共查询到20条相似文献,搜索用时 31 毫秒
1.
Comparison of the structure of ribosomal 5S RNA from E. coli and from rat liver using X-ray scattering and dynamic light scattering 总被引:2,自引:0,他引:2
J. J. Müller T. N. Zalkova D. Ziwer R. Misselwitz K. Gast I. N. Serdyuk H. Welfle G. Damaschun 《European biophysics journal : EBJ》1986,13(5):301-307
The structures of eukaryotic ribosomal 5S RNA from rat liver and of prokaryotic 5S RNA from E. coli (A-conformer) have been investigated by scattering methods. For both molecules, a molar mass of 44,500±4,000 was determined from small angle X-ray scattering as well as from dynamic light scattering. The shape parameters of the two rRNAs, volume V
c, surface O
c, radius of gyration R
s, maximum dimension of the molecule L, thickness D, and cross section radius of gyration R
sq, agree within the experimental error limits. The mean values are V
c=57±3 nm3, O
c=165±10 nm2, R
s=3.37±0.05 nm, L=10.8±0.7 nm, D=1.57±0.07 nm, R
sa=0.92±0.01 nm.Identical structures for the E. coli 5S rRNA and the rat liver 5S rRNA at a resolution of 1 nm can be deduced from this agreement and from the comparison of experimental X-ray scattering curves and of experimental electron distance distribution function. The flat shape model derived for prokaryotic and eukaryotic 5S rRNA shows a compact region and two protruding arms. Double helical stems are eleven-fold helices with a mean base pair distance of 0.28 nm. Combining the shape information obtained from X-ray scattering with the information about the frictional behaviour of the molecules, deduced from the diffusion coefficients D
20,w
0
=(5.9±0.2)·10-7 cm2s-1 and (6.2±0.2)·10-7 cm2s-1 for rat liver 5S rRNA and E. coli 5S rRNA, respectively, a solvation shell of about 0.3 nm thickness around both molecules is determined. This structural similarity and the consensus secondary structure pattern derived from comparative sequence analyses suggest that all 5S rRNAs may indeed have conserved essentially the same type of folding of their polynucleotide strands during evolution, despite having very different sequences. 相似文献
2.
G. M. Pavlov 《European biophysics journal : EBJ》1997,25(5-6):385-397
The relationship between the sedimentation coefficient s0 and its concentration coefficient ks obtained in experiments on velocity sedimentation for polysaccharides is discussed. The values of s0, ks and an independently determined molecular weight reported by different authors for different polysaccharides are considered.
It was established that the scaling relation. ks∼ s0
v
unambiguously relates to the scaling relation s0∼ Mb. The values of the sedimentation parameter β
s introduced on the basis of Svedberg's equation for s0 and on the basis of the expression ks = B 〈h2〉3/2M–1 are discussed and the generalized Wales-van Holde-Rowe equation MKS = (NA/β
S)3/2[s]3/2 kS
1/2 is used for evaluation of the molecular weights of polysaccharides. The adequacy of this evaluation is illustrated by taking
as an example the determination of the unit length weight of an extra-rigid polysaccharide chain and of the equilibrium rigidity
of rigid-chain, semi-rigid-chain and flexible-chain polysaccharides. The pair of experimental values s0 and kS obtained in a single series of experiments give the same information as may be obtained from the other pairs of hydrodynamic
values such as [η] and s0 or [η] and D0, where [η] is the intrinsic viscosity and D0 is the translational diffusion coefficient.
Accepted: 11 December 1996 相似文献
3.
Kluge MG Ullrich R Scheibner K Hofrichter M 《Applied microbiology and biotechnology》2007,75(6):1473-1478
Agrocybe aegerita peroxidase (AaP) is a versatile heme-thiolate protein that can act as a peroxygenase and catalyzes, among other reactions,
the hydroxylation of aromatic rings. This paper reports a rapid and selective spectrophotometric method for directly detecting
aromatic hydroxylation by AaP. The weakly activated aromatic compound naphthalene served as the substrate that was regioselectively
converted into 1-naphthol in the presence of the co-substrate hydrogen peroxide. Formation of 1-naphthol was followed at 303 nm
(ɛ
303 = 2,010 M−1 cm−1), and the apparent Michaelis–Menten (K
m) and catalytic (k
cat) constants for the reaction were estimated to be 320 μM and 166 s−1, respectively. This method will be useful in screening of fungi and other microorganisms for extracellular peroxygenase activities
and in comparing and assessing different catalytic activities of haloperoxidase–peroxygenases. 相似文献
4.
