The structural diversity of metal binding sites in bacterial metalloproteins: the disordered iron-binding coil of iron-sulfur cluster protein and the stable zinc ribbon motif of the carboxyltransferase subunit of acetyl-coa carboxylase

Abstract

This dissertation describes the crystal structures of two distinct metal-binding proteins: Escherichia coli Iron-sulfur cluster protein A and the carboxyltransferase subunit of the acetyl-coA carboxylase enzymes from Staphylococcus aureus and Escherichia coli. Iron-sulfur cluster protein A (IscA) belongs to an ancient family of proteins responsible for iron-sulfur cluster assembly in essential metabolic pathways preserved throughout evolution. The crystal structure of Escherichia coli IscA reveals a novel fold in which mixed beta-sheets form a compact alpha-beta sandwich domain. In contrast to the highly mobile secondary structural elements within the bacterial Fe-S scaffold protein IscU, a protein which is thought to have a similar function, the great majority of the amino acids which are conserved in IscA homologues are located in elements which constitute a well-ordered fold. However, the 10-residue C-terminal tail segment which contains two invariant cysteines critical for the Fe-S binding function of IscA is not ordered. In addition, the crystal packing reveals a helical assembly which is constructed from two possible tetrameric oligomers of IscA. The rates of severe, multi-drug resistant bacterial infections, including those caused by pathogens previously confined to the hospital setting, have increased dramatically in both hospital and community populations. Acetyl-coA carboxylase is a central metabolic enzyme that catalyzes the committed step in fatty acid biosynthesis: biotin-dependent conversion of acetyl-coA to malonyl-coA. This work presents the structures of the bacterial carboxyltransferase subunits from two prevalent nosocomial pathogens, Staphylococcus aureus and Escherichia coli. Both structures reveal a small, independent zinc-binding domain that appears to shield the active site during the catalytic process. The zinc domain of bacterial carboxyltransferase, which lacks a complement in the primary sequence or structure of the eukaryotic homologue, is a feature that yields promise for the structure-based design and development of new, selective antimicrobial classes.