BIOCHEMICAL

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Biochemical and structural basis for trans-to-cis isomerization of retinoids : 

Biochemical and structural basis for trans-to-cis isomerization of retinoids

Retina showing Rods & Cones : 

Retina showing Rods & Cones

Rhodopsin : 

Rhodopsin

Structure of Rhodopsin : 

Structure of Rhodopsin

Rhodopsin : 

Rhodopsin Photoreceptor Comprise a class of GPCR Heptahelical in structure Consists: a. Opsin[apoprotein], b. Retinylidene chromophore moiety, c.Protonated schiff -base linkage to lysine residue . Functional & active Functional has 11-cis-stereoisomer. Active has all-trans-stereoisomer.[META II] META II is catalytically active binds TRANSDUCIN(Gt).

Rhodopsin with its chromophore pocket : 

Rhodopsin with its chromophore pocket

Process of vision : 

Process of vision

Absorbtion,metabolism,transport : 

Absorbtion,metabolism,transport

Regeneration of cis-chromophore : 

Regeneration of cis-chromophore Mammals Insects Activation of Rhodopsin to META II METAII binds to Transducin (Gt) Signal amplifying pathway involving cGMP Results in hyperpolarisation of membrane VISUAL CYCLE Regeneration of CIS-chromophore by a Enzymatic pathway Retinoid Isomerase (RPE65) Transducin(Gt) binding Phosphoinositide signaling Opening of TRP channels Depolarisation of membrane Insects lacks enzymatic pathway.

Regeneration of cis-chromophore : 

Regeneration of cis-chromophore

Recycling of retinoids in RPE : 

Recycling of retinoids in RPE

Cone specific visual cycle : 

Cone specific visual cycle

Key steps in retinoid metabolsm in insects and mammals : 

Key steps in retinoid metabolsm in insects and mammals

Visual cycle : 

Visual cycle

Structural insights –RPE65 : 

Structural insights –RPE65 Membrane bound enzyme Seven bladed β-propeller Cofactor non-haem ferrous iron Four conserved His-residues One tunnel for active site Stretch of hydrophobic residues Same tunnel for substrate&product Narrow for water molecule transport

RPE65 with its tunnels : 

RPE65 with its tunnels Blue mesh-substrates/product Red mesh-water molecules

RBP4 : 

RBP4 Single domain proteins Lipocalin family Cup shaped Ligand binds-antiparallel β-barrels Major plasma transporter to target tissues Binds all-trans-retinol Buried deep into hydrophobic cavity β-inone ring positioned to the base

CRBP1 : 

CRBP1 Belongs to intracellular lipid-binding proteins Similar to RBP in all aspects β-inone ring positioned near entrance

IRBP : 

IRBP

IRBP : 

IRBP Soluble lipoglycoprotein,has 4 modules-each-300 aminoacid residue Largest among the retinoid binding proteins Each module contains 2 qualitatively different ligand binding domains Mediates bidirectional transport of several retinoid Assists transport in four types of cells Rods,Cones,RPE,Muller cells Two hydrophobic cavities one in domain-A,other in domain-B Module conformation helps to protect retinoid from degradation Module has role in targeting retinoid to intended cell type

CRALBP : 

CRALBP

CRALBP : 

CRALBP Water soluble protein Found in Retina&pineal gland Family-CRAL-TRIO Carries 11-cis-retinal (or) retinol Contains a highly basic patch of amino acids Hydrogen bonds with Y180&E202 residues π stacking interaction with F161 It has a lipid-exchange loop undergoing conformational changes

Open conformation participates in ligand association & dissociation : 

Open conformation participates in ligand association & dissociation Closed conformation serves as a stable carrier of Retinoid protecting them from the cellular hydrophilic environment

Unimolecular nucleophilic substitution : 

Unimolecular nucleophilic substitution

Bimolecular nucleophilic substitution : 

Bimolecular nucleophilic substitution

Saturation/desaturation : 

Saturation/desaturation

Retinal diseases : 

Retinal diseases

Concluding remarks: : 

Concluding remarks: Defects in any component of the visual cycle can cause inherited diseases. Complete knowledge about the structure of proteins involved may help in Gene therapy against blinding diseases. Those diseases acquired due to accumulation of toxic retinoid can be treated by inhibiting visual cycle or limiting the supply of Vitamin A to eyes. Future perspectives: Pathway for chromophore production in Cone pigments. Function of non-visual light-sensory protein melanopsin.

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