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1.
Juozas Kulys Kastis Krikstopaitis Arturas Ziemys 《Journal of biological inorganic chemistry》2000,5(3):333-340
N -substituted phenothiazines (PTs) and phenoxazines (POs) catalyzed by fungal Coprinus cinereus peroxidase and Polyporus pinsitus laccase were investigated at pH 4–10. In the case of peroxidase, an apparent bimolecular rate constant (expressed as k
cat/K
m) varied from 1 ×107 M−1 s−1to 2.6×108 M−1 s−1 at pH 7.0. The constants for PO oxidation were higher in comparison to PT. pH dependence revealed two or three ionizable
groups with pK
a values of 4.9–5.7 and 7.7–9.7 that significantly affected the activity of peroxidase. Single-turnover experiments showed
that the limiting step of PT oxidation was reduction of compound II and second-order rate constants were obtained which were
consistent with the constants at steady-state conditions. Laccase-catalyzed PT and PO oxidation rates were lower; apparent
bimolecular rate constants varied from 1.8×105 M−1 s−1 to 2.0×107 M−1 s−1 at pH 5.3. PO constants were higher in comparison to PT, as was the case with peroxidase. The dependence of the apparent
bimolecular constants of compound II or copper type 1 reduction, in the case of peroxidase or laccase, respectively, was analyzed
in the framework of the Marcus outer-sphere electron-transfer theory. Peroxidase-catalyzed reactions with PT, as well as PO,
fitted the same hyperbolic dependence with a maximal oxidation rate of 1.6×108 M−1 s−1 and a reorganization energy of 0.30 eV. The respective parameters for laccase were 5.0×107 M−1 s−1 and 0.29 eV.
Received: 20 September 1999 / Accepted: 24 February 2000 相似文献
2.
Morphogenesis and regeneration from stomatal guard cell complexes of cotton (Gossypium hirsutum L.) 总被引:1,自引:0,他引:1
The development of a regeneration system from cotton stomatal guard cells directly on epidermal strips is described. The
most important factors affecting embryogenic callus initiation in both of the varieties tested (Coker 312 and 315) were the
source of the epidermal tissue, including plant age (4–5 months old), the developmental stage of the flower (opening flower
stage) from which bracts were obtained, the composition of the culture medium and light irradiance. The flower developmental
stage was critical for callus formation, which was observed only from bracts obtained from opening flowers. In addition, epidermal
strips excised from the bract basal region were more responsive in culture than those obtained from the top region. Improved
callus initiation was obtained on epidermal strips which had their cuticle in contact with the culture medium. Light irradiance
was a limiting factor for embryogenic callus formation, which was observed only in calluses cultured under the lower light
irradiance (15.8 μmol m–2 s–1). Somatic embryogenesis was observed on callus cultures subcultured consecutively to a culture medium containing naphthalene
acetic acid (10.7 μM) and isopentenyladenine (4.9 μM). Histodifferentiation of somatic embryos was improved on a medium containing naphthaleneacetic acid (8.1 μM)+isopentenyladenine (2.5 μM) and abscisic acid (0.19–0.38 μM). Somatic embryo germination and plantlet development were obtained using established protocols with few modifications. On
average, one fully developed plant was obtained from the culture of circa 100 epidermal strips in both cultivars.
Received: 19 May 2000 / Revision received: 25 August 2000 / Accepted: 29 August 2000 相似文献
3.
To establish an efficient asparagus microspore culture system, experiments were conducted to investigate the effects of medium
components, period of cold pretreatment for flower buds, and period of anther co-culture on culture response. All factors
affected the frequency of asparagus microspore division and the yields of microspore-derived calli. The best results were
obtained by pretreating genotype G459 flower buds at 4 °C for 7–9 days, co-culturing anthers with shed microspores for 14
days, and including 6% sucrose, 2 mg l–1α-naphthaleneacetic acid and 1 mg l–1 N6-benzylaminopurine in the culture medium. After 4 days of culture, most shed microspores contained starch-like bodies and
died. The 2% of shed microspores lacking these structures divided to produce microcalli. For the best treatments in the different
experiments, about 140 calli per 100 anthers were recovered. Cultured on four different regeneration media, 19.6–21% and 3.9–8.0%
of microspore-derived calli produced shoots and embryos, respectively, and ultimately plantlets, among which 49% were haploid,
34% diploid, 4% triploid and 11% tetraploid.
