Heteroproteins

The heteroproteínas are proteins formed by the binding of a protein group (apoprotein) with another non - protein, called prosthetic group.

According to their prosthetic group, heteroproteins are classified as:

Chromoproteins

The chromoproteins have prosthetic group as a colored substance, so also called pigments. According to the nature of the prosthetic group, they are divided into:

• Porphyrin pigments. Its prosthetic group is a tetrapyrrolic or porphyrin ring, with a metal cation in the center of this ring.
  • If it is a ferrous cation (Fe²⁺), the porphyrin is called the heme group. This group has it, for example:
    • The hemoglobin, which carries oxygen in the blood.
    • The myoglobin, which carries oxygen in the muscles.
  • If it is a ferric cation (Fe³⁺), the porphyrin is called the hemino group, and it appears, for example, in the enzymes peroxidases and catalases. In some molecules, such as cytochromes, the ferrous ion can be oxidized to ferric and this ion, in turn, can be reduced.
• Non-porphyric pigments. For instance:
  • The hemocyanin, respiratory pigment blue, copper containing and involved in the transport of oxygen in crustaceans and molluscs.
  • The hemerythrin, respiratory pigment having iron found in the annelids marine and branchiopods.

Glycoproteins

The glycoproteins have, as a prosthetic group, a glucide. The union is carried out by means of a covalent bond between a hydroxyl radical of the carbohydrate and an amino radical of the protide. They belong to this group:

• The prothrombin, involved in blood clotting.

• The immunoglobulins or antibodies.

• The hormone stimulating follicle (FSH), the stimulating hormone the thyroid (TSH), and luteinizing hormone (LH).

• The proteoglycans that appear in mucous secretions in synovial fluids and constituting tendons, bones and cartilage.

• The constituent glycoproteins of the glycocalyx in cell membranes.

• Some enzymes, such as ribonucleases.

The different blood groups are caused by the variability of the glycoprotein chain of erythrocyte membrane glycoproteins.

Lipoproteins

They are heteroproteins whose prosthetic group is a lipid. They appear in the structure of plasma membranes. A special group is constituted by blood lipoproteins, since they are water soluble and are responsible for transporting lipids through the blood from their place of absorption, the intestine, to the target tissues.

Nucleoproteins

Its prosthetic group is a nucleic acid. An example of nucleoproteins, the associations of histones or protamines with nucleic acids, forming the chromatin fibers. Another example, the ribonucleoproteins that make up ribosomes.

Phosphoproteins

Its prosthetic group is phosphoric acid (H₃PO₄). Milk casein and yolk , which is found in the yolk of eggs, belong to this group.