?
A
Acetyl-CoA carboxylase (ACC)
• insulin activates insulin-dependent protein phosphatase
• phosphatase dephosphorylates and activates ACC (acetyl-CoA carboxylase)
• ACC catalyzes formation of malonyl-CoA, which is the 1st step in FA anabolism
22
Q
What inhibits carnitine acyltransferase I?
A
Malonyl-CoA
This prevents fatty acid entry into mitochondria for catabolism.
23
Q
What starts fatty acid and glycogen catabolism?
A
Low [glucose] in blood = start FA and glycogen catabolism
• glucagon turns on PKA which phosphorylates and inactivates ACC
• FAs enter mitochondria via carnitine shuttling and undergo beta oxidation
24
Q
What hormone activates PKA to phosphorylate and inactivate ACC?
A
Glucagon.
25
How do fatty acids enter mitochondria?
Via **carnitine shuttling** (and undergo beta oxidation)
26
What is the first step in **fatty acid anabolism?**
Carboxylation of acetyl-CoA to **form** of **malonyl-CoA**.
27
What does malonyl-CoA inhibit?
Carnitine acyltransferase I.
28
What happens when there is high [glucose] in blood?
Stop FA and glycogen catabolism, and start FA anabolism.
29
What role does insulin play in fatty acid synthesis?
Activates insulin-dependent **protein phosphatase**.
• **phosphatase dephosphorylates and activates ACC** (acetyl-CoA carboxylase)
• **ACC catalyzes formation of malonyl-CoA**, which:
• is the **1st step in FA anabolism**
• inhibits carnitine acyltransferase I, preventing FA entry into mitochondria = **prevents FA catabolism**
• high glucose in blood = stop FA and
glycogen catabolism, and start FA anabolism
30
What occurs during a no-carb diet regarding oxaloacetate?
It is used for **gluconeogenesis**
Note: pyruvate is depleted because
there is no glycolysis — so it **cannot** be used to make more OAA via an **anaplerotic (replenishing) reaction**
31
What happens to acetyl-CoA when oxaloacetate is depleted?
It cannot be oxidized.
32
Under what conditions are ketone bodies formed?
During fasting, carbohydrate-restrictive diets, prolonged exercise, type 1 diabetes, and starvation.
Note: under these conditions, **acetyl-CoA** is **converted** to **ketone bodies** through a series of steps **in the liver**
• “bodies” means insoluble, but these
molecules are actually **soluble**
• **acetone** = **toxic byproduct that is exhaled (can help diagnose diabetes)**
33
What are the two types of ketone bodies exported from the liver?
**Acetoacetate and D-β hydroxybutyrate**. (via blood to other organs, where they are converted to acetyl-CoA for citric acid cycle)
Note:
**ketone bodies formed in liver cells, and exported to other tissues for energy**
• **ketone bodies formed when [acetyl-CoA] is high, but citric acid cycle is slow**
• glucose in blood is low (or poor insulin signaling!)
• so **citric acid cycle intermediates (remember: “amphibolic”) are pulled away for gluconeogenesis**
• **slows down oxidation of acetyl-CoA in citric acid cycle**
• **removing excess acetyl-CoA allows for continuing FA beta oxidation to meet the body’s energy needs**
34
What is the chemiosmotic theory?
**Transmembrane differences** in **proton concentration** are the **reservoir** for the **energy extracted** from **biological oxidation reactions**
explains how cellular respiration and photosynthesis generate ATP (adenosine triphosphate), the cell's energy currency. It involves the creation of a proton gradient across a membrane, which then drives ATP synthesis through a protein complex called ATP synthase.
35
What is the function of the cristae in mitochondria?
Serves as a **platform for many copies of different enzymes to assemble in proximity for substrate shuttling**
Note:
• these leaflets are home to tens of thousands of proteins responsible for the electron-transfer system, and ATP synthase
• the **mitochondria of heart muscle**, a muscle under constant activity, **contains >3x the amount of electron-transfer systems than the liver** mitochondrion.
36
What type of proteins are found in the electron-transfer system of mitochondria?
**NADH Dehydrogenase** (Complex I): Receives electrons from NADH and transfers them to ubiquinone.
**Succinate Dehydrogenase** (Complex II): Receives electrons from FADH2 and transfers them to ubiquinone.
**Cytochrome bc1 Complex** (Complex III): Receives electrons from ubiquinone(Q) and transfers them to cytochrome c.
**Cytochrome c** (Complex IV): Receives electrons from cytochrome c and donates them to oxygen, forming water.
**Ubiquinone (CoQ)**: A mobile electron carrier that shuttles electrons between Complex I/II and Complex III.
**ATP Synthase** (Complex V): While not directly part of the ETS, it uses the proton gradient created by the ETS to synthesize ATP.
