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Malika Biochem MCAT > Chapter 9 > Flashcards

Chapter 9 Flashcards

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MCAT flashcards
1
Q

How does insulin increase affect the rate of glucose transport into the cell?

A

As insulin increases, the rate of glucose transport increases.

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2
Q

What is the main glucose transporter involved in detecting glucose concentration in peripheral blood?

A

GLUT 4 is the main glucose transporter present in adipose & muscle tissue which responds to blood concentration.

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3
Q

What happens to the GLUT 4 transporters when glucose levels are higher than normal?

A

The GLUT 4 transporters become saturated, so more of these transporters are recruited to the periphery.

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4
Q

True or false:
Increased insulin causes an increase in the number of GLUT 4 transporters on the plasma membrane.

A

True.

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5
Q

What are the functions of GLUT 2 transporter?

A

It has low binding affinity to glucose, therefore in the liver it only captures excess glucose for storage.
In the beta cells of the pancreas, GLUT 2 serves as a glucose sensor for insulin release.

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6
Q

True or false:
Only some cells can carry out glycolysis.

A

Falls, all cells carry out glycolysis.

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7
Q

What is the pentose phosphate pathway?

A

A pathway which occurs in the cytoplasm of all cells and serves to create the precursor for nucleotide synthesis.

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8
Q

What is the 1st step of the pentose phosphate pathway (PPP)?

A

Glucose is phosphorylated to glucose-6-phosphate.

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9
Q

What are the oxidative and non-oxidative phases of the PPP?

A

The oxidative phase is when glucose is oxidized and this is irreversible. The non-oxidative phase is reversible.

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10
Q

What are the reactants & products of the oxidative phase?

A

Reactants: Glucose-6-phosphate, H2O
Products: 2 NADPH, 1 CO2, ribulose-5-phosphate

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11
Q

What are the products of the non-oxidative phase?

A

Reactants: ribulose-5-phosphate
Main Product: ribose-5-phosphate –> can be converted into sugars and amino acids

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12
Q

What is the importance of the pentose phosphate pathway?

A

It creates ribose-5-phosphate which is a precursor for many important macromolecules in the body. Also the creation of NADPH allows building of fatty acids & cholesterol and donation of electrons to fight ROS.

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13
Q

The oxidative phase of the PPP is composed of 2 steps. What happens in step 1?

A

Glucose-6-phosphate is oxidized to lactone which utilizes H2O to form a linear sugar 6-phosphogluconate.

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14
Q

The oxidative phase of the PPP is composed of 2 steps. What happens in step 2?

A

6-phosphogluconate has a carbon cleaved releasing CO2 and forming the final product: ribulose-5-phosphate.

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15
Q

The nonoxidative phase is also composed of 2 steps. What happens in step 3?

A

Ribulose-5-phosphate is converted to cyclic ribose-5-phosphate.

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16
Q

The nonoxidative phase is also composed of 2 steps. What happens in step 4?

A

With ribose-5-phosphate as a precursor, it can be used to convert to various molecules like DNA, RNA, amino acids, etc via different reactions.

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17
Q

What is the definition of glycolysis?

A

A cytoplasmic pathway which converts glucose into 2 pyruvate molecules, while releasing energy as byproducts.

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18
Q

What are the enzymes responsible for phosphorylating glucose in the first step of glycolysis?

A

Hexokinase and glucokinase

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19
Q

What is the difference between hexokinase and glucokinase?

A

Hexokinase - produced in various tissues, inhibited by its product glucose-6-phosphate.
Glucokinase - produced in liver and beta pancreas cells, stimulated by insulin

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20
Q

What is the rate-limiting enzyme in glycolysis? What is it inhibited by?

A

phosphofructokinase-1 (PFK-1)
Inhibited by ATP and citrate

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21
Q

What does PFK-1 do?

A

It utilizes ATP to convert fructose 6-phosphate into fructose-1,6-biphosphate

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22
Q

PFK-2 is an enzyme that can convert fructose-6-phosphate into another product called fructose-2,6-biphosphate. What is the importance of this product?

A

Fructose-2,6-biphosphate activates the enzyme PFK-1, allowing it override inhibition caused by ATP and carry out the next step of glycolysis.

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23
Q

PFK-1 is activated by the product fructose-2,6-biphosphate. How is the PFK-2 enzyme activated though?

