10 Flashcards
(202 cards)
1
Q
What are the two main components of ATP synthase?
A
F₀ component (membrane-spanning, proton channel)
F₁ component (catalytic domain that synthesizes ATP)
2
Q
How does the F₀ component of ATP synthase contribute to ATP production?
A
protons flow through F₀ due to a gradient created by the electron transport chain, generating torque that causes the central shaft (γ subunit) to rotate.
3
Q
What happens in the F₁ component of ATP synthase during rotation?
A
The rotation causes conformational changes in the F₁ catalytic sites, which drives the synthesis of ATP from ADP and inorganic phosphate (Pi).
4
Q
What is the energy source that drives ATP synthesis in oxidative phosphorylation?
A
The proton motive force (PMF), a gradient of H⁺ ions across the mitochondrial inner membrane, created by the electron transport chain.
5
Q
Why is oxygen such an effective terminal electron acceptor in aerobic respiration?
A
It is abundant on Earth.
It diffuses easily across membranes.
Its diradical structure allows it to accept electrons readily.
6
Q
What is a disadvantage of oxygen’s high electron affinity?
A
It can lead to the formation of reactive oxygen species (ROS), which are toxic metabolites that damage cells.
7
Q
Where is the electron transport chain (ETC) located in eukaryotic cells?
A
in the inner mitochondrial membrane.
8
Q
define chemiosmosis.
A
the process by which energy from electron flow is used to pump protons, creating a proton gradient that powers ATP synthase.
8
Q
What is the role of reduced coenzymes in the ETC?
A
donate electrons to the chain:
NADH and FADH₂ are oxidized, providing high-energy electrons for transport.
9
Q
What are reactive oxygen species (ROS)? Give examples.
A
Toxic by-products of oxygen metabolism, including:
Superoxide anion (O₂⁻)
Hydrogen peroxide (H₂O₂)
Hydroxyl radical (*OH)
10
Q
How do cells defend themselves against ROS?
A
by using antioxidant enzymes, such as:
Superoxide dismutase (SOD)
Catalase
Glutathione peroxidase
11
Q
What is the terminal electron acceptor in aerobic respiration?
A
Oxygen (O₂) — it accepts electrons and protons to form water (H₂O).
12
Q
What drives the rotation of ATP synthase’s shaft?
A
electrochemical gradient of protons (H⁺) moving through F₀.
13
Q
Where is the electron transport chain (ETC) located in eukaryotic cells?
A
In the inner mitochondrial membrane.
14
Q
What is the ETC also known as?
A
the electron transport system.
15
Q
In what order are electron carriers arranged in the ETC?
A
increasing electron affinity
16
Q
What is the overall change in standard reduction potential (ΔE°) when NADH donates electrons to oxygen?
A
+1.14 V, calculated as +0.82 V − (−0.32 V).
17
Q
What are the sources of reduced coenzymes that supply electrons to the ETC?
A
Glycolysis
Citric acid cycle
Fatty acid oxidation
18
Q
How does the energy from electron transfer in the ETC contribute to ATP synthesis?
A
By coupling the energy to proton pumping across the membrane, creating a proton gradient that drives ATP synthase.
19
Q
How many protein complexes make up the ETC?
A
4
20
Q
What other processes are driven by electron transport, besides ATP synthesis?
A
Calcium ion (Ca²⁺) transfer into the mitochondrial matrix via MAMs (mitochondria-associated membranes)
Heat production in brown adipose tissue
21
Q
What type of enzymes are ETC complexes classified as?
A
Oxidoreductases
22
Q
What is the role of coenzyme Q (ubiquinone or UQ) in the ETC?
A
transfers electrons from Complex I and II to Complex III.
23
Q
What is the role of cytochrome c in the ETC?
A
transfers electrons from Complex III to Complex IV.
24
What is the function of Complex I in the ETC?
transfers electrons from NADH to UQ and pumps protons across the membrane.
25
What are the two structural domains of Complex I (NADH dehydrogenase)?
Hydrophilic arm (binds NADH, FMN, UQ, and has iron-sulfur clusters)
Membrane domain (contains 4 proton-translocating channels)
26
What is the molecular weight of Complex I in bovine mitochondria?
