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Biochemistry > Exam 3 November 11 Lecture > Flashcards

Exam 3 November 11 Lecture Flashcards

1
Q

The conversion of lactate to pyruvate and vice versa is what kind of process?

A

anaerobic process → no oxygen, no mitochondria

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2
Q

What is an overview of glycolysis?

A
  1. a 10 step pathway starting with glucose and ending with 2 pyruvate molecules
  2. occurs in 3 main steps: priming (3 steps), splitting (2 steps), and energy extraction (5 steps)
  3. occurs in the cytosol
  4. 2 ATP and 2 NADH are harvested
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3
Q

Why is the commitment step not the first irreversible step in glycolysis?

A

glucose-6-phosphate can enter different pathways so it is not committed to glycolysis

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4
Q

What are the three enzymes involved with the priming step?

A
  1. hexokinase
  2. phosphoglucose isomerase (PGI)
  3. phosphofructokinase (PFK)
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5
Q

What are the 2 enzymes involved with the splitting step?

A
  1. aldolase

5. triose phosphate isomerase (TPI)

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6
Q

What are the 5 enzymes for the energy extraction step?

A
  1. glyceraldehyde-3-phosphate dehydrogenase
  2. phosphoglycerate kinase
  3. phosphoglycerate mutase
  4. enolase
  5. pyruvate kinase
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7
Q

What is the overall purpose of glycolysis?

A

to make ATP

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8
Q

How is glycolysis regulated during the priming step?

A
  1. 2 of 3 reactions are irreversible and highly regulated
  2. reactions coupled with ATP hydrolysis are usually irreversible
  3. glucose-6-phosphate is also used in glycogen synthesis and pentose phosphate pathway
  4. production of fructose-1,6-bisphosphate is the commitment step of glycolysis
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9
Q

What is a negative regulator of hexokinase?

A

glucose-6-phosphate → hexokinase is less active when there is an abundant amount of glucose-6-phosphate

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10
Q

What are some positive regulators of PFK-1?

A

AMP, fructose 2,6-bisphosphate (hormonal regulation in liver)

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11
Q

What are some negative regulators of PFK-1?

A

ATP, citrate, H+ (feedback inhibition mechanism)

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12
Q

Why is the reaction by PFK-1 so important?

A

non-phosphorylated (aka non-charged) cells are able to leave the cell so PFK-1 is crucial or else we would lose half of the glucose

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13
Q

Glucokinase is also known as what?

A

hexokinase IV

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14
Q

What cells have glucokinase instead of hexokinase for glycolysis?

A

liver and pancreatic β cells

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15
Q

When does glucokinase utilize glucose?

A

when the concentration is sufficiently high (like after a meal) → has high Km values

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16
Q

How is the cellular glucose concentration maintained to be high?

A

the futile cycle formed by glucokinase and glucose-6-phosphatase

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17
Q

The Km values of hexokinase I-III are ____ compared to glucokinase?

A

lower

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18
Q

Glucokinase and hexokinase I-III have the same mechanism but how are they different?

A

they are activated at different glucose levels

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19
Q

What is the commitment step of glycolysis?

A

the production of fructose-1,6-bisphosphate by PFK-1 (has a large ΔG from the hydrolysis of ATP)

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20
Q

What are the relative levels of ATP, ADP, and AMP in the cytosol?

A

[ATP]&raquo_space; [ADP]&raquo_space; [AMP]

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21
Q

When is glycolysis needed?

A

when ATP levels are low and AMP levels are high

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22
Q

A small % decrease in [ATP] is a ____ % increase in [AMP]

A

large → cells use AMP to sense energy status

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23
Q

What does high ATP allosterically inhibit?

A

hexokinase, PFK-1, and pyruvate kinase

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24
Q

What does elevated AMP allosterically activate?

A

PFK-1 and pyruvate kinase

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25
Q

What happens when AMPK is activated?

A

pathways that consume ATP are turned off while pathways that produce ATP are turned on

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26
Q

Are reversible steps regulated?

A

rarely → enzymes keep the reactions at near equilibrium

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27
Q

Fructose 1,6-bisphosphate activates what?

