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MCAT Biochemisty > Chapter 10- Carbohydrate Metabolism II > Flashcards

Chapter 10- Carbohydrate Metabolism II Flashcards

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MCAT flashcards
1
Q

2 other names for the citric acid cycle

A

krebs cycle and tricarboxylic acid (TCA) cycle

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2
Q

main function of citric acid cycle?

A

oxidation of acetyl-CoA to CO2 and H2O

-this cycle also produces NADH and FADH2

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3
Q

what happens to pyruvate after its formed

A
  1. enters mitochondrion via active transport

2. pyruvate is oxidized and decarboxylated by pyruvate dehydrogenase complex (multienzyme complex)– becomes acetyl-CoA

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4
Q

enzymes in pyruvate dehydrogenase complex

A
  1. pyruvate dehydrogenase (PDH): oxidizes pyruvate into a 2 carbon molecule, yielding CO2 with TPP (coenzyme) and Mg2+
  2. dihydrolipoyl transacetylase: 2 carbon molecule gets bonded to TPP and transferred to lipoic acid (coenzyme)… end result is lipoic acid in the reduced form
  3. dihydrolipoyl dehydrogenase: FAD used as coenzyme to oxidize lipoic acid to help acetyl-CoA formation… FAD reduces to FADH2

*first three convert pyruvate to acetyl-CoA

  1. pyruvate dehydrogenase kinase
  2. pyruvate dehydrogenase phosphatase

*4 and 5 regulate actions of PDH

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5
Q

coenzyme A (CoA)

A

CoA-SH is a thiol (has an SH group)— this is a thioester which has very high-energy properties

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6
Q

what are different ways of forming acetyl-CoA (other than glycolysis forming pyruvate to turn into acetyl-CoA)

A
  1. fatty acid oxidation (B-oxidation)
  2. amino acid catabolism
  3. ketones
  4. alcohol
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7
Q

B-oxidation (pre-steps)

A

aka. fatty acid oxidation. before B-oxidation can occur the fatty acid must go through activation which causes a thioester bond to form between carboxyl groups of fatty acids and CoA. once carnitine brings the complex into the inner membrane of the mitochondria then acyl-CoA is formed which then undergoes B-oxidation.

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8
Q

carnitine

A

brings CoA-SH from the cytosol (intermembrane space) to the inner membrane of the mitochondria

cytosolic CoA-SH —> mitochondrial CoA-SH

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9
Q

amino acid catabolism

A

only with certain amino acids (ketogenic aa). lose their amino group via transamination. the carbon skeleton then forms ketone bodies.

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10
Q

ketones forming acetyl-CoA

A

ketone bodies are essentially transportable molecules of acetyl-CoA

acetyl-CoA is typically used to produce ketones when pyruvate dehydrogenase complex is inhibited the reverse reaction can occur as well to produce acetyl-CoA. (DONT NEED TO KNOW ENZYMES)

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11
Q

alcohol

A

when alcohol is consumed in moderate amounts the enzymes alcohol dehydrogenase and acetaldehyde dehydrogenase convert it to acetyl-CoA (primarily used for fatty acid synthesis b/c the krebs cycle is inhibited due to NADH buildup from alcohol consumption)

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12
Q

overall reaction of pyruvate dehydrogenase complex?

A

pyruvate + CoA-SH + NAD+ —> acetyl-CoA + CO2 + NADH + H+

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13
Q

general krebs cycle

A

acetyl-CoA (2C) goes into the cycle and reacts with oxaloacetate (OAA, 4C) using citrate synthase (enzyme that aids in condensation reaction). this forms citrate. through a variety of reactions citrate

makes 3NADH, 1GTP, 1FADH2 per pyruvate

each NADH = 2.5ATP
each FADH2 = 1.5ATP

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14
Q

synthases

A

enzymes that form new covalent bonds without needing significant energy

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15
Q

citrate formation

A

KREBS STEP 1: acetyl-CoA + Oxaloacetate + H2O —> Citrate + CoA-SH + H+

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16
Q

citrate isomerized to isocitrate

A

KREBS STEP 2: citrate (aconitase) and releases H2O cis-aconitate D-Isocitrate (aconitase) and adds water in a different place as before

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17
Q

a-ketoglutarate and CO2 formation

A

KREBS STEP 3: isocitrate (isocitrate dehydrogenase) —> produces NADH + oxalosuccinate —> releases CO2 and adds H+ + a-Ketoglutarate

*Note: isocitrate dehydrogenase is the rate-limiting enzyme for the citric acid cycle.
The first NADH is produced and CO2 is released here as well.

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18
Q

Succinyl-CoA and CO2 formation

A

KREBS STEP 4: a-ketoglutarate (a-ketoglutarate dehydrogenase complex) + CoA-SH + NAD+ —> Succinyl-CoA + CO2

*Note: NADH produced.

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19
Q

dehydrogenases

A

subtype of oxidoreductases (enzymes that catalyze redox reactions). transfer hydride ion (H-) to an electron acceptor, ususally NADH or FADH2.

