Vitreoscilla hemoglobin promoter is not responsive to nitrosative and oxidative stress in Escherichia coli

Advances in Escherichia coli Nissle 1917 as a customizable drug delivery system for disease treatment and diagnosis strategies

With the in-depth and comprehensive study of bacteria and their related ecosystems in the human body, bacterial-based drug delivery system has become an emerging biomimetic platform that can retain the innate biological functions. Benefiting from its good biocompatibility and ideal targeting ability as a biological carrier, Escherichia coli Nissle 1917 (ECN) has been focused on the treatment strategies of inflammatory bowel disease and tumor. The advantage of a bacterial carrier is that it can express exogenous protein while also acting as a natural capsule by releasing drug slowly as a result of its own colonization impact. In order to survive in harsh environments such as the digestive tract and tumor microenvironment, ECN can be modified or genetically engineered to enhance its function and host adaptability. The adoption of ECN carries or expresses drugs which are essential for accurate diagnosis and treatment. This review briefly describes the properties of ECN, the relationship between ECN and inflammation and tumor, and the strategy of using surface modification and genetic engineering to modify ECN as a delivery carrier for disease treatment.

Recent Advances in the Physicochemical Properties and Biotechnological Application of Vitreoscilla Hemoglobin

Vitreoscilla hemoglobin (VHb), the first discovered bacterial hemoglobin, is a soluble heme-binding protein with a faster rate of oxygen dissociation. Since it can enhance cell growth, product synthesis and stress tolerance, VHb has been widely applied in the field of metabolic engineering for microorganisms, plants, and animals. Especially under oxygen-limited conditions, VHb can interact with terminal oxidase to deliver enough oxygen to achieve high-cell-density fermentation. In recent years, with the development of bioinformatics and synthetic biology, several novel physicochemical properties and metabolic regulatory effects of VHb have been discovered and numerous strategies have been utilized to enhance the expression level of VHb in various hosts, which greatly promotes its applications in biotechnology. Thus, in this review, the new information regarding structure, function and expressional tactics for VHb is summarized to understand its latest applications and pave a new way for the future improvement of biosynthesis for other products.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

The single-domain globin of Vitreoscilla: augmentation of aerobic metabolism for biotechnological applications

Extensive studies have revealed that large-scale, high-cell density bioreactor cultivations have significant impact on metabolic networks of oxygen-requiring production organisms. Oxygen transfer problems associated with fluid dynamics and inefficient mixing efficiencies result in oxygen gradients, which lead to reduced performance of the bioprocess, decreased product yields, and increased production costs. These problems can be partially alleviated by improving bioreactor configuration and setting, but significant improvements have been achieved by metabolic engineering methods, especially by heterologously expressing Vitreoscilla hemoglobin (VHb). Vast numbers of studies have been accumulating during the past 20 years showing the applicability of VHb to improve growth and product yields in a variety of industrially significant prokaryotic and eukaryotic hosts. The global view on the metabolism of globin-expressing Escherichia coli cells depicts increased energy generation, higher oxygen uptake rates, and a decrease in fermentative by-product excretion. Transcriptome and metabolic flux analysis clearly demonstrate the multidimensional influence of heterologous VHb on the expression of stationary phase-specific genes and on the regulation of cellular metabolic networks. The exact biochemical mechanisms by which VHb is able to improve the oxygen-limited growth remain poorly understood. The suggested mechanisms propose either the delivery of oxygen to the respiratory chain or the detoxification of reactive nitrogen species for the protection of cytochrome activity. The expression of VHb in E. coli bioreactor cultures is likely to assist bacterial growth through providing an increase in available intracellular oxygen, although to fully understand the exact role of VHb in vivo, further analysis will be required.

Production of L-DOPA and dopamine in recombinant bacteria bearing the Vitreoscilla hemoglobin gene

Given the well-established beneficial effects of Vitreoscilla hemoglobin (VHb) on heterologous organisms, the potential of this protein for the production of L-DOPA and dopamine in two bacteria, Citrobacter freundii and Erwinia herbicola, was investigated. The constructed recombinants bearing the VHb gene (vgb(+)) had substantially higher levels of cytoplasmic L-DOPA (112 mg/L for C. freundii and 97 mg/L for E. herbicola) than their respective hosts (30.4 and 33.8 mg/L) and the vgb(-) control strains (35.6 and 35.8 mg/L). Further, the vgb(+) recombinants of C. freundii and E. herbicola had 20-fold and about two orders of magnitude higher dopamine levels than their hosts, repectively. The activity of tyrosine phenol-lyase, the enzyme converting L-tyrosine to L-DOPA, was well-correlated to cytoplasmic L-DOPA levels. As cultures aged, higher tyrosine phenol-lyase activity of the vgb(+) strains was more apparent.

Intrinsic non-symbiotic and truncated haemoglobins and heterologous Vitreoscilla haemoglobin expression in plants

To date, haemoglobins (Hbs) have been shown to exist in all kingdoms of life. The least studied and understood groups are plant non-symbiotic haemoglobins (nsHbs) and the recently found plant truncated Hbs (trHbs). From a biotechnological point of view, the best characterized and almost exclusively applied Hb is the bacterial Vitreoscilla haemoglobin (VHb). In this review, the present state of knowledge of structural features and ligand binding kinetics of plant nsHbs and trHbs and their proposed roles as oxygen carriers, oxygen sensors, and for oxygen storage, in nitric oxide (NO) detoxification, and in peroxidase activity are described. Furthermore, in order to predict the functioning of plant Hbs, their characteristics will be compared with those of the better known bacterial globins. In this context, the effects of heterologous applications of VHb on plants are reviewed. Finally, the challenging future of plant Hb research is discussed.

