N-l -Carboxybiotin

Carboxybiotin. The structure of biotin suggested that bicarbonate might be incorporated reversibly into its position 2. However, this proved not to be true and it remained for F. Lynen and associates to obtain a clue from a "model reaction." They showed that purified P-methylcrotonyl-CoA carboxylase promoted the carboxylation of free biotin with bicarbonate (H14C03 ) and ATP. While the carboxylated biotin was labile, treatment with diazomethane (Eq. 14-6) gave a stable dimethyl ester of N-l -carboxybiotin.53 54 The covalently bound biotin at active sites of enzymes was also successfully labeled with 14C02 Treatment of the labeled enzymes with diazomethane followed by hydrolysis with trypsin and pepsin gave authentic N-l -carboxybiocytin. It was now clear that the cleavage of ATP is required to couple the C02 from HCOs to the biotin to form carboxybiotin. The enzyme must... 

Biotin (see Scheme 40) is the prosthetic group associated with the biotin-dependent carboxylases and transcarboxylases. The role of biotin is as a carboxyl group carrier. At the first active site, biotin is carboxylated at the N(l) position to form A-carboxybiotin, which acts as CO2 transport. At the second subsite, the carboxyl group is transferred from A-carboxybiotin to substrate. The mechanism of neither half-reaction has been fully defined (for recent discussions see Blonski et al., 1987 Thatcher et ai, 1986). 

Kinetics and reaction mechanism for CO2 exchange in 2-imidazolidinone-l-carboxyhc acid lithium salt (Li 20) were investigated by Lihs and Caudle [134]. N-Carboxyimidazolidone anion, 20, was probed as an analogue for N -carboxybiotin and synthesized as the lithium salt by deprotonation of 2-imidazolidone 21 with phenyllithium and further reaction of the resulting lithium amide with carbon dioxide. The study was addressed to ascertain the viability of unimolecular CO2 elimination from... 

At the same period, X-ray work on the active (-l-)-form revealed that the two rings are cis fused, and the hydrogens of all three asymmetric carbons are in a cis relationship. The structure shows that the N-3 of the ureido function is hindered from reaction by the five carbon side chain of valeric acid. The distance between N-3 and C-6 is only 0.28 nm. Only then did the mechanism of action of biotin become clear a C02-transfer reaction is achieved through the reversible formation of T-Af-carboxybiotin. 

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