Pigments Found in the Blood of Animals

The following points highlight the four main types of pigments found in the blood of animals. The types are: 1. Haemoglobin 2. Haemocyanin 3. Chlorocruorin 4. Haemerythrin.

Type # 1. Haemoglobin:

A haemoglobin (Hb) molecule is a conju­gated protein, because it consists of a simple protein and with a non-protein part. The non-protein part is called prosthetic group. The protein part of the haemoglobin is called globin (96%) and a porphyrin ring with an iron atom at its centre, called haem or haematin (4%). The globin part consists of 4 polypeptide chains-two alpha (α) chains and two beta (β) chains.

Porphyrins are heterocyclic ring structure containing haem or magnesium. The heterocyclic ring struc­ture is composed of four pyrrole rings which are linket by methine bridges.

The globin part helps to prevent oxygen from binding tightly to haem; when globin is present, oxygen binds reversibly to haem and can be released to the tissues. In respiratory organs the haemoglobin combines with O₂ which form Oxyhaemoglobin at normal tempera­ture and pressures.

At the low pressure the oxyhaemoglobin dissociates as oxygen and haemoglobin (HbO₂ ↔ Hb + O₂)- Haemo­globin is involved in vertebrates in the trans­port of respiratory CO₂ (about 10% of the total) as carbamino-haemoglobin in which CO₂ is bound to the globin protein. The molecular weight of a haemoglobin mol­ecule is 64,500 daltons.

The oxygenated form of haemoglobin is scarlet and deoxygenated form is bluish-red. The haemoglobin is present in the erythrocytes in almost all vertebrates except a few Antarctic fish. In the invertebrates they are found in the plasma, coelomic fluid and haemoglobin-containing cells.

In Annelida, the pigment is found in polychaeta (different kinds of respiratory pigments), in Oligochaeta (e.g., Pheretima, Lumbricus, Tubifex etc.), and in Hirudinea (e.g., Hirudo, Hirudinaria, etc.).

In Mollusca the haemoglobin is found in the plasma of Gastropods (e.g., Planorbis), and in Bivalvia (e.g., Solen, Area, etc.). In Chiton and in some prosobranchs the haemoglobin is present in the muscles of radula as My­oglobin. In Crustacea the pigment is present in small-sized animals (e.g., Artemia, Daphnia, Triops, etc.).

Type # 2. Haemocyanin:

A blue-green copper containing respira­tory pigments found in some crustaceans, xiphosurans, myriapodes, and in some gas­tropods, bivalves and cephalopods. Haemo­cyanin is always found in dissolved condi­tion in plasma. It occurs in two forms— oxidized and reduced forms, and in reduced forms the prism-shaped or needle-shaped crystals are soluble in water.

The oxygen carrying capacity of haemocyanin is lesser than haemoglobin. The mol. weight is vari­able in different groups of animals. In some crustaceans the mol. weight is about 4,00,000 daltons and in some gastropods it is 13,00,00,000.

Haemocyanin binds a molecule of oxygen between a pair of copper atoms linked to amino acid side chains. In oxygen­ated condition the haemocyanin is bluish- green but it is colourless in deoxygenated state.

Type # 3. Chlorocruorin:

It is a green coloured respiratory pig­ment containing iron found in the plasma of some polychaetes (e.g., Serpulid, spirorbid, sabellid fanworms). It is also found in oxy­genated and reduced forms. The metalloprotein of chlorocruorin is similar to haemoglobin except one vinyl group (CH₂ = CH—) is replaced by formyl (HCO—) group. The mol. weight is 30,00,000 daltons and generally it functions as oxygen carrier.

Type # 4. Haemerythrin (Haemo-erythrin):

It is an iron containing respiratory pig­ment found in the blood corpuscles of some invertebrates (e.g., Sipunculans, Priapulids and inarticulate Brachiopods). It is pink or violet coloured in oxygenated state and colourless in deoxygenated state. The mol. weight varies from 40,000 to 108,000 daltons and plays the role of oxygen storage.

A few other less common respiratory pigments are:

(i) Pinnaglobin—a brown coloured manganese containing pigment found in the plasma of Pinna (Lamellibranchs).

(ii) Vanadium—It is a green coloured vanadium containing pigment found in the vanadocytes of some sea squirts (Ascidians).

(iii) Molpadin pigment is present in Holothurian Molpadia, and

(iv) Echinochrome is known in sea ur­chins of echinoderms.