LECTURE 22 - MIDTERM 3 Flashcards
What is step 2 of the ETC?
– QH2 transfers electrons to cytochrome c - the Q cycle
– complex III – multiple electron transfer centers within the protein/enzyme complex
T or F, the heme groups found on Complex III are the same as those found in hemoglobin
True
Describe the first half of the Q cycle.
– First half: QH2 enters enzyme and gives up 2 electrons. 1 flows to cytochrome c, which then diffuses away. The 2nd flows to a distinct molecule of Q bound at a distal site – this converts distal Q (Q that is bound to Q 1 site) to the semiquinone intermediate, Q
– The Q-cycle is initiated when QH2 diffuses through the bilipid layer to the QH2 binding site (Q0 site).
Describe the second half of Q cycle.
Another QH2 enters enzyme and gives up 2 electrons. 1 flows to cytochrome c, which then diffuses away, and 2nd completes the reduction of the distal Q- intermediate to form QH2
What is the key concept of the Q cycle?
– key concept is that the protons released during oxidation of the 2 QH2 molecules go to cytoplasm/intermembrane space. In addition, during reduction of distal Q new protons are pulled from matrix
– Note: while 2 different QH2 molecules are oxidized during the cycle, the distal Q is reduced to QH2 and thus there is a net metabolism of 1 molecule of QH2
– 4 protons are being pumped into the IMS
– when QH2 donates its electrons its protons are pumped into IMS
What is 3rd step of the ETC?
– Reduction of O2 by Complex IV
– reactions of Complex III generate 2 molecules of reduced cytochrome c for every 1 molecule of QH2 that is reduced to Q. Q is a 2-electron carrier, cytochrome c is a single electron carrier
– the final step involves reduction of molecular oxygen to form water
– catalyzed by cytochrome c oxidase or COMPLEX IV
– this reaction is why we need to breather air in order to live
Describe electron flow within Complex IV.
- electron eneters from cytochrom c into CuA/CuA then moves to
- —> Heme A
- —> then ends up at CuB
- two molecules of cytochrome c sequentially transfer electrons to reduce CuB and heme a3
- ——> have porifirin ring structures that are good for oxygen
– two reduced CuB and Fe in heme a3 bind O2, which forms a peroxide bridge (iron picks up one electron and copper picks up other)
– the addition of two more electrons and two more protons cleaves the peroxide bridge
– the addition of two more protons leads to the release of water
NOTE: these reactions extract 4 protons from the matrix, and all 4 end up in water –> proton gradient being formed
What is the Complex IV influence on proton gradient?
– Reduction of O2 to water releases a great deal of energy, some of which is harnessed by Complex IV to pump an additional 4 protons from the matrix to the cytoplasm
Net reaction:
4 Cyt c + 8H+ + O2 –> 4 Cyt C + 2 H2O + 4 H+
Note: 1 NADH reduces 2 Cyt C so we need 4 Cyt C to reduce molecular oxygen
– we would need 2 NADH molecules to provide 4 electrons necessary to reduce molecular oxygen
What is a brief summary of the ETC?
– 10 protons are going to be contributed to ETC from single NADH
—> 4 protons in Complex I, 4 in Complex III and 2 in Complex IV
– use of oxygen as electron acceptor can adverse consequences
– FADH2 is going to pump 6 protons
– also there are 6 NADH molecules per one glucose molecule
T or F, a single FADH is going to pump 6 protons
True
Describe the reduction of O2.
– sequential transfer of 4 electrons to O2 yields a safe product (water)
– partial reduction yield dangerous products –> causes oxidative damage
– reactive oxygen species or ROS cause oxidative damage implicated in aging as well as a growing list of diseases
– this is because when it’s partially reduced its free electrons bind to other molecules and create reactions that aren’t supposed to happen
– cytochrome c oxidase (complex IV) hold O2 tightly until it’s fully reduced in most cases, but sometimes ROS are released
What is the defense systems against Reactive Oxygen Species ?
– Super Oxide Dismutase (ROS-SOD) reaction mechanism
– it turns free radicals into oxygen
– the enzyme itself becomes reduced during this reaction
– in its reduced form, it catalyzes a different reaction
T or F, catalase scavanges H2O2
– catalyzes the reduction of hydrogen peroxide
2H2O2 –> O2 + 2 H2O
– help combat issue of oxygen being partially reduced
What is the chemiosmotic theory?
- electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane –> gradient is needed for ATP synthesis
- proposed by Peter Mitchell
What are Mitchell’s postulates for chemiosmotic theory?
– Intact inner mitochondrial membrane is required
– electron transport through ETC generates a proton gradient
– ATP synthase catalyzes the phosphorylation of ADP in a reaction driven by movement of H+ across the inner membrane into the matrix
T or F, gradients have the ability to do work, called the proton-motive force
– True, 2 components; chemical (pH) gradient and charge gradient
– intermembrane space is more acidic bc there are more protons there whereas the matrix is more basic since there’s a low amount of protons
T or F, The proton gradient that is established by the electron transport chain flows back through an ion channel that powers ATP synthesis
True; ATP Synthase
– protons ejected by electron flow through respiratory complexes
What is the ATP synthase specifically the structure and mechanism ?
– originally called “mitochondrial ATPase” because its capacity to catalyze the reverse reaction was discovered first
– macromolecular organization
– 2 assemblies, F0 and F1
F0 complex - sits in the membrane and forms the proton channel
F1 complex - contains the catalytic activity, and protrudes into mitochondrial matrix
these two are held together by a central stalk
– stator stabilizes entire enzyme
Describe the F1 subunit.
– 5 proteins in the following stoichiometry: a3, B3, gamma, delta, epsilon.
– alpha and beta subunits arranged alternately in a hexameric ring structure
– gamma, delta and epsilon form a central stalk with gamma extending down into ring structure of the alpha/beta hexaner
– importantly makes a different type of contact with each individual Beta subunit, and this distinguishes the beta subunits conformation
What is the difference between the L, O and T conformations?
– L; loosely binds to ADP –> ADP + APi
– Tight –> at tight place is where ATP is being made
— Open this is the place where ADP enters
Describe the F0 subunit.
– 10 -14 subunits, forming a ring-llike channel
– single a subunit binds the outside of the ring
– F0 and F1 are connected by the gamma/epsilon stalk as well as the a/b2/delta complex
– mechanically, the F0 subunit rotates and the F1 assembly is stationary during catalysis
– catalyzes following reaction:
ADP3 + HPO42- + H+ ----> ATP4- + H2O