37
What do **dehydrogenases** do in **catabolic pathways**?
**Collect electrons** and **funnel** them **into universal electron acceptors**:
• **nicotinamide nucleotides (NAD+ or NADP+)**
• **flavin nucleotides: FMN (flavin mononucleotide), and FAD (flavin adenine dinucleotide**), **cofactors** derived from **riboflavin (vitamin B₂)**
The **respiratory chain = series of electron carriers**
38
Flavoproteins
39
What type of reactions do nicotinamide nucleotide-linked dehydrogenases catalyze?
**Reversible reactions**.
40
How many hydrogen atoms are removed from substrates during electron transfer?
**Two** hydrogen atoms.
–One is transferred as a hydride ion (:H-, a hydrogen atom with an extra
electron = 1 proton and 2 electrons) to NAD(P)+
–One is released as H+ in the medium
41
Ubiquinone role in Complexes
• electrons from a variety of sources are passed to Q (ubiquinone)
• **Complex I** uses **electrons** from **NADH + H+**
• **Complex II** uses the **FADH2** generated in the **oxidation of succinate to fumarate** (citric acid cycle reaction 6!)
• **glycerol 3-phosphate dehydrogenase** uses **FADH2** generated during **glycerol oxidation**
• electrons from **FADH2 generated in the first step of fatty acid oxidation is passed to Q**
• **role of Q is to transport electrons**
• Q remains in and travels across membrane bilayer
• extensive hydrophobic tail allows for membrane association
• can freely diffuse within membrane
42
Mechanisms of Electron transfer in Oxidative Phosphorylation
In oxidative phosphorylation, electrons are transferred via three mechanisms:
1. **direct transfer, such as reduction of Fe+3 to Fe+2 (e.g. cytochrome)**
2. **transfer as proton H+ and electron e- (e.g. flavins, FADH2)**
3. **transfer as hydride ion :H- (e.g. dehydrogenases, NADH)**
• five types of electron-carrying molecules in the respiratory chain:
- NAD (nicotinamide nucleotides)
- Flavoproteins (contain a very tightly, sometimes covalently, bound flavin nucleotide (FMN or FAD)
- ubiquinone (coenzyme Q or Q)
- Cytochromes
- iron-sulfur proteins
43
What type of gradient does the electron transport chain generate?
**Proton gradient** across the **mitochondrial membrane**.
44
What is the role of **ATP synthase**?
**Condense ADP and Pi to ATP** using the **proton gradient**.
45
Porphyrin
Found in **cytochromes**:
• porphyrin-carrying proteins, similar to Hb and Mb
• **tightly, sometimes covalently, associated with flavoprotein**
• **Iron-containing heme** prosthetic group
• **Fe3+ converted to Fe2+** during reduction. Absorption spectra of cytochrome c
46
Iron-sulfur proteins (centers)
Iron-sulfur proteins
• **iron associated with Cys or His AA**, and **inorganic sulfur**
• Participate in 1 e- transfer → **one iron atom is oxidized or reduced**
• the reduction potential of the clusters vary by their environment (number of sulfurs and irons present, the geometry, and the neighbors)
• **Rieske iron-sulfur proteins: One Fe atom is coordinated to 2 His rather than 2 Cys**
47
Additional prosthetic groups and ligands involved in electron transfer
• NADH = reduced
• NAD+ = oxidized
• water soluble and released into solution by enzyme
• carries :H-
• H+ released into solution
• FADH2 = reduced
• FAD = oxidized
• either tightly or covalently bound to flavoproteins (e.g. succinate dehydrogenase in citric acid cycle)
48
What happens to the **iron** in **cytochromes** during **reduction**?
**Fe3+ is converted to Fe2+**
49
What types of **electron-carrying molecules** are found in the **respiratory chain**?
* NAD (nicotinamide nucleotides)
* Flavoproteins
* Ubiquinone (Q)
* Cytochromes
* Iron-sulfur proteins.
50
What does the **high reduction potential** of a molecule indicate?
It wants electrons.
51
What is the primary product of oxidative phosphorylation?
ATP.
52
What is the role of coenzyme Q in electron transport?
To **transport electrons across the membrane bilayer**
## Footnote
Coenzyme Q **remains in the membrane and can freely diffuse within it.**
53
What is Complex I also known as?
NADH dehydrogenase
## Footnote
Complex I consists of 42 different polypeptide chains and contains 8 Fe-S clusters/centers.
54
What are the coupled processes carried out by Complex I?
coupled processes carried out by Complex I:
• **exergonic transfer of 2e** from **NADH to Q**, and **2H+* from matrix to Q**
• **endergonic transfer of 4H+**from **matrix (N side) to intermembrane space (P side)**
## Footnote
The purpose is to **generate QH2 and pump H+ into the intermembrane space.**
55
What enzyme is Complex II known as?