Received: 3 September 1998 / Revision received: 25 November 1998 / Accepted: 5 December 1998 相似文献
4.
Tiecheng Qiao Robert Witkowski Robin Henderson G. McLendon 《Journal of biological inorganic chemistry》1996,1(5):432-438
The kinetics of methemoglobin reduction by cytochrome b
5 has been studied by stopped-flow and saturation transfer NMR. A forward rate constant k
f = 2.44×104 M–1 s–1 and a reverse rate constant k
b = 540 M–1s–1 have been observed at 10 mm, pH 6.20, 25 °C. The ratio k
f/k
b = k
eq = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b
5 and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b
5 to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic strengths supports such
collisional mechanism. It is also found that the reaction rate strongly depends on the conformations of methemoglobin.
Received: 20 February 1996 / Accepted: 4 June 1996 相似文献
5.
Microspores of Brassica napus L. cv. Topas, undergo embryogenesis when cultured at 32.5 °C for the first 18–24 h and then at 25 °C. The first division
in heat-treated microspores is a symmetric division in contrast to the asymmetric division found after the first pollen mitosis
in-planta or in microspores cultured continuously at 25 °C. This asymmetric division is unique in higher plants as it results
in daughter cells separated by a non-consolidated wall. The cytoskeleton has an important role in such morphological changes.
We examined microtubule (MT) organization during the first 24 h of heat induction in the embryogenic B. napus cv. Topas and the non-embryogenic B. napus breeding line 0025. Preprophase bands (PPBs) of MTs appeared in cv. Topas microspores in late uninucleate microspores and
in prophase figures after 4–8 h of heat treatment. However, more than 60% of the PPBs were not continuous bands. In contrast,
PPBs were never observed in pollen mitosis; MT strands radiated from the surface of the nuclear envelope throughout microspore
maturation to the end of prophase of pollen mitosis I, during in-planta development and in microspores cultured at 25 °C.
Following 24 h of heat treatment, over 95% of the microspores appeared to have divided symmetrically as indicated by the similar
size of the daughter nuclei, but only 7–16% of the microspores eventually formed embryos. Discontinuous walls were observed
in more than 50% of the divisions and it is probable that the discontinuous PPBs gave rise to such wall abnormalities which
may then obstruct embryo development. Preprophase bands were not formed in heat-treated microspores of the non-embryogenic
line 0025 and the ensuing divisions showed discontinuous walls. It is concluded that the appearance of PPBs in heat-induced
microspores marks sporophytic development and that continuous PPBs are required for cell wall consolidation and embryogenesis.
It follows that induced structures with two equally condensed nuclei, do not necessarily denote symmetric divisions.
Received: 22 October 1998 / Accepted: 28 November 1998 相似文献
6.
Elžbieta Milewska-Pawliczuk 《Biologia Plantarum》1987,29(4):295-298
Six thousand anthers ofSecale cereale cv. Dańkowskie Zlote and inbred lines L214 L215 and L258c/5, in the uninuclear microspore were cultured over a period of
three months on Murashige and Skoog nutrient medium supplemented with IAA (1–2μg1−1), kinetin (0.2 to (1.0μg1−1) and 2,4-D (0.5–2.0μg1−1). First divisions of microspore were observed after 7 days. In the course of 10 weeks, 4 albino tic embryos at the cotyledon
stage were observed that died away in the course of further culture. The course of androgenesis was regular in inbred lines
and irregular in rye cv. Dańkowskie Ziote. The efficiency of androgenesis and the amount of observed globular structures in
anthers were also dependent on the genetic potency of the material. Inbred lines did not show any greater viability of embryos. 相似文献
7.
