Kinetics of MgATP-dependent iron chelation from the Fe-protein of the Azotobacter vinelandii nitrogenase complex. Evidence for two states

Chelation of Fe from the Fe-protein component (Av2) of Azotobacter vinelandii nitrogenase has been investigated. The chelation, which requires MgATP binding by Av2, is best described as a two-exponential process. The rates for the two phases differed by approximately 10-fold and increased as the concentration of MgATP was increased. The rates for both phases were 50% of maximum at approximately 1.5 mM MgATP. At MgATP concentrations greater than 100 microM, the more rapid phase represented approximately 25% of the total Fe chelated from Av2. However, below 100 microM MgATP, the proportion of the faster phase decreased until at 20 microM MgATP, only a single phase could be detected. The properties of Av2 were studied at various stages of Fe chelation. The partially chelated protein was isolated from the reaction by gel filtration and was subjected to a second MgATP-dependent Fe chelation. Material isolated after the completion of the first phase regained biphasic kinetics in subsequent chelation reactions. However, if MgATP was present during the isolation of Av2, then only a single phase was observed in the subsequent chelation studies. In addition, the enzymatic activity of Av2 decreased concomitantly with total Fe chelation. To account for these observations, a model is presented in which Av2 exists in two conformers. Fe chelation is proposed to occur from either conformer but only when two MgATP are bound. Both conformers bind MgATP with the same affinity but are distinguished by a 10-fold difference in chelation rate. The two conformers are in equilibrium and can interconvert only in the absence of MgATP. That is, MgATP binding prevents the conversion of the two conformational states.     

Thiol reactivity of the nitrogenase Fe-protein from Azotobacter vinelandii

A procedure has been developed to examine some of the functional roles of the 14 cysteinyl residues in the nitrogenase Fe-protein (Av2) from Azotobacter vinelandii. The reduced form of Av2 was alkylated with iodo[2-14C]acetic acid under a variety of experimental conditions, e.g. reaction in the presence of nucleotides, alpha,alpha'-dipyridyl and nucleotides, or denaturants. The labeled cysteinyl residues were identified and quantified using an analytical DEAE-Sepharose ion exchange chromatography peptide mapping technique based upon the known amino acid sequence (Hausinger, R. P., and Howard, J. B. (1982) J. Biol. Chem. 257, 2483-2490). From the results of the labeling experiments, the following features of the Av2 structure have been proposed. 1) Av2 contains no disulfides, hyperreactive thiols, or surface thiols as defined by reaction with iodoacetic acid. 2) Cysteines 97 and 132 are the probable ligands for the Av2 Fe:S center which is bound symmetrically between subunits. 3) MgATP partially protects cysteine 85 from carboxymethylation by iodoacetic acid and may be part of the nucleotide-binding site. 4) Of the five nonligand thiols only cysteines 5 and 184 are completely alkylated when Av2 is denatured in hexamethylphosphoramide, whereas all five nonligand thiols appear to rapidly exchange at the Fe:S center if the protein is denatured in the absence of alkylating reagents. 5) Both Av2 and apo-Av2 appear to undergo a reversible conformational change upon binding MgATP.     

On the prosthetic group(s) of component II from nitrogenase. EPR of the Fe-protein from Azotobacter vinelandii

The EPR spectrum of the reduced Fe-protein from nitrogenase has been reinvestigated. The dependences on temperature, microwave power, and microwave frequency all suggest that the observed signal represents a magnetically isolated [4Fe-4S]1+(2+;1+) cluster. Also, the signal can be simulated assuming a simple, g-strained S = 1/2 system. However, the integrated intensity amounts to no more than 0.2 spins per protein molecule. It is, therefore, impossible that Fe-protein preparations contain a single type of [4Fe-4S] cluster.     

Characterization of the gene for the Fe-protein of the vanadium dependent alternative nitrogenase of Azotobacter vinelandii and construction of a Tn5 mutant

Summary A sequence homologous to the conventional nifH gene has been cloned from a different region of the Azotobacter vinelandii genome. Tn5 insertions were obtained in this clone and the mutagenized plasmid was used for marker exchange with A. vinelandii strain CA12 (nifHDK) to obtain Tn5 mutants. These mutants exhibited a Nif^- phenotype in the presence of vanadium, unlike CA12 which was Nif⁺ on vanadium-containing medium. The gene in the cloned nifH-like region is therefore apparently involved in the vanadium dependent alternative pathway of nitrogen fixation. This gene, nifH2, has been sequenced and encodes a protein of 289 amino acids that is similar to nifH in nucleotide sequence, deduced amino acid sequence, predicted secondary structure and hydrophobicity profile. A second open reading frame downstream of nifH2 codes for a protein of 64 amino acids, similar to the ferredoxin (Fd)-like protein encoded downstream of nifH ^* in A. chroococcum. Sequence analysis suggests that the nifH2 and Fd-like genes are in a single operon.     

