Dissimilatory sulfate reduction in the intestinal sulfate-reducing bacteria
The process of reducing dissimilation sulfate and the accumulation of hydrogen sulphide, role in inflammatory bowel diseases, including ulcerative colitis. The characteristics of these bacteria and their mechanism for reducing dissimilation sulfate.
| Рубрика | Биология и естествознание |
| Вид | статья |
| Язык | украинский |
| Дата добавления | 01.12.2017 |
| Размер файла | 160,8 K |
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Reverse electron transfer is important in anaerobic respiration where it is often a necessity and using reduced equivalents at low redox potential or regulation of the redox reactions. If bacteria D. vulgaris metabolize organic substrates such as pyruvate (E° = -500 mV) or CO (E° = -520 mV) by oxidation of reduced ferredoxin, then two hydrogenases are involved in the formation of H2. For example, the reduced cytoplasmic NADP hydrogenase is found in bacteria D. fructosovorans. However, it has not been detected in D. vulgaris [63].
Bacteria D. vulgaris Hildenborough produces a burst of metabolites such as H2, formate and CO (Fig. 19). This observation led to a proposal of the hydrogen-cycling model which tries to explain the growth of this microorganism despite the energetic constraints that are associated with sulfate reduction [72].
Zhou et al. (2011) proposed a model of hydrogen-cycling. According to this model, hydrogen equivalents that are generated by the oxidation of organic compounds are hypothesized to be cycled to the periplasm via the activities of the cytoplasmic hydrogenases E. coli hydrogenase 3 (Ech) [56] and CO-dependent hydrogenase (Coo). In the periplasm, the H2 is re-oxidized to protons and electrons by the periplasmic hydrogenases, such as the iron-only hydrogenase, and the electrons pass through the cytochrome c3 network. From here, electrons are proposed to be transferred to the mena- quinone-linked quinone reductase complex (Qrc), then to the quinone-interacting membrane-bound oxidoreductase (Qmo) complex and finally to the adenosine phosphosul- phate reductase for sulfate reduction. Concurrently, electrons are passed by an unknown mechanism to the dissimilatory sulphite reductase (Dsr) transmembrane complex and then to bisulphite reductase. In this way, sufficient electrons are made available for complete reduction of sulfate to hydrogen sulfide. The process is made energetically favourable by the activity of inorganic pyrophosphatase which removes the pyrophosphate that is generated by sulphate activation. Protons that are generated in the periplasm produce the proton-motive force that is necessary for the generation of additional ATP for growth. CO is metabolized in the cytoplasm by CO dehydrogenase, and formate is cycled to the periplasm, where it is metabolized by formate dehydrogenase, EC 1.2.1.2 (Fdh). Hydrogen cycling is not necessary when H2 is used as the electron donor, as periplasmic metabolism of H2 directly establishes the electrochemical gradient that is necessary for ATP synthesis [72] (Fig. 19).
Thus, dissimilatory sulfate reduction is a process consisting of many stages, including transport of sulfate in the SRB cells and its activation, the formation of APS and its reduction to sulfite, periplasmic oxidation of H2, transmembrane transport of electrons, and cytoplasmic oxidation of H2.
In previous studies, the intestinal SRB Desulfovibrio piger Vib-7 and Desulfomicro- bium sp. Rod-9 were isolated from the healthy human large intestine and identified as described [25, 26]. The strains have since been kept in the collection of microorganisms at the Department of Molecular Biology and Pharmaceutical Biotechnology of Pharmacy Faculty at the University of Veterinary and Pharmaceutical Sciences Brno (Czech Republic) and their physiological and biochemical properties have been studied [21-24, 27-30, 32-35]. The activity and kinetic analysis of main enzymes involved in dissimilatory sulfate reduction and the enzymes of antioxidant system including catalase [37] and superoxide dismutase [38] in the intestinal sulfate-reducing bacteria D. piger Vib-7 and Desulfomicro- bium sp. Rod-9 were studied in detail. For each of these enzymes were determined: activity (A, Uxmg-1 protein), initial (instantaneous) reaction rate (V0, ^molxmin-1xmg-1 protein), the maximum of enzymatic reaction (Vmax, |amolxmin-1xmg-1 protein), Michaelis constant (Km) determined by the concentration of substrate (Table).
The activity of these enzymes is significantly higher in D. piger Vib-7 than in Desul- fomicrobium sp. Rod-9. The peaks of the enzymatic activity occurred at the temperature of +35°C. Maximum activity of the enzymes was at pH 8.0 which is consistent with the condition of the human colon. Obviously, such conditions provide their intensive development. The initial and maximum rates of enzymatic reactions and the maximum amount of product were significantly higher in D. piger Vib-7 compared to Desulfomicro- bium sp. Rod-9. Probably, D. piger Vib-7 can be more dangerous and have some pathogenic role in the development of inflammatory bowel disease, the dissimilation of sulfate and lactate and, accordingly, accumulating acetate and sulfide with a higher rate.