1 MDa (1000 kDa), composed of at least 45 subunits.
27
What is the function of iron–sulfur (Fe–S) clusters in Complex I?
mediate one-electron transfer reactions.
28
What are the specific electron flow steps in the ETC?
Complex I: NADH → UQ
Complex II: Succinate (FADH₂) → UQ
Complex III: UQH₂ → Cytochrome c
Complex IV: Cytochrome c → O₂
29
What is the ultimate electron acceptor in the mitochondrial ETC?
Oxygen (O₂), which is reduced to water (H₂O).
30
What is the primary function of the ETC?
ransfer electrons to oxygen while pumping protons into the intermembrane space, generating the proton gradient required for ATP synthesis.
31
What organism is used as a model to study Complex I in this section?
thermus thermophilus (a type of archaea).
32
What happens when NADH binds in the distal segment of Complex I’s peripheral arm?
hydride ion is transferred from NADH to FMN, forming FMNH₂.
33
How many electrons are transferred from FMNH₂ to the iron–sulfur clusters in Complex I?
Two electrons, transferred one at a time through seven iron–sulfur clusters.
34
What is the product when both electrons are transferred to ubiquinone (UQ)?
UQH₂ (ubiquinol).
35
What physical process is triggered by electron transfer in Complex I, leading to proton pumping?
mini electric current lasting about 5 milliseconds triggers conformational changes, opening proton channels to pump 4 protons.
36
What causes the opening of proton channels in Complex I?
Conformational changes due to altered pKa values of specific side chains from electron flow.
37
What mechanism allows rapid electron transfer between Fe-S clusters?
Electron tunneling, a quantum-mechanical process allowing electrons to bypass activation barriers.
38
What structural conditions enable electron tunneling?
Short edge-to-edge distances (≤ 14 Å)
Precise polypeptide orientation
Internal water molecules
39
What is the role of Complex II in the ETC?
It transfers electrons from succinate to ubiquinone (UQ).
40
What is Complex II also known as?
Succinate ubiquinol reductase.
41
Shda (complex II subunit)
FMN with FADH2 binding site and covalently bound FAD
42
ShdC and ShdD (complex II subunit)
Integral membrane proteins forming UQ and heme binding sites
42
ShdB (complex II subunit)
Contains 3 Fe-S clusters
43
What is the function of the heme group in Complex II?
To suppress electron leakage that could form reactive oxygen species (ROS).
44
Does Complex II translocate protons across the mitochondrial membrane?
No, it does not pump protons.
45
Which flavoprotein enzyme on the outer face of the IMM transfers electrons from cytoplasmic NADH?
Glycerol-3-phosphate dehydrogenase.
46
What protein acts as an electron acceptor in fatty acid and amino acid oxidation?
Electron-transferring flavoprotein (ETF).
47
What does ETF transfer its electrons to?
QO (electron-transferring flavoprotein ubiquinone oxidoreductase), which then reduces UQ.
48
Which enzyme in fatty acid oxidation donates electrons directly to UQ?
Acyl-CoA dehydrogenase.
49
Name at least four amino acids whose oxidation involves ETF.
Proline
Leucine
Isoleucine
Valine
50
What is the redox reaction catalyzed by Complex II?
Succinate is oxidized to fumarate, and FAD is reduced to FADH₂.
51
Which subunit of Complex II contains the FAD cofactor?
ShdA (a flavoprotein with a succinate-binding site and covalently bound FAD).
52
How are electrons transferred within Complex II?
Electrons from FADH₂ are transferred one at a time to three iron-sulfur clusters, then to UQ, forming UQH₂.
53
What intermediate is formed after the first electron is transferred to UQ?
Ubisemiquinone (a one-electron reduced form of UQ).
54
What is the function of the heme group in Complex II?
It acts as an electron sink to suppress reactive oxygen species (ROS) formation.
54
Does Complex II pump protons across the membrane?
No, Complex II does not translocate protons.
55
Name two other FADH₂-producing dehydrogenases that transfer electrons to UQ.
Glycerol-3-phosphate dehydrogenase
Acyl-CoA dehydrogenase
56
What is the function of Complex III?
To transfer electrons from UQH₂ to cytochrome c.
57
How many subunits does each monomer of Complex III have?
11 subunits, including:
Cytochromes: bL, bH, c1
One iron–sulfur cluster in the Rieske protein
58
What is cytochrome c, and where is it located?