**Succinate dehydrogenase**
## Footnote
This enzyme is involved in the citric acid cycle.
56
What is the purpose of heme b in Complex II?
To protect against the formation of **reactive oxygen species (ROS)**
## Footnote
Heme b is not part of the electron pathway.
• = succinate dehydrogenase (enzyme 6 in citric acid cycle)
• **couple oxidation of succinate at one site with reduction of ubiquinone at another site** (~40 )
• 4 polypeptide chains
• 5 prosthetic groups
• FAD is tightly bound to enzyme
• transmembrane domains (heme b and and Q binding site) with cytoplasmic extensions
• transfers electrons to FAD from oxidation of succinate → fumarate
• heme b is not part of the electron pathway, but rather acts to protect against formation of reactive oxygen species (ROS) by electrons that escape the pathway and interact with O2
• **purpose is to generate QH2**
• no H+ are transported
57
What is the function of FADH2 in fatty acid oxidation?
FADH2 needs to be **oxidized** to FAD to reduce Q
first step in beta oxidation involves generation of FADH2:
• FADH2 needs to be oxidized to FAD
• FAD cannot be released into solution
• electrons are passed from one flavoprotein to another (from **FAD in the acyl-CoA hydrogenase to ETF-electron transferring flavoprotein to ETF oxidoreductase**)
• the final flavoprotein passes the electrons to Q to make QH2
## Footnote
Electrons are passed from one flavoprotein to another until they reach Q.
58
What is Complex III also known as?
**Cytochrome bc1 complex**
On the P side, in each stage 1QH2 donates:
• 1e- to the 2Fe-2S center → cyt c1 → cyt c
• 1e to heme bL → heme bu → a different Q on the N side
• 2H+ to the intermembrane space to increase the electrochemical potential
• On the N side:
• in stage 1, Q accepts 1e from chain of hemes, forming •Q semiquinone radical
• in stage 2, the same •Q accepts another 1e- from the chain of hemes and 2H+ from the matrix, forming QH2
• thus, overall 2QH2 pass 4H* to the intermembrane space and pull 2H* from the matrix
## Footnote
Complex III is a **homodimer with 11 polypeptide chains.**
59
What is the goal of Complex III?
To **transfer electrons from QH2 to soluble cytochrome c and pump H+ to the intermembrane space**
a.k.a. cytochrome bc complex
• Complex is a homodimer, each with 11 polypeptide chains of 2 cytochrome b (with two hemes brand bu) and c1, and the Rieske iron-sulfur cluster (2Fe-2S center)
• Q and hemes bh, bL, and cytochrome c1 shuttle electrons
• Rieske iron-sulfur protein swivels between heme bL and c1
• 2Fe-2S receives electrons from Qp when it's close to Qp
• 2Fe-2S donates electrons when it's closer to heme C1
• goal of Complex III:
**transfer electrons from QH to soluble cytochrome c**
• pump H+ to intermembrane space
## Footnote
The Q cycle involves complex electron transfers.
60
What is the net equation for Complex IV?
4 cyt c (reduced) + 8H+N + O2 → 4 cyt c (oxidized) + 4H+p + 2H20
## Footnote
This involves the transfer of electrons and pumping of H+.
61
What are reactive oxygen species (ROS)?
**Flow of electrons** that results in the **formation** of **superoxide free radicals**
## Footnote
Under normal conditions, 0.1-4% of O2 is converted to superoxide.
62
What is the role of superoxide dismutase?
To eliminate superoxides
## Footnote
It **reduces hydrogen peroxide to water using glutathione.**
63
What does the proton-motive force generate?
An electrochemical gradient
## Footnote
This gradient is between the mitochondrial matrix and intermembrane space.
• electron passing/transporting and proton pumping generate an electrochemical gradient
• the gradient is between the mitochondrial matrix and intermembrane space (NOT cytosol!)
• composed of chemical and electrical potential energy
• energy can be converted to
АТР
64
What is the function of ATP synthase?
To synthesize ATP
## Footnote
• **F1 hydrolyzes ATP**
• **Fo destroys electrochemical gradient**
• **F1 and Fo together synthesize ATP**
• **ATP synthase binds ATP (Kd = 10^-12 M) more tightly than ADP+Pi (10^-5 M)
• favorable binding energy for ATP drives reaction toward formation of
ATP (which would be **endergonic in solution without the enzyme**)
• **protons** are responsible for **releasing ATP from ATP synthase**
65
What happens during the binding of ATP in ATP synthase?
ATP is bound more tightly than ADP+Pi
## Footnote
This binding energy drives the formation of ATP.
66
What is the significance of the asymmetrical structure of ATP synthase?
It allows for **three non-equivalent binding sites**: **ATP, ADP+Pi, and empty**
## Footnote
This is crucial for the synthesis of ATP.
