Biochemistry of Visual Process- Ocular Biochemistry

Optometry

1. Biochemistry
of the vision
process
-By Sunny Biswas

2. Introduction
•Visible lightis electromagnetic
radiation(400–750 nm). It spreads through
the environment, refracts and absorbs in
different spectra. It reaches
theretinathrough the optical environment
of theeye. The photoreceptors of the retina
convert light energy into atomic motion, the
chemical change translating into a nerve
impulse propagating to the brain.
•Purkinje shift: shifting of sensitivity of
eye from photopic to scotopic vision
■ Photopic vision: →Day light vision
due to cones →Color vision
→Brightness above 1mA
■ Scotopic vision: →Dim light vision
due to rods Below 0.001 mA
■ Mesopic vision: →Full moonlight
vision both rods & cones

3. Dietary sources of retinol
•Animal foodscontain vitamin A as such (i.e. retinol); some foods are much richer in retinol than
others.
•Eg: Liver,Fishoils,Milk,Eggs
•Plant foodsdo not contain vitamin A as such but in the form of precursors the carotenoid
pigments (carotenes).
•Eg: Leafy green vegetables (kale, spinach, broccoli), orange and yellow vegetables (carrots,sweet
potatoes, pumpkin and otherwinter squash, summer squash),Tomatoes,Red bell
pepper,Cantaloupe, mango.
•The carotenes cannot be used directly in the photochemical process but must be converted into
vitamin A (retinol) by metabolic activity in the wall of the small intestine. Three types of carotenes—
the alpha, beta and gamma—are present in plant food. The beta carotenes yield 2 molecules of
vitamin A, while the alpha and gamma yield one molecule each.

4. Absorption and storage
•In the intestine, the vitamin A is esterified and
reaches the bloodstream through the intestinal
lymphatics.
•Most of this retinol from the bloodstream is
transported to the liver, where it is stored. In the
liver, retinol becomes bound with the retinol-
binding protein (RBP).
•It is quite stable in this combination.

5. Vitamin A cycle from its
dietary intake to its
transport to the eyes

6. Transport from liver to the
eye
•The retinol-protein complex enters the circulation
and reaches the target tissues where it is utilized.
•In the retina, it becomes attached to the specific
receptors present on the basal surfaces of the
retinal pigment epithelial (RPE) cells.
•Then, it is assumed that the RBP is left outside and
the retinol is transported by a specific transport
protein inside the RPE cells

7. Utilization of vitamin A for
synthesis of rhodopsin (Neogenesis)
•Inside the RPE cells, there occurs no change in the retinol (vitamin
A) passes through into the outer segments of the photoreceptor’s.
•Inside the photoreceptor’s outer segment, the retinol is oxidized to
retinene by the enzyme retinene reductase.
•The retinene then combines immediately with the protein opsin to
form the rhodopsin.
•The NAD oxidative system (present in the RPE) supports the
reaction of rhodopsin formation by removing hydrogen.
•Therefore, for the formation of rhodopsin, it is essential that the RPE
and photoreceptor outer segment must be closely opposed to each
other.
•The freshly formed rhodopsin molecule is then incorporated into
the newly forming double discs, which then assume their place in
the innermost portion of outer segment of photoreceptors
Fig- Utilization of vitamin A for synthesis of
rhodopsin

8. Photoreceptor cells of the
retina and Visual Pigments
•The retina is composed of interconnected nerve cells, whose
arrangement allows histological differentiation of 10 layers, light
passing through the upper layers to the layer of two types of
luminous cells -rods and cones.
•Their distribution in the retina is not uniform.
•Their function can be impaired by, among other things,retinal
detachment.

9. Rods
•They provide vision even in low light intensity -scotopic
vision.
•Provide black white vision
•Can't detectcolours.
•Present almost -130 million innumbers.
•More concentrated in the peripheral parts of the retina.
•Theinner segment'is highly metabolically active, producing
abundantATPand protein.
•Theouter segment'forms densely stacked discs.
•Rod Cell's membrane contains the
chromophoreRHODOPSIN(so-called visual purple).