A

Insulin activates PFK-2 enzyme.

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24
Q

What is the function of glyceraldehyde-3-phosphate dehydrogenase?

A

Responsible for catalyzing oxidation of glyceraldehyde-3-phosphate as well as adding another phosphate group to it.

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25
Q

During the step with the glyceraldehyde enzyme, oxidation occurs. What about reduction?

A

If glycolysis is under aerobic conditions, reduction occurs when NAD+ is reduced to NADH.

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26
Q

What is the function of 3-phosphoglycerate kinase?

A

It creates ATP by taking away the phosphate group from 1,3-biphosphoglycerate. This process is called substrate-level phosphorylation.

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27
Q

What is the function of pyruvate kinase?

A

It converts phosphoenolpyruvate (high energy intermediate) to pyruvate by dephosphorylating it. This creates a molecule of ATP.

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28
Q

What are the two high energy intermediates + their enzymes that participate in substrate level phosphorylation in glycolysis? (creation of ATP)

A

1,3-BPG and 3-phosphoglycerate kinase. Phosphoenolpyruvate and pyruvate kinase.

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29
Q

Give an example of how feed forward activation (feedback loop) is demonstrated in glycolysis.

A

One of the earlier intermediates fructose-1,6-bisphosphate activates an enzyme pyruvate kinase in a later reaction.

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30
Q

What is the rate-limiting enzyme in glycolysis?

A

phosphofructokinase-1

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31
Q

Why is phosphofructosekinase-1 a rate limiting enzyme?

A

Because it is dependent on insulin or glucagon release which either stimulates or inhibits its activity.

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32
Q

In the last step when obtaining the final product pyruvate, what happens if O2 is absent in the cell?

A

Instead of pyruvate being converted into acetyl-CoA, it is reduced to lactate.

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33
Q

Even though reducing pyruvate to lactate (another form of energy) doesn’t require O2, the reaction is still cost effective? What does the reduction require instead?

A

In order to reduce pyruvate, NADH should be present & oxidized to NAD+.

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34
Q

What is the enzyme responsible for creating lactate?

A

Lactate dehydrogenase

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35
Q

Which steps/enzymes are irreversible in glycolysis?

A

Hexokinase + glucokinase, PFK-1, pyruvate kinase

36
Q

What is dihydroxyacetone-phosphate (DHAP)?

A

It is an alternate intermediate, formed in earlier steps of glycolysis from fructose-1,6-bisphosphate.

37
Q

What is the significance of DHAP?

A

DHAP can be converted to glycerol which is used in triglyceride synthesis.

38
Q

Do RBCs perform aerobic or anaerobic glycolysis?

A

Anaerobic glycolysis.

39
Q

How many molecules of ATP are created from 1 molecule of glucose in glycolysis?

A

2 ATP

40
Q

In mammals, why is lactic acid fermentation important?

A

It regenerates NAD+, which is utilized in glycolysis.

41
Q

What is the definition of substrate level phosphorylation?

A

The process by which a phosphate group is transferred to ADP/GDP from a phosphorylated intermediate. This creates ATP/GTP.

42
Q

Which processes out of the list use substrate level phosphorylation to generate ATP/GTP:
Citric Acid Cycle
Glycolysis
Pentose Phosphate Pathway
Electron Transport Chain

A

Citric Acid Cycle, Glycolysis, Pentose Phosphate Pathway

43
Q

Anaerobic respiration typically refers to what processes?

A

Fermentation, glycolysis in absence of O2, and lactic acid cycle in muscles

44
Q

Aerobic respiration typically refers to what processes in the cell?

A

Glycolysis w/ O2 present, citric acid cycle, ETC

45
Q

Define obligate aerobe and obligate anaerobe.

A

Obligate means “must use”.
Obligate aerobe - only survives under O2 presence.
Obligate anaerobe - only survives under conditions without O2

46
Q

Define facultative aerobe and anaerobe.

A

Facultative means the organism uses whichever respiration is available, but prefers one over the other.

47
Q

How does an increase in 2,3-BPG affect the hemoglobin binding affinity curve?

A

An increase in BPG decreases hemoglobin binding affinity, so more O2 can reach the tissues.

48
Q

What are other types of metabolic processes that can feed into glycolysis?

A

fructose metabolism, galactose metabolism, glycogenolysis

49
Q

What is glycogenolysis?

