Lecture 21 - MIDTERM 3

Question 1

How do we get energy?

Answer 1

– glycolysis and CAC produce little ATP, but we do generate reduced electron carriers, NADH and FADH2

– these molecules are re-oxidized in the mitochondria during cellular respiration by passing electrons through a series of electron carriers, called the electron transport chain

– finally reducing O2 to H2O

– free energy released during these redox reactions is coupled to ATP synthesis, called oxidative phosphorylation

Question 2

Describe oxidative phosphorylation.

Answer 2

– the majority of ATP recycling occurs via oxidative phosphorylation

– Ox-Phos is the process by which ATP is formed as a result of transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers

– Complete oxidation of a glucose to CO2 and H2O generates 32-36 molecules of ATP, and ox-phos is responsible for 26

Question 3

How do NADH and FADH contribute to the proton gradient?

Answer 3

– NADH and FADH2 generated during glycolysis the TCA cycle, and other pathways are reduced O2 to H2O

– this creates a proton gradient across the membrane

– the energy in the gradient, the proton-motive force, powers the phosphorylation of ADP to ATP

Question 4

Where does ox-phos take place?

Answer 4

– in the mitochondria

Question 5

Describe Mitochondria organization.

Answer 5

– two membranes; Outer membrane surrounds the organelle. Inner membrane is folded in an elaborate manner to create cristae –> folding increases surface area of inner membrane, more sites for ox-phos

– space between outer and innner membrane = intermembrane space

– space within inner membrane = matrix

– outer membrane –> permeable to most small molecules and ions

– inner membrane –> site of ox-phos reactions; highly impermeable

Question 6

Describe mitochondria organization and location of respiratory chain.

Answer 6

– NADH or FADH2 are oxidized on the inner mitochondrial membrane by enzyme complexes of the respiratory chain

– 5 separate complexes, I-IV are involved in redox reactions (ETC) and complex V is an ATP synthase (phosphorylation)

Question 7

What is the purpose of Complex V?

Answer 7

– it takes energy from proton gradient and that drives ATP synthesis

Question 8

What is the mobile that carries electrons from complexes 1, 2 and 3?

Answer 8

– Coenzyme Q

Question 9

T or F, 8 electrons from CAC move to inner membrane of mitochondria

Answer 9

True; 6 from NADH and 2 from FADH2 ( 3 NADH and 1 FADH2 each of which carry 2 electrons)

Question 10

T or F, NADH and FADH2 enter in different places in the ETC

Answer 10

True, NADH goes to Complex I and FADH2 goes to Complex II

Question 11

What is the key concept of the ETC?

Answer 11

– electron transfer through the chain is driven by differences in the electron-transfer potential of the individual components

– NADH and FADH2 donate these electrons to carriers that ultimately transfer them to O2 to form water

Question 12

What is the difference between strong reducing agents and strong oxidizing agents?

Answer 12

– strong reducing agent (e.g. NADH) is inclined to donate e- and has neg E’0 value

– strong oxidizing agent (e.g. O2) is inclined to accept e- and has pos E’0 value

Question 13

How many ATP can 1 molecule NADH drive in the synthesis of ATP?

Answer 13

– 2.5 molecules of ATP

– electrons on NADH drive synthesis of ATP

Question 14

How does the reduction of oxygen to water by NADH occur?

Answer 14

– it happens gradually and involves several carriers

– electrons are transported down a chain

– NADH –> Q –> cytochrome c –> O2

– note that E’O gets increasingly positive as electrons move down the chain

– the more positive the value, the more able a substance is to accept electrons

– redox potential thus determines the direction of electron flow

Question 15

What happens to the redox potential as electrons from NADH move down ETC?

Answer 15

– the potential becomes increasingly positive

– the more positive the value the easier it is to accept those electrons which helps move/drive electrons in a unidirectional fashion

Question 16

T or F, large enzyme complexes catalyze electron transfer and harness the released energy to pump protons

Answer 16

True; electron transfer is catalyzed by enzymes located in the inner mitochondrial membrane

– these are massive complexes

Question 17

What role do prosthetic groups have in the ETC?