Nicola D’Amelio Luca Fontanive Fulvio Uggeri Furio Suggi-Liverani Luciano Navarini 《Food biophysics》2009,4(4):321-330
Caffeine complexation by chlorogenic acid (3-caffeoylquinic acid, CAS Number [327-97-9]) in aqueous solution as well as caffeine–chlorogenate
complex in freshly prepared coffee brews have been investigated by high-resolution 1H-NMR. Caffeine and chlorogenic acid self-associations have also been studied and self-association constants have been determined
resorting to both classical isodesmic model and a recently introduced method of data analysis able to provide also the critical
aggregation concentration (cac). Furthermore, caffeine–chlorogenate association constant was measured. For the caffeine, the
average value of the self-association constant determined by isodesmic model (K
i = 7.6 ± 0.5 M−1) is in good agreement with the average value (K
a = 10 ± 1.8 M−1) determined with the method which permits the determination of the cac (8.43 ± 0.05 mM). Chlorogenic acid shows a slight
decreased tendency to aggregation with a lower average value of association constants (K
i = 2.8 ± 0.6 M−1; K
a = 3.4 ± 0.6 M−1) and a critical concentration equal to 24 ± 1 mM. The value of the association constant of the caffeine–chlorogenate complex
(30 ± 4 M−1) is compatible with previous studies and within the typical range of reported association constants for other caffeine–polyphenol
complexes. Structural features of the complex have also been investigated, and the complex conformation has been rediscussed.
Caffeine chemical shifts comparison (monomeric, complexed, coffee brews) clearly indicates a significant amount of caffeine
is complexed in beverage real system, being chlorogenate ions the main complexing agents. 相似文献
8.
Jose Neptuno Rodriguez-Lopez Andrew T. Smith R. N. F. Thorneley 《Journal of biological inorganic chemistry》1996,1(2):136-142
Horseradish peroxidase isoenzyme C (HRPC) mutants were constructed in order to understand the role of two key distal haem
cavity residues, histidine 42 and arginine 38, in the formation of compound I and in substrate binding. The role of these
residues as general acid-base catalysts, originally proposed for cytochrome c peroxidase by Poulos and Kraut in 1980 was assessed for HRPC. Replacement of histidine 42 by leucine [(H42L)HRPC*] decreased
the apparent bimolecular rate constant for the reaction with hydrogen peroxide by five orders of magnitude (k
1 = 1.4×102 M–1s–1) compared with both native-glycosylated and recombinant forms of HRPC (k
1 = 1.7×107 M–1s–1). The first-order rate constant for the heterolytic cleavage of the oxygen-oxygen bond to form compound I was estimated to
be four orders of magnitude slower for this variant. Replacement of arginine 38 by leucine [(R38L)HRPC*] decreased the observed
pseudo-first-order rate constant for the reaction with hydrogen peroxide by three orders of magnitude (k
1 = 1.1×104 M–1s–1), while the observed rate constant of oxygen bond scission was decreased sixfold (k
2 = 142 s–1). These rate constants are consistent with arginine 38 having two roles in catalysing compound I formation: firstly, promotion
of proton transfer to the imidazole group of histidine 42 to facilitate peroxide anion binding to the haem, and secondly,
stabilisation of the transition state for the heterolytic cleavage of the oxygen-oxygen bond. These roles for arginine 38
explain, in part, why dioxygen-binding globins, which do not have an arginine in the distal cavity, are poor peroxidases.
Binding studies of benzhydroxamic acid to (H42L)HRPC* and (R38L)HRPC* indicate that both histidine 42 and arginine 38 are
involved in the modulation of substrate affinity.
Received: 21 July 1995 / Accepted: 27 November 1995 相似文献
9.
