Physiological function of soluble cytochrome c-552 from alkaliphilic Pseudomonas alcaliphila AL15-21(T)

Extremophilic Microorganisms as a Source of Emerging Enzymes for the Food Industry: A Review

Modern-day consumers are interested in highly nutritious and safe foods with corresponding organoleptic qualities. Such foods are increasingly subjected to various processing techniques which include the use of enzymes. These enzymes like amylases, lipases, proteases, xylanases, laccases, pullulanase, chitinases, pectinases, esterases, isomerases, and dehydrogenases could be derived from extremophilic organisms such as thermophiles, psychrophiles, acidophiles, alkaliphiles, and halophiles. As these organisms can grow under severe environmental conditions, they can produce functional enzymes (extremozymes) used in producing safe foods (such as gluten-free, lactose-free, lower acrylamide, or lower trans-fat products). The extremozymes also enhance nutrient bioavailability and bioaccessibility (e.g., predigested nourishments like baby formulae), and improve nourishment functionalities such as surface, sensory, and bioactive properties. Therefore, exploring alternative sources of enzymes for better compatibility and long-term adaptability in the processing stages is a promising approach for obtaining novel food products. This review will establish novel discovery methods of extremozymes from psychrophiles, thermophiles, acidophiles, alkaliphiles, and halophiles, the enzymes' types, mechanisms of action, and their food applications. It will also contribute to their commercial relevance and the furtherance of their discovery.

Molecular level insight of thiocyanate degradation by Pseudomonas putida TDB-1 under a high arsenic and alkaline condition

It is a big challenge to bioremediate thiocyanate pollution in the gold extraction heap leaching tailings and surrounding soils with high contents of arsenic and alkali. Here, a novel thiocyanate-degrading bacterium Pseudomonas putida TDB-1 was successfully applied to completely degrade 1000 mg/L thiocyanate under a high arsenic (400 mg/L) and alkaline condition (pH = 10). It also leached the contents of thiocyanate from 1302.16 to 269.72 mg/kg in the gold extraction heap leaching tailings after 50 h. The maximum transformation rates of S and N in thiocyanate to the two finial products of SO₄^2- and NO₃^- were 88.98 % and 92.71 %, respectively. Moreover, the genome sequencing confirmed that the biomarker gene of thiocyanate-degrading bacterium, CynS was identified in the strain TDB-1. The bacterial transcriptome revealed that critical genes, such as CynS, CcoNOQP, SoxY, tst, gltBD, arsRBCH and NhaC, etc. in the thiocyanate degradation, S and N metabolisms, and As and alkali resistance were significantly up-regulated in the groups with 300 mg/L SCN^- (T300) and with 300 mg/L SCN^- and 200 mg/L As (TA300). In addition, the protein-protein interaction network showed that the glutamate synthase encoding by gltB and gltD served as central node to integrate the S and N metabolism pathways with thiocyanate as substrate. The results of our study provide a novel molecular level insight for the dynamic gene expression regulation of thiocyanate degradation by the strain TDB-1 with a severe arsenic and alkaline stress.

Formation of Proton Motive Force Under Low-Aeration Alkaline Conditions in Alkaliphilic Bacteria