Answer 17

– they help the transfer of electrons to ultimately pump protons

Question 18

What is the key concept of electron transfer?

Answer 18

– electron transfer drives the pumping of protons across the inner membrane, from the matrix to the intermembrane space, against a concentration gradient.

Question 19

Describe Step 1 of NADH oxidation

Answer 19

– coenzyme Q reduction – proton pumping

– catalyzed by NADH Q oxidoreductase (Complex I)

– electrons transferred to coenzyme Q (Q) – a quinone derivative that exists in several different oxidation states

– another important feature of Q is that is it mobile within the inner membrane of the mitochondria

Question 20

What is the electron transport route through complex I?

Answer 20

– NADH enters complex I and is picked up by FMN which then transfers them to iron sulfur complexes; protons reduce FMN to FMNH2 in the process

– Quinon (Q) picks them up from iron-sulfur complexes and becomes reduced to QH2 and then exits complex I

– as electrons exit there is a conformational change in Complex I

Question 21

What is FMN and what is it’s role in complex I in ETC?

Answer 21

– FMN is Flavin mononucleotide (oxidized) (FMN)

– NADH reduces it to FMNH2 (w proton)

– it is involved with the first transfer step –> it picks up electrons from NADH

– 2 electrons are transferred

Question 22

What are the Iron- Sulfur clusters?

Answer 22

– Iron boun to Cysteine’s sulfhydral groups

– also have Iron bound to Cys sulfhydral groups and inorganic sulfides

– the sulfhydryl groups help to bind iron together

– iron-sulfur clusters are important bc they help electrons move along through being good accepters and transferers

Question 23

T or F, after Q exits complex I the protons diffuse back into the intermembrane space

Answer 23

False; mitochondrial matrix

Question 24

Describe coenzyme Q as an electron carrier.

Answer 24

– Coenzyme Q is a versatile cofactor because it is a soluble electron carrier in the hydrophobic bilipid layer of the inner mitochondrial membrane.

– Like flavoproteins, CoQ can accept/donate electrons one at a time or two at a time

– Long hydrophobic tail anchors Q in the membrane, but it’s mobile

– exists in different oxidative states

Question 25

What is the importance of Q’s varied redox states?

Answer 25

– can exist stabling a semiquinone state ( can pick up 1 electron)

– important if the need arises to transfer one electron at a time

– can also pick up one or two electrons at one time

– also high energy electron carrier; picking up electrons but not being reduced

– Q can pick up one or two electrons depending on who is donating

Question 26

T or F, Q accepts protons exclusively from the matrix

Answer 26

True

Question 27

What happens when there is a conformational change of Complex I?

Answer 27

– Another 4 protons are “pumped,” due to the conformation change which comes from electrons exiting complex I

Question 28

What is the net reaction of NADH oxidation in complex 1?

Answer 28

– electron transfer from NADH through complex I provides QH2 for the Q pool and pumps protons

– NADH + Q + 5protons matrix –> NAD+ +QH2 + 4protons intermembrane space

Question 29

T or F, another source of QH2 are reactions catalyzed by Complex II, or succinate-Q reductase

Answer 29

True

Question 30

Describe the role succinate in complex II.

Answer 30

– succinate dehydrogenase is part of complex II with succunate-Q reductase

– FADH2 never leaves the complex instead it is used to reduce Q to QH2 and thereby increase QH2 content in the Q pool

– Although QH2 is generated, no protons are pumped in this reaction

Question 31

How does FADH2 drop off its electrons?

Answer 31

– FADH drops off electrons at Q-cytocrhome c oxidoreductase

– Q comes and picks it up and this is a different Q that is also then reduced to QH2

Question 32

T or F, Q being reduced to QH2 induces conformational change?

Answer 32

True

Question 33

T or F, Coenzyme Q can also receive electrons from other sources

Answer 33

True; but all enter the same “Q pool”

– from here QH2 transfers electrons to Complex III

Question 34

T or F, FAD functions as 2 electron acceptor and a 1 electron donor

Answer 34

true; final step of this Complex is transfer of 2 electrons one at a time to coenzyme Q to produce CoQH2