Conformational studies of Escherichia coli pyruvate oxidase

Sulfhydryl chemistry detects three conformations of the lipid binding region of Escherichia coli pyruvate oxidase

Site-specific disulfide cross-linking experiments detected a conformational change within the C-terminal segment of Escherichia coli pyruvate oxidase (PoxB), a lipid-activated homotetrameric enzyme, upon substrate binding [Chang, Y.-Y., & Cronan, J. E., Jr. (1995) J. Biol. Chem. 270, 7896-7901]. The C-terminal lipid binding regions were cross-linked only in the presence of the substrate, pyruvate, and the thiamine pyrophosphate cofactor, indicating close proximity of a pair of C termini. We have now systematically substituted cysteine at 18 additional amino acid positions within the C-terminal region to obtain a panel of 21 proteins each having a single residue changed to cysteine. These proteins have been studied by disulfide cross-linking and by accessibility of the cysteine side chain to a variety of sulfhydryl agents. In the absence of pyruvate, the cysteine residues of the modified PoxB proteins failed to form disulfide bonds, generally failed to react with a large and rigid hydrophilic sulfhydryl reagent, 4-acetamido-4'-[(iodoacetyl)amino]stilbene-2,2'-disulfonic acid (IASD), and in some cases reacted weakly with a smaller more hydrophobic reagent, N-ethylmaleimide. Therefore, in this conformation, the C termini appear fixed in a rigid environment having limited exposure to solvent. In the presence of pyruvate, all of the C-terminal cysteine residues (except the two most distal from the C terminus) reacted with both sulfhydryl reagents and readily formed disulfide cross-linked species, indicating conversion to a structure having a high degree of conformational freedom. In the presence of lipid activators, Triton X-100 or dipalmitoylphosphatidylglycerol, a subset of the cysteine-substituted proteins no longer reacted with the membrane-impermeable IASD reagent, indicating penetration of these protein segments into the lipid micelles. For most of the proteins, similar extents of disulfide formation were seen upon addition of an oxidizing agent in the presence or absence of lipid activators. An exception was PoxB D560C which was much more readily cross-linked in the presence of lipid. Moreover, a subset of PoxB proteins that cross-linked to lower extents in the presence of lipids was found. The behavior of these proteins provides strong support for the model in which two C termini associate to form the functional lipid binding domain. These data are discussed in terms of three distinct PoxB conformers and the known crystal structure of a highly related protein.

Detection by site-specific disulfide cross-linking of a conformational change in binding of Escherichia coli pyruvate oxidase to lipid bilayers

Escherichia coli pyruvate oxidase, a peripheral membrane homotetrameric flavoprotein, exposes its C-terminal lipid binding site in the presence of substrate pyruvate and co-factor thiamine pyrophosphate Mg2+ and binds tightly to phospholipid bilayers during catalysis. Using site-specific disulfide cross-linking, we demonstrate that disulfide cross-links are formed between C termini of D560C pyruvate oxidase and that the degree of cross-linking is greatly increased by the presence of substrate and co-factors indicating a conformational change that results in juxtaposition of two subunit C termini. The cross-linked oxidase is enzymatically active and remains able to associate with lipid micelles. These results argue strongly that lipid bilayer binding of pyruvate oxidase involves pairing of the C termini of two subunits.

Nucleotide sequence and deduced amino acid sequence of Escherichia coli pyruvate oxidase, a lipid-activated flavoprotein

The entire nucleotide sequence of the poxB (pyruvate oxidase) gene of Escherichia coli K-12 has been determined by the dideoxynucleotide (Sanger) sequencing of fragments of the gene cloned into a phage M13 vector. The gene is 1716 nucleotides in length and has an open reading frame which encodes a protein of Mr 62,018. This open reading frame was shown to encode pyruvate oxidase by alignment of the amino acid sequences deduced for the amino and carboxy termini and several internal segments of the mature protein with sequences obtained by amino acid sequence analysis. The deduced amino acid sequence of the oxidase was not unusually rich in hydrophobic sequences despite the peripheral membrane location and lipid binding properties of the protein. The codon usage of the oxidase gene was typical of a moderately expressed protein. The deduced amino acid sequence shares homology with the large subunits of the acetohydroxy acid synthase isozymes I, II, and III, encoded by the ilvB, ilvG, and ilvI genes of E. coli.

Reconstitution of the membrane-bound, ubiquinone-dependent pyruvate oxidase respiratory chain of Escherichia coli with the cytochrome d terminal oxidase

Pyruvate oxidase is a flavoprotein dehydrogenase located on the inner surface of the Escherichia coli cytoplasmic membrane and coupled to the E. coli aerobic respiratory chain. In this paper, the role of quinones in the pyruvate oxidase system is investigated, and a minimal respiratory chain is described consisting of only two pure proteins plus ubiquinone 8 incorporated in phospholipid vesicles. The enzymes used in this reconstitution are the flavoprotein and the recently purified E. coli cytochrome d terminal oxidase. The catalytic velocity of the reconstituted liposome system is about 30% of that observed when the flavoprotein is reconstituted with E. coli membranes. It is also shown that electron transport from pyruvate to oxygen in the liposome system generates a transmembrane potential of at least 180 mV (negative inside), which is sensitive to the uncouplers carbonyl cyanide p-(tri-chloromethoxy)phenylhydrazone and valinomycin. A trans-membrane potential is also generated by the oxidation of ubiquinol 1 by the terminal oxidase in the absence of the flavoprotein. It is concluded that (1) the flavoprotein can directly reduce ubiquinone 8 within the phospholipid bilayer, (2) menaquinone 8 will not effectively substitute for ubiquinone 8 in this electron-transfer chain, and (3) the cytochrome d terminal oxidase functions as a ubiquinol 8 oxidase and serves as a "coupling site" in the E. coli aerobic respiratory chain. These investigations suggest a relatively simple organization for the E. coli respiratory chain.