In Mitchell’s chemiosmotic theory, a proton (H⁺) motive force across the membrane (Δp), generated by the respiratory chain, drives F₁F_o-ATPase for ATP production in various organisms. The bulk-base chemiosmotic theory cannot account for ATP production in alkaliphilic bacteria. However, alkaliphiles thrive in environments with a H⁺ concentrations that are one-thousandth (ca. pH 10) the concentration required by neutralophiles. This situation is similar to the production of electricity by hydroelectric turbines under conditions of very limited water. Alkaliphiles manage their metabolism via various strategies involving the cell wall structure, solute transport systems and molecular mechanisms on the outer surface membrane. Our experimental results indicate that efficient ATP production in alkaliphilic Bacillus spp. is attributable to a high membrane electrical potential (ΔΨ) generated for an attractive force for H⁺ on the outer surface membrane. In addition, the enhanced F₁F_o-ATPase driving force per H⁺ is derived from the high ΔΨ. However, it is difficult to explain the reasons for high ΔΨ formation based on the respiratory rate. The Donnan effect (which is observed when charged particles that are unable to pass through a semipermeable membrane create an uneven electrical charge) likely contributes to the formation of the high ΔΨ because the intracellular negative ion capacities of alkaliphiles are much higher than those of neutralophiles. There are several variations in the adaptation to alkaline environments by bacteria. However, it could be difficult to utilize high ΔΨ in the low aeration condition due to the low activity of respiration. To explain the efficient ATP production occurring in H⁺-less and air-limited environments in alkaliphilic bacteria, we propose a cytochrome c-associated “H⁺ capacitor mechanism” as an alkaline adaptation strategy. As an outer surface protein, cytochrome c-550 from Bacillusclarkii possesses an extra Asn-rich segment between the region anchored to the membrane and the main body of the cytochrome c. This structure may contribute to the formation of the proton-binding network to transfer H⁺ at the outer surface membrane in obligate alkaliphiles. The H⁺ capacitor mechanism is further enhanced under low-aeration conditions in both alkaliphilic Bacillus spp. and the Gram-negative alkaliphile Pseudomonas alcaliphila.

Bioenergetics and the role of soluble cytochromes C for alkaline adaptation in gram-negative alkaliphilic Pseudomonas

Very few studies have been conducted on alkaline adaptation of Gram-negative alkaliphiles. The reversed difference of H⁺ concentration across the membrane will make energy production considerably difficult for Gram-negative as well as Gram-positive bacteria. Cells of the alkaliphilic Gram-negative bacterium Pseudomonas alcaliphila AL15-21^T grown at pH 10 under low-aeration intensity have a soluble cytochrome c content that is 3.6-fold higher than that of the cells grown at pH 7 under high-aeration intensity. Cytochrome c-552 content was higher (64% in all soluble cytochromes c) than those of cytochrome c-554 and cytochrome c-551. In the cytochrome c-552-dificient mutant grown at pH 10 under low-aeration intensity showed a marked decrease in μ_max⁡ [h⁻¹] (40%) and maximum cell turbidity (25%) relative to those of the wild type. Considering the high electron-retaining abilities of the three soluble cytochromes c, the deteriorations in the growth of the cytochrome c-552-deficient mutant could be caused by the soluble cytochromes c acting as electron storages in the periplasmic space of the bacterium. These electron-retaining cytochromes c may play a role as electron and H⁺ condenser, which facilitate terminal oxidation at high pH under air-limited conditions, which is difficult to respire owing to less oxygen and less H⁺.

Relationship between rates of respiratory proton extrusion and ATP synthesis in obligately alkaliphilic Bacillus clarkii DSM 8720(T)

To elucidate the energy production mechanism of alkaliphiles, the relationship between the rate of proton extrusion via the respiratory chain and the corresponding ATP synthesis rate was examined in obligately alkaliphilic Bacillus clarkii DSM 8720(T) and neutralophilic Bacillus subtilis IAM 1026. The oxygen consumption rate of B. subtilis IAM 1026 cells at pH 7 was approximately 2.5 times higher than that of B. clarkii DSM 8720(T) cells at pH 10. The H⁺/O ratio of B. clarkii DSM 8720(T) cells was approximately 1.8 times higher than that of B. subtilis IAM 1026 cells. On the basis of oxygen consumption rate and H⁺/O ratio, the rate of proton translocation via the respiratory chain in B. subtilis IAM 1026 is expected to be approximately 1.4 times higher than that in B. clarkii DSM 8720(T). Conversely, the rate of ATP synthesis in B. clarkii DSM 8720(T) at pH 10 was approximately 7.5 times higher than that in B. subtilis IAM 1026 at pH 7. It can be predicted that the difference in rate of ATP synthesis is due to the effect of transmembrane electrical potential (Δψ) on protons translocated via the respiratory chain. The Δψ values of B. clarkii DSM 8720(T) and B. subtilis IAM 1026 were estimated as -192 mV (pH 10) and -122 mV (pH 7), respectively. It is considered that the discrepancy between the rates of proton translocation and ATP synthesis between the strains used in this study is due to the difference in ATP production efficiency per translocated proton between the two strains caused by the difference in Δψ.