Physiological Response To Light Absorption example essay topic

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, crypto chromes, flavoprotein's 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 proto nated 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 chromophore 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 th prosthetic group in its native state. It is photoisomerised by green-orange light (lmax = 587 nm) to the deprotonated 13-cis state (lmax = 370 nm). Reisomerisation to the all-trans state is accelerated by absorption at 370 nm. A response is elucidated in the bacterium by a pumping of protons by the rhodopsin. Sensory rhodopsin I causes the halobacteria to show a photo tactic response to green light (needed for bacteriorhodopsin function), and a photophobic response to UV light (causes cell damage).

Sensory rhodopsin II (photo rhodopsin) 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 38 kDa. 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 440 nm (in methanol) to 500 nm (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 photocycle 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 thermos table, thus doesnt bleach in the retina. FTIR data has suggested that the interaction of the chromophore with opsin in the bath state is very different to bovine rhodopsin. Fly visual sense cells have a sensitizing pigment 3-hydroxy retinol, which binds non-covalently to the rhodopsin. The sensitizing pigment absorbs in the UV, then transfers the energy to 11-cis 3-hydroxy retinal via radiation less dipole-dipole interactions.

This allows flys to receive visual information from wavelengths in the UV (lmax = 350 nm). 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 retinas 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 channels phosphodiesterase phosphodiesterase close 5' GMP hyper polarised electrical signal slowing of neurotransmitter release at synaptic terminal The cGMP phosphodiesterase is activated by the G-protein Transducins a subunit. Transducin is activated by the binding of Metarhodopsin II (the photo excited state of rhodopsin). This transduction cascade allows a large amplification of the original photon absorption into a transmittable electrical signal. One Metarhodopsin II molecule can activate many Tas before the retinal dissociates from the opsin apo-protein. One Ta will remove the inhibition from one phosphodiesterase, which can hydrolyze up to 1000 cGMP molecules per second. Cryo-electron microscopy has delivered structural information about rhodopsin and the intermediates of the photocycle that has allowed the changes in structure on photo excitation to be elucidated.

A motion of helix relative to helix IV has been identified this would mean a change in the conformation of the third cytoplasmic loop, which is the region that interacts with Ta. Retinal directly interacts with helix in the region of Glu 121. Isomerisation of retinal results in a rearrangement in hydrogen bonding between Glu 134, Tyr 223, Trp 265, Lys 296 and Tyr 306. Breakage of the salt bridge between Lys 296 and Glu 113 allows activation to take place i.e. metarhodopsin II can form. Metarhodopsin II is deactivated by phosphorylation and arrestin binding. Arrestin binds to Ser 334, Ser 338, Ser 343 near the C terminus of opsin.

Ta deactivates itself by its own GTPase activity. Rhodopsin kinase is inhibited by Ca 2+ bound recover in, so when the cytosolic [Ca 2+] decreases rhodopsin kinase becomes more active. Phosphodiesterase recombines with its inhibitory subunits. The drop in cytosolic calcium concentration from 0.5 to 0.1 mM after a light flash stimulates guanylate cyclase which results in the reopening of cation channels and the dissipation of the electrical signal.

The regeneration of rhodopsin after photo bleaching starts with the dissociation of all-trans retinal from opsin and its conversion to all-trans retinol. An isomerase converts all-trans to 11-cis retinol, which is then dehydrogenated to 11-cis retinal. This mechanism would not be fast enough to maintain the rhodopsin content of the membrane, so it only occurs occasionally. There is instead a fast light mediated interconversion between metarhodopsin and rhodopsin, i.e. rhodopsin is regenerated by the absorption of light by metarhodopsin and subsequent reisomerisation of retinal. In invertebrates the retinal does not dissociate fron the opsin, an exchange of chromophore occurs between two pigment systems, rhodopsin and retinochrome by a retinal binding protein. Retinochrome is found associated with the inner segment.

It consists of an apo-protein of Mr 24000 and bound retinal (all-trans). Absorbance of light (lmax = 496 nm) causes isomerisation of all-trans to 11-cis retinal. There are two known retinal disorders related to rhodopsin, Retinis pigmentosa and congenital night blindness. 70 different mutations in the rhodopsin gene have been identified that can cause retinis pigmentosa, either by producing a mis folded opsin or producing one which is unable to bind retinal.

Congenital night blindness is an inability of the retina to adapt to dark conditions. Two disease causing mutations have been identified Ala 292 to Glu and Gly 90 to Asp. The phytochrome light detection and signaling pathway has a wide range of physiological roles within plants including phototropism of seedlings, ion fluxes, leaf orientation, intracellular movements and day length dependent processes. The phytochrome protein has a Mr 0 f 120,000 an exists as a dimer. Little sequence homology is seen between phytochromes in different plants, for example only 65% homology between oat and zucchini. However the hydropathy profiles between different phytochromes are very similar.

Light absorption by the tetrapyrrole chromophore causes structural changes in the chromophore which are transmitted to the surrounding apo-protein. CD studies carried out in the UV spectrum have revealed that large conformational changes occur near the N-terminus upon photo transformation of Pr to Pfr and vice versa. Absorption in the red band of the spectrum (lmax = 666 nm) converts the inactive Pr to the physiologically active Pfr. Absorption in the far red (lmax = 730 nm) will reconvert the phytochrome.

Pr Lumi-R Meta-Ra Meta-RcP fr response Biosynthesis Degradation Absorption at 666 nm causes the isomerisation of the C 15-C 16 bond from cis to trans. The structures of the two forms of the tetrapyrrole chromophore are shown below. The chromophore is linked to the protein via a thioester linkage, although the nature of the overall chromophore-protein interaction is still unclear, it is thought that hydrophobic interactions might be important. The apo-protein and chromophore synthesis are regulated separately-only Pr is synthesised and Pfr is degraded 100 x faster than Pr, thus functioning as a mechanism of replenishing Pr. The biochemical mechanism for Pfr elucidating its response is not known, but a kinase activity has been found in phytochrome preparations, so it could be by phosphorylation. It is thought that Pfr binds to operators on the DNA sequence and effects the rate of transcription.

Pfr thus regulates gene expression in a tissue specific manner. It can also elicit a response by regulating enzyme activity. The physiological response could be under control of one of a range of light factors measured by phytochrome; light quality (spectral distribution), light quantity, direction of light, duration of light and polarisation of light. It is likely that phytochrome regulates enzymes by phosphorylating them, for example NT Pase activity can be shown to be light controlled. Intracellular movement is regulated by the Ca 2+ gradient across the cell, which in turn is generated by the Pr / Pfr gradient across the cell.

Phytochrome is oriented in the membrane, and can therefore cause a response to the direction of light. The direction of light falling on a leaf will cause a specific Pr / Pfr gradient to be set up across the cell, which will effect actin / myosin such that the leaf is directed at 90 o to the plane of light. Porphyrin are derivatives of por phin such as harm or uroporphirnogen VII. Evidence for their participation in photo biological phenomena relies on the similarity in spectral nature between the absorption spectra of the porphyrin and the action spectra of the biological response. The spectral nature or a particular porphyrin depends on the side chain proto nation of N atoms and the chelation of metal of metal ions. They typically have a strong absorption band in the far violet called the Sort band.