Biotin
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Biotin
Biotin is a coenzyme belonging to vitamin B2 group which is an essential growth factor for yeast and many microorganisms, but is also required by higher organisms. It was isolated by kogl and Du Vigneaud from liver extracts. Biotin has a strong affinity for a protein of egg white, called avidin. Avidin binds with biotin and acts as an inhibitor of biotin catalyzed reactions. The vitamin is widely distributed in nature but liver is the best source, where it is usually found bound to protein through ε- N-lysine residue . The structure of biotin and the bound form, biocytin are given below.
Biochemical function Biotin is a water soluble vitamin and participates in transfer of carboxyl groups. Some enzymes that carry out carboxylation reactions are pyruvate carboxylase and acetyl CoA carboxylation. Pyruvate carboxylase is a high molecular weight enzyme which contains four molecules of biotin covalently attached to the enzyme protein through peptide bond with ε-amino group of lysine residue at the active site. Biotin and lysine are linked through peptide bond.
All biotin dependent carboxylases require ATP in all reactions. We can take the specific example of pyruvate decarvboxylases which catalyzes the reactions in two steps. In the first step the biotin-bound enzyme proteins removes CO2 to form carboxyl biotinyl enzyme:
All biotin dependent carboxylases require ATP in all reactions. We can take the specific example of pyruvate decarvboxylases which catalyzes the reactions in two steps. In the first step the biotin-bound enzyme proteins removes CO2 to form carboxyl biotinyl enzyme:
In the next step the CO2 moiety is transferred to an appropriate acceptor molecule depending upon the type of transcarboxylase involved. Pyruvate carboxylases requires an α-keto acid as an acceptor molecule.
The enzyme acetyl CoA carboxylases requires acetyl CoA as an acceptor molecule, a reaction that is common in Escherichia coli.
Charging of the biotin enzyme with Co2 is an endergonic process requiring ATP, and the CO2 that is attached to the nitrogen of biotin is the active form of carbon dioxide that carries out carboxylation reaction.
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