Membrane-surfactant interactions. The role of surfactant in mitochondrial complex III-phospholipid-Triton X-100 mixed micelles

Complex III (ubiquinol-cytochrome c reductase) was purified from beef heart mitochondria in the form of protein-phospholipid-Triton X-100 mixed micelles (about 1:80:100 molar ratio). Detergent may be totally removed by sucrose density gradient centrifugation, and the resulting lipoprotein complexes retain full enzyme activity. In order to understand the role of surfactant in the mixed micelles, and the interaction of Triton X-100 with integral membrane proteins and phospholipid bilayers, both the protein-lipid-surfactant mixed micelles and the detergent-free lipoprotein system were examined from the point of view of particle size and ultrastructure, enzyme activity, tryptophan fluorescence quenching, 31P NMR, and Fourier transform infrared spectroscopy. The NMR and IR spectroscopic studies show that surfactant withdrawal induces a profound change in phospholipid architecture, from a micellar to a lamellar-like phase. However, electron microscopic observations fail to reveal the existence of lipid bilayers in the absence of detergent. We suggest that, under these conditions, the lipid:protein molar ratio (80:1) is too low to permit the formation of lipid bilayer planes, but the relative orientation and mobility of phospholipids with respect to proteins is similar to that of the lamellar phase. Protein conformational changes are also detected as a consequence of surfactant removal. Fourier transform infrared spectroscopy indicates an increase of peptide beta-structure in the absence of Triton X-100; changes in the amide II/amide I intensity ratio are also detected, although the precise meaning of these observations is unclear. Tryptophanyl fluorescence quenching by acrylamide shows that a significant fraction of the Trp residues sensing the quencher become less readily available to it in the absence of surfactant. The temperature dependence of enzyme activity (expressed in the form of Arrhenius plots) is also different in the presence and absence of detergent. The effects of surfactant removal do not appear to be readily reversible upon readdition of Triton X-100.     

Properties of adenylate kinase after modification of Arg-97 by phenylglyoxal.

Adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) isolated from porcine skeletal and heart muscle and from rabbit muscle are inactivated when a single arginine residue is modified. In adenylate kinase from pig the modified residue was identified as Arg-97 by peptide-mapping. In native adenylate kinase Arg-97 is located at the bottom of the active site cleft. The protein fluorescence of modified adenylate kinase is reduced. Whereas the addition of AMP, ADP and MgATP quench the fluorescence of native adenylate kinase, the fluorescence of phenylglyoxal-modified adenylate kinase is only affected by ADP and MgATP. This finding is discussed in connection with the structural isomerization observed in native adenylate kinase by X-ray diffraction analysis.     

Histone-DNA interactions and their modulation by phosphorylation of -Ser-Pro-X-Lys/Arg- motifs.

The sea urchin sperm-specific histones H1 and H2B are multiply phosphorylated in spermatids, dephosphorylated in the final stages of spermatogenesis to give mature sperm, and rephosphorylated upon fertilization. Phosphorylation in spermatids, and probably at fertilization, occurs at repeated -Ser-Pro-X-Basic-motifs in the distinctive N-terminal basic domains of both histones and at the end of the much longer C-terminal domain of H1. Here we identify the consequences of multiple phosphorylation through comparison of some physical and biochemical properties of spermatid (phosphorylated) and sperm (dephosphorylated) chromatin and histones. Study of the DNA binding properties of the intact histones and isolated basic domains suggests that phosphorylation at three dispersed sites in the C-terminal tail of H1 has little effect on its overall DNA binding affinity, whereas, strikingly, binding of the N-terminal domains of H2B and H1 is abolished by phosphorylation at four or six tandemly repeated sites respectively. Together with the relative timing of events in vivo, this suggests that phosphorylation/dephosphorylation of the N-terminal (and distal end of the C-terminal) tail of H1, and/or the N-terminal tail of H2B, effectively controls intermolecular interactions between adjacent chromatin filaments, and hence chromatin packing in the sperm nucleus.     

Conformational changes in the nitrogenase complex in vivo by preincubation under acetylene.

