Citric Acid Cycle

Question 1

What are the final products of the citric acid cycle ?

Answer 1

CO2 and water

Question 2

Name the steps of the citric acid cycle.

Answer 2

1. Condensation of the acetyl group of Acetyl-CoA (2 Cs) with the keto acid Oxaloacetate (4 Cs), forming Citrate (6 Cs)
2. Citrate (6 Cs) converted to isocitrate (6 Cs)
3. Isocitrate (6 Cs) oxidised into Oxalosuccinate (6 Cs)
   (simultaneously, NAD+ reduced to NADH)
4. Oxalosuccinate (6 Cs) decarboxylated into alpha-Ketoglutarate (5 Cs)
5. More stages of oxidation (uses reduction of NAD+ to NADH and FAD to FADH2), finally result in formation of a new molecule of the original 4-carbon compound (oxaloacetate)

Question 3

What are products and byproducts of the citric acid cycle ?

Answer 3

• 2 molecules of CO2
• 1 molecule of GTP
• 3 molecules of NADH
• 1 molecule of FADH2
• Water

Question 4

How is water formed ?

Answer 4

The remaining hydrogen (not used by NAD+ or FAD) is combined with molecular O2 in a process coupled to ATP synthesis

Question 5

What are possible results of deficiency of riboflavin ?

Answer 5

Lesions of the mucous membranes

cracked, sore lips and corners of mouth; similar lesions in genital regions; tongue swollen, tender, magenta

Question 6

Where does the citric acid cycle take place ?

Answer 6

In the matrix of the mitochondria

Question 7

What are other names for the citric acid cycle ?

Answer 7

Krebs Cycle, Tricarboxylic Acid Cycle

Question 8

What molecule is FLAVIN ADENINE DINUCLEOTIDE (FAD) formed from ?

Answer 8

Riboflavin (vitamin B2)

Question 9

Explain the major role of flavin adenine dinucleotide (FAD) in oxidation.

Answer 9

It is an electron/proton acceptor so get reduced (accepts 2 protons) to allow for the oxidation of the 6-carbon molecule (citrate)

Question 10

How is pyruvate converted to acetyl-Coenzyme A ?

Answer 10

1. Translocation of Pyruvate into the mitochondrial matrix
2. Decarboxylation (loss of CO2) and oxidation (reduction of NAD+ into NADH) (in this process, coenzyme A forms a high energy bond with it)

Question 11

What are the main structural features of coA ?

Answer 11

Nucleotide that contains the vitamin pantothenic acid

= vitamin B5

Question 12

What is the bonding group of CoA ?

Answer 12

HS

Question 13

What property of ketoacids allows them to be decarboxylated ?

Answer 13

Their instability

Question 14

Where does oxidative phosphorylation take place ?

Answer 14

On macromolecular assemblies of proteins in the inner mitochondrial membrane

Question 15

What is another name for electron transfer chain ?

Answer 15

Respiratory chain

Question 16

What are the components of the respiratory chain ?

Answer 16

Four large protein complexes linked together by molecules that act as electron shuttles

Question 17

What are some chemicals which may target the respiratory chain ?

Answer 17

Rotenone (Insecticide, piscicide) 
Antimycin A (Antibiotic)
Cyanide and carbon monoxide

Question 18

Which step of the respiratory chain do cyanide and carbon monoxide block ?

Answer 18

They block the final step by which hydrogen combines with oxygen

Question 19

What are the names of the three main protein complexes in the respiratory chain ?

Answer 19

Complex I- NADH-Q-reductase
Complex III- Cytochrome-reductase
Complex IV- Cytochrome-oxidase

Question 20

Describe the main steps of the respiratory chain.

Answer 20

1. NADH transfer its electrons directly to complex I, turning back into NAD+ As electrons move through complex I, energy is released, and the complex uses this energy to pump protons from the matrix into the intermembrane space.
2. FADH2 feeds electrons into the transport chain through complex II, which does not pump protons across the membrane (Because of this “bypass,” each FADH2 molecule causes fewer protons to be pumped (and contributes less to the proton gradient) than an NADH).
3. Beyond the first two complexes, electrons from NADH and FADH2 travel exactly the same route.
   Both complex I and complex II pass their electrons ubiquinone (Q) electron carrier, which is reduced to form QH2 and travels through the membrane, delivering the electrons to complex III. As electrons move through complex III, more H+ ions are pumped across the membrane, and the electrons are ultimately delivered to another cytochrome C (cyt C) electron carrier. Cyt C carries the electrons to complex IV, where a final batch of H+ ions is pumped across the membrane.
4. Complex IV passes the electrons to O2 which splits into two oxygen atoms and accepts protons from the matrix to form water. Four electrons are required to reduce each molecule of O2 and two water molecules are formed in the process.
5. Movement of electrons down the respiratory chain generated a H+ ion (pH) gradient across the inner mitochondrial membrane (high H+ concentration in the intermembrane space relative to the mitochondrial matrix)
   This gradient is used to drive ATP synthesis by way of ATP synthase (= H+-ATPase) (ATP synthase, an enzyme embedded in the membrane, pumps protons into the mitochondrial matrix, thus driving the hydrolysis of ATP from ADP + Pi) = CHEMIOSMOSIS

Question 21

What is the function of NADH and FADH2 in the respiratory chain ?

Answer 21

They feed electrons into the system at different places

Question 22

What are the names of the electron shuttles in the respiratory chain ?

Answer 22

Cytochrome C and Ubiquinone

Question 23

How much ATP is generated by the oxidation of each molecule of NADH ? FADH2 ?

Answer 23

Oxidation of each molecule of NADH produces about 3 molecules of ATP.
Oxidation of each molecule of FADH2 generates 2 ATP molecules.

Question 24

Does the H+-ATPase work in the same or opposite direction from the sodium pump (Na-K ATPase) in the surface membrane ?

Answer 24

Opposite direction (The sodium pump uses ATP to generate an ion gradient, while the ATP synthase uses an ion gradient to generate ATP)

Question 25

How many molecules of ATP are produced by breakdown of one molecule of glucose by way of glycolysis ? In the entire process of aerobic metabolism (incl. glycolysis, citric acid cycle and respiratory chain) ? In the process of anaerobic metabolism ?

Answer 25

Glycolysis: Net +2 ATP (and +2 of NADH)
Aerobic metabolism (glycolysis + citric acid cycle + oxidative phosphorylation): Net +30-32 ATP
Anaerobic metabolism: Net +2 ATP

Question 26

What are the main functions of the respiratory chain ?

Answer 26

1. Regenerates electron carriers. NADH and FADH2 pass their electrons to the electron transport chain, turning back into NAD+ and FAD. The oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running.
2. Makes a proton gradient. The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H+ in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy, and it can be used to make ATP.