Biochemistry of Vision

Question 1

summarize the steps of the visual cycle

Answer 1

• 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

Question 2

rhodopsin

• is what type of protein?
• has what structure?
• is found where?

Answer 2

• 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

Question 3

rhodopsin

• is actived by what ligand?
• to undergo what molecular changes?
• leading to what next step?

Answer 3

• 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

Question 4

what is R*?

how long is it present in the system?

how is its activity shut down?

what will this lead to?

Answer 4

• 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

Question 5

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

Answer 5

• 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

Question 6

transducin is

• is what kind of protein?
• is located where?
• has what structure?
• is activated how?
• does what?

Answer 6

• 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)

Question 7

how is transducin inactivated?

what would this lead to?

Answer 7

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

Question 8

PDE-6

• is what kind of protein?
• has what structure?
• is activated how?
• does what?

Answer 8

• 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

Question 9

how is PDE-6 inactivated?

what will this do?

Answer 9

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

Question 10

cyclic nucleotide gated ion (CNG) channels

• are what kind of channels?
• are mediated how?
• do what?

Answer 10

• 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

Question 11

guanylyl cyclase

• is what kind of protein?
• has what structure?
• is activated how?
• does what?

Answer 11

• 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+

Question 12

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

Answer 12

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

Question 13

what is the product of the retinoid cycle?

what happens to it?

Answer 13

11-cis-retinal

is taken back to rods and cones

Question 14

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

• covanetly bound lipids
• isoprenylation

Answer 14

• transducin
• PDE-6

Question 15

which component of the visual cycle has

• 7 membrane spanning domains
• a membrane spanning helix?

Answer 15

• rhodopsin
• guanylyl cyclase

Question 16

color vision is

• are encoded by what genes? on what chromosomes?
• is generated
  
  • from what pigments?
  • by what structures?
  • how?

Answer 16

• 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

Question 17

discuss the characteristics of lipids in the optic disc membrane

why is this important?

Answer 17

• 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

Question 18

retinitis pigmentosa

• defintion
• pathogenesis
• presentation

Answer 18

• 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

Question 19

leber congenital amaurosis

• definition
• presentation
• presentation
• treatment

Answer 19

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

Question 20

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

Answer 20

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

Question 21

bradyopsia

• definition
• pathogenesis
• presentation

Answer 21

• 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

Question 22

diabetic retionopathy

• definition
• pathogenesis
• presentation

Answer 22

• 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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

hyperglycemia -> sorbital accumulation d/t aldolase

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

Question 25

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

Answer 25

• 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

Question 26

list the each gene mutation associated with

• retinitis pigmentosa
• leber congenital amaurosis (LCA)
• bradyopsia

Answer 26

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