67
How many protons are needed per 120° turn of ATP synthase?
Approximately 3 H+
## Footnote
This rotation is necessary for ATP release and synthesis.
68
What is the transport mechanism for NADH in liver, kidney, and heart tissues?
**Malate-aspartate shuttle**
* cytosolic NADH used to reduce OAA to malate
* malate transported across inner membrane
* malate oxidized back to OAA (= last reaction of citric acid cycle), regenerating NADH in the matrix
* cytosolic OAA regenerated by other reactions
* Goal: **oxidize the NADH generated during glycolysis**
* **NADH/NAD diffuse through mitochondrial outer membrane, but not across inner membrane**
## Footnote
**Cytosolic NADH reduces OAA to malate, which is transported across the inner membrane.**
69
Transport and reoxidation of NADH in skeletal muscle and brain
* in **skeletal muscle and brain: glycerol 3-phosphate shuttle**
* cytosolic isozyme of GPDH reduces DHAP to glycerol 3P with NADH, thus regenerating NAD+
* mitochondrial isozyme of GPDH, associated with inner membrane, oxidizes glycerol 3P to DHAP to create FADH2
* FADH2 passes electrons to Q
* skips Complex I, so loses out on 4 H+ pumped
* in these tissues, accounting of net ATP per glucose changes (Q: how?)
## Footnote
This regenerates NAD+ for mitochondrial isozyme of GPDH.
70
True or False: The old integer model assumed 3 ATPs per NADH.
True
* Old, WRONG, integer numbers before chemiosmotic model:
* Roughly 2-3 ATPs, assumed to be 3 ATPs, per NADH
* Roughly 1-2 ATPs, assumed to be 2 ATPs, per FADH2 from succinate
* New, CORRECT, fractional numbers with chemiosmotic model:
* 10 protons pumped per NADH (Complex I, III, IV)
* 6 protons pumped per FADH2 from succinate (Complex III, IV)
* 4 protons “spent” per ATP (3 for ATP synthase, 1 for ATP/ADP/Pi transport)
* 10/4 = **2.5 ATPs per NADH**
* 6/4 = **1.5 ATPs per FADH2 from succinate**
## Footnote
The new fractional numbers indicate 2.5 ATPs per NADH.
71
Fill in the blank: The chemiosmotic model allows for _______ ATP accounting.
[non-integer]
## Footnote
This contrasts with the previous assumption of integer values.
72
What does the **cytosolic isozyme of GPDH** do?
Reduces DHAP to glycerol 3P with NADH, thus **regenerating NAD+**
73
What is the role of the **mitochondrial isozyme of GPDH**?
Oxidizes glycerol 3P to DHAP to create FADH2
74
What happens to FADH2 in the electron transport chain?
It passes electrons to Q and skips Complex I, losing out on 4 H+ pumped
75
What occurs under low O2 (hypoxia) conditions?
Under low O2 (hypoxia):
* H+ pumping slows, so proton-motive force collapses
* ATP synthase runs in reverse, wasting ATP
* pH drops in the cytosol and mitochondria (Q: why?)
* to prevent this:
* IF1 in the matrix dimerizes at pH ≤ 6.5
* dimer of IF1 inhibits ATP synthase
76
What is the effect of low pH in the cytosol and mitochondria?
It causes a drop in pH, which can inhibit ATP synthase
77
What does IF1 do in the matrix?
**Dimerizes at pH ≤ 6.5 to inhibit ATP synthase**
78
What regulates O2 consumption?
**Regulated by [ADP] and by mass-action ratio [ATP]/[ADP][Pi]**
* this works in conjunction with many other points of regulation to control metabolic flux
* low-energy molecules (ADP, AMP, Pi) promote catabolism
* high-energy molecules (ATP, NADH, citrate, acetyl-CoA) disfavor further catabolism
79
What promotes **catabolism**?
Low-energy molecules such as **ADP, AMP, and Pi**
80
What disfavor further catabolism?
High-energy molecules like **ATP, NADH, citrate, and acetyl-CoA**
81
How do fatty acids get to cells that need energy from diet?
* from diet: bile salts, chylomicrons, …
* from adipocytes: lipase cascade,
From the diet: Dietary fats are emulsified by bile salts in the small intestine, then broken down by pancreatic lipase into free fatty acids and monoglycerides. These are absorbed by intestinal cells and re-esterified into triglycerides, then packaged into chylomicrons. Chylomicrons enter the lymphatic system and then the bloodstream, where lipoprotein lipase (LPL) on capillary endothelial cells breaks them down again into free fatty acids, which can be taken up by tissues for energy.
