Tetrapyrroles

The bilins are linear tetrapyrroles produced by ring cleavage of protoporphyrins. They have been identified in most major groups of animals. In mammals they are known as bile pigments. They are found in Phasmida, Mantida, Orthoptera and Lepidoptera. The green colour of many grasshoppers and lepidopteran larvae is due to bilins of the biliverdin type (Figure 8.20).

Biosynthesis of the tetrapyrroles begins with a Claisen condensation between glycine attached to pyridoxal phosphate (Chapter 2) and suc-cinyl CoA. After cleavage from the pyridoxal, 5-aminolaevulinic acid is obtained. Condensation of two molecules of the latter gives the first pyrrole, porphobilinogen (Figure 8.20). Porphobilinogen molecules are coupled by deamination to give a dimer, trimer and tetramer, which can be cyclized to give a variety of tetrapyrroles, the important one here being protoporphyrin IX. Oxidative ring opening at the a-meso position of protoporphyrin IX (between rings A and D) gives the blue-green pigment biliverdin. Reduction of the central -CH= gives the orange coloured bilirubin (Figure 8.20). Chironomus larvae store both biliverdin and bilirubin in their fat body Cleavage of protoporphyrin IX between rings C and D gives pterobilin, first isolated from the larvae and pupae of Pieris brassicae (cabbage white butterfly), but has been found in the majority of 100 species of Lepidoptera examined. Irradiation of pterobilin causes rotation about the -CH= joining rings A and B and then reaction of the vinyl group on ring B with ring A. Further reaction between the vinyl group of ring A and ring D also occurs to give sarpedobilin. More details of the biosynthesis of this group can be found in Torssell or Britton (see list of further reading).

Figure 8.20 A brief summary of the biosynthesis of some insect bilins, starting from 5-aminolaevulinic acid. Pterobilin is found only in the wings ofpierid butterflies, while phorcabilin is found in the wings ofpapilionid, arctid and microlepidoptera together with isophorcabilin and sarpedobilin (not illustrated)

Figure 8.20 A brief summary of the biosynthesis of some insect bilins, starting from 5-aminolaevulinic acid. Pterobilin is found only in the wings ofpierid butterflies, while phorcabilin is found in the wings ofpapilionid, arctid and microlepidoptera together with isophorcabilin and sarpedobilin (not illustrated)

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