Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Vitamins and Trace Elements: Their Role in Enzyme Function
Biotin is an active component of biocytin, the prosthetic group of certain enzymes that catalyze carboxylation reactions

In 1935, the Dutch biochemist Fritz Kögl isolated about 1 mg of crystalline Yeast growth factor, a feat that required 250 kg of dried egg yolks. It was later discovered that this substance is also essential for the growth of rats fed large amounts of raw egg white. The new growth factor was named biotin. Although chicken eggs are a highly nutritious food containing biotin, feeding animals large quantities of raw egg whites leads to a biotin deficiency. This paradoxical fact is explained by the presence of a protein called Avidin in egg white, which binds biotin extremely tightly, thereby preventing the intestinal absorption of this vitamin.

In biotin-dependent Enzymes, the biotin molecule is covalently attached to the enzyme protein via an amide bond formed by the ε-amino group of a reactive Lysine residue located at the active center of the enzyme. This biotinyl-lysine residue, known as biocytin, can be isolated from biotin-containing enzymes following their acid or Enzymatic Hydrolysis (Fig. 10.11). Biotin acts as a carrier of carboxyl (—COO-) groups in numerous enzymatic carboxylation reactions that proceed with the participation of ATP. The carboxyl group of the acid is reversibly bound to the nitrogen atom of the bicyclic system of biotin. A classic example of a biotin-dependent carboxylation reaction is the process catalyzed by Pyruvate carboxylase, in which pyruvate is converted into oxaloacetate (Fig. 10-11).