Jingyuan Ai John A. Broadwater Thomas M. Loehr Joann Sanders-Loehr B. G. Fox 《Journal of biological inorganic chemistry》1997,2(1):37-45
The stearoyl-acyl carrier protein Δ9 desaturase (Δ9D) uses an oxo-bridged diiron center to catalyze the NAD(P)H– and O2–dependent desaturation of stearoyl-ACP. Δ9D, ribonucleotide reductase, and methane monooxygenase have substantial similarities
in their amino acid primary sequences and the physical properties of their diiron centers. These three enzymes also appear
to share common features of their reaction cycles, including the binding of O2 to the diferrous state and the subsequent generation of transient diferric-peroxo and diferryl species. In order to investigate
the coordination environment of the proposed diferric-peroxo intermediate, we have studied the binding of azide to the diiron
center of Δ9D using optical, resonance Raman (RR), and transient kinetic spectroscopic methods. The addition of azide results
in the appearance of new absorption bands at 325 nm and 440 nm (k
app≈3.5 s–1 in 0.7 M NaN3, pH 7.8). RR experiments demonstrate the existence of two different adducts: an η1–terminal structure at pH 7.8 (14N3
– asymmetric stretch at 2073 cm–1, resolved into two bands with 15N14N2
–) and a μ-1,3 bridging structure at pH<7 (14N3
– asymmetric stretch at 2100 cm–1, shifted as a single band with 15N14N2
–). Both adducts also exhibit an Fe–N3 stretching mode at ≈380 cm–1, but no accompanying Fe–O–Fe stretching mode, presumably due to either protonation or loss of the oxo bridge. The ability
to form a μ-1,3 bridging azide supports the likelihood of a μ-1,2 bridging peroxide as a catalytic intermediate in the Δ9D
reaction cycle and underscores the adaptability of binuclear sites to different bridging geometries.
Received: 23 August 1996 / Accepted: 4 October 1996 相似文献
10.
S. C. J. Meskers C. Dennison Gerard W. Canters Harry P. J. M. Dekkers 《Journal of biological inorganic chemistry》1998,3(6):663-670
The dynamic quenching of the luminescence of racemic Eu(III)(pyridine-2,6-dicarboxylate=dpa)3
3– by the title proteins is investigated and the enantioselectivity of the proteins in the quenching of the Δ and Λ enantiomers
of Eu(dpa)3
3– is determined. The two diastereomeric quenching rate constants pertaining to azurin (k
q
Δ=3.3×106, k
q
Λ=2.7×106 M–1 s–1, pH 7.2, ionic strength I=22 mM) are lower than for its Met→44Lys mutant (k
q
Δ=1.9×107, k
q
Λ=1.4×107 M–1 s–1, same pH and I), indicating that energy transfer occurs from Eu(dpa)3
3– to the Cu(II) centre when the luminophore is bound to the hydrophobic patch of the protein near residue 44. The enantioselectivity
remains unaltered by the mutation: k
q
Δ/k
q
Λ=1.27±0.04, so Lys44 is probably not in direct contact with the Eu chelate. The I and pH dependence of k
q indicate that the lysine residue interacts electrostatically with Eu(dpa)3
3–. For plastocyanin the quenching rates are of the order of 106 M–1 s–1; for amicyanin they are two orders of magnitude larger (k
q
Δ=12×107, k
q
Λ=11×107 M–1 s–1, pH 7.2, I=22 mM). The variation of k
q is attributed to differences in the charge distribution on the proteins, which influences the binding of the luminophore
to the protein surface. For amicyanin the anion binding site near Lys59 and Lys60 may be involved in the energy transfer.
Received: 16 June 1998 / Accepted: 18 September 1998 相似文献
11.
Polymerization of guaiacol by lignin-degrading manganese peroxidase from Bjerkandera adusta in aqueous organic solvents 总被引:1,自引:0,他引:1
Lignin-degrading manganese (II) peroxidase (MnP) purified from the culture of a wood-rotting basidiomycete, Bjerkandera adusta, was used in the polymerization of guaiacol. MnP was found to catalyze polymerization of guaiacol in 50% aqueous acetone,
dimethyl formamide, methanol, ethanol, dioxane, acetonitrile, ethylene glycol and methylcellosolve. Maximum yield of polyguaiacol
was achieved in 50% aqueous acetone. The weight average molecular weight (M
w) of the polymer was estimated to be 30 300 by gel permeation chromatography. However, matrix-assisted laser desorption ionization
time of flight mass spectroscopy (MALDI-TOF-MS) analysis gave a more reliable M
w of 1690. IR, 13C-NMR, MALDI-TOF-MS and pyrolysis GC-MS analyses showed the presence of C–C and C–O linkages and quinone structure in polyguaiacol.