A

pyruvate formation (feedforward control) → transported to mitochondria for the conversion to acetyl CoA

28
Q

Pyruvate is used to make what?

A

alanine (through the alanine cycle) and glucose through gluconeogenesis

29
Q

What is the positive regulators of pyruvate kinase?

A

fructose 1,6-bisphosphate

30
Q

What are the negative regulators of pyruvate kinase?

A

ATP and alanine

31
Q

When oxygen is not available, what cannot occur?

A

oxidative phosphorylation (oxidation of NADH)

32
Q

What is the overview of anaerobic fermentation?

A

location: cytosol
input: glucose
output: 2 lactate molecules
energy harvested: 2 ATP

32
Q

What is the overview of anaerobic fermentation?

A

location: cytosol
input: glucose
output: 2 lactate molecules
energy harvested: 2 ATP

33
Q

How is glycolysis continued with anaerobic fermentation?

A

NAD+ is regenerated from NADH by reducing pyruvate to lactate which is then released into the blood circulation → ATP is generated without oxygen and makes less than aerobic metabolism (2 ATP compared to 32 ATP)

34
Q

Why is anaerobic fermentation the only way to generate ATP in red blood cells?

A

they do not have mitochondria (no oxygen available)

35
Q

What enzyme regenerates NAD+?

A 
lactate dehydrogenase (LDH)
(pyruvate + NADH + H+ ↔ L-Lactate + NAD+)
36
Q

What does homolactic fermentation allow?

A

continued production of ATP as long as glucose is available → L-lactate (as lactic acid) can leave cells which is what causes the sensation of muscle fatigue

37
Q

What does the formation of lactate do?

A

decrease the intracellular pH (more acidic) which inhibits glycolysis (at PFK-1)

38
Q

How is lactate transported to the bloodstream?

A

lactate and H+ are transported to the bloodstream by the lactate-H+ symporter → lactate can also be transported to the liver via the Cori cycle in order to be a substrate for gluconeogenesis

39
Q

What happens when lactate is overproduced or underutilized?

A

blood pH may decrease and cause lactic acidosis (examples include tissue hypoxia due to circulatory and pulmonary failure, vigorous exercise, and liver disease)

40
Q

What effect does fructose have on blood insulin levels?

A

does not increase blood insulin levels (also requires different kinases from glycolysis)

41
Q

What happens when there is an excess amount of fructose?

A

fructose-1-phosphate will accumulate in liver cells, deplete ATP and phosphate which can then cause severe liver damage

42
Q

What are the 3 unique enzymes only expressed in the liver?

A
  1. fructokinase
  2. fructose-1-phosphate aldolase (aldolase B)
  3. glyceraldehyde kinase
43
Q

Why does the main metabolism of fructose occur in the liver?

A

fructokinase is present in the liver (only organ to have it) and fructokinase is very efficient in metabolizing fructose (has a Km value of 0.5 mM versus 12 mM for hexokinase in muscle and adipose tissue)

44
Q

Why does high intake of fructose increase the risk of obesity?

A

fructose metabolism bypasses the regulated PFK-1 step in glycolysis → reaction is unstoppable in addition to overproduction of triglyceride formation which increases the risk of obesity

45
Q

When is gluconeogenesis needed?

A

when glucose levels are low and if we need glucose in tissues during fasting or starvation

46
Q

What are the carbon and energy sources for gluconeogenesis to make glucose?

A

carbon sources: lactate, amino acids, glycerol, fructose

energy source: ATP from fatty acid oxidation

47
Q

Are the enzymes used in glycolysis the same for gluconeogenesis?

A

yes except for the three irreversible steps since the three glycolysis reaction with large negative ΔG values cannot run in reverse direction in cells (aka hexokinase, PFK-1, and pyruvate kinase)

48
Q

What are the changes between glycolysis and gluconeogenesis?

A

glucose-6-phosphate → glucose
fructose-1,6-bisphosphate → fructose-6-phosphate
pyruvate → phosphoenolpyruvate

49
Q

Why does gluconeogenesis only occur when the need is great?

