The universal response of ACAC(A,B) proceeds by way of a -step mechanism. The first response is performed by means of BC and includes the ATP-established carboxylation of biotin with bicarbonate serving because the source of CO2. The carboxyl institution is transferred from biotin to acetyl CoA to shape malonyl CoA inside the 2d reaction, which is catalyzed through CT.
In the lively web site, the reaction proceeds with enormous interaction of the residues Glu296 and undoubtedly charged Arg338 and Arg292 with the substrates. 2 Mg2+ are coordinated by using the phosphate groups on the ATP, and are required for ATP binding to the enzyme. Bicarbonate is deprotonated by means of Glu296, although in answer, this proton switch is not likely because the pKa of bicarbonate is 10.3. The enzyme seemingly manipulates the pKa to facilitate the deprotonation of bicarbonate. The pKa of bicarbonate is reduced with the aid of its interplay with positively charged facet chains of Arg338 and Arg292. Furthermore, Glu296 interacts with the aspect chain of Glu211, an interplay that has been proven to reason an increase in the plain pKa. Following deprotonation of bicarbonate, the oxygen of the bicarbonate acts as a nucleophile and assaults the gamma phosphate on ATP. The carboxyphosphate intermediate quickly decomposes to CO2 and PO43−. The PO43− deprotonates biotin, creating an enolate, stabilized by Arg338, that subsequently attacks CO2 ensuing inside the manufacturing of carboxybiotin. The carboxybiotin translocates to the carboxyl transferase (CT) energetic web page, where the carboxyl organization is transferred to acetyl-CoA. In comparison to the BC domain, little is known approximately the reaction mechanism of CT. A proposed mechanism is the release of CO2 from biotin, which sooner or later abstracts a proton from the methyl group from acetyl CoA carboxylase. The resulting enolate attacks CO2 to shape malonyl CoA. In a competing mechanism, proton abstraction is concerted with the attack of acetyl CoA.