Ethylene receptor site

Fruit tissues respond to ethylene by exhibiting increases in the activities of enzymes that catalyze ripening reactions, and in some cases, the increases in enzyme activity probably are the result of de novo synthesis, rather than activation of preexisting enzymes. Other target tissues respond similarly to ethylene. But it is not known whether ethylene acts directly to evoke new enzyme production. Interpretation of results with inhibitors of RNA and protein synthesis is inconclusive, because it could be merely that RNA and protein synthesis are essential to maintain the cells in a state competent to respond to ethylene. Moreover, there are some responses to ethylene, besides fruit ripening, which occur under conditions which apparently do not directly involve RNA and protein synthesis (e.g., membrane permeability changes). It has been proposed that the in vivo ethylene receptor site contains a metal such as copper (34,35). 

MCP or 1 -Methylcyclopropene Hydrocarbon Blocks ethylene receptor sites Delay of ripening, conservation of fruits, flowers, and so on 11-40... 

Ethylene plays an essential role in the development of plants, for example in germination, growth and fruit ripening. Carbons 3 and 4 of methionine appear to be the most important source of the gas in vivo, but the effect is also induced by externally applied gas. Acetylene and carbon monoxide are competitive with ethylene. This suggests that a metal ion is present at the ethylene receptor site, a view confirmed by the inhibition of alkene binding by dithiocarbamate. The possibility that this metal is copper is supported by the preparation of copper(I)-monoalkene complexes that show the tight binding of monoalkenes characteristic of the ethylene receptor sites of plants. ... 

Ethylene action depends on binding to a receptor site that has a low affinity for other olefines. Oxygen is required for the response, which is competitively inhibited by carbon dioxide. 

Ethylene receptors and regulatory control. The mode of action of ethylene at the molecular level is unknown. Some attempts, however, have been made to determine the receptor sites for ethylene (54) as well as their characteristics (55). There appears to be very little incorporation of ethylene applied to tissues (only about 0.05%). The - - C ethylene incorporated into pea seedling tissues which responded physiologically to the gas was metabolized to C02 and water-soluble metabolites (55). Metabolism of the incorporated ethylene by pea seedlings and other tissues was inhibited by high levels of CO2 (7-10%) and Ag+ ions (10-500 ppm) (56). Ag+ ions prevented the incorporation of ethylene into water-soluble tissue metabolites and... 

Carbon dioxide and Ag+ ions clearly suppressed senescence, as determined by chlorophyll loss. Aminoethoxy vinylglycine (AVG), the inhibitor of ethylene biosynthesis, also significantly suppressed senescence, as determined by preservation of chlorophyll in the leaf disks aging in the dark.Combinations of C0-, Ag ions and AVG were especially effective on preserving chlorophyll, presumably by suppressing both ethylene biosynthesis and action at the two receptor sites. After 6 days aging at 25° in the dark, the controls contained only 7% of the chlorophyll present at the start, whereas 84% of the chlorophyll was retained by the leaf disks treated with a combination of CO-, Ag and AVG. 

Advances in ethylene biochemistry and physiology have preceded along a number of fronts. Firstly the biosynthetic pathway from methionine to ethylene has been further clarified and intermediates identified. Secondly some progress has been made in recognising two possible receptor sites which are inhibited by Ag ions and C0 , respectively. Thirdly the localization of ethylene production has been shown to be associated with membranes in studies with protoplasts. 

By far the most exciting development of this work was the demonstration by Schaller and Bleecker [31] that transformation of yeast with the wild type ETR gene conferred the ability to bind ethylene in a saturable and reversible manner and with an appropriate Kp (2.4xl0 M). These results taken together with those outlined above provide almost conclusive evidence that the ETR gene product is indeed an ethylene receptor. In the same work expression of truncated forms of ETRI in yeast provided further evidence that the N-terminal hydrophobic domain of the protein is the site of ethylene binding. 

Another series of hydrocarbons exemplified by ethylene (ethene=C3H, CH2=CHj) and propene (CjH, CH3-CH=CHj) has a more reactive site, i.e., C=C double bond, so that they may significantly interact with a receptor site. They do indeed smeU. 

Ethylene is a potent inductor of Striga spp. germination.26 It has been hypothesized as a possible mechanism for parasitic seed germination that attachment of the inductor to the receptor s site triggers a cascade of biosynthetic reactions leading to the synthesis of the enzymes necessary for ethylene production.12... 

The concept of bridging has also been applied to antagonists bivalent ligands containing /3-naltrexamine pharmacophores (p. 67) linked by an oligoethylene glycol spanner have been described that differentially block /x, k, and S-opioid receptors.(198) Thus, 10a (with a six-ethylene-units spanner) most effectively blocks S-receptors (DADL on MVD), while 10b (with shorter spanner) acts preferentially at k-sites (ethylketazocine on GPI). 

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