From adipocytes: In the fasting state, hormone-sensitive lipase (HSL) is activated by a cascade involving glucagon and epinephrine via cAMP signaling. This mobilizes stored triglycerides into free fatty acids and glycerol. The free fatty acids bind to albumin in the blood and are transported to tissues like muscle and liver, where they are taken up, converted to acyl-CoA, and enter the mitochondria via the carnitine shuttle for β-oxidation, producing ATP
82
How do fatty acids get to cells that need energy from adipocytes?
Via a lipase cascade
83
What is necessary for preparing fatty acids for beta oxidation?
CoA and carnitine
84
What is the chemical strategy of beta oxidation?
🔬 Chemical Strategy of β-Oxidation:
Activate the fatty acid in the cytosol:
Fatty acid + CoA + ATP → Fatty acyl-CoA (requires 2 ATP equivalents)
This step traps the fatty acid in an "activated" high-energy state.
Transport into mitochondria via the carnitine shuttle:
Fatty acyl-CoA is converted to acyl-carnitine to cross the inner mitochondrial membrane.
Once inside, it's converted back to acyl-CoA.
Cycle of four enzyme reactions in the mitochondrial matrix:
Each round removes two carbons as acetyl-CoA and generates one FADH₂ and one NADH:
Oxidation by acyl-CoA dehydrogenase → introduces a trans double bond (makes FADH₂)
Hydration by enoyl-CoA hydratase → adds water across the double bond
Oxidation by β-hydroxyacyl-CoA dehydrogenase → makes NADH
Thiolysis by thiolase → cleaves 2-carbon acetyl-CoA, leaving a shortened acyl-CoA
💡 Strategic Logic:
β-oxidation is a repetitive oxidative cleavage process that turns long-chain fatty acids into:
Acetyl-CoA → enters TCA cycle (produces 3 NADH, 1 FADH₂, 1 GTP)
NADH and FADH₂ → donate electrons to the ETC for ATP production
85
How many cycles of beta oxidation occur for N # of carbons?
Number of β-oxidation cycles = (N / 2) – 1
Each β-oxidation cycle removes 2 carbons as 1 acetyl-CoA.
The final round cleaves the last 4-carbon chain into 2 acetyl-CoA, so you only need (N/2) – 1 cycles to fully break down the chain.
86
Which of the following statements about processing of dietary lipids is true?
A. The primary role of lipases is to bind to free fatty acids
B. Chylomicrons primarily carry free fatty acids in the blood
C. Apolipoproteins cleave triglycerides into free fatty acids plus glycerol
D. Bile salts solubilize dietary fats in the intestine
E. Triglycerides are cleaved only once during the process of digestion, transport through the blood, and import into myocytes
Bile salts solubilize dietary fats in the intestine
87
What is involved in both fatty acid catabolism and carbohydrate metabolism?
A. glucagon
B. cAMP
C. PKA
D. glyceraldehyde 3-phosphate
E. all of these answers are correct
all of these answers are correct
88
How many cycles of beta oxidation are needed to break down the 14-carbon, saturated fatty acid myristoyl-CoA into 7 acetyl-CoA molecules?
A. 6
B. 7
C. 14
D. 15
E. 28
6
89
When and why are ketone bodies made?
During periods of **low carbohydrate** availability
90
What happens to the glycerol that results from breakdown of triglycerides?
A. It enters glycolysis as glycerol
B. It enters glycolysis as glyceraldehyde 3-phosphate
C. It enters the citric acid cycle as oxaloacetate
D. It enters beta-oxidation as glycerol-CoA
E. It enters the pyruvate dehydrogenase complex as pyruvate
It enters glycolysis as glyceraldehyde 3-phosphate
91
What do NAD, FAD, and Q have in common?
A. They all can carry 2e-
B. They all can carry 2H+
C. They all include adenosine nucleotide motifs
D. They are all reduced during the citric acid cycle
E. none of the above
They all can carry 2e-
92
What is the order of reduction potential along the electron transport chain?
A. Reduction potential increases (becomes more positive) along the chain
B. Reduction potential is random along the chain
C. Reduction potential decreases (becomes more negative) along the chain
D. Reduction potential increases (becomes more positive) until Complex IV, then decreases (becomes more negative) when the electrons reach O2
E. None of the other answers is correct
Reduction potential increases (becomes more positive) along the chain
93
What is Q's role in the electron transport chain?
Carries electrons and gets reduced
94
How many ATP molecules are generated for one full 360°cycle for ATP synthase?
A. 1
B. 1.5
C. 2.5
D. 3
E. 6
3
A. 1—this is how many ATPs are made per 120°turn of ATP synthase
B. 1.5—this is how many ATPs are made per FADH2
C. 2.5—this is how many ATPs are made per NADH
D. 3—1 ATP per 120°turn, so 3 ATPs per 360°cycle
95
Which statement is false regarding the processing of dietary lipids in vertebrates?