It was also indicated that polyguaiacol has a methoxy-phenyl group as the terminal moiety. This suggests that polyguaiacol
is a branched polymer in which guaiacol units are cross-linked at the phenolic group. Thermal gravimetric and differential
scanning calorimetric analyses were also carried out. MnP also catalyzed the polymerization of o-cresol, 2,6-dimethoxyphenol and other phenolic compounds and aromatic amines. M
w of these polymers ranged from around 1000 to 1500.
Received: 2 August 1999 / Received revision: 10 December 1999 / Accepted: 4 January 2000 相似文献
12.
The anthers of three genotypes ofLycopersicon esculentum, viz. cv. HS-101, cv. HS-102 and an F1 hybrid (Montfavet 63-4xHS-101) in different stages of development were cultured in various defined nutritive media. Only
anthers containing microspores in the early uninucleate stage were found to respond with the culture medium in the formation
of androgenic callus. The DGII medium with 2 mg l−1 NAA and 1 mg 1−1 kinetin was found to be best for callus induction but MS medium supplemented with 2 mg l−1 2,4-D and 0.1 mg 1−1 BAP favoured proliferation and growth of the callus. The androgenic microspores followed the ‘B’ type pathway of androgenesis
in the formation of callus.
Induction of tracheids in the callus could be achieved by supplementing the basal medium with NAA and kinetin or 2,4-D and
BAP. Initiation of vessel elements and cambium were favoured by addition of NAA and kinetin and that of the phloem in the
presence of 2,4-D and BAP in the basal medium, suggesting that the hormonal requirements for production of different elements
of the vascular system in androgenic callus are different. Although roots could be induced from the callus, shoot differentiation
could not be achieved under cultural conditions. 相似文献
13.
Multiple shoot regeneration of kenaf (Hibiscus cannabinus L.) from a shoot apex culture system 总被引:1,自引:0,他引:1
M. Srivatanakul S. H. Park J. R. Sanders M. G. Salas R. H. Smith 《Plant cell reports》2000,19(12):1165-1170
In this research, a medium was developed that would stimulate multiple shoot initiation from shoot apex explants of Hibiscus cannabinus L. (kenaf). Adventitious shoot formation on a shoot induction media supplemented with combinations of 2,4-dichlorophenoxyacetic
acid (2,4-D) (0, 0.5, 2.3 μmol·l–1) and thidiazuron (N-phenyl-N′-1,2,3-thiadiazol-5-ylurea; TDZ) (0, 1, 5, 20 μmol·l–1) was evaluated. Multiple shoot induction medium with 1 μmol·TDZ l–1 resulted in the highest number of regenerated shoots per explant for all three kenaf cultivars tested (Tainung 1, Tainung 2,
and Everglades 71). The 2,4-D did not enhance multiple shoot formation. Additionally, kenaf cultivars 7N and SF459 also produced
multiple shoots on the induction medium with 1 μmol·TDZ l–1. Multiple shoot clumps formed after 2 weeks in culture without callus formation. Shoots elongated and rooted within 2 weeks
after subculture on a plant growth regulator-free medium. A histological study demonstrated the de novo regeneration of shoots
from the shoot apex.
Received: 2 February 2000 / Revision received: 30 March 2000 / Accepted: 22 June 2000 相似文献
14.
