A Menagarie of Molecules
(Appendix, from Life Under the Sun, 2001, Yale University Press)
Photoisomerization of rhodopsin ("Vision at the Threshold").
Photoisomerization (light-induced structural change) of rhodopsin is the primary event of vision in the many organisms that have rhodopsin-based vision (P. Buser and M. Imbert, 1995, Vision, Cambridge, MIT Press). The rhodopsin of humans uses the "bent" 11-cis-retinal as the light-absorbing component, which is bound to the protein opsin. Upon light absorption, rhodopsin changes to the "straight" all-trans-retinal and free opsin. A series of enzymatic reactions regenerates rhodopsin. Compare this reaction with the photocycle of bacteriorhodopsin (see below).
Light and dark activation of rhodopsin ("Vision at the Threshold").
Vision at very low light levels is limited by "noise" in photoreceptor cells. Rhodopsin, the visual pigment in photoreceptor cells, normally consists of the "bent" 11-cis-retinal bound to the protein opsin with a positively charged bond (protonated Schiff base). Light rapidly activates rhodopsin (~10-12 sec) and changes the structure of retinal. A novel hypothesis proposes that photoreceptor noise is caused by the dark isomerization of a rare and unstable form of rhodopsin that lacks the positive charge (R.B. Barlow et al., 1993, Nature 366, 64-6).
Cyclobutane pyrimidine dimer ("A Burning Issue").
Ultraviolet radiation is harmful to most organisms because it can damage their DNA. The most common type of DNA damage caused by ultraviolet radiation is the cyclobutane pyrimidine dimer (CPD) (S.E. Freeman et al., 1989, Proc Natl Acad Sci, USA 86, 5605-9). The formation of the CPD is shown in detail on the top and diagrammatically on the bottom.
Melatonin synthesis ("A SAD Tale").
Disruption in the levels of melatonin and serotonin, compounds synthesized in the pineal gland, has been implicated in causing SAD, Seasonal Affective Disorder (M. Shafii and S.L. Shafii, 1990, Biological Rhythms, Mood Disorders, Light Therapy, and the Pineal Gland, Washington D.C., American Psychiatric Press). The amino acid tryptophan (top) moves from the blood stream into the pineal gland, and is converted to serotonin and then melatonin (bottom). During the day, light blocks norepinepherine release, inhibiting melatonin synthesis and increasing the level of serotonin. At night, nerve fibers connected to the pineal gland release norepinepherine, stimulating the synthesis of melatonin and decreasing the level of serotonin. Melatonin is transported into the blood stream and is eventually broken down in the liver and then excreted.
The final steps of heme synthesis ("The Purple Disease").
Heme is a red, iron-containing pigment that is an important component of hemoglobin, the oxygen-carrying protein of red blood cells. The last two steps of heme synthesis are conversion of protoporphyrinogen IX to protoporphyrin IX, and of protoporphyrin IX to heme. Several researchers have suggested that George III (1738-1820), King of England, Scotland, and Ireland, suffered from variegate porphyria, a disease in which the gene for synthesis of protoporphyrin IX (protoporphrinogen oxidase) is defective (J.C.G. Rohl et al., 1998, Purple Secret, London, Bantam; see Table).
Photoisomerization of phytochrome ("A Novel Method of Weed Control").
A red/far-red light reversible photoreaction controls many physiological and developmental responses in plants. The photoisomerization (light-induced structural change) of the open-chain tetrapyrole of phytochrome at the 15,16 double bond is responsible for this effect (D.L. Farrens et al., 1989, J Am Chem Soc 111, 9162-9). Red light, which is strongly absorbed by Pr, causes accumulation of the far-red absorbing form of phytochrome (Pfr); far-red light, which is strongly absorbed by Pfr, causes accumulation of the red-absorbing form (Pr). In the semi-extended conformations of the two forms of phytochrome depicted here, note that the right-most pyrole ring "flips" back and forth.
Sun-struck reaction ("Light and Beer").
In making beer, a brewer boils the sweet wort with hops prior to fermentation. During the wort boil, the alpha-acids of hops are transformed into bitter-tasting iso-alpha-acids which balance the residual sweetness in the fermented beer. Exposure of iso-alpha-acids to ultraviolet radiation leads to the formation of prenyl mercaptan, a "skunky" smelling compound (J. Templar et al., 1995, Brewers Digest May, 18-25).