A. Dietary lipids are emulsified by bile salts in the intestine.
B. Triacylglycerols in mixed micelles in the intestine diffuse into cells of the intestinal mucosa.
C. Ultimately, dietary lipids are oxidized as fuel by muscle or stored as triacylglycerols in adipose tissue.
D. Dietary lipids are packaged in lipoprotein aggregates known as chylomicrons, which are then exported to the lymph system.
Triacylglycerols in mixed micelles in the intestine diffuse into cells of the intestinal mucosa.
Water-soluble lipases in the intestine convert triacylglycerols to monoacylglycerols, diacylglycerols, and free fatty acids. These products of lipase action diffuse or are transported into the epithelial cells lining the intestinal surface (the intestinal mucosa), where they are reconverted to triacylglycerols.
96
Which factor would stimulate movement of fatty acids to muscle and the liver when blood glucose levels fall?
A. insulin
B. glucagon
C. an increase in protein kinase C in activity
D. an increase in phospholipase C activity
E. an increase in citric acid cycle activity in adipose
B. glucagon
Low levels of glucose in the blood trigger the release of glucagon. Binding of this hormone to a G protein–coupled receptor on the adipocyte plasma membrane triggers the mobilization of stored triacylglycerol.
97
Which molecule can be produced rapidly from glycerol in only three steps, allowing an interaction between carbohydrate and lipid metabolism?
A. acetyl-CoA
B. glucose
C. pyruvate
D. glyceraldehyde 3-phosphate
E. phosphoenolpyruvate
D. glyceraldehyde 3-phosphate
The glycerol released by lipase action is phosphorylated by glycerol kinase, and the resulting glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate. The glycolytic enzyme triose phosphate isomerase converts this compound to glyceraldehyde 3-phosphate, which is oxidized via glycolysis.
98
Where does β oxidation occur?
A. in the cytosol
B. in the mitochondrial matrix
C. in the ER lumen
D. on the Golgi apparatus membrane
E. at the plasma membrane
B. in the mitochondrial matrix
The enzymes of fatty acid oxidation in animal cells are located in the mitochondrial matrix.
99
Why does β oxidation occur in the mitochondrial matrix?
A. to allow coordinated regulation with fatty acid synthesis
B. to coordinate production of acetyl-CoA with the introduction into the citric acid cycle
C. to compartmentalize
D. because necessary oxidative enzymes are present
E. All of the answers are correct.
E. All of the answers are correct.
The mitochondrial matrix contains all the enzymes necessary for the β-oxidation pathway and the citric acid cycle. If β oxidation occurred in the cytosol, the simultaneous synthesis and degradation of fatty acids could occur, resulting in a wasteful and futile cycle.
100
How do fatty acids get into the mitochondrial matrix?
A. spontaneously
B. via the malate shuttle
C. via carnitine palmitoyltransferase
D. via palmitoyl-CoA transferase
E. via the citrate shuttle
C. via carnitine palmitoyltransferase
In a transesterification catalyzed by carnitine acyltransferase 1, CAT1 (also called carnitine palmitoyltransferase 1, CPT1) in the outer mitochondrial membrane, the fatty acyl–CoA is transiently attached to the hydroxyl group of carnitine to form fatty acyl–carnitine. The fatty acyl–carnitine ester then diffuses across the intermembrane space and enters the matrix by passive transport through the acyl-carnitine/carnitine cotransporter of the inner mitochondrial membrane.
101
Vertebrate processing of dietary lipids
• Bile salts emulsify fats
• Lipases degrade triacylglycerols
• The intestinal mucosa absorbs fatty acids and other remnants
• They are reassembled into triacylglycerols
• Combined with cholesterol &
apolipoproteins into chylomicrons and sent to the blood
• Near target cells, new lipases degrade triacylglycerols again
• Fatty acids enter cells
• **myocytes use for oxidation**
• **adipocytes use for storage**
102
Steps to **Mobilizing triacylglycerols from adipocyte for catabolism**
1. **glucagon binds receptor on adipocyte** (needs of metabolic energy)
2. stimulates adenylyl cyclase, via a G protein, to produce cyclic AMP
• **cAMP activates PKA** (protein kinase A)
3. **PKA phosphorylates HSL (lipase)**, sending it to the lipid droplet
4. **PKA phosphorylates perilipin** protein on surface of lipid droplet, **allowing access to the droplet**
5. **CGI (released from perilipin) activates ATGL (lipase)**
6. **ATGL hydrolyzes tri- into diacylglycerols**
7. **HSL hydrolyzes di- into monoacylglycerols**
8. **MGL (lipase) hydrolyzes monoacylglycerols to FAs**
9. **FAs leave adipopocyte, bind serum albumin** (lipoprotein) in blood
10. **FAs enter myocyte via transporter**
11. **FAs are oxidized to CO2 to generate ATP**
103
An important theme in Biochemistry is interaction among metabolic pathways. Which pathway would obviously be MOST affected by increased β oxidation of fatty acids?