*when you see a dehydrogenase look for a high-energy electron carrier being formed.

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20
Q

Succinate Formation

A

KREBS STEP 5: Succinyl-CoA + CoA-SH (succinyl-CoA synthetase) —> Succinate

*Note: GDP becomes GTP in this step as well, this GTP undergoes nucleosidediphosphate kinase (also does GDP to GTP) which catalyzes the phosphate transfer from GTP to ATP (ONLY time in krebs cycle where ATP is produced directly).

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21
Q

whats the difference between a synthetase and a synthase?

A

sythetases create new covalent bonds with energy input.

synthases don’t use energy.

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22
Q

Fumarate Formation

A

KREBS STEP 6: only step that takes place in the inner membrane of the mitochondria instead of the matrix.
succinate is oxidized to yield fumarate by succinate dehydrogenase– considered a flavoprotein b/c its covalently bonded to FAD (which is reduced to FADH2 during this process).

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23
Q

how many ATP are produced for each molecule of FADH2 and each molecule of NADH?

A

FADH2 —> 1.5 ATP

NADH —> 2.5 ATP

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24
Q

Malate Formation

A

KREBS STEP 7: fumarase catalyzes the hydrolysis of the alkene bond in fumarate (giving rise to malate, only the L conformation)

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25
Q

Oxaloacetate Formed Anew

A

KREBS STEP 8: oxidation of malate to oxaloacetate by malate dehydrogenase. NAD+ is reduced to NADH as well.

26
Q

Substrates for each step of the Citric acid cycle

A

Please, Can I Keep Selling Sex For Money, Officer?

  1. Pyruvate
  2. Citrate
  3. Isocitrate
  4. a-Ketoglutarate
  5. Succinyl-CoA
  6. Succinate
  7. Fumarate
  8. Malate
  9. Oxaloacetate
27
Q

what are the products (results) of the pyruvate dehydrogenase complex?

A

acetyl-CoA
NADH
CO2
H+

28
Q

what the producs (results) of the citric acid cycle?

A

Important products

  • 3 NADH
  • FADH2
  • GTP

other products

  • 2CO2
  • CoA-SH
  • 3 H+
29
Q

How many total ATP are made after the citric acid cycle (including pyruvate dehydrogenase)

A

12.5 ATP per pyruvate and a total of 25 ATP per glucose

30
Q

how does the pyruvate dehydrogenase complex regulate the citric acid cycle?

A

when levels of ATP rise, phosphorylating the pyruvate dehydrogenase kinase inhibits acetyl-CoA production.

This enzyme is then reactivated by pyruvate dehydrogenase phosphatase in response to high levels of ADP.

*Note: overall the citric acid cycle regulation is determined by ATP/ADP and NADH/NAD+ ratios.

31
Q

what are the 3 control points of the citric acid cycle?

A
  1. citrate synthase (ATP and NADH function as allosteric inhibitors of citrate synthase, both citrate and succinyl-CoA can also inhibit citrate synthase directly)
  2. isocitrate dehydrogenase (this enzyme catalyzes the citric acid cycle and is inhibited by ATP and NADH. it is activated by ADP and NAD+)
  3. a-Ketoglutarate dehydrogenase complex (inhibited by ATP, NADH, and succinyl-CoA. stimulated by ADP and Ca2+ ions)
32
Q

1 fact thats important to note about the electron transport chain

A

it is not the flow of electrons but the proton gradient that ultimately produces ATP.

33
Q

where do glycolysis/fermentation and the citric acid cycle occur?

A
  • anaerobic processes occur in the cytosol

- aerobic processes occur in the mitochondira, the citric acid cycle occurs in the mitochondrial matrix

34
Q

what is the purpose of cristae in the mitochondria?

A

maximizes surface area.

the inner membrane is essential for generating ATP using the proton-motive force (electrochemical proton gradient)

35
Q

final step in aerobic respiration

A
  1. electron transport along the inner mitochondrial membrane
  2. generation of ATP via ADP phosphorylation

*separate yet coupled processes. NADH/FADH2 transfer their electrons to carrier proteins along the inner mitochondrial membrane. these electrons are then given to oxygen in the form of hydride ions (H-) and water is formed.

36
Q

energy and electron transport chain

A

formation of ATP - endergonic
electron transport - exergonic

*by coupling these reactions, the energy yielded by one reaction can fuel the other

37
Q

Complex I (NADH-CoQ oxidoreductase)

A

first membrane bound complex in transport chain. transfer of electrons from NADH to coenzyme Q (CoQ, ubiquinone) is catalyzed in this first complex.
over 20 subunits (2 important: iron-sulfur cluster and a flavoprotein (w/ coenzyme flavin mononucleotide-FMN- that oxidizes NADH)

-adds 4 protons to gradient

net effect: passing high-energy electrons from NADH to CoQ to form CoQH2.
NADH + H+ + CoQ —> NAD+ +CoQH2

38
Q

Complex II (Succinate-CoQ oxidoreductase)

A

transfers electrons to coenzyme Q (like Complex I), but Complex II receives electrons from succinate (citric acid cycle intermediate).