DNA binding by trans-[(H2O)(Pyr)(NH3)4RuII]2+ (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, 4-bnpy) is highly selective for G7 with K
G=1.1×104 to 2.8×104, with the more hydrophobic Pyr ligands exhibiting slightly higher binding. A strong dependence on ionic strength indicates
that ion-pairing with DNA occurs prior to binding. At μ=0.05, d[RuII-DNA]/dt=k[RuII][DNA], where k=0.17–0.21 M–1 s–1 with the various Pyr ligands. The air oxidation of [(py)(NH3)4RuII]
n
-DNA to [(py)(NH3)4RuIII]
n
-DNA at pH 6 occurs with a pseudo-first-order rate constant of k
obs=5.6×10–4 s–1 at μ=0.1, T=25 °C. Strand cleavage of plasmid DNA appears to occur by both Fenton/Haber-Weiss chemistry and by base-catalyzed routes,
some of which are independent of oxygen. Base-catalyzed cleavage is more efficient than O2 activation at neutral pH and involves the disproportionation of covalently bound RuIII and, in the presence of O2, Ru-facilitated autoxidation to 8-oxoguanine. Disproportionation of [py(NH3)4RuIII]
n
-DNA occurs according to the rate law: d[RuII–GDNA]/dt=k
0[RuIII–GDNA]+k
1[RuIII–GDNA][OH–], where k
0=5.4×10–4 s–1 and k
1=8.8 M–1 s–1 at 25 °C, μ=0.1. The appearance of [(Gua)(py)(NH3)4RuIII] under argon, which occurs according to the rate law: d[RuIII–G]/dt=k
0[RuIII–GDNA]+k
1[OH–][RuIII–GDNA] (k
0=5.74×10–5 s–1, k
1=1.93×10–2 M–1 s–1 at T=25 °C, μ=0.1), is consistent with lysis of the N-glycosidic bond by RuIV-induced general acid hydrolysis. In air, the ratio of [Ru-8-OG]/[Ru-G] and their net rates of appearance are 1.7 at pH 11,
25 °C. Small amounts of phosphate glycolate indicate a minor oxidative pathway involving C4′ of the sugar. In air, a dynamic
steady-state system arises in which reduction of RuIV produces additional RuII.
Received: 11 November 1998 / Accepted: 3 March 1999 相似文献
15.
Andreas Erkens Klaus Schneider A. Müller 《Journal of biological inorganic chemistry》1996,1(2):99-110
In this study we confirmed the previous observation that the cytoplasmic NAD-linked hydrogenase of Alcaligenes eutrophus H16 is EPR-silent in the oxidized state. We also demonstrated the presence of significant Ni-EPR signals when the enzyme
was either reduced with the natural electron carrier NADH (5–10 mM) or carefully titrated with sodium dithionite to an intermediate,
narrow redox potential range (–280 to –350 mV). Reduction with NADH under argon atmosphere led to a complex EPR spectrum at
80 K with g values at 2.28, 2.20, 2.14, 2.10, 2.05, 2.01 and 2.00. This spectrum could be differentiated by special light/dark treatments
into three distinct signals: (1) the "classical" Ni-C signal with g values at 2.20, 2.14 and 2.01, observed with many hydrogenases in the reduced, active state; (2) the light-induced signal
(Ni-L) with g values at 2.28, 2.10 and 2.05 and (3) a flavin radical (FMN semiquinone) signal at g = 2.00. The assignment of the Ni-EPR signal was clearly confirmed by EPR spectra of hydrogenase labeled with 61Ni (nuclear spin I = 3/2) yielding a broadening of the Ni spectra at all g values and a resolved 61Ni hyperfine splitting into four lines of the low field edge in the case of the light-induced Ni-EPR signal. The redox potentials
determined at pH 7.0 for the described redox components were: for FMN –170 mV (midpoint potential, Em, for appearance), –200 mV (EPR signal intensity maximum) and –230 mV (Em for disappearance); for the Ni centre (Ni-C), –290 mV (Em for appearance), –305 mV (signal intensity maximum) and –325 mV (Em for disappearance). Exposure of the NADH-reduced hydrogenase to carbon monoxide led to an apparent Ni-CO species indicated
by a novel rhombic EPR signal with g values at 2.35, 2.08 and 2.01.
Received: 19 July 1995 / Accepted: 10 September 1995 相似文献
16.