Preincubation of $\text{Anabaena}$ L-31 under acetylene results in conformational changes in the nitrogenase enzymecomplex. In particular, the site common to both nitrogen and acetylene is modified such that the affinity towards acetylene increases considerably and there is a corresponding decrease in the affinity towards nitrogen. The site for azide remains unaltered. The binding site for carbon-monoxide (CO) also undergoes change as revealed by the remarkable decrease in the affinity for CO. The results explain the enhancement in acetylene reduction upon preincubation under acetylene.     

Effect of amino-acid substitutions on Alzheimer's amyloid-beta peptide-glycosaminoglycan interactions.

One of the major clinical features of Alzheimer's disease is the presence of extracellular amyloid plaques that are associated with glycosaminoglycan-containing proteoglycans. It has been proposed that proteoglycans and glycosaminoglycans facilitate amyloid fibril formation and/or stabilize these aggregates. Characterization of proteoglycan-protein interactions has suggested that basic amino acids in a specific conformation are necessary for glycosaminoglycan binding. Amyloid-beta peptide (Abeta) has a cluster of basic amino acids at the N-terminus (residues 13-16, His-His-Gln-Lys), which are considered critical for glycosaminoglycan interactions. To understand the molecular recognition of glycosaminoglycans by Abeta, we have examined a series of synthetic peptides with systematic alanine substitutions. These include: His13-->Ala, His14-->Ala, Lys16-->Ala, His13His14Lys16-->Ala and Arg5His6-->Ala. Alanine substitutions result in differences in both the secondary and fibrous structure of Abeta1-28 as determined by circular dichroism spectroscopy and electron microscopy. The results demonstrate that the His-His-Gln-Lys region of Abeta, and in particular His13, is an important structural domain, as Ala substitution produces a dysfunctional folding mutant. Interaction of the substituted peptides with heparin and chondroitin sulfate glycosaminoglycans demonstrate that although electrostatic interactions contribute to binding, nonionic interactions such as hydrogen bonding and van der Waals packing play a role in glycosaminoglycan-induced Abeta folding and aggregation.     

Protein-protein interactions in the synaptonemal complex.

In mammalian systems, an approximately M(r) 30,000 Cor1 protein has been identified as a major component of the meiotic prophase chromosome cores, and a M(r) 125,000 Syn1 protein is present between homologue cores where they are synapsed and form the synaptonemal complex (SC). Immunolocalization of these proteins during meiosis suggests possible homo- and heterotypic interactions between the two as well as possible interactions with yet unrecognized proteins. We used the two-hybrid system in the yeast Saccharomyces cerevisiae to detect possible protein-protein associations. Segments of hamsters Cor1 and Syn1 proteins were tested in various combinations for homo- and heterotypic interactions. In the cause of Cor1, homotypic interactions involve regions capable of coiled-coil formation, observation confirmed by in vitro affinity coprecipitation experiments. The two-hybrid assay detects no interaction of Cor1 protein with central and C-terminal fragments of Syn1 protein and no homotypic interactions involving these fragments of Syn1. Hamster Cor1 and Syn1 proteins both associate with the human ubiquitin-conjugation enzyme Hsubc9 as well as with the hamster Ubc9 homologue. The interactions between SC proteins and the Ubc9 protein may be significant for SC disassembly, which coincides with the repulsion of homologs by late prophase I, and also for the termination of sister centromere cohesiveness at anaphase II.     

Ionic and nonionic interactions in adenoviral nucleoprotein complexes.

Ionic and nonionic interactions between the adenoviral histone-like proteins and DNA were examined by determining effects of ionic strength and urea concentration on disruption of viral nucleoprotein. The viral proteins were as susceptible to dissociation by salt in the presence of urea as histones. Nonionic interactions between viral proteins appeared more extensive than those between histones.     

Fully active Fe-protein of the nitrogenase from Azotobacter vinelandii contains at least eight iron atoms and eight sulphide atoms per molecule

A purified preparation of kidney basolateral membrane vesicles is capable of ATP-dependent Ca2+ uptake. The reaction has high affinity for Ca2+ (Km about 0.1 microM) and a V of 5.8 nmol Ca2+ X mg-1 protein X min-1 in the predominantly right-side-out vesicular preparation used. It is inhibited by vanadate (K0.5 about 5 microM) and by anti-calmodulin drugs. A stimulatory effect of calmodulin is visible in membranes depleted of the activator. Exposure of basolateral membranes to 125I-azido-modified calmodulin results in the specific labeling of a membrane protein of Mr 141 000, which is tentatively suggested to be the Ca2+-pumping ATPase.     

