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Neuro Exam I > Biochemistry of Vision > Flashcards

Biochemistry of Vision Flashcards

1
Q

summarize the steps of the visual cycle

A
  • photon hits rhodopsin (11-cis retinal) converting it -> metarhodospin II (all-trans)
  • metarhodopsin II converts GDP -> GTP
  • GTP converts transducin to its active form (By + a)
  • transducin a subunit converts PDE-6 to its active form (aB2y)
  • PDE-6 hydrolyzes cGMP to 5-GMP
  • the decrease in cGMP decreases Na+ & Ca+ influx
  • decrease in Na+ & Ca+ influx leads to hyperpolarization
  • hyperpolarization leads to glutamate release
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2
Q

rhodopsin

  • is what type of protein?
  • has what structure?
  • is found where?
A
  • a GCPR (has 7 membrane spanning domains)
  • is found on the optic disc: makes up
    • 90% of all disc proteins
    • 1/2 of disc volume
    • 1/3 of disc surface area
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3
Q

rhodopsin

  • is actived by what ligand?
  • to undergo what molecular changes?
  • leading to what next step?
A
  • a photon serves as the “ligand” for rhodospsin
  • the photon stimulates key conformational change of the K296 residue:
    • 11-cis retinal -> all-trans retinal
      • ​all trans-retinal defines metarhodopsin II (R*), the active form of rhodopsin
  • metarhodopsin-II activates transducin
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4
Q

what is R*?

how long is it present in the system?

how is its activity shut down?

what will this lead to?

A
  • metarhodopsin, the active form of rhodopsin (all-trans retinal)
  • present in system for only 60s
  • is inactivated via phosphorylation by rhodospin kinase (RK), which:
    1. reduces its affinity for transducin
    2. increaes its affinity for arrestin.
  • halts activation of transducin
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5
Q

after inactivation of R*, how is rhodopsin “regenerated” so that it may receive the next photon?

A
  • RKs actions are reserved
    1. dephosphorylated by a phosphotase
    2. removal of arrestin
  • 11-cis retinal residue is restored
    • all-trans retinal removed by hydrolysis
    • 11-cis retinol + opsin = rhodopsin
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6
Q

transducin is

  • is what kind of protein?
  • is located where?
  • has what structure?
  • is activated how?
  • does what?
A
  • heterotrimeric G-protein: alpha, beta, gamma subunit
  • is linked to disc membrane by covalently bound lipis
  • as a heterotrimeric G-protein, it is activated by GTP.
    • GTP, produced by, metarhodospin (R*) replaces GDP on the heterotrimer, converting the aby trimer (inactive form) into By + a (active form)
    • the alpha subunit then goes to activate phosphodiesterase-6 (PDE-6)
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7
Q

how is transducin inactivated?

what would this lead to?

A
  • via GTP hydrolyis
    • regulated by GTPase accelerated proteins (GAPs)
    • the “slow step” of rod vision”
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8
Q

PDE-6

  • is what kind of protein?
  • has what structure?
  • is activated how?
  • does what?
A
  • is a heterotrimeric phosphodiesterase
  • aB subunits are subunits are attached to the disc membrane by isopreynylation
  • activated by transducin (a subunit), which
    • converts aB + 2y (inactive form) into aB2y (active form)
    • active PDE-6 converts hydolyzes cGMP
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9
Q

how is PDE-6 inactivated?

what will this do?

A
  • inactivated by the inactivation of transducin: i.e., hydrolysis of Galpha - GTP
  • this leads to:
    • reduction of PDE activity to dark lectures
    • a halt in the activation of cGMP hydrolysis
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10
Q

cyclic nucleotide gated ion (CNG) channels

  • are what kind of channels?
  • are mediated how?
  • do what?
A
  • and Na+, Ca+ channels
  • mediation:
    • opened by cGMP
    • thus, closed by PDE-6, which hydrolyzes cGMP to GMP
  • when closed:
    • Na+, Ca+ influx decreases
    • this hyperpolarizes the membrane
    • hyperpolarization leads to -> decreased glutamate relesase
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11
Q

guanylyl cyclase

  • is what kind of protein?
  • has what structure?
  • is activated how?
  • does what?
A
  • a membrane photoreceptor
  • contains a transmembrane helix
  • activated by low levels of Ca+
  • restores cGMP levels -> depolarization
    • by cleaving GTP into cGMP, which
    • activates CNGS, which
    • which permit influx of Na+, C+
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12
Q

after all trans retinal is removed from R* during the regernation of retinol, what happens to it?