A mobile electron carrier loosely associated with the outer face of the inner mitochondrial membrane.
59
What are the substrates and products of Complex III?
Substrates: UQH₂ and oxidized cytochrome c (cyt c_ox, Fe³⁺)
Products: UQ and reduced cytochrome c (cyt c_red, Fe²⁺)
60
How many protons are pumped into the intermembrane space per UQH₂ oxidized?
Four protons are pumped (2 from UQH₂ and 2 from the matrix via the Q cycle).
61
What is the purpose of the Q cycle in Complex III?
To efficiently transfer two electrons from UQH₂ to cytochrome c, while pumping four protons across the inner mitochondrial membrane
62
What happens to the first electron from UQH₂ in the Q cycle?
It is transferred to the Rieske Fe–S protein, then to cyt c₁, and finally to cytochrome c.
63
What happens to the second electron from UQH₂?
It is transferred to cyt bL, then to cyt bH, and finally reduces UQ to ubisemiquinone.
64
How is UQH₂ regenerated in the second round of the Q cycle?
A second UQH₂ donates an electron to cyt c, while the ubisemiquinone from round 1 accepts the second electron and 2 protons from the matrix, forming UQH₂.
65
what is the net result of one full Q cycle?
2 electrons transferred to cyt c
4 protons pumped into intermembrane space
1 UQH₂ oxidized, 1 UQ reduced back to UQH₂
66
What allows UQ molecules to move between Complex I/II and III?
UQ is lipid-soluble and diffuses within the inner mitochondrial membrane.
67
What is the main function of Complex III?
transfers electrons from UQH₂ to cytochrome c, while pumping protons across the inner mitochondrial membrane.
68
How many UQH₂ molecules are oxidized to reduce 2 cytochrome c molecules?
two UQH₂ molecules are oxidized sequentially.
69
Where does the first electron from UQH₂ go in Complex III?
To the Fe-S protein, then to cytochrome c₁, then to cytochrome c.
70
What happens to the second electron from the first UQH₂ molecule?
It goes to cyt bL → cyt bH, then to UQ, forming ubisemiquinone.
71
What happens in the second round of the Q cycle?
second UQH₂ donates an electron to cyt c, and the ubisemiquinone from round one picks up that second electron and 2 matrix protons to reform UQH₂.
72
What is the net result of one Q cycle in terms of protons and electron carriers?
4 protons pumped into intermembrane space
2 electrons transferred to 2 cytochrome c molecules
1 UQ oxidized, 1 UQH₂ regenerated
73
Does Complex III contribute to the proton gradient?
Yes, by transferring 4 protons per 2 electrons into the intermembrane space.
74
What is the main role of Complex IV?
catalyzes the four-electron reduction of O₂ to H₂O, and pumps protons into the intermembrane space.
75
What’s the full balanced redox reaction at Complex IV?
O₂ + 4H⁺ (matrix) + 4e⁻ → 2H₂O
76
Why can O₂ only accept one electron at a time?
Due to spin restrictions: both valence electrons in O₂ have the same spin quantum number.
77
What cofactors are involved in Complex IV?
Heme a and heme a₃ (iron centers)
CuA (binuclear copper center)
CuB (forms a Fe–Cu center with heme a₃)
78
What is the electron path through Complex IV?
Cytochrome c → CuA → heme a → heme a₃/CuB → O₂
79
How many electrons are needed to reduce one O₂ molecule?
Four electrons
80
What role does Subunit III of Complex IV play?
It facilitates the transport of four protons from the matrix to the intermembrane space.
81
How many protons are involved total in Complex IV activity?
4 protons used in H₂O formation
4 additional protons pumped to the intermembrane space
82
What is the total number of protons pumped by Complex IV per O₂ reduced?
4 protons pumped, 4 used in water formation = 8 matrix protons removed from the matrix side.
83
What regulates electron transport through cytochrome c and Complex IV?
high ATP levelsinhibit Complex IV and cytochrome c by binding to ATP-binding sites, decreasing electron transport activity.
84
How many protons are pumped across the IMM by Complex IV?
4 protons per O₂ reduced are pumped into the intermembrane space.
85
Why doesn’t Complex IV contribute to reactive oxygen species (ROS) formation like other complexes?