10. Rhodopsin
•Rhodopsin'-
•It's a covalently bound complex of the proteinOPSIN'and11-cis-RETINAL(a derivative ofvitamin A).
•Thecomplex is formed by the reaction of the aldehyde group of retinal with the NH
2group of the
lysine residue of theopsin molecule (= Schiff base).
•'Opsin-a protein 348 amino residues in the membrane of the disc of the outer segment of
the rod, composed of 7 helices =7times passing through the membrane, protruding on both
sides.
•'11-cis-retinal'-low molecular mass dye, binds roughly in the middle of the membrane
betweenthehelices of opsin.

11. Rhodopsin
•In the dark''the outer segment is strongly penetratedbyNa
+
ions'through
specificmembranechannels,the sodium pump of the inner segment (Na
+
, K
+
ATPase) maintains a
highconcentrationgradient(theresulting potential is about -40 mV). The Na
+
channel is kept open
bycyclicguanosinemonophosphate(cGMP).
•It hasmol. wt. of41000
•It absorbs primarily yellow wavelength of light, transmitting violet and red to appear purple by
transmittedlight;it is, therefore, also called visual purple.

12. •The chemical
basisof rodvision

13. Rhodopsin Bleaching
(Photodecomposition)
•When light hits the retina, it isabsorbed by the rods
•Absorption leads toexcitationof the membrane, which results inisomerization of 11-cis-retinaltoALL-
TRANS-RETINAL. The energy of the photon is thus transformed into atomic motion.
•Within milliseconds, a series of photochemical reactions take place
whoseintermediates(bathorhodopsin,lumirhodopsin,metarhodopsinI,metarhodopsinII) show
differentmaxima from 500 to 380nm.

14. Isomerization of 11-cis-retinaltoALL-
TRANS-RETINAL.

15. RhodopsinBleaching
(Photodecomposition)
•The trans-isomer no longer fits into the binding site. Rhodopsin thus breaks down into opsin and all-
trans-retinal.
•This activated rhodopsin furtheractivates the G-proteinTRANSDUCIN.
•The cascade continues with the activation ofPHOSPHODIESTERASE (PDE),
whichhydrolyzescGMPtoNON-cyclic form (5´-GMP).

16. Fig-TheschemeforreactionstriggeredbyrhodopsinbleachingwhichaffectcGMP:A,light-induced
conversionofrhodopsin(R)intotheactiveform(R*);B,activationofG-protein(G),GTP/GDP
exchangeandactivationofcGMPphosphodiestrase(PDE)protein;C,phosphorylationofphotolysed
rhodopsin(R’).

17. RhodopsinBleaching
(Photodecomposition)
•The originally openchannel for Na
+
ions closes, the ion flow stops.
•The consequence isHYPERPOLARIZATION of the membrane, it becomes more
negative(hyperpolarization is only -35 mVhere, because the resting membrane potential is -30 mV ).
•Hyperpolarization spreads to thesynapse, allowing the transmissionof excitations further along
thevisual pathway(beginning of the nerve impulse.)
•From the ganglion cells, the signal continues as depolarization.
•The valueof hyperpolarization depends on the intensity of illumination. In addition, the signal sent
from a single photon is greatlyamplified byhyperpolarization.

18. Figure -Dark current and light response.
(Left) In the dark, rhodopsin is inactive; the cyclic
nucleotide–gated (CNG) channels in the outer
segment are open; and the rod is depolarized with a
steady release of glutamate from its axonal terminal.
(Right) Rhodopsin is activated by light, which 356
leads to closing of the CNG channels, rod membrane
hyperpolarization, and inhibition of glutamate release
from the axon termina

19. Regenerationof
Rhodopsin
•The released trans-retinal is partly carried by the blood to theliver, where it ishydrogenatedto the
alcoholTRANS-RETINOLandisomerizedtoCIS-RETINOL, the latter being carried by the blood back
to the retina, where it must beoxidizedto 11-cis-retinal.
•Blood transfer is enabled by binding to the transport proteinretinol-binding protein (RBP).
•A sufficient amount ofvitamin A (retinol)and its provitamin β-carotene are required for proper retinal
function.