A

it takes more energy to undergo gluconeogenesis than what is gained from glycolysis

50
Q

What phosphatases reverse the hexokinase and the PFK-1 reaction?

A

glucose-6-phosphatase reverses the hexokinase reaction and fructose bisphosphate phosphatase reverses the PFK-1 reaction

51
Q

Why is the liver the only organ that can complete gluconeogenesis?

A

it’s the only organ that expresses glucose-6-phosphatase

52
Q

How many reactions are needed to produce PEP from pyruvate?

A

2 reactions in which two high energy bonds are needed (2 per reaction) → pyruvate to oxaloacetate (by pyruvate carboxylase) and oxaloacetate to PEP aka phosphoenolpyruvate (by PEP carboxykinase)
(oxaloacetate is also an intermediate in the TCA cycle)

53
Q

What is the rate limiting step of PEP production?

A

the step catalyzed by phosphoenolpyruvate (PEP) carboxykinase (from oxaloacetate to PEP)

54
Q

Where does gluconeogenesis occur?

A

mainly in the liver since needed enzymes are expressed → but gluconeogenesis from lactate can occur in muscle and other tissues (all cells express lactate dehydrogenase) but glucose-6-phosphate made that way stays inside the cells in which it was made

55
Q

What are some carbon sources for gluconeogenesis?

A

for pyruvate: lactate and amino acids
for oxaloacetate: amino acids and propionate
for triose phosphates: glycerol and fructose

56
Q

What is the alanine cycle?

A
  1. pyruvate in muscle is converted to alanine by alanine transaminase and released into the blood
  2. in the liver, alanine is transaminated back to pyruvate and reconverted to glucose (as a substrate for glycolysis)
    (amino acid like glutamate + pyruvate ↔ α-keto acid like α-ketoglutarate + alanine)
57
Q

Where does the alanine that is a negative regulator of pyruvate kinase come from?

A

the alanine cycle

58
Q

What is the Cori cycle?

A

gluconeogenesis from lactate in which lactate is released from red blood cells which lack mitochondria into the bloodstream → highly exercised (hypoxic) muscle can also release lactate into the blood
(pyruvate + NAD + H+ ↔ L-lactate + NAD+)

59
Q

How is PEP transported between the cytosol and mitochondria?

A

by oxaloacetate transport (oxaloacetate is an intermediate in the TCA cycle in mitochondria and a substrate in gluconeogenesis) → however, there are no transporters for oxaloacetate on the mitochondrial membrane

60
Q

For oxaloacetate to be transported, it can be converted to what?

A
  1. aspartate (by aspartate aminotransferase)

2. malate (by malate dehydrogenase)

61
Q

What are the 3 main ways in which oxaloacetate can be transported if we wanted to perform gluconeogenesis from oxaloacetate (since gluconeogenesis takes place in the cytosol and the TCA cycle takes place in the mitochondria)?

A
  1. oxaloacetate can be converted to PEP since PEP can be transported between the cytosol and mitochondria
  2. oxaloacetate can be converted to aspartate using aspartate aminotransferase
  3. oxaloacetate can be converted to malate using malate dehydrogenase
62
Q

Glycolysis should be ON and gluconeogenesis should be OFF when:

A
  1. ATP is low
  2. AMP is high
  3. citrate and acetyl-CoA are low
63
Q

What are some negative regulators of glycolysis and gluconeogenesis?

A

negative regulator of glycolysis: ATP

negative regulators of gluconeogenesis: AMP and ADP

64
Q

When does glucose-6-phosphate accumulate?

A

only during gluconeogenesis which inhibits hexokinase

65
Q

Regulation of glycolysis and gluconeogenesis are all what?

A

allosteric!

66
Q

What happens with alcohol and gluconeogenesis?

A
  1. oxidation of ethanol by the liver depletes NAD+ in the cytosol so enzymes that depend on NAD+ → NADH are suppressed
  2. excess NADH converts pyruvate to lactate which may cause alcohol induced lactic acidosis
  3. gluconeogenesis by the liver is suppressed which can cause hypoglycemia
  4. more severe when glycogen stores in the liver is depleted under malnutrition or after strenuous exercise

Biochemistry (18 decks)

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