A

undergoes the retinoid pathway

  • all-trans retinal is reduced to all trans retinol by retinol dehydrogenase
  • all trans-retinol is then transported to RPE via binding protein
  • within the RPE, all-trans retinol is then esterified by LRAT into all trans retinyl ester, where it can either:
    1. remain stored in the IPE
    2. be converted to 11-cis retinyl, which is then
      • converted to 11-cis-retinol, which is
      • converted to 11-cis retinal, which is
        • transported back to rods and cones
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13
Q

what is the product of the retinoid cycle?

what happens to it?

A

11-cis-retinal

is taken back to rods and cones

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14
Q

which component of the visual cycle is attached to the disc via

  • covanetly bound lipids
  • isoprenylation
A
  • transducin
  • PDE-6
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15
Q

which component of the visual cycle has

  • 7 membrane spanning domains
  • a membrane spanning helix?
A
  • rhodopsin
  • guanylyl cyclase
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16
Q

color vision is

  • are encoded by what genes? on what chromosomes?
  • is generated
    • from what pigments?
    • by what structures?
    • how?
A
  • genes:
    • rhodospin -> chromosome 3
    • blue -> chromosome 7
    • red, green -> chromosome X
  • generated from:
    • red, blue & green pigments
    • by cones
    • that produces a spectral response via different amino acid compositions
17
Q

discuss the characteristics of lipids in the optic disc membrane

why is this important?

A
  • lipids consist of a high conentration of PUFA (polyunsaturated fatty acids), which makes the membrane highly fluid (double bonds = more fluid)
    • this is key because of premits rapid movements of visual cycle components, which allows arpid transmission of vision
18
Q

retinitis pigmentosa

  • defintion
  • pathogenesis
  • presentation
A
  • defintion: rod / cone dystrophy
  • pathogenesis: mutations in either -
    1. rhodospin (many variations)
    2. peripherin - a disc membrane protein
  • presentation: night blindness is followed by decreasing visual fields
19
Q

leber congenital amaurosis

  • definition
  • presentation
  • presentation
  • treatment
A
  • definition: early onset retinal degeneration
  • pathogenesis: mutations in
    • RPE-65: mediates retinoid recycling
  • treatment: insertion of RPE-65 gene via adeno-associated viral vector
20
Q

efficacy of REP-65 gene insertion for LCA (leber congenital amaurosis)

A

effective for at least 2 yrs (sometimes 3) typically followed by decline

21
Q

bradyopsia

  • definition
  • pathogenesis
  • presentation
A
  • definition: inability to rapidly shut off phototransduction cascade
  • pathogenesis: mutations in GTPase accelerating proteins (GAPs)
    • RG29
    • R9AP
  • presentation: inability to transition from light to dark
22
Q

diabetic retionopathy

  • definition
  • pathogenesis
  • presentation
A
  • definition: retinopathy d/t chronic hyperglycemia
  • pathogenesis: 3 major theories
    • hyperglycemia leads to:
      • impaired autoregulation of retinal blood flow
      • sorbital accumulation in retinal cells
      • advanced glycation end products
23
Q

explain how impaired blood flow would cause diabetic retinopathy & how it would present

A

diabetic

  • increased retinal blood flow leads to increased shear
    • which results in
      1. vasoactive subustances
      2. vascular leakage
    • both of which lead to: macular edema
24
Q

explain how sorbital accumulation would lead to diabetic retinopathy & how it would present

A

hyperglycemia -> sorbital accumulation d/t aldolase

  • sorbital builds up in retinal cells
  • this build-up increases omsolarity
  • increased osmolarity -> swelling
25
Q

how do accumulation of advanced glycation end products (RAGE) lead to diabetic retinopathy & how does it present?

A
  • excess glucose combines with free amino acids to form ->AGE
  • AGES accumulate in tissue leads to:
    • dysfunction & delayed turnover
    • AGE binding to receptors (= RAGE), which ledas to generation or ROS -> inflammation
26
Q

list the each gene mutation associated with

  • retinitis pigmentosa
  • leber congenital amaurosis (LCA)
  • bradyopsia
A
  • retinitis pigmentosa
    • rhodopsin
    • perpherin
  • LCA: RPE-65 (retinoid recycling machinery)
  • bradyopsia: GTPase accelerating protreins
    • R9AP
    • RGS9

Neuro Exam I (15 decks)

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