Electrons are tightly controlled and do not leak out of Complex IV, minimizing ROS generation.
86
How many ATP are generated per NADH oxidized in the ETC?
approximately 2.5 ATP per NADH.
87
How many ATP are generated per FADH₂ oxidized in the ETC?
Approximately 1.5 ATP per FADH₂.
88
Why does NADH produce more ATP than FADH₂?
NADH donates electrons to Complex I, pumping more protons, while FADH₂ donates to Complex II, which doesn’t pump protons.
89
What does the fluid state model suggest about electron transfer in the ETC?
Electron carriers like UQ and cytochrome c diffuse freely, and transfer occurs via random collisions between complexes.
90
Why is the solid state model more accepted now?
Structural studies show physical proximity of carrier binding sites (e.g., UQ binding in I near III), and the high protein density of the IMM limits random diffusion.
91
What is the solid state model of the ETC?
ETC components are arranged in supercomplexes (e.g., I–III₂–IV₁–₂), called respirasomes, enabling efficient electron transfer via short diffusion distances
92
What complexes are part of the respirasome?
Complex I, III dimer (III₂), and IV₁–₂.
93
which complex is notably not part of the respirasome?
complex II
94
What does antimycin A inhibit in the ETC?
Inhibits cytochrome b in Complex III.
95
What happens upstream and downstream of an ETC inhibitor site?
Upstream (non-O₂ side): carriers become reduced
Downstream (O₂-reducing side): carriers become oxidized
96
Which inhibitors block Complex I?
Rotenone and amytal block NADH dehydrogenase (Complex I).
97
Which inhibitors block Complex IV?
Cyanide (CN⁻), carbon monoxide (CO), and azide (N₃⁻).
98
What is the effect of ETC inhibition on oxygen consumption?
O₂ consumption drops or stops completely because electron flow halts.
99
What is oxidative phosphorylation?
It is the process where energy from electron transport is conserved by the phosphorylation of ADP to ATP.
100
What generates the proton gradient in oxidative phosphorylation?
The electron transport chain (ETC) pumps H⁺ from the matrix to the intermembrane space across the inner mitochondrial membrane (IMM).
101
What is the electrochemical gradient called in mitochondria?
The proton-motive force (Δp) — includes membrane potential (Ψ) and ΔpH.
102
What are the approximate values of membrane potential and ΔpH?
What are the approximate values of membrane potential and ΔpH?
103
What is ATP synthase?
A protein complex (Complex V) that synthesizes ATP using the proton gradient.
104
How do protons return to the matrix to drive ATP synthesis?
They flow down their gradient through the proton channel in ATP synthase.
105
Why can protons not diffuse freely across the inner mitochondrial membrane?
Because the IMM is impermeable to ions, including protons.
106
How much ATP does a typical human cell use per second?
10 million ATP molecules per second.
107
What happens to mitochondrial pH when O₂ is added?
pH drops in suspension due to proton expulsion into the intermembrane space.
108
What do uncouplers like dinitrophenol do?
they collapse the proton gradient by equalizing proton concentrations across the membrane, dissipating energy as heat.
108
What happens to ATP synthesis when the IMM is disrupted?
ATP synthesis stops, even though electron transport may continue.
109
What are ionophores? Give an example.
Hydrophobic molecules that form ion channels in membranes.
Example: Gramicidin allows passage of H⁺, K⁺, Na⁺.
110
What are the two main uses of the proton gradient?
ATP synthesis
Biological work like heat generation and metabolite transport (e.g., ADP/ATP, phosphate)
111
What happens to matrix pH and charge during proton flow?
Matrix becomes more alkaline (higher pH) and more negatively charged.
112
What are the lollipop-shaped structures seen in early EM images of mitochondria?
They are ATP synthase complexes on the inner mitochondrial membrane (IMM)
113
What are the two major components of ATP synthase?
F₁ unit (catalytic ATPase)
F₀ unit (proton channel)
114
What is the subunit composition of the F₁ unit?
α₃:β₃:γ:δ:ε
115
What is the subunit composition of the F₀ unit?
a:b₂:c₁₀–₁₂
116
Which antibiotic inhibits ATP synthase, and how?
Oligomycin, by blocking the proton channel via binding to the a subunit of F₀.
