Classification of Chromoproteins | Proteins

As indicated by their name, these are coloured proteins, their prosthetic group often contains a metal element (Fe, Cu, Mg). These compounds are generally classified on the basis of their prosthetic group having, or not having, the tetrapyrrol structure called porphyrin and according to their respiratory or non-respiratory physiological role. 

1. Porphyrin Chromoproteins:

A. Respiratory Pigments:

They all contain iron.

a) Hemoglobins or respiratory pigments of Vertebrates and various Inver­tebrates;

b) Cytochromes, electron transporters;

c) Cruorins, pigments of Invertebrates

d) Heme Enzymes:

i. The catalases, which decompose peroxides:

H₂O₂ → H₂O + ½O₂

ii. The peroxidases, which catalyze the oxidation of a substrate at the cost of a peroxide:

RH₂ + H₂O₂ → R + 2H₂O

B. Non-Respiratory Pigments:

a) Chloroplastins, found in chloroplasts (where photosynthesis takes place); their prosthetic group is a chlorophyll with a porphyrin ring somewhat similar to that of hemoglobin but containing magnesium (see fig. 3-10).

b) Phycobilins, pigments of blue algae (Phycocyanins) and red algae (Phycoerythrins).

2. Non Porphyrin Chromoproteins:

A. Respiratory Pigments:

a) Hemerythrins, pigments of certain Invertebrates; they contain iron.

b) Hemocyanins, blue pigments (containing copper) of certain Crustaceans and Molluscs.

B. Non-Respiratory Pigments:

a) Carotenoproteins, containing a carotenoid (see fig. 5-8), found for ex­ample in the retina.

b) Flavoproteins, enzyme proteins; we will see later their role in oxidation- reduction phenomena.

C. Metalloproteins:

1. Containing IRON:

i. Ferritin, enabling the storage of iron in the liver and spleen,

ii. Siderophilin, enabling the transfer of iron into the plasma.

2. Containing COPPER:

i. Hemocuprein, in red blood cells,

ii. Ceruleoplasmin, in the plasma.