¦ Outer membrane - Inner membrane
Cytoplasm
Enzymatic characteristics of the intestinal SRB strains Ензиматичні характеристики кишкових штамів СВБ
|
Enzymes of dissimilatory sulfate reduction |
||||
|
Strains of intestinal sulfate-reducing bacteria |
||||
|
Kinetic characteristics [reference] |
D. piger Vib-7 |
Desulfomicrobium sp. Rod-9 |
||
|
А |
16.11±1.87 |
7.31±0.98** |
||
|
ATPase [36] |
V0 |
15.95±1.58 |
10.69±0.93*** |
|
|
Vmax |
36.10±2.87 |
16.64±1.73*** |
||
|
Km |
2.24±0.21 |
2.06±0.18 |
||
|
А |
2.26±0.231 |
0.98±0.0082** |
||
|
ATP sulfurylase [42] |
V0 |
5.48±0.57 |
4.12±0.38 |
|
|
Vmax |
4.87±0.55 |
2.11±0.22** |
||
|
Km |
1.98±0.21 |
1.07±0.12* |
||
|
А |
0.34±0.029 |
0.11±0.012** |
||
|
APS reductase [41] |
V0 |
0.675±0.062 |
0.231±0.022*** |
|
|
Vmax |
0.862±0.084 |
0.282±0.027*** |
||
|
Km |
4.33±0.47 |
3.57±0.32 |
||
|
А |
0.032±0.0026 |
0.028±0.0022 |
||
|
Sulfite reductase [34] |
V0 |
0.351±0.033 |
0.138±0.012*** |
|
|
V max |
0.067±0.0053 |
0.045±0.0039 |
||
|
Km |
3.53±0.334 |
3.86±0.341 |
||
|
А |
24.27±2.47 |
8.16±0.82*** |
||
|
Pyrophosphatase [39] |
V0 |
18.24±1.92 |
5.81±0.52*** |
|
|
V max |
43.86±4.24 |
13,74±1,32*** |
||
|
Km |
2.53±0.27 |
2.60±0.21 |
||
|
А |
1421.4±123.7 |
568.7±45.6*** |
||
|
Periplasmic |
V0 |
205.67±18.91 |
58.16±5.38*** |
|
|
hydrogenase [31] |
V max |
2500±219 |
1111±107*** |
|
|
Km |
864±73 |
669±62 |
||
|
А |
0.472±0.037 |
0.153±0.014*** |
||
|
Lactate |
V0 |
0.114±0.012 |
0.026±0.022*** |
|
|
dehydrogenase [40] |
Vmax |
1.20±0.11 |
0.65±1.73*** |
|
|
Km |
0.83±0.07 |
1.54±0.14*** |
Based on the described activities of the enzymes in D. piger Vib-7 and Desulfomi- crobium sp. Rod-9 and the kinetic analysis of enzymatic reactions, the generalized scheme showing the hypothetical model of dissimilatory sulfate reduction in intestinal SRB and the activity of the enzymes in their cells at each stage of this process was demonstrated for the first time. The proposed scheme summarizes already existing data on sulfate reduction and it is especially important for a more detailed understanding of the dissimilation of sulfate in the human and animal intestines.
Summarizing the above described studies based on literature data and own research, it can be stated that sulfate-reducing bacteria belong to the human and animal intestinal microflora. The number of these microorganisms in the intestine depends on the diet. The presence of sulfate induces the increased SRB level which can cause an excessive production of hydrogen sulfide. This compound is the final product of the dis- similatory sulfate reduction and may be mutagenic and toxic. The biochemical and physiological properties of intestinal SRB, their possible role in inflammatory bowel diseases, including ulcerative colitis, are summarized and analyzed. This is important for the understanding of the mechanisms behind these diseases and for the evaluation of the effectiveness of the therapy based on inhibition of SRB growth, accordingly, their production of hydrogen sulfide and acetate in gut.
Sulfate-reducing bacteria carry out the dissimilatory sulfate reduction. Sulfate is used in this process as a final electron acceptor. Organic compounds which enter the human and animal intestine and are formed in the fermentation process can be electron donors for SRB in the dissimilatory sulfate reduction. This process is complex and multi-staged.
The oxidation of organic substrates leads to a change of redox potentials. It depends on the nature of compounds that are oxidized or reduced, as well as the environmental conditions. SRB can grow in a wide range of oxidation-reduction potentials.
Lactate, pyruvate, acetate and ethanol are the most widespread substrates for SRB in the gut. The presence of sulfate in the food in the increased concentration can stimulate SRB growth and their competition with methanogenic organisms by the substrate in the gut.
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