117
What is the function of the c ring in ATP synthase?
It rotates as protons move through it, transmitting motion to the central shaft (γ and ε subunits).
118
What keeps the α,β hexamer stationary during rotation?
The stator, composed of b₂ and δ subunits.
119
How many protons are needed to make one ATP molecule?
3 protons for ATP synthesis, plus 1 more for transporting ATP out and ADP/Pi in
120
How fast does the γ shaft rotate?
150 rotations per second.
121
Which subunits make up the rotor and the stator?
Rotor: ε, γ, c ring (c₁₂)
Stator: a, b₂, δ, α₃, β₃
122
Where do protons enter and exit the c ring?
They enter via a half-channel in subunit a from the IMS, bind to Asp/Glu in c, then exit into the matrix via a second half-channel.
123
What conformational states do β subunits cycle through?
L (loose) → T (tight) → O (open)
124
What happens at each β-subunit conformation?
L: ADP + Pi bind
T: ATP is synthesized
O: ATP is released
125
What drives the conformational changes in β subunits?
Rotation of the asymmetric γ subunit inside the α₃β₃ hexamer.
126
Why is ATP not released unless ADP + Pi are bound?
Because the O state only opens after the T site is occupied, ensuring tight coupling between substrate binding and product release.
127
In what organisms or conditions can ATP synthase run in reverse?
E. coli in anaerobic conditions
Lactic acid bacteria
128
What happens when ATP synthase runs in reverse?
ATP is hydrolyzed to pump protons out, creating a proton gradient for processes like flagella movement or nutrient transport.
129
What is the P:O ratio, and what does it indicate?
The P:O ratio reflects the number of moles of Pi used per O atom reduced to H₂O and indicates the efficiency of ATP synthesis.
130
What is the P:O ratio for NADH and FADH₂?
NADH: 2.5
FADH₂: 1.5
131
What is "respiratory control"?
It refers to the regulation of electron transport by ADP levels—oxygen consumption increases when ADP is added, linking respiration to ATP demand.
132
What regulates ATP synthase activity?
High [ATP]/([ADP][Pi]) inhibits ATP synthase;
High ADP + Pi activates it.
133
What proteins control ADP and Pi availability in mitochondria?
ADP–ATP translocator (ANT): exchanges ATP⁴⁻ (out) for ADP³⁻ (in)
Phosphate translocase: symports H₂PO₄⁻ with H⁺ into the matrix
134
How many protons are needed per ATP synthesis cycle?
4 protons:
3 for ATP synthase
1 for phosphate import via phosphate translocase
135
Why is the ADP–ATP exchange energetically favorable?
Because of the negative membrane potential (matrix side is negative) and ATP has one more negative charge than ADP.
136
What problem does the CK/PCr shuttle solve?
ATP diffuses too slowly in the cell; PCr (smaller and faster) transports energy more efficiently to where ATP is needed.
137
What is the reaction catalyzed by creatine kinase (CK)?
ATP + Cr ⇌ ADP + PCr
(Phosphocreatine acts as a temporary high-energy store.)
138
Why is phosphocreatine a good energy carrier?
It has a high phosphoryl transfer potential:
ΔG°′ = −43.1 kJ/mol
139
What are the four CK isoenzymes and their locations?
MMCK: muscle
BBCK: brain
MBCK: heart
mtCK: mitochondria (ubiquitous and sarcomeric forms)
140
Where is mitochondrial CK (mtCK) located?
In the intermembrane space, bound to cardiolipin, ANT, and VDAC.
141
What is the function of VDAC?
A diffusion pore in the outer mitochondrial membrane (OMM) for small hydrophilic molecules like PCr and Cr.
142
How does the CK/PCr shuttle function?
-mtCK transfers phosphate from ATP (from ANT) to creatine → PCr
-PCr diffuses through VDAC into cytoplasm
-Cytoplasmic CK regenerates ATP near ATPases (e.g., Na⁺/K⁺ pump)
-Cr returns to mitochondria to be rephosphorylated
143
What is the resting ratio of PCr:Cr and ATP:ADP in cells?
PCr:Cr = 2:1
ATP:ADP ≈ 100:1
144
What is the average number of ATP molecules produced from the complete oxidation of one molecule of glucose?
