However, a recent studyon plant genomics found that Brassica BCCPs might play a key role in abiotic and biotic stress responses.

Two different protein subassemblies, along with BCCP, are required for this two step reaction to be successful: biotin carboxylase (BC) and carboxyltransferase (CT).

However, most studies of this protein have been conducted on the E. coli form of the enzyme, where all three components exist as three separate complexes rather than being united on one polypeptide chain.

The connection to the rest of the enzyme at the N-terminus of BCCP core is located at the opposite end of the structure from the biotin moiety.

[8] The structures of biotin-accepting domains from E. coli BCCP-87 and the 1.3S subunit of P. shermanii TC were determined by both X-ray crystallography and nuclear magnetic resonance studies.

At the other end of the molecule, the biotinyl- or lipoyl-accepting lysine resides on a highly exposed, tight hairpin loop between β4 and β5 strands.

[10] The structure of the Biotin-accepting domain consists of BCCP-87 which contains a seven-amino-acid insertion common to certain prokaryotic acetyl-CoA carboxylases but not present in other biotindomains (Chapman-Smith and Cronan, 1999).

It has been proposed that this thumb may function as a mobile lid for either, or possibly both, the biotin carboxylase or carboxyl- transferase active sites in the biotin-dependent enzyme (Cronan, 2001).

The significance of this requires further investigation but it is possible that the mechanism employed by the biotin enzymes may involve noncovalent interactions between the protein and the prosthetic group.