J. B. K. Leonard J. F. Norieka B. Kynard S. D. McCormick 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1999,169(4-5):287-295
To assess the energetics of migration in an anadromous fish, adult American shad (Alosa sapidissima) were swum in a large respirometer at a range of speeds (1.0–2.3 body lengths (BL) s−1, 13–24 °C). Metabolic rate (MO2) was logarithmically related to swimming speed (Bl s−1; r
2 = 0.41, slope = 0.23 ± 0.037) and tailbeat frequency (beats × min−1; r
2 = 0.52, slope = 0.003 ± 0.0003). Temperature had a significant effect on metabolic rate (r
2 = 0.41) with a Q10 of 2.2. Standard metabolic rate (SMR), determined directly after immobilization with the neuroblocker gallamine triethiodide,
ranged from 2.2–6.2 mmolO2 kg−1 h−1 and scaled with mass (W) such that SMR = 4.0 (±0.03)W0.695(±0.15). Comparison of directly determined and extrapolated SMR suggests that swimming respirometry provides a good estimate of SMR
in this species, given the differences in basal activity monitored by the two methods. Overall, American shad metabolic rates
(MO2 and SMR) were intermediate between salmonids and fast-swimming perciforms, including tunas, and may be a result of evolutionary
adaptation to their active pelagic, schooling life history. This study demonstrates variability in metabolic strategy among
anadromous fishes that may be important to understanding the relative success of different migratory species under varying
environmental conditions.
Accepted: 3 March 1999 相似文献
17.
This article reports rate constants for thiol–thioester exchange (k
ex), and for acid-mediated (k
a), base-mediated (k
b), and pH-independent (k
w) hydrolysis of S-methyl thioacetate and S-phenyl 5-dimethylamino-5-oxo-thiopentanoate—model alkyl and aryl thioalkanoates, respectively—in water. Reactions such as
thiol–thioester exchange or aminolysis could have generated molecular complexity on early Earth, but for thioesters to have
played important roles in the origin of life, constructive reactions would have needed to compete effectively with hydrolysis
under prebiotic conditions. Knowledge of the kinetics of competition between exchange and hydrolysis is also useful in the
optimization of systems where exchange is used in applications such as self-assembly or reversible binding. For the alkyl
thioester S-methyl thioacetate, which has been synthesized in simulated prebiotic hydrothermal vents, k
a = 1.5 × 10−5 M−1 s−1, k
b = 1.6 × 10−1 M−1 s−1, and k
w = 3.6 × 10−8 s−1. At pH 7 and 23°C, the half-life for hydrolysis is 155 days. The second-order rate constant for thiol–thioester exchange
between S-methyl thioacetate and 2-sulfonatoethanethiolate is k
ex = 1.7 M−1 s−1. At pH 7 and 23°C, with [R″S(H)] = 1 mM, the half-life of the exchange reaction is 38 h. These results confirm that conditions
(pH, temperature, pK
a of the thiol) exist where prebiotically relevant thioesters can survive hydrolysis in water for long periods of time and
rates of thiol–thioester exchange exceed those of hydrolysis by several orders of magnitude. 相似文献
18.
Ribitsch D Karl W Wehrschütz-Sigl E Tutz S Remler P Weber HJ Gruber K Stehr R Bessler C Hoven N Sauter K Maurer KH Schwab H 《Applied microbiology and biotechnology》2009,81(5):875-886
In the course of a microbial screening of soil samples for new oxidases, different enrichment strategies were carried out.
With choline as the only carbon source, a microorganism was isolated and identified as Arthrobacter nicotianae. From this strain, a gene coding for a choline oxidase was isolated from chromosomal DNA. This gene named codA was cloned in Escherichia coli BL21-Gold and the protein (An_CodA) heterologously overexpressed as a soluble intracellular protein of 59.1 kDa. Basic biochemical characterization of
purified protein revealed a pH optimum of 7.4 and activity over a broad temperature range (15–70 °C). Specific activities
were determined toward choline chloride (4.70 ± 0.12 U/mg) and the synthetic analogs bis(2-hydroxyethyl)-dimethylammonium
chloride (0.05 ± 0.45 × 10–2 U/mg) and tris-(2-hydroxyethyl)-methylammonium methylsulfate (0.01 ± 0.12 × 10–2 U/mg). With increasing number of oxidizable groups, a significant decrease in activity was noted. Determination of kinetic
parameters in atmorspheric oxygen resulted in K
M = 1.51 ± 0.09 mM and V
max = 42.73 ± 0.42 mU/min for choline chloride and K
M = 4.77 ± 0.76 mM and V
max = 48.40 ± 2.88 mU/min for the reaction intermediate betaine aldehyde respectively. Nuclear magnetic resonance spectroscopic
analysis of the products formed during the enzyme reaction with choline chloride showed that in vitro the intermediate betaine
aldehyde exists also free in solution. 相似文献
19.