30 ATP molecules
145
What is the net reaction for the complete oxidation of glucose?
C₆H₁₂O₆ + 6O₂ + 30ADP + 30Pi → 6CO₂ + 6H₂O + 30ATP
146
How does the inner mitochondrial membrane affect cytoplasmic NADH oxidation?
It is impermeable to NADH, requiring shuttle systems to transfer electrons to the ETC.
147
What glycolytic intermediate is reduced to form glycerol-3-phosphate in the glycerol-3-phosphate shuttle?
Dihydroxyacetone phosphate (DHAP).
148
What enzyme oxidizes glycerol-3-phosphate on the mitochondrial inner membrane?
Mitochondrial glycerol-3-phosphate dehydrogenase.
149
What is the electron acceptor in the mitochondrial glycerol-3-phosphate dehydrogenase reaction?
FAD, which is reduced to FADH₂.
150
How much ATP is generated per molecule of cytoplasmic NADH using the glycerol-3-phosphate shuttle?
1.5 ATP.
151
What is oxaloacetate (OAA) converted into for transport into the mitochondrial matrix?
malate
152
What happens to malate inside the mitochondrial matrix?
It is reoxidized to oxaloacetate, producing NADH.
153
Why is oxaloacetate converted into aspartate in the matrix?
Because OAA cannot cross the inner mitochondrial membrane; aspartate can.
154
What are the two key transport proteins involved in the malate–aspartate shuttle?
Glutamate–aspartate transport protein
Malate–α-ketoglutarate transport protein
155
How much ATP is produced per cytoplasmic NADH using the malate–aspartate shuttle?
2.25 ATP
156
What is the function of uncoupling proteins in mitochondria?
They translocate protons across the inner membrane, reducing ATP synthesis and dissipating energy as heat.
157
What is UCP1 also known as, and where is it found?
Thermogenin; found in brown adipose tissue mitochondria.
158
How is UCP1 activated?
By binding to fatty acids released during fat hydrolysis.
159
What neurotransmitter regulates nonshivering thermogenesis in brown fat?
Norepinephrine.
160
What is the physiological role of UCP2 and UCP3?
UCP2: Controls reactive oxygen species (ROS) formation.
UCP3: Regulates fatty acid oxidation and reduces ROS in skeletal muscle and brown fat.
161
Where are UCP4 and UCP5 expressed, and what is known about their function?
Expressed in central nervous system neurons; their functions are not well understood
162
Why is the oxidation of cytoplasmic NADH via the ETC preferred over lactate formation during glycolysis?
Because it produces more ATP. NADH oxidation by the ETC yields up to 2.5 ATP per molecule, whereas lactate formation regenerates NAD⁺ without producing additional ATP.
163
Why can’t cytoplasmic NADH directly enter the mitochondrial matrix?
The inner mitochondrial membrane is impermeable to NADH.
164
What are the two main shuttles that transfer electrons from cytoplasmic NADH to the mitochondrial ETC?
The glycerol-3-phosphate shuttle and the malate–aspartate shuttle.
165
How does the glycerol-3-phosphate shuttle work?
DHAP is reduced by NADH to glycerol-3-phosphate, which is then oxidized by mitochondrial glycerol-3-phosphate dehydrogenase using FAD, forming FADH₂ that enters the ETC.
166
How much ATP is generated per NADH molecule via the glycerol-3-phosphate shuttle?
Approximately 1.5 ATP.
167
How does the malate–aspartate shuttle transfer reducing equivalents into the matrix?
OAA is reduced to malate by NADH in the cytosol; malate enters the matrix and is reoxidized to OAA, regenerating NADH inside the matrix.
168
Why must OAA be converted to aspartate in the malate–aspartate shuttle?
Because OAA cannot cross the inner mitochondrial membrane
169
What is the ATP yield per NADH molecule using the malate–aspartate shuttle?
Approximately 2.25 ATP (slightly lower than 2.5 due to proton cost of transport).
170
What are reactive oxygen species (ROS) and how are they formed in mitochondria?
ROS are unstable oxygen-containing molecules formed when O₂ partially accepts electrons during the ETC, especially at complexes I and III.
170
What is the total ATP yield from one glucose molecule using the ETC and shuttle systems?
Between 29.5 and 31 ATP depending on the shuttle used.