Invertebrate hemoglobins and nitric oxide: how heme pocket structure controls reactivity

Hemoglobins (Hbs), generally defined as 5 or 6 coordinate heme proteins whose primary function is oxygen transport, are now recognized to occur in virtually all phyla of living organisms. Historically, study of their function focused on oxygen as a reversibly bound ligand of the ferrous form of the protein. Other diatomic ligands like carbon monoxide and nitric oxide were considered "non-physiological" but useful probes of structure-function relationships in Hbs. This investigatory landscape changed dramatically in the 1980s when nitric oxide was discovered to activate a heme protein, cyclic guanylate cyclase. Later, its activation was likened to Perutz' description of Hb's allosteric properties being triggered by a ligand-dependent "out-of-plane/into-plane" movement of the heme iron. In 1996, a functional role for nitric oxide in human and mammalian Hbs was demonstrated and since that time, the interest in NO as a physiologically relevant Hb ligand has greatly increased. Concomitantly, non-oxygen binding properties of Hbs have challenged the view that Hbs arose for their oxygen storage and transport properties. In this focused review we discuss some invertebrate Hbs' functionally significant reactions with nitric oxide and how strategic positioning of a few residues in the heme pocket plays an large role in the interplay of diatomic ligands to ferrous and ferric heme iron in these proteins.

Construction of highly efficient E. coli expression systems containing low oxygen induced promoter and partition region

A series of high-copy-number Escherichia coli expression vectors equipped with an oxygen-sensitive promoter P(vgb) of Vitreoscilla hemoglobin (encoded by the vgb gene) were constructed and characterized. Plasmid pKVp containing P(vgb) was inducible by low oxygen tension, while plasmid pKVpP containing a partition (par) region from plasmid pSC101 ligated to P(vgb) provided inheritable stability for the vectors in the absence of ampicillin. Plasmid pKVpV had the Vitreoscilla hemoglobin operon vgb ligated to P(vgb), while a construct containing P(vgb), the vgb operon and a par region constituted plasmid pKVpPV. Shake-flask studies demonstrated that plasmids pKVpV and pKVpPV expressed higher levels of Vitreoscilla hemoglobin under low aeration condition (5% air saturation in water) compared with the levels observed under strong aeration (20% air saturation in water). Introduction of either the enhanced green fluorescent protein (eGFP) gene egfp or the toluene dioxygenase (TDO) gene tod into either pKVpV (P(vgb), vgb operon) or pKVpPV (P(vgb), vgb operon, par) slightly attenuated (approximately 30%) the strong expression of VHb under low aeration. However, all displayed approximately a three-fold increase versus that observed for strong aeration. Recombinant E. coli harboring either pKVp-E (P(vgb), egfp) or pKVpP-E (P(vgb), par, egfp) displayed at least a two-fold increase in eGFP expression under conditions of low aeration and absence of antibiotic, compared with that under strong aeration after 24 h of cultivation. Strong expression of TDO was also observed using low aeration in recombinant E. coli harboring pKVpPV-T (P(vgb), vgb operon, par, tod) or pKVpP-T (P(vgb), par, tod). Plasmids containing the par region were stable over 100 generations. These results indicate that the novel expression system combining plasmid stability over the cell growth phase and a promoter inducible by low oxygen tension will be very useful for high-density production of foreign proteins.

Role of an inducible single-domain hemoglobin in mediating resistance to nitric oxide and nitrosative stress in Campylobacter jejuni and Campylobacter coli

Campylobacter jejuni expresses two hemoglobins, each of which exhibits a heme pocket and structural signatures in common with vertebrate and plant globins. One of these, designated Cgb, is homologous to Vgb from Vitreoscilla stercoraria and does not possess the reductase domain seen in the flavohemoglobins. A Cgb-deficient mutant of C. jejuni was hypersensitive to nitrosating agents (S-nitrosoglutathione [GSNO] or sodium nitroprusside) and a nitric oxide-releasing compound (spermine NONOate). The sensitivity of the Cgb-deficient mutant to methyl viologen, hydrogen peroxide, and organic peroxides, however, was the same as for the wild type. Consistent with the protective role of Cgb against NO-related stress, cgb expression was minimal in standard laboratory media but strongly and specifically induced after exposure to nitrosative stress. In contrast, the expression of Cgb was independent of aeration and the presence of superoxide. In the absence of preinduction by exposure to nitrosative stress, no difference was seen in the degree of respiratory inhibition by NO or the half-life of the NO signal when cells of the wild type and the cgb mutant were compared. However, cells expressing GSNO-upregulated levels of Cgb exhibited robust NO consumption and respiration that was relatively NO insensitive compared to the respiration of the cgb mutant. Based on similar studies in Campylobacter coli, we also propose an identical role for Cgb in this closely related species. We conclude that, unlike the archetypal single-domain globin Vgb, Cgb forms a specific and inducible defense against NO and nitrosating agents.

The Role of the Flavodiiron Proteins in Microbial Nitric Oxide Detoxification

The flavodiiron proteins (first named as A-type flavoproteins) constitute a large superfamily of enzymes, widespread among anaerobic and facultative anaerobic prokaryotes, from both the Archaea and Bacteria domains. Noticeably, genes encoding for homologous enzymes are also present in the genomes of some pathogenic and anaerobic amitochondriate protozoa. The fingerprint of this enzyme family is the conservation of a two-domain structural core, built by a metallo-beta-lactamase-like domain, at the N-terminal region, harbouring a non-heme diiron site, and a flavodoxin-like domain, containing one FMN moiety. These enzymes have a significant nitric oxide reductase activity, and there is increasing evidence that they are involved in microbial resistance to nitric oxide. In this review, we will discuss available data for this novel family of enzymes, including their physicochemical properties, structural and phylogenetic analyses, enzymatic properties and the molecular genetic approaches so far used to tackle their function.