A

It is the breakdown of glycogen in the liver to form glucose.

50
Q

What is the rate-limiting enzyme for glycogenolysis? Why is it rate-limiting?

A

Glycogen phosphorylase. It’s rate-limiting because its activated by glucagon release from the pancreas, as well as activated by AMP & epinephrine in skeletal muscles. (It is inhibited by ATP).

51
Q

What is the function of glycogen phosphorylase?

A

It breaks down glycogen to glucose-1-phosphate using inorganic phosphate (instead of hydrolysis w/ water). Specifically, it cleaves alpha-1,4-glycosidic bonds.

52
Q

Since glycogen is a branched polymer of glucose, how are the branches broken down?

A

A debranching enzyme complex hydrolyzes an oligosaccharide to cut down the branch point to only 1 monomer. Then, the branch point connected via a-1,6 glycosidic bond is hydrolyzed, forming a free glucose molecule.

53
Q

After glycogen is broken down into glucose-1-phosphate, what happens?

A

It is converted to glucose-6-phosphate by phosphoglucomutase. Then, it is funneled into 2nd step of glycolysis.

54
Q

Where does glycogenolysis occur? What purpose does it serve in these locations?

A

Liver - maintain blood glucose levels
Muscles - provide glucose to muscles during exercise

55
Q

What is glycogenesis? Where does it occur?

A

It is the creation of glycogen stores. Liver and skeletal muscle.

56
Q

What is the rate-limiting enzyme for glycogenesis? Why is it rate-limiting?

A

Glycogen synthase because it is stimulated by insulin & glucose-6-phosphate. Its inhibited by epinephrine & glucagon.

57
Q

Which enzyme is responsible for creating the linear 1,4 polyglucose chain of glycogen?

A

Glycogen synthase

58
Q

How are the branches of glycogen created?

A

They are created by a branching enzyme complex which makes branches and attaches it to the linear polymer via 1,6-glycosidic bonds.

59
Q

What are the steps leading up to glycogen synthase incorporating glucose into the polymer chain to create glycogen?

A

Glucose –> glucose-6-phosphate –> glucose-1-phosphate –> glucose-1-phosphate couples to uridine diphosphate (UDP) –> UDP-glucose is created –> glucose can now be polymerized

60
Q

What is glycogenolysis? What are the two enzymes involved in the release of glucose monomers?

A

Glycogenolysis is the break down of glycogen stores to create glucose.
The 2 enzymes are glycogen phosphorylase and debranching enzyme.

61
Q

Galactose comes from what disaccharide?

A

Lactose

62
Q

Steps of galactose metabolism

A

Galactose (phosphorylated by galactokinase) –> galactose-1-phosphate –> converted to glucose-1-phosphate (by galactose-1-phosphate uridyltransferase)

63
Q

Sucrose is a disaccharide, when hydrolyzed, creates which two sugars?

A

Glucose and fructose

64
Q

Steps of fructose metabolism in liver

A

Fructose (phosphorylated by fructokinase) –> fructose-1-phosphate –> cleaved into DHAP & glyceraldehyde (by aldolase)

65
Q

Steps of fructose metabolism in muscles & kidneys

A

fructose –> converted to fructose-6-phosphate (by hexokinase) funneled into 3rd step of glycolysis

66
Q

What is the pyruvate dehydrogenase complex (PDH) reaction?

A

It is an irreversible reaction, occurring in the mitochondria, which converts pyruvate to acetyl-CoA.

67
Q

If pyruvate isn’t converted to acetyl-CoA, what else can occur to it?

A

Alternatively, it can be converted to lactate by lactate dehydrogenase, or converted to oxaloacetate by pyruvate carboxylase.

68
Q

Where does PDH occur in the body? What exactly activates this reaction to occur?

A

In the liver; insulin release (individual in well-fed state) which signals that glucose needs to be burned for energy

69
Q

What products are created in the PDH reaction?

A

1 NADH, 1 CO2, 1 Acetyl-CoA

70
Q

Once Acetyl-CoA is created, what can it be used for?

A

Used in the citric acid cycle to create more ATP or used in fatty acid synthesis.

71
Q

How does the liver maintain blood glucose levels during the fasting state?