20. Regenerationof
Rhodopsin
•Lightcausesaphotoisomerizationof11-cisretinalintoanintermediateformthat,aftergoingthrough
aseriesofspontaneouschanges,finallyisconvertedintoall-transretinal,Theall-transretinal
dissociatesfromtheopsinandthenisconvertedintoall-transretinol(vitaminA).
•All-transretinolisenzymaticallyisomerizedto11-cisretinolandthenenzymaticallyoxidizedto11-
cisretinal.
•The11-cisretinalthenspontaneouslycombineswithopsin,reformingrhodopsin.
•(1)Theconversionoftheretinoltoretinalisdependentonthepigmentepithelium.
•(2)DeficiencyofvitaminApreventstheregenerationofrhodopsin.Forthisreason,lackofvitaminA
leadstonightblindness.

21. Bleachingand
Regeneration
of Rhodopsin
Rhodopsin
Bleaching
Reformation

22. Walds
Visual
Cycle

23. Cones
•They provide vision in good light conditions -photopic
vision.
•They perceive colours.
•There are almost 20 times fewer of them than rods -7
million.
•Their greatest concentration is in theyellow spots.
•Thereareseveraltheoriesexplainingtheircolour
sensitivity.Themostwidelyaccepteddistinctionisthatof
conesinto3typesaccordingtotheirsensitivityto
wavelength.
•3idopsininconepigmentsare:
•Erythrolabe(Rcones):red,570nm
•Cyanolabe(Bcones):blue,440nm
•Chlorolabe(Gcones):green,540nm

24. •The chemical
basisofcone vision

25. The chemicalbasisof
cone vision
•Like rhodopsin, cone pigments also consist of the protein opsin (called photopsin) and the
retinene (11-cis-retinal). Photopsin differs slightly from the scotopsin(rhodopsin).
•There are three classes of cone pigments: red-sensitive (erythrolabe), green-sensitive
(chlorolabe) and bluesensitive(cyanolabe), which have different absorption spectra.
•It has been assumed that when light strikes the cones, the photochemical changes occur in
the cone pigments which are very similar to those of rhodopsin.
•However, it has been noted that, nearly total rod bleaching occurs before significant
bleaching can be observed in the cones.
•This differential bleaching quality sets aside the scotopic rod portion of the visual system
from the photopic portion which functions during brightly lighted conditions.

26. Retinal
photoreceptor
disorders

27. ColorBlindness:
Missing or defective pigment proteins for certain cone cell types -~8% of men,
rare in women.
Types of Color Blindness -Achromatopsia -black and white vision.
-Dichromacy -2 functioning cone types.
-Anomalous Trichromacy -shifted cone absorption.
Non-Genetic Causes -Disease, Accidents, Medication.

28. Daltonism-deficiency of photoreceptor protein absorbing green
or red, common (1-2% of population).
Protanopia-impaired vision of red colour.
Deuteranopia-impaired green vision.
Tritanopia-blue vision impairment.

29. Night Blindness (Nyctalopia):
The most common cause of nyctalopia is retinitis pigmentosa, a disorder in
which the rod cells in the retina gradually lose their ability to respond to the
light.
Patients suffering from this genetic condition have progressive nyctalopia
and eventually their daytime vision may also be affected.

30. Retinitis pigmentosa-mutation of the gene for rhodopsin, inherited,
light-harvesting cells die, leading to blindness.
Avitaminosis A-not only limits rhodopsin regeneration -glaucoma,
causes morphological changes to destruction of receptors.

31. •Thank You
!!!