Vanadium nitrogenase of Azotobacter chroococcum. MgATP-dependent electron transfer within the protein complex.

The kinetics of MgATP-induced electron transfer from the Fe protein (Ac2V) to the VFe protein (AclV) of the vanadium-containing nitrogenase from Azotobacter chroococcum were studied by stopped-flow spectrophotometry at 23 degrees C at pH 7.2. They are very similar to those of the molybdenum nitrogenase of Klebsiella pneumoniae [Thorneley (1975) Biochem. J. 145, 391-396]. Extrapolation of the dependence of kobs. on [MgATP] to infinite MgATP concentration gave k = 46 s-1 for the first-order electron-transfer reaction that occurs with the Ac2V MgATPAclV complex. MgATP binds with an apparent KD = 230 +/- 10 microM and MgADP acts as a competitive inhibitor with Ki = 30 +/- 5 microM. The Fe protein and VFe protein associate with k greater than or equal to 3 x 10(7) M-1.s-1. A comparison of the dependences of kobs. for electron transfer on protein concentrations for the vanadium nitrogenase from A. chroococcum with those for the molybdenum nitrogenase from K. pneumoniae [Lowe & Thorneley (1984) Biochem. J. 224, 895-901] indicates that the proteins of the vanadium nitrogenase system form a weaker electron-transfer complex.     

Dinitrogenase reductase (Fe-protein) of nitrogenase in the cyanobacterial symbionts of three Azolla species: Localization and sequence of appearance during heterocyst differentiation

Transmission electron microscopy and immunocytological labeling were used to study the distribution and ontological occurrence of dinitrogenase reductase (Fe-protein) of nitrogenase in cyanobacterial symbionts within young leaves of the water-ferns Azolla filiculoides Lamarck, A. caroliniana Willdenow, and A. pinnata R. Brown. Rabbit anti-dinitrogenase reductase antisera and goat anti-rabbit-immunoglobulin G antibody conjugated to colloidal gold were used as probes. Western blot analyses showed that a polypeptide of approx. 36 kDa (kdalton) was recognized in the symbionts of all three Azolla species and that the polyclonal sera used were monospecific. In all symbionts, nitrogenase was immunologically recognizable within heterocysts. It was absent from vegetative cells, and also from the akinetes of the A. caroliniana and A. pinnata symbionts. The differentiation of vegetative cells into heterocysts in all three symbionts was initiated by formation of additional external cell-wall layers and narrowing of the neck followed by loss of glycogen, mild vesiculation of thylakoid membranes, and the appearance of polar nodules. No nitrogenase was detected at these early stages, but it appeared in the intermediate proheterocyst stage concomitantly with the formation of contorted membranes, and reached the strongest labeling in mature heterocysts, containing extensive tightly packed membranes. Nitrogenase was evenly distributed throughout heterocysts except at the polar regions, which contained honey-comb configurations and large polar nodules. With increased age of the A. caroliniana and A. pinnata symbionts, heterocysts became highly vesiculated, with a concomitant decrease in the amount of nitrogenase detected.Abbreviations IgG Immunoglobulin G - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate - TEM transmission electron micrograph     

Activity, reconstitution, and accumulation of nitrogenase components in Azotobacter vinelandii mutant strains containing defined deletions within the nitrogenase structural gene cluster.

The Azotobacter vinelandii genes encoding the nitrogenase structural components are clustered and ordered: nifH (Fe protein)-nifD (MoFe protein alpha subunit)-nifK (MoFe protein beta subunit). In this study various A. vinelandii mutant strains which contain defined deletions within the nitrogenase structural genes were isolated and studied. Mutants deleted for the nifD or nifK genes were still able to accumulate significant amounts of the unaltered MoFe protein subunit as well as active Fe protein. Extracts of such nifD or nifK deletion strains had no MoFe protein activity. However, active MoFe protein could be reconstituted by mixing extracts of the mutant strains. These results establish an approach for the purification of the individual MoFe protein subunits. Mutants lacking either or both of the MoFe protein subunits were still able to synthesize the iron-molybdenum cofactor (FeMo-cofactor), indicating that in A. vinelandii the FeMo-cofactor is preassembled and inserted into the MoFe protein. In contrast, a mutant strain lacking both the Fe protein and the MoFe protein failed to accumulate any detectable FeMo-cofactor. The further utility of specifically altered A. vinelandii strains for the study of the assembly, structure, and reactivity of nitrogenase is discussed.