A

Glycogenolysis (first 12 hrs of fasting) and gluconeogenesis (primary source after 12 hrs)

72
Q

In gluconeogenesis, pyruvate is the main substrate used in the beginning steps to eventually create glucose. However, pyruvate cannot go back to to its previous intermediate, phosphoenol form (PEP). What happens to pyruvate instead so it can be shuttled through the pathway?

A

Pyruvate is converted into oxaloacetate (OAA) by the enzyme pyruvate carboxylase.

73
Q

What activates the enzyme pyruvate carboxylase?

A

Acetyl-CoA activates this mitochondrial enzyme. Biotin is necessary for it to function.

74
Q

In gluconeogenesis, where is the acetyl-CoA derived from? From glycolysis/pyruvate dehydrogenase reaction or B-oxidation of fatty acids? What is the point of having Acetyl-CoA for gluconeogenesis, if it can’t be converted back to pyruvate?

A

Acetyl-CoA is derived from the B-oxidation of fatty acids for gluconeogenesis because the point is NOT to waste more glucose (therefore glycolysis wouldn’t occur)
Acetyl-CoA is simply there to stimulate the enzyme pyruvate carboxylase, which converts pyruvate into OAA.

75
Q

Where does the pyruvate come from that is shuttled into the gluconeogenesis pathway? (two different sources)

A

1) It comes from the amino acid alanine (which is converted to pyruvate by alanine aminotransferase).
2)It comes from lactate which is converted into pyruvate by lactate dehydrogenase.

76
Q

Instead of pyruvate being directly used in gluconeogenesis, it is converted to what molecule which is then shuttled to another intermediate? Where is it converted exactly?

A

Pyruvate is converted into OAA in the mitochondria.

77
Q

Once pyruvate is converted into OAA, what happens?

A

OAA is shuttled through the malate-aspartate shuttle and travels to the cytosol where the enzyme phosphoenolpyruvate carboxykinase converts OAA to phosphoenolpyruvate using GTP.

78
Q

Phosphoenol pyruvate goes through 3 reversible reactions (which are present in glycolysis as well in reverse, list them here as well for a reminder) to get to what important intermediate?

A

Phosphoenolpyruvate converts –> 2-phosphoglycerate –> 3-phosphoglycerate –> 1,3-bisphosphoglycerate –> important intermediate is glyceraldehyde-3-phosphate

79
Q

Glyceraldehyde-3-phosphate is converted into what intermediate in gluconeogenesis?

A

Fructose-1,6-bisphosphate

80
Q

What is the rate-limiting step/enzyme of gluconeogenesis? What controls the activation/inhibition of the enzyme?

A

The conversion of fructose-1,6-bisphosphate into fructose-6-phosphate by the enzyme fructose-1,6-bisphosphatase.
Activated by ATP, glucagon, and citrate. Inhibited by AMP, insulin, and fructose-2,6-bisphosphate.

81
Q

Name the four irreversible reactions catalyzed by enzymes during gluconeogenesis starting with pyruvate.

A
  1. Pyruvate to oxaloacetate by mitochondrial pyruvate carboxylase
  2. Oxaloacetate to PEP by mitochondrial or cytosolic PEP carboxykinase
  3. Fructose 1,6-bisphosphate to fructose 6-P by fructose 1,6-bisphosphatase
  4. Glucose 6-P to glucose by glucose 6-phosphatase
82
Q

Tip: think of glycolysis and gluconeogenesis as opposing cycles. Whatever activates glycolysis & its rate-limiting enzyme, inhibits gluconeogenesis and vice versa.

A
83
Q

Fructose-6-phosphate is converted into what intermediate?

A

glucose-6-phosphate

84
Q

The last step of gluconeogenesis involves…

A

Converting glucose-6-phosphate into glucose utilizing the enzyme glucose-6-phosphatase.

85
Q

Attribute these two hormones: insulin and glucagon to these two processes: glycolysis and gluconeogenesis.

A

Insulin –> glycolysis (during meals)
Glucagon –> gluconeogenesis (fasting state)

86
Q

How do the roles of the skeletal muscle and liver differ in (glucose release/supply)?

A

The liver releases the majority of glucose into the blood from glycogenolysis and gluconeogenesis.
Even though, skeletal muscle contains twice as many glycogen stores, it only supplies glucose to the muscle and doesn’t release it into the blood.

Malika Biochem MCAT (11 decks)

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