The Biochemical Nature Oflight Detection And Emission

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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