-only complex of ETC that does not contribute to proton gradient.

net effect: passing high-energy electrons from succinate to CoQ to form CoQH2.
succinate + CoQ + 2H+ —> fumarate + CoQH2

39
Q

Complex III (CoQH2 - cytochrome c oxidoreductase)

A

aka. cytochrome reductase. facilitates transfer of electrons from coenzyme Q to cytochrome c (through oxidation/reduction of cytochromes)
- adds 4 protons to gradient

overall reaction
CoQH2 + 2cytochrome c [with Fe3+] —> CoQ + 2cytochrome c [with Fe2+] + 2H+

40
Q

cytochromes

A

proteins with heme groups in which iron is reduced to Fe2+ and reoxidized to Fe3+

41
Q

Q cycle

A

Complex III’s main contribution to the proton-motive force is through this cycle.

  • 2 electrons are shuttled from ubiquinol (CoQH2) near the intermembrane space to a olecule of ubiquinone (CoQ) near the mitochondrial matrix.
42
Q

Complex IV (cytochrome c oxidase)

A

facilitates the culminating step of the electron transport chain: transfer of electrons from cytochrom c to oxygen (final e- acceptor).

subunits of cytochrome a, a3, and Cu2+ ions.
cytochrome oxidase = cytochrome a and a3

-adds 2 protons to gradient

overall reaction:
2 cytochrome c [with Fe2+] + 2H+ + 1/2O2 —> 2 cytochrome c [with Fe3+] +H20

43
Q

proton-motive force

A

H+ increases in the intermembrane space which causes the pH to drop in this space and the voltage difference between the intermembrane space and matrix increases due to proton pumping…. together these two create an electrochemical gradient.

*any electrochemical gradient stores energy and it will be the responsibility of ATP synthase to harness this energy to form ATP from ADP + inorganic phosphate

44
Q

cyanide

A

inhibitor to cytochrome c oxidase subunits a and a3. it attaches to iron and prevents transfer of electrons.

45
Q

net ATP yield per glucose

A

ranges between 30-32 b/c efficiency of aerobic respiration varies between cells.

46
Q

what causes the 30-32 variability of ATP per glucose?

A

variable efficiency is due to the fact that cytosolic NADH formed though glycolysis cannot directly cross into the mitochondrial matrix. it needs to use different shuttle mechanisms.

47
Q

different shuttle mechanisms

A

DEFINITION: transfers the high-energy electrons of NADH to a carrier that can cross the inner mitochondrial membrane.

  1. Glycerol 3-phosphate shuttle: ultimately moves electrons to mitochondrial FAD. DHAP is a major player, generates 1.5ATP per molecule of cytosolic NADH,
  2. Malate-aspartate shuttle: ultimately moves electrons to mitochondrial NAD+ and no energy is lost. major players- malate dehydrogenase, oxaloacetate into aspartate; generates 2.5ATP per molecule of NADH
48
Q

which complexes are associated with pumping a proton into the intermembrane space?

A

I, III, IV

49
Q

which complexes are associated with acquiring electrons from NADH?

A

I

50
Q

which complexes are associated with acquiring electrons from FADH2?

A

II

51
Q

which complexes are associated with having the highest reduction potential?

A

IV, reduction potentials increase along ETC

52
Q

what role does the electron transport chain play in generating ATP?

A

it generates a proton-motive force and electrochemical gradient across the inner mitochondrial membrane, which provides energy for ATP synthase to function.

53
Q

ATP synthase

A

link between electron transport and ATP synthesis. this protein complex spans the entire inner mitochondrial membrane and protrudes into the matrix.

54
Q

F0 portion of ATP synthase

A

interacts with proton-motive force and functions as an ion channel, so proteons travel though F0 along their gradient back into the matrix.

55
Q

chemiosmotic coupling

A

allows the chemical energy of the gradient to be harnessed as a means of phosphorylating ADP, thus forming ATP. describes a direct realtionship b/w the proton gradient and ATP synthesis. (main accepted mechanism for oxidative phosphorylation)

*ETC generates high concentration of protons in intermembrane space, which then flow though F0 ion channel of ATP synthase back into the matrix

56
Q

F1 portion of ATP synthase

A

utilizes the energy released from electrochemical gradient to phosphorylate ADP to ATP.

57
Q

ATP synthase reaction

A

ATP synthase generates ATP from ADP and inorganic phosphate by allowing high-energy protons to move down the concentration gradient created by the electron transport chain.

58
Q

conformational coupling

A
  • another mechanism to describe oxidative phosphorylation
  • suggests that the relationship between the proton gradient and ATP synthesis is indirect.
  • ATP is released by the synthase as a result of conformational change caused by the gradient.
  • F1 is like a turbine spinning within a stationary compartment to facilitate the harnessing of the gradient energy for chemical bonding.
59
Q

key regulators of oxidative phosphorylation

A

O2 and ADP

60
Q

uncouplers

A

inhibit ATP synthesis without affecting the ETC

MCAT Biochemisty (13 decks)

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