BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.6. Protein Diversity and Classification

5.6.2. Classification of Proteins Based on Chemical Structure

Some Proteins consist solely of polypeptide chains made up of amino acid residues. These are known as simple proteins. A classic example is Histones, the core proteins of Chromatin, which are rich in basic amino acid residues such as Lysine and Arginine, whose radicals carry a positive charge. The Extracellular matrix protein Elastin is also classified as a simple protein.

Many proteins contain a non-protein component attached to The polypeptide chains via weak or covalent bonds. This non-protein moiety may consist of Metal Ions, or various low- or high-molecular-weight organic molecules. Such proteins are referred to as conjugated (or complex) proteins. A non-protein moiety that is tightly bound to the protein is called a prosthetic group and can vary greatly in chemical nature. For instance, proteins combined with a heme group are known as Hemoproteins. In addition to Hemoglobin and Myoglobin, which were discussed earlier, hemoproteins include Enzymes such as Cytochromes, catalase, and peroxidase. When attached to different protein structures, heme performs specific Functions tailored to each protein (for instance, transporting _O2 in hemoglobin, or transferring electrons in cytochromes).

Proteins associated with phosphoric acid residues are called Phosphoproteins. The phosphate groups are linked via ester bonds to the hydroxyl groups of Serine, Threonine, or Tyrosine residues, a reaction mediated by enzymes known as protein Kinases.

Carbohydrate residues are frequently incorporated into protein structures. They impart additional Specificity to proteins and partially protect them against enzymatic proteolysis. Such proteins are designated as Glycoproteins. Many Plasma Proteins, as well as Cell-surface receptor proteins, belong to the glycoprotein family.

Proteins that function in complexes with Lipids are called Lipoproteins, while those complexed with metals are known Metalloproteins.

A conjugated protein comprising both a protein moiety (apoprotein) and a non-protein moiety (prosthetic group) is referred to as a holoprotein.