A. glycolysis
B. the citric acid cycle
C. the glyoxylate pathway
D. the pentose phosphate pathway
E. gluconeogenesis
B. the citric acid cycle
In the second stage of fatty acid oxidation, the acetyl groups of acetyl-CoA are oxidized to CO2 in the citric acid cycle, which also takes place in the mitochondrial matrix.
104
Which sequence of electron carriers transfers electrons from a fatty acyl–CoA to the mitochondrial respiratory chain?
A. ETF → ubiquinone → ETF:ubiquinone oxidoreductase → FADH2
B. FADH2 → ETF → ETF:ubiquinone oxidoreductase → ubiquinone
C. ETF → FADH2 → ETF:ubiquinone oxidoreductase → ubiquinone
D. FADH2 → ETF:ubiquinone oxidoreductase → ubiquinone → ETF
B. FADH2 → ETF → ETF:ubiquinone oxidoreductase → ubiquinone
Each molecule of FADH2 formed during oxidation of the fatty acyl–CoA donates a pair of electrons to the electron transfer flavoprotein (ETF). Electrons move from ETF to a second flavoprotein, ETF:ubiquinone oxidoreductase, and through ubiquinone into the mitochondrial respiratory chain.
105
The reactions of mitochondrial β oxidation do NOT include:
A. a hydratase
B. a thiolase
C. a dehydrogenase
D. an oxidase
D. an oxidase
The four enzymes of mitochondrial β oxidation are:
* acyl-CoA dehydrogenase
* enoyl-CoA hydratase
* β-hydroxyacyl-CoA dehydrogenase
* acyl-CoA acetyltransferase (thiolase)
106
Which product from oxidation of fatty acids CANNOT feed into the citric acid cycle?
A. acetyl-CoA
B. succinyl-CoA
C. succinate
D. NADP+
E. FAD
D. NADP+
The acetyl-CoA resulting from all the β-oxidation pathways, the succinyl Co-A formed by the oxidation of odd-number fatty acids, and the succinate resulting from the oxidation of polyunsaturated fatty acids can all feed into the citric acid cycle. However, the NADP+ resulting from the reductase step in the oxidation of polyunsaturated fatty acids cannot enter the citric acid cycle.
107
Acetone resulting from ketone body production is:
A. removed from the body by exhalation.
B. used as fuel in tissues other than the liver.
C. not a ketone body.
D. produced by nonenzymatically and enzymatically by decarboxylation of D‐β‐hydroxybutyrate.
A. removed from the body by exhalation.
Acetone, a volatile compound produced in smaller quantities than the other ketone bodies, is exhaled. The exhaled acetone imparts a characteristic odor to the breath.
108
Starvation and uncontrolled diabetes mellitus can both result in:
A. ketosis, implying that the body of a person with uncontrolled diabetes mellitus is acting metabolically as though it is starving.
B. ketosis, implying that ketoacidosis is a result of uncontrolled diabetes mellitus but not starvation.
C. ketosis, implying that high levels of ketone bodies in the blood must be well-tolerated.
D. ketosis, though neither case is due to overproduction of ketone bodies by the liver, but rather because cells are impaired in uptake of ketone bodies.
A. ketosis, implying that the body of a person with uncontrolled diabetes mellitus is acting metabolically as though it is starving.
When the insulin level is insufficient, extrahepatic tissues cannot take up glucose efficiently from the blood. Fatty acid oxidation increases, but the resulting acetyl-CoA cannot pass through the citric acid cycle because cycle intermediates have been drawn off for use as substrates in gluconeogenesis. The accumulation of acetyl-CoA accelerates the formation of ketone bodies.
109
Which ion, atom, or molecule constitutes one reducing equivalent?
A. proton (H+)
B. hydrogen atom (H+ + e−)
C. hydride ion (:H−)
D. NADH
B. hydrogen atom (H+ + e−)
The term reducing equivalent is used to designate a single electron equivalent transferred in an oxidation-reduction reaction. A proton cannot act as a reducing equivalent because it has no electrons. A hydrogen atom acts as one reducing equivalent. A hydride ion or NADH molecule acts as two reducing equivalents.
110
Which electron-carrier complex in the respiratory chain oxidizes ubiquinone?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV
C. Complex III
Complex III carries electrons from reduced ubiquinone to cytochrome c.
111
Which statement is false about Complex I?
A. It consists of more than 40 different polypeptide chains.
B. It has an FMN-containing flavoprotein.
C. It catalyzes the transfer of a hydride from NADH and a proton from the matrix to ubiquinone (Q).
D. Its activity makes the matrix more positively charged.
D. Its activity makes the matrix more positively charged.
Complex I transfers four protons from the matrix to the intermembrane space. The loss of positively charged protons makes the matrix more negative.