Photostable hop compounds ("Light and Beer").
Chemists can prepare numerous bitter tasting compounds from hops that are stable in the light and protect beer from the sun-struck reaction (J. Templar et al., 1995, Brewers Digest May, 18-25). For example, rho-iso-alpha-acids, which make beer much less vulnerable to the sun-struck reaction, can be prepared by hydrogenation (reaction with Sodium borohydride, NaBH4) of iso-alpha-acids. Tetra-hydroisoalpha-acids and hexa-hydroisoalpha-acids can also be easily prepared from hop iso-alpha-acids. These bitter-tasting hop compounds make beer totally safe from the sun-struck reaction. Apparently, tetra-hydro-iso-alpha-acids can be destroyed by light, but do not form foul-smelling mercaptans.
Riboflavin and roseoflavin ("Phycomyces - The Fungus that Sees").
Phycomyces is a primitive fungus that bears reproductive spores on the tip of long and slender reproductive stalks called sporangiophores. These sporangiophores display pronounced phototropism toward ultraviolet and blue light. Based on experiments with riboflavin and roseoflavin, a flavin is implicated as the pigment that controls this phototropic reaction (M.K. Otto et al., 1981, Proc Natl Acad Sci, USA 78, 266-9). Riboflavin and roseoflavin have similar structures, but riboflavin absorbs blue and ultraviolet radiation more strongly and roseoflavin absorbs yellow light more strongly.
Biochemical basis of Dictyostelium amoeba migration ("Dictyostelium - The Amoeba and the Slug").
Dictyostelium is a slime mold that consists of independent cells (amoebas) which aggregate into a slug and then form an erect spore-bearing structure (P.R. Fisher, 1997, BioEssays 19, 397-407). When a population of Dictyostelium amoebas are deprived of food, certain cells break down adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) and then secrete this compound. In response, nearby cells migrate toward the source of cAMP, and then secrete their own cAMP.
Hypericin and stentorin ("High Hopes for Hypericin").
Hypericin, classified by chemists as a meso-napthodianthrone-type molecule, is a naturally occurring pigment found in Saint John's wort and is a potential therapeutic agent for various diseases. Stentorin, a hypericin-like pigment, is the photoreceptive pigment that controls light-induced movement responses in the ciliated microorganism, Stentor coeruleus; a similar pigment, blepharismin, controls movement responses in the related species, Blepharisma japonicum (P.S. Song, 1995, J Photoscience 2, 21-35). Hypericin, stentorin, and blepharismin all strongly absorb red and ultraviolet radiation.
Gonyaulax bioluminescence ("Blue Moons and Red Tides").
Gonyaulax polyedra is a photosynthetic marine microorganism that emits blue bioluminescent light at night. The compound responsible for this bioluminescence is called Gonyaulax luciferin (H. Nakamura et al., 1989, J Am Chem Soc 111, 7607-11), an open chain tetrapyrole that is derived from the breakdown of chlorophyll. When Gonyaulax luciferin is oxidized by the enzyme luciferase (note the boxed in "=O" in the lower compound) it emits blue bioluminescent light.
The photocycle of bacteriorhodopsin ("Photosynthesis and the Great Salt Lake").
In Halobacterium, light rapidly changes the "straight" all-trans form of retinal in bacteriorhodopsin into the "bent" 13-cis form of retinal, with the concomitant release of a proton outside the cell (M.P Krebs and G. Khorana, 1993, J Bacteriol 175, 1555-60). The 13-cis form spontaneously returns to the all-trans form in a light-independent reaction, with the concomitant uptake of a proton from within the cell. Compare this reaction with the photocycle of rhodopsin (see above).
The Xanthophyll Cycle ("Too much of a Good Thing").
Plants have developed several strategies for minimizing damage caused by excess light. The xanthophyll cycle allows plants to accumulate zeaxanthin under bright light and violaxanthin under dim light. Zeaxanthin accepts some of the energy in light-excited chlorophyll so this energy cannot cause photooxidative stress (B. Demmig-Adams and W.W. Adams, 1996, Trends Plant Sci 1, 21-26).
Peter A. Ensminger, ensmingr@twcny.rr.com
|
|