| Summary: | Acetyl-CoA carboxylase is the first and committed step of de novo fatty acid synthesis in all organisms. In Escherichia coli, the enzyme is expressed as separate proteins for the three functional components: a biotin carboxylase, a biotin carboxyl carrier protein, and a carboxyltransferase. The carboxyltransferase enzyme has an α2β2 heterotetrameric quaternary arrangement. The crystal structure of the β subunit revealed a zinc-binding domain, a feature common among nucleic acid-binding proteins. Carboxyltransferase preferentially binds mRNA coding for its two subunits over other nucleic acids, suggesting a means by which the enzyme can regulate its own expression. In the first study, the role played by the zinc-binding motif in carboxyltransferase is revealed through site-directed mutagenesis of the four coordinating cysteinyl residues. Results indicate that the zinc-binding domain is involved in both enzymatic activity of the enzyme as well as mediating binding of the enzyme to its own subunit mRNA. In this utility, the zinc-binding domain as a structural feature physically links the two functional aspects of the enzyme, possibly as a means to evolutionally conserve the capacity to regulate its own translation. In the second study, the individual interactions of carboxyltransferase with substrate and carboxyltransferase with mRNA are representated by mathematical modeling in an effort to validate these interactions function as a single system in regulating the activity and expression of carboxyltransferase in response to the metabolic state of the cell. Comparison of experimental and simulation results validate the model while also suggesting a more complex mechanism of carboxyltransferase translational regulation not captured by the current model.
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