112
Which electron carrier or prosthetic group would NOT function after site-directed mutagenesis substituted Pro for Cys in succinate dehydrogenase?
A. cytochrome c
B. iron-sulfur center
C. flavin adenine dinucleotide
D. ubiquinone
E. FMN
B. iron-sulfur center
Subunits A and B contain three iron-sulfur (2Fe-2S) centers, in which the iron is present in association with the sulfur atoms of Cys residues in the protein.
113
Which statement is false about Complex III?
A. It holds ubiquinone on the matrix side of the inner mitochondrial membrane throughout the redox process.
B. Its functional unit consists of two dimers made up of cytochrome b, cytochrome c1, and the Rieske iron-sulfur protein.
C. It couples the oxidation of two molecules of reduced ubiquinone (QH2) with the reduction of two molecules of cytochrome c.
D. It catalyzes the net movement of two protons from the N side to the P side of the inner mitochondrial membrane.
A. It holds ubiquinone on the matrix side of the inner mitochondrial membrane throughout the redox process.
The two cytochrome b monomers surround a cavern in the middle of the membrane in which ubiquinone is free to move from the matrix side of the membrane (site QN on one monomer) to the intermembrane space (site QP on the other monomer) as it shuttles electrons and protons across the inner mitochondrial membrane.
114
Which statement regarding the proton-motive force is false?
A. It is a result of electron flow through the respiratory chain.
B. It is used by ATP synthase to synthesize ATP.
C. It results from an [H+] gradient across the outer mitochondrial membrane.
D. It is both chemical and electrical potential energy.
C. It results from an [H+] gradient across the outer mitochondrial membrane.
The proton-motive force results from an [H+] gradient across the inner, not the outer, mitochondrial membrane.
115
Why are reactive oxygen species (ROS) generated?
A. Molecular oxygen recombines with hydrogen to produce dihydrogen oxide.
B. Stray electrons bind to oxygen, creating a free radical oxygen species.
C. There are high ADP levels.
D. There is a high NAD+/NADH ratio.
E. Transfer of a H+ across the inner mitochondrial membrane is uncoupled from ATP synthase.
B. Stray electrons bind to oxygen, creating a free radical oxygen species.
Some intermediates in the electron-transfer system, such as the partially reduced ubisemiquinone (*QH), can react directly with oxygen to form the superoxide radical (*O2−) as an intermediate.The superoxide radical forms when a single electron is passed to O2 in the reaction:
O2 + e− → *O2−
116
The chemiosmotic model:
A. is supported by the observation that, when isolated mitochondria have an artificial electrochemical gradient (higher pH in the matrix) imposed on them, electrons spontaneously flow through the respiratory chain.
B. requires an intact outer mitochondrial membrane.
C. requires that mitochondrial ATP synthesis and electron flow through the respiratory chain be obligately coupled.
D. is supported by the observation that isolated mitochondria that are actively respiring cause the pH of the solution they are in to increase.
C. requires that mitochondrial ATP synthesis and electron flow through the respiratory chain be obligately coupled.
The chemiosmotic model, proposed by Peter Mitchell, is the paradigm for energy coupling. Here, “coupling” refers to the obligate connection between mitochondrial ATP synthesis and electron flow through the respiratory chain.
117
What is the major energy barrier for ATP synthase?
A. binding of ADP to the enzyme
B. binding of Pi to the enzyme
C. formation of ATP
D. release of ATP from the enzyme
D. release of ATP from the enzyme
In a typical enzyme-catalyzed reaction, reaching the transition state between substrate and product is the major energy barrier to overcome. In the reaction catalyzed by ATP synthase, release of ATP from the enzyme, not formation of ATP, is the major energy barrier.
118
With each rotation of 120°, the γ subunit of FoF1 comes into contact with a different β subunit, forcing that β subunit:
A. into the β-empty conformation.
B. into the β-ADP conformation.
C. into the β-ATP conformation.
D. to catalyze the synthesis of ATP.
A. into the β-empty conformation.
The β subunit changes to the β-empty conformation when it comes into contact with the γ subunit. Because this conformation has very low affinity for ATP, the newly synthesized ATP leaves the enzyme surface.
119
Which statement regarding ATP synthase is false?
A. The γ subunit is stationary as the αβ dimers rotate around it.
B. Protons flow through the a and c subunits.
C. When the F1 domain is isolated, it functions as an ATPase.
D. It is considered to have an Fo domain and an F1 domain.
A. The γ subunit is stationary as the αβ dimers rotate around it.
The streaming of protons through the Fo pore causes the c ring and the attached γ subunit to rotate about the long axis of γ, which is perpendicular to the plane of the membrane. The γ subunit passes through the center of the α3β3 spheroid, which is held stationary relative to the membrane surface.