171
What is the first ROS formed during oxygen reduction?
a The superoxide radical (O₂⁻*)
172
What enzyme converts superoxide radicals into hydrogen peroxide (H₂O₂)?
Superoxide dismutase (SOD); SOD1 in the intermembrane space and SOD2 in the matrix.
173
Why is H₂O₂ less dangerous than superoxide or hydroxyl radicals?
H₂O₂ is not a radical and is less reactive; it can diffuse and act as a signaling molecule.
174
What reaction produces the hydroxyl radical (*OH), and why is it dangerous?
Fenton reaction: H₂O₂ + Fe²⁺ → *OH + OH⁻ + Fe³⁺. The hydroxyl radical is extremely reactive and damages any biomolecule it contacts.
175
What is the function of uncoupling protein 1 (UCP1)?
UCP1 dissipates the proton gradient as heat instead of synthesizing ATP, enabling nonshivering thermogenesis in brown fat.
176
What activates UCP1?
Fatty acids generated by norepinephrine-stimulated lipolysis.
177
What is the role of glutathione (GSH) in redox homeostasis?
GSH acts as a reducing agent, helping to detoxify ROS and maintain a reducing environment.
178
What is oxidative stress?
a condition where ROS levels exceed the cell’s antioxidant capacity, leading to damage.
179
What types of damage are associated with oxidative stress?
Enzyme inactivation, DNA breakage, membrane lipid peroxidation, and polysaccharide degradation.
180
Name diseases associated with oxidative stress.
Cancer, cardiovascular diseases, Parkinson’s, Alzheimer’s, ALS.
181
what are reactive nitrogen species (RNS), and name examples.
Nitrogen-containing radicals linked to ROS; examples include nitric oxide (*NO), nitrogen dioxide (*NO₂), and peroxynitrite (ONOO⁻).
182
What roles does nitric oxide (*NO) play in physiology?
Blood pressure regulation, blood clot inhibition, immune response, and cell death signaling.
183
How does *NO cause protein damage?
By nitrosylating thiol groups (forming SNO derivatives) and damaging iron–sulfur clusters.
184
Describe lipid peroxidation.
It begins when a fatty acid is oxidized to a lipid radical, which reacts with O₂ to form lipid peroxyl radicals, propagating chain reactions that damage membranes.
185
How are lipid peroxidation chain reactions initiated and sustained?
Initiated by ROS and transition metals like Fe²⁺, sustained as peroxyl radicals extract H atoms from other lipids.
186
What are some non-enzymatic physical defenses organisms use to prevent oxidative stress?
Melanin protects against photooxidative stress, and chromatin protects DNA from oxidative attack.
187
Once ROS are formed in excess, how are they primarily neutralized?
By antioxidant enzymes like superoxide dismutase, glutathione peroxidase, peroxiredoxin, and catalase.
187
What is the reaction catalyzed by SOD enzymes?
2O₂⁻* + 2H⁺ → H₂O₂ + O₂
187
What mechanisms repair oxidative biomolecular damage?
DNA repair mechanisms and autophagy.
188
What disease is associated with a mutation in the SOD1 gene?
Amyotrophic lateral sclerosis (ALS), due to mitochondrial dysfunction caused by H₂O₂ accumulation.
189
What are the three human isoforms of superoxide dismutase (SOD)?
SOD1 (Cu-Zn): Found in cytoplasm and mitochondrial intermembrane space.
SOD2 (Mn): Located in the mitochondrial matrix.
SOD3 (Cu-Zn): Found extracellularly
190
What does glutathione peroxidase (GPx) do, and what cofactor does it require?
It reduces H₂O₂ and organic peroxides using GSH; it requires selenium.
191
What does GPx4 specifically act on?
Lipid hydroperoxides.
192
What is the catalytic mechanism of peroxiredoxin (PRX)?
Peroxide oxidizes a cysteine thiol group to sulfenic acid (RSOH), which is then reduced by thioredoxin (TRX).
192
How is reduced glutathione (GSH) regenerated from oxidized glutathione (GSSG)?
by glutathione reductase, using NADPH as the electron donor.
193
What are the primary cellular sources of NADPH for GSH regeneration?
The pentose phosphate pathway, isocitrate dehydrogenase, and the malic enzyme.
194
