Classification and Biological Functions of Proteins

Chromoproteins and Respiratory Pigments

Certain molecules also belong to the chromoproteins group.

Hemocyanin

Hemocyanin is very similar to hemoglobin and shares the same function as a gas carrier. The metal present in its prosthetic group is copper. It appears blue when combined with O2 and colorless when reduced. It is primarily found in invertebrates.

Chlorophyll

Chlorophyll is a photosynthetic pigment characterized by containing a magnesium (Mg) atom. It is specialized to capture photons and releases electrons when light rays impinge upon it.

Holoproteins: Simple Proteins

Holoproteins are simple proteins formed exclusively by amino acids. They serve various biological functions:

• Albumins: These lack glycine. Examples include ovalbumin (egg), lactalbumin (milk), and serum albumins.
• Globulins: Involved in protection and defense. Examples include ovoglobulins, lactoglobulins, and serum globulins (immune antibodies).
• Protamines: Essential for sperm functionality. Examples include clupeine (herring/sardines) and salmine (salmon).
• Histones: These have a structural function, forming the covering of chromosomes.
• Scleroproteins: Found in connective, bone, and cartilaginous tissues.
  • Keratin: Found in structures such as hair, feathers, and nails. It has a high concentration of the amino acid cysteine, which produces a sulfur smell.
  • Collagen: Provides elasticity to tissues and is often used in skincare.

Heteroproteins: Conjugated Proteins

Heteroproteins consist of a protein portion combined with a non-protein prosthetic group:

• Glycoproteins: The prosthetic group consists of carbohydrates. The most representative are mucins, which provide lubricating and antibacterial properties in mucus (bronchial phlegm, vaginal fluids, etc.).
• Phosphoproteins: Contain phosphorus. The most representative is caseinogen, the milk protein that causes coagulation.
• Nucleoproteins: Contain nucleic acids as the prosthetic group and are found as part of cell nuclei.
• Chromoproteins: Contain a metal-porphyrin prosthetic group.
  • Hemoglobin: Contains iron in a bivalent state. One valence binds to globin, one binds to O2 or CO2, and the other two are associated with nitrogen. Hemoglobin is bright red when oxidized (arterial) and dark red when reduced (venous).