Question

Which enzyme contains the coenzyme biotin? What is the function of this coenzyme and how is it covalently attached to the enzyme? Show the steps in the mechanism of this enzyme.

Answer 1

• Carboxylase enzyme contains the coenzyme biotin.
• Function of BIOTIN: Biotin serves an important role in several enzyme systems, namely the decarboxylases of oxalacetate and succinate, the deaminases of aspartic acid, serine, and threonine, succinic acid dehydrogenase, and the synthesis of citrulline. In addition to these functions, this vitamin is somehow related to carbohydrate and fatty acid metabolism and to the amino acid oxidases. Delwiche has presented data demonstrating that biotin is concerned with the decarboxylation of succinic acid to propionic acid. He employed a strain of P. pentosaceum grown on a chemically defined medium deficient in biotin. Resting cell suspensions prepared from organisms grown under these conditions were deficient in the production of carbon dioxide from added succinate, and the addition of biotin consistently more than doubled the decarboxylation rate. Hydrolysis of bound biotin yields a substance not identical with biotinic acid and exhibiting a profound stability to acid treatment.
• The cofactor, itself, is composed of a ureido ring, a tetrahydrothiophene ring, and a valeric acid side chain. It is the ureido ring that functions as the CO2 carrier. The adenine moiety of ATP interacts with a loop of residues delineated by Glu 201 to Leu 204. The exocyclic amine forms hydrogen bonds with the carboxylate group of Glu 201 and the main chain carbonyl oxygen of Lys 202, whereas the peptidic nitrogen of Leu 204 interacts with N-1? of the adenine ring. In addition, Tyr 203 participates in a T-shaped stacking interaction with the adenine ring. Both Tyr 203 and Leu 204 form part of the hinge that allows the B-domain to rotate over the active site