Water conductance of the cuticular membrane (CM) of mature sweet cherry fruit (Prunus avium L. cv. Sam) was investigated by monitoring water loss from segments of the outer pericarp excised from the cheek of the fruit.
Segments consisted of epidermis, hypodermis and several cell layers of the mesocarp. Segments were mounted in stainless-steel
diffusion cells with the mesocarp surface in contact with water, while the outer cuticular surface was exposed to dry silica
(22 ± 1 °C). Conductance was calculated by dividing the amount of water transpired per unit area and time by the difference
in water vapour concentration across the segment. Conductance values had a log normal distribution with a median of 1.15 × 10−4 m s−1 (n=357). Transpiration increased linearly with time. Conductance remained constant and was not affected by metabolic inhibitors
(1 mM NaN3 or 0.1 mM carbonylcyanide m-chlorophenylhydrazone) or thickness of segments (range 0.8–2.8 mm). Storing fruit (up to 42 d, 1 °C) used as a source of
segments had no consistent effect on conductance. Conductance of the CM increased from cheek (1.16 ± 0.10 × 10−4 m s−1) to ventral suture (1.32 ± 0.07 × 10−4 m s−1) and to stylar end (2.53 ± 0.17 × 10−4 m s−1). There was a positive relationship (r2=0.066**; n=108) between conductance and stomatal density. From this relationship the cuticular conductance of a hypothetical astomatous
CM was estimated to be 0.97 ± 0.09 × 10−4 m s−1. Removal of epicuticular wax by stripping with cellulose acetate or extracting epicuticular plus cuticular wax by dipping
in CHCl3/methanol increased conductance 3.6- and 48.6-fold, respectively. Water fluxes increased with increasing temperature (range
10–39 °C) and energies of activation, calculated for the temperature range from 10 to 30 °C, were 64.8 ± 5.8 and 22.2 ± 5.0 kJ
mol−1 for flux and vapour-concentration-based conductance, respectively.
Received: 23 March 2000 / Accepted: 28 July 2000 相似文献
20.
G. J. Grobben W. H. M. van Casteren H. A. Schols A. Oosterveld G. Sala M. R. Smith J. Sikkema J. A. M. de Bont 《Applied microbiology and biotechnology》1997,48(4):516-521
The exopolysaccharides produced by Lactobacillus delbrueckii subsp. bulgaricus NCFB 2772 grown in defined medium were investigated. At equal cell densities, the strain produced 95 mg l−1 exopolysaccharides with glucose and 30 mg l−1 with fructose as the carbohydrate source. High-performance size-exclusion chromatography of the exopolysaccharides produced
on glucose showed the presence of two fractions with relative molecular masses (M
r) of 1.7 × 106 and 4 × 104 in almost equal amounts. The exopolysaccharides produced on fructose contained mainly a fraction of low M
r of 4 × 104. The high-M
r fraction of the purified exopolysaccharides produced on glucose appeared to have a sugar composition of galactose, glucose
and rhamnose in the molar ratio of 5:1:1, whereas the low-M
r weight fraction contained galactose, glucose and rhamnose in the molar ratio of approximately 11:1:0.4. The purified exopolysaccharide
fractions produced on fructose showed comparable ratios. The high-molecular-mass fractions contained terminally linked galactose,
1,2,3-linked galactose, 1,3,4-linked galactose, 1,3-linked glucose and terminally linked rhamnose. The low-molecular-mass
fractions contained mainly 1,3-linked galactose and 1,6-linked galactose and lower amounts of other sugar linkages. The production
of the high-M
r fractions appeared to be dependent on the carbohydrate source, whereas the low-M
r fractions were produced more continuously.
Received: 30 April 1997 / Received revision: 11 June 1997 / Accepted: 14 June 1997 相似文献