Respiratory pigments | respiratory pigments in different phylogenetic group | biology of invertebrat

Respiratory Pigments in Invertebrates:

In order to facilitate the transport of oxy­gen to different parts of the body, most ani­mals have developed respiratory pigments. Respiratory pigments are of profound physi­ological importance, especially in large sized animals, where the uniform distribution of oxygen to all parts of the body by way of simple diffusion would have been a difficult process.

In general, respiratory pigments are coloured proteins that contain a metallic element in their constitution and have the property of forming loose combination with oxygen and sometimes with carbon dioxide.

Four different (biochemically) respiratory pigments are recognized – haemoglobin, chlorocruorin, haemocyanin, and haemerythrin. Even in the same phylum there may be several distinct pigments, and more than one distribution of four pigments in the animal pigment may exist in the same animal.

Haemoglobin:

It is the most efficient respiratory pigment. It is widely distributed in the animal kingdom, starting from some protozoa like Paramoecium to almost all verte­brates except eel larvae and some Antarctic fishes. Some invertebrate phyla viz., Porifera, Cnidaria and Ctenophora, totally lack it.

It also occurs in the plant world. In vertebrates, it is always contained inside erythrocytes of blood. In invertebrates, it may remain dis­solved in blood plasma, (e.g. Oligochaetes, Hirudinea, Arenicola, Nereis etc.) or may be contained inside erythroid coelomocytes (e.g., Holothurians, some polychaetes like Capitella, Glycera etc.).

Vertebrate haemo­globins have molecular weights varying between 64,000-68,000 Daltons. Intracellular haemoglobins of invertebrates generally have low molecular weights, ranging between 15,000-17,000 Daltons. But extra­cellular haemoglobins of invertebrates have high molecular weight which vary between 1 to 3 million Daltons.

Haemoglobin is made up of an iron por­phyrin compound, haeme, associated with a protein, globin. Haeme is composed of four pyrrole rings linked by methenyl-bridges (—CH=) to form a super-ring with an atom of ferrous iron in the centre attached to the pyrrole nitrogen .

It also occurs in the plant world. In vertebrates, it is always contained inside erythrocytes of blood. In invertebrates, it may remain dis­solved in blood plasma, (e.g. Oligochaetes, Hirudinea, Arenicola, Nereis etc.) or may be contained inside erythroid coelomocytes (e.g., Holothurians, some polychaetes like Capitella, Glycera etc.).

Haemoglobin is made up of an iron por­phyrin compound, haeme, associated with a protein, globin. Haeme is composed of four pyrrole rings linked by methenyl-bridges (—CH=) to form a super-ring with an atom of ferrous iron in the centre attached to the pyrrole nitrogen

In the tetramer, a molecule of heme is linked to one of the histidine residues of each chain; the four chains are folded together into an intri­cate molecule which has been mapped in detail by X-ray analysis

Haemocyanin:

Among various copper- proteins occurring in nature, only haemo­cyanin can reversibly combine with oxygen and thus, serves as a transport pigment. It is found in Chitons, some gastropods and cephalopods amongst the molluscs and in crustaceans and Limulus amongst the arthro­pods. It always remain dissolved in the plasma.

The molecular weight is very high, varying from 4,00,000 Daltons in some crustaceans to 13,00,00,000 Daltons in some gastropods. The number of subunits vary from a few to many in haemocyanin.

Generally, molluscan haemocyanins are larg­er than the arthropodan pigment and the for­mer consists of a greater number of subunits as well. Haemocyanin has no porphyrin. In this metalloprotein, the metallic part is cop­per (Cu++), which remains directly linked to the protein part.

Chlorocruorin:

This green coloured metalloprotein is found in the plasma of certain polychaet families, viz., Sabellidae, Serpulidae etc. It is a metalloprotein with the metal being iron (Fe++); the metalloporphyrin is similar to heme of haemoglobin except that one vinyl (CH = CH2) group is replaced by formyl (0=CH) group in chlorocruorin (Fig. 2.51). The porphyrin is called chlorocruoheme.

Haemerythrin:

This violet coloured pigment is found inside the corpuscles of animals belonging to the phyla Sipunculida, Priapuloidea and Brachiopoda, and also in the polychaete worm Magelona. It is also an iron containing metalloprotein but has no porphyrin.

Pinna globin:

This brown coloured, manganese containing pigment is present in the plasma of the Lamellibranch and Pinna.



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