Bioluminescence, the ability of living organisms to emit light, is widely distributed throughout more primitive orders. It occurs in insects (fireflies and glow-worms) among the Collembola, Hemiptera, Diptera, Lepidoptera and Coleoptera. It is found particularly in three Coleoptera families: Lampydidae, Elateridae and Phengodidae. All the insects examined share the same system of luciferin and luciferase to produce light.

Luciferin is synthesized by the fusion of quinone and cysteine to give first 6-hydroxybenzothiazine-2-carboxylic acid. This condenses with a second molecule of cysteine before oxidation and re-arrangment in an unexplained way to give luciferin (Figure 9.22). Luciferin is oxidized enzymically to oxyluciferin, which is formed in two excited states (Figure 9.23). To reach ground state, one form emits red light, the other green, the resultant effect is close to white light. One third of the molecules that undergo oxidation emit light (White, Miano and Umbreit, Journal of the American Chemical Society, 1975, 97, 198). Insects emitting light make an easy target for prey. It is not surprising that lampyrid beetles also employ chemicals, lucibufagins (Chapter 10), to make themselves unpalatable.

S luciferin

Figure 9.22 The biosynthesis of luciferin. The carboxyl group of cysteine was labelled with 14 C

luciferase + ATP , q ' [ luciferin - luciferase - ATP] + pyrophosphate

luciferase + ATP

1180-180 T 18Q-180

red hv J green hv oxyluciferin, ground state

Figure 9.23 The light-emitting reaction of luciferin. When 18 02 gas was used, labelling was found equally in C02 and oxyluciferin. The asterisks indicate the excited electronic states

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