LECTURE 22 - MIDTERM 3

Question 1

What is step 2 of the ETC?

Answer 1

– QH2 transfers electrons to cytochrome c - the Q cycle

– complex III – multiple electron transfer centers within the protein/enzyme complex

Question 2

T or F, the heme groups found on Complex III are the same as those found in hemoglobin

Answer 2

True

Question 3

Describe the first half of the Q cycle.

Answer 3

– 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).

Question 4

Describe the second half of Q cycle.

Answer 4

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

Question 5

What is the key concept of the Q cycle?

Answer 5

– 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

Question 6

What is 3rd step of the ETC?

Answer 6

– 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

Question 7

Describe electron flow within Complex IV.

Answer 7

• • 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

Question 8

What is the Complex IV influence on proton gradient?

Answer 8

– 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

Question 9

What is a brief summary of the ETC?

Answer 9

– 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

Question 10

T or F, a single FADH is going to pump 6 protons

Answer 10

True

Question 11

Describe the reduction of O2.

Answer 11

– 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

Question 12

What is the defense systems against Reactive Oxygen Species ?

Answer 12

– 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

Question 13

T or F, catalase scavanges H2O2

Answer 13

– catalyzes the reduction of hydrogen peroxide

2H2O2 –> O2 + 2 H2O

– help combat issue of oxygen being partially reduced

Question 14

What is the chemiosmotic theory?

Answer 14

• • 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

Question 15

What are Mitchell’s postulates for chemiosmotic theory?

Answer 15

– 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

Question 16

T or F, gradients have the ability to do work, called the proton-motive force

Answer 16

– 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

Question 17

T or F, The proton gradient that is established by the electron transport chain flows back through an ion channel that powers ATP synthesis

Answer 17

True; ATP Synthase

– protons ejected by electron flow through respiratory complexes

Question 18

What is the ATP synthase specifically the structure and mechanism ?

Answer 18

– 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

Question 19

Describe the F1 subunit.

Answer 19

– 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

Question 20

What is the difference between the L, O and T conformations?

Answer 20

– 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

Question 21

Describe the F0 subunit.

Answer 21

– 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