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Biochemistry I > Chapter 5: Allosteric Enzymes > Flashcards

Chapter 5: Allosteric Enzymes Flashcards

1
Q

Allosteric enzymes

A
  • Oligomeric proteins

- Catalyze reactions toward the beginning of or following a branch point in metabolic pathways

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2
Q

Sigmoidal curve

A
  • Allosteric enzyme plot
  • Does not obey M/M kinetics
  • V vs [S]
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3
Q

Active site

A
  • Binding site for substrate
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4
Q

Allosteric site

A
  • Binding site for small biochemical molecules

- Non-covalent binding

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5
Q

Allosteric effectors

A
  • Binding site causes a subtle change in enzyme structure/shape
  • Influences the ability of enzyme to bind substrate
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6
Q

Cooperativity

A
  • The influence that the binding of a ligand to one subunit has on the binding of another ligand to a second subunit of an oligomeric protein
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7
Q

Homotropic effect

A
  • The subtle conformational change in the second subunit may be induced by an allosteric modulator or by the substrate itself
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8
Q

Binding of ligand

A
  • Causes changes in the quaternary structure of the protein

- Small changes in tertiary structure as well

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9
Q

T-State

A
  • Less active conformation of allosteric enzymes
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10
Q

R-State

A
  • More active conformation of allosteric enzymes
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11
Q

Sigmoidal curve when V vs [S] is plotted

A
  • Generated due to the cooperative nature of substrate binding to an allosteric enzyme
  • Binding of first substrate molecule enhances further substrate binding in homotropic way
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12
Q

Activators

A
  • Heterotropic allosteric effectors

- Increase catalytic activity

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13
Q

Inhibitors

A
  • Effectors that reduce/prevent substrate binding to active site
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14
Q

Allosteric activators

A
  • The purine ATP in the case of ATCase
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15
Q

Allosteric inhibitors

A
  • The pyrimidine CTP in the case of ATCase
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16
Q

Aspartate transcarbamoylase (ATCase)

A
  • Composed of two catalytic trimers and three regulatory dimers
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17
Q

ATCase

A
  • Catalyzes the first step in the pathway of pyrimidine nucleotide biosynthesis
  • Committed step in bacteria
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18
Q

Cytidine triphosphate (CTP)

A
  • End product of pyrimidine nucleotide biosynthesis pathway
  • Inhibits ATCase
  • Feedback/end-product inhibition
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19
Q

Phosphofructokinase 1 (PFK-1)

A
  • Tetrameric allosteric enzyme

- Phosphorylates fructose 6-phosphate to fructose 1, 6-bisphosphate (committed step in glycolytic pathway)

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20
Q

Fructose 6-phosphate

A
  • Substrate that binds in a positively, cooperative manner
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21
Q

PFK-1 kinetics

A
  • Sigmoidal kinetics

- Sensitive to the energy level of the cell

22
Q

PFK-1 most active

A
  • At low levels of ATP
23
Q

PFK-1 most inhibited

A
  • At high levels of ATP
24
Q

Curve when PFK-1 inhibited

A
  • Substrate-binding curve shifts to the right

- Becomes more sigmoidal

25
Q

ATP effects on PFK-1

A
  • Serves as both a substrate and an allosteric inhibitor of PFK-1
26
Q

Fructose 2, 6-bisphosphate

A
  • The most potent allosteric activator of PFK-1
  • Favors the T –> R transition
  • Enhances the enzyme’s affinity for its substrate (fructose 6-phosphate)
27
Q

Glycogen phosphorylase

A
  • Catalyzes a key regulatory and irreversible step in glycogenolysis
  • Active a-form
  • Less active b-form
  • Found in liver and muscle tissue
28
Q

Glucose

A
  • Acts as an allosteric inhibitor of glycogen phosphorylase in the liver
  • Reduces the breakdown of glycogen when blood sugar levels are high
29
Q

AMP in muscle tissue

A
  • Serves as a potent allosteric activator of the enzyme (glycogen phosphorylase) favoring glycogenolysis
30
Q

Protein Kinase A

A
  • Tetrameric allosteric enzyme
  • Two separate catalytic subunits (38kD)
  • Two regulatory subunits that are catalytically inactive (49kD)
31
Q

Intracellular cAMP

A
  • Stimulates protein kinase A when elevated

- Causes tetramer dissociation into a regulatory dimer and two catalytically active monomers

32
Q

Hemoglobin

A
  • Tetrameric allosteric protein
  • Consists of two structurally similar alpha and beta globin subunits
  • Held together by hydrogen bonds and electrostatic interactions
  • Each subunit: 8 alpha helical domains and a heme prosthetic group
33
Q

Hemoglobin function

A
  • Present only in erythrocytes

- Serves as an oxygen delivery system

34
Q

Hemoglobin affinity

A
  • Low affinity for oxygen in actively respiring tissues (readily giving oxygen up)
  • High affinity for oxygen in the lungs
35
Q

Myoglobin

A
  • Monomeric protein

- Facilitates oxygen storage in actively respiring muscle

36
Q

Oxygen-dissociation curve for myoglobin

A
  • Hyperbolic curve

- Expected kinetics of a monomeric protein with a single heme oxygen-binding site that does not exhibit cooperativity

37
Q

Molecular oxygen

A
  • Binds reversibly to the heme group causing conformational changes
  • Changes hemoglobin from T to R state
38
Q

T-State of oxygen

A
  • Deoxygenated state
39
Q

R-State of oxygen

A
  • Oxygenated state
40
Q

Each hemoglobin subunit

A
  • Binds one molecule of oxygen to the ferrous iron of the heme prosthetic group
41
Q

Positive cooperativity of hemoglobin

A
  • The binding of a single oxygen molecule to a subunit of deoxyhemoglobin increases the likelihood that subsequent oxygen molecules will bind to the adjacent hemoglobin subunits
42
Q

Oxygen-dissociation curve for hemoglobin

A
  • Plots fractional saturation (Yo2) on the y-axis

- Plots the partial pressure of oxygen on the x-axis (pO2)

43
Q

Increasing pH on hemoglobin plot

A
  • Shifts sigmoidal curve to the left
44
Q

Lowering pH on hemoglobin plot

A
  • Hemoglobin gives off oxygen more readily

- Shifts sigmoidal oxygen-dissociation curve to the right

45
Q

Purified (stripped) hemoglobin

A
  • Has a high affinity for oxygen
46
Q

Negative allosteric effectors of purified hemoglobin

A
  • D-2
  • 3-bisphosphoglycerate (BPG)
  • Carbon dioxide
  • Hydrogen ions
47
Q

Effect of negative allosteric effectors of purified hemoglobin

A
  • Reduce its affinity for oxygen
  • Stabilize the T-form (deoxygenated)
  • Shift the sigmoidal oxygen-dissociation curve for purified hemoglobin to the right
48
Q

Allosteric enzymes

A
  • Possess two physically distinct binding sites that are cooperative
49
Q

Allosteric effectors

A
  • May be either activators or inhibitors, structurally unrelated to the enzyme substrate
50
Q

Sigmoidal Vo vs [S] plot

A
  • Shape of allosteric enzyme curve

- Indicates that the enzyme does not obey M/M kinetics

51
Q

Important allosteric enzymes

A
  • Phosphofructokinase-1 (of glycolysis)
  • Glycogen phosphorylase
  • cAMP-dependent protein kinase (protein kinase A)
52
Q

Hemoglobin

A
  • Not an enzyme

- Exhibits characteristics of allosteric enzymes

Biochemistry I (34 decks)

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