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The Biochemical Nature Of?light Detection And Emission Essay, Research Paper

In this essay I aim to describe the range of biochemical pathways and

mechanisms used by living organisms both to detect and to emit light.? I will discuss general principles employed,

and illustrate the range of different biochemistry involved by the use of many

specific examples.Light Detection ????

I will discuss the mechanism and function of light detection by five

groups of light detecting molecule.? The

biggest of these is the rhodopsin group of proteins, I will also look at the

role of phytochromes, cryptochromes, flavoproteins and porphirins in light

detection.????

Rhodopsins are found in a diverse array of organisms, all featuring a

retinoid prosthetic group linked to a an apo-protein, opsin via a protonated

schiff base linkage.? Electrons from the

schiff base lone pair occupy an extra orbital?

(the ?n orbital?), therefore electrons can undergo a n-p* transition as

well as a p-p* transition.? ?Retinal proteins were first discovered in 1876 by Bell, who

observed a reddish pigment that bleaches on exposure to light, which he called

visual purple.? Most rhodopsins contain

retinal as the prosthetic group, but some have one of the other chromophores as

shown below.?? For example freshwater

fish have a rhodopsin containing 3,4-didehydroretinal, which has a red shifted

UV absorption band.?? The opsins found

in all organisms show strong homology for one another.??

All rhodopsins seem to be involved in light detection, with the notable

exception of bacteriorhodopsin, which pumps protons using energy from light

photons in order to generate ATP in anaerobic conditions i.e. is not a light

sensing protein. ????

Halobacteria do however have two sensory rhodopsins.? Sensory rhodopsin I (archaeorhodopsin) has

all trans retinal as the prosthetic group in its native state.? It is photoisomerised by green-orange light

(lmax

= 587 nm) to the deprotonated 13-cis state (lmax = 370nm).? Reisomerisation to the all-trans state is

accelerated by absorption at 370nm.? A response is elucidated in the bacterium

by a pumping of protons by the rhodopsin.?

Sensory rhodopsin I causes the halobacteria to show a phototactic

response to green light (needed for bacteriorhodopsin function), and a

photophobic response to UV light (causes cell damage).?? Sensory rhodopsin II (photorhodopsin) also

has the retinal chromophore in the all-trans state.? Light absorption causes chloride ions to

be pumped across the membrane, triggering a photophobic response to blue-green

light. ???

???

Bovine rhodopsin is the most extensively studied of mammalian

rhodopsins.? It is a single polypeptide

of 348 amino acids which forms 7 TM helices and has a Mr of approximately

38kDa.? Upon absorption of light it

follows the photocycle pictured below.????

The retinal chromophore shows a bathochromic shift on attachment to an

opsin.? This can be explained by an

interaction with two carboxylate groups which act as counter ions, shifting the

lmax

from 440nm (in methanol) to 500nm (in rhodopsin).?? The different absorption maximums of the cone cells of the

retina can be explained by differing counter ion structure in their opsins.? Glu 113 has been determined as a counter ion

by site directed mutagenesis experiments. ????

The photocycles of rhodopsins have been studied using time resolved

laser spectroscopy.? The intermediates

have been isolated by low-temperature spectroscopy, i.e. rapid cooling thus

blocking the normal decay of the intermediates.? For example the photocycle of Octopus rhodopsin was

elucidated.?? It was found that

metarhodopsin is thermostable , thus doesn?t bleach in the retina.? FTIR data has suggested that the interaction

of the chromophore with opsin in the batho state is very different to bovine

rhodopsin.????

Fly visual sense cells have a sensitizing pigment ? 3-hydroxyretinol,

which binds non-covalently to the rhodopsin.??

The sensitizing pigment absorbs in the UV, then transfers the energy to

11-cis 3-hydroxyretinal via radiationless dipole-dipole interactions.?? This allows flys to receive visual

information from wavelengths in the UV (lmax = 350nm). ????

????

The physiological response to light absorption has been studied in

detail in higher animals.?? In mammals

the rhodopsin molecules are found in the membrane of the outer segment of the

retina?s rod (or cone) cells.? In the

dark sodium and calcium ions are able to enter the outer segment through cGMP

gated channels.? This inward movement

balances the outward flux of cations caused by the sodium-potassium pump. Upon

absorption of a photon and the isomerisation of retinal, the following

transduction cascade occurs. ??????????????????????????????????????????????????????????????????????????????????????

cGMP???????????????? inactive cGMP???????????????????????? active cGMP???????????????????????????????????? cation channnels phosphodiesterase??????????????????? phosphodiesterase??????????????????????????? close ???????????? ????????????????????????????????????????????????????????????????????????????????????

5` GMP ? ?????????????????????????????????????????????????????????????????????

??????????????????????????????????????hyperpolarised

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