The translational diffusion coefficient D
20,w
0
, of monomeric human immunoglobulin G (IgG) has been studied by photon-correlation spectroscopy as a function of pH and protein concentration. At pH 7.6, we find D
20,w
0
=3.89×10–7±0.02 cm2/sec, in good agreement with the value determined by classic mehods. This value corresponds to an effective hydrodynamic radius R, of 55.1±0.3 Å. As pH is increased to 8.9; with the same ionic strength, the molecule appears to expand slightly (3.5% increase in hydrodynamic radius). The concentration dependence of the IgG diffusion constant is interpreted in terms of solution electrostatic effects and shows that long-range repulsive interactions are negligible in the buffer used. The diffusion coefficient for dimeric IgG has also been determined to be D20,w=2.81×10–7±0.04 cm2/sec at 1.6 mg/ml, which corresponds to a hydrodynamic radius of 75 Å. For light-scattering studies of protein molecules in the dimension range of 5–10 nm (Mr=105–107) we find monomeric horse spleen ferritin well suited as a reference standard. Ferritin is a spherical molecule with a hydrodynamic radius R of 6.9±0.1 nm and is stable for years in our standard Tris-HCl-NaCl buffer even at room temperature. 相似文献
5.
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
6.
Salmon sperm DNA platination has been conducted under strictly pseudo-first-order conditions with cisplatin (1) and rac-{(1S,2S,4S)-exo-2-(aminomethyl)-2-amino-7-bicyclo[2.2.1]heptane}dichloroplatinum(II) (2). An aquation step first occurs for both complexes, with the rate constants k
1 = 1.12(0.02)×10–4 s–1 and 1.47(0.02)×10–4 s–1 respectively for 1 and 2 at 37 °C, values in agreement with those previously reported. It is followed by the actual platination step whose second-order
rate constant has been determined for the first time by physicochemical techniques. The values for 1 and 2 respectively are: k
2 = 2.08(0.07) M–1 s–1 and 3.9(0.4) M–1 s–1. These kinetic data are discussed in the context of a comparison of several biological properties of the two complexes.
Received: 15 May 1998 / Accepted: 26 June 1998 相似文献
7.
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
8.
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
9.
Sara Dodd Graham A. Place R. L. Hall Stephen E. Harding 《European biophysics journal : EBJ》1998,28(1):38-47
We have used two different approaches to determine hydrodynamic parameters for mucins secreted by guinea-pig tracheal epithelial
cells in primary culture. Cells were cultured under conditions that promote mucous cell differentiation. Secreted mucins were
isolated as the excluded fraction from a Sepharose CL-4B gel filtration column run under strongly dissociating conditions.
Biochemical analysis confirmed the identity of the high molecular weight material as mucins. Analytical ultracentrifugation
was used to study the physical properties of the purified mucins. The weight average molecular mass (M
w
) for three different preparations ranged from 3.3×106 to 4.7×106 g/mol (corresponding to an average structure of 1 – 2 subunits), and the sedimentation coefficient from 25.5 to 35 S. Diffusion
coefficients ranging from 4.5×10–8 to 6.4×10–8 cm2/s were calculated using the Svedberg equation. A polydispersity index (M
z
/M
w
) of ∼1.4 was obtained. Diffusivity values were also determined by image analysis of mucin granule exocytosis captured by
videomicroscopy. The time course of hydration and dissolution of mucin was measured and a relationship is presented which
models both phases, each with first order kinetics, in terms of a maximum radius and rate constants for hydration and dissolution.
A median diffusivity value of 8.05×10–8 cm2/s (inter-quartile range = 1.11×10–7 to 6.08×10–8 cm2/sec) was determined for the hydration phase. For the dissolution phase, a median diffusivity value of 6.98×10–9 cm2/s (inter-quartile range = 1.47×10–8 to 3.25×10–9 cm2/sec) was determined. These values were compared with the macromolecular diffusion coefficients (D
20,w
) obtained by analytical ultracentrifugation. When differences in temperature and viscosity were taken into account, the resulting
D
37,g
was within the range of diffusivity values for dissolution. Our findings show that the physicochemical properties of mucins
secreted by cultured guinea-pig tracheal epithelial cells are similar to those of mucins of the single or double subunit type
purified from respiratory mucus or sputum. These data also suggest that measurement of the diffusivity of dissolution may
be a useful means to estimate the diffusion coefficient of mucins in mucus gel at the time of exocytosis from a secretory
cell.
Received: 10 March 1998 / Accepted: 27 March 1998 相似文献
10.
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
11.
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
12.
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
13.
The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied
in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m−2 s−1]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ)
and minimal fluorescence (SV0), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that
this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2
(LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected
fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV0, and the PS2 photochemical efficiencies (FV/FM and FV′/FM′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %. 相似文献
14.
The antioxidant properties of hydroxycinnamic acid derivatives (HCA) were studied by laser flash photolysis. The transient species with maximum absorption at 360 nm were assigned to the phenoxyl radical of HCA. The SO4•− induced oxidation of HCA was also investigated. It was shown that the interaction of SO4•− with HCA resulted in the formation of HCA phenoxyl radicals with rate constants of 2.0–3.9×109 M−1 s−1. The reactions of HCA with triplet state of benzophenone were analyzed and quenching rate constants of 4.3–7.8×109 M−1 s−1 were determined. 相似文献
15.
Amalia Muñoz Frank Laib David H. Petering C. F. Shaw III 《Journal of biological inorganic chemistry》1999,4(4):495-507
The synthetic peptide fragment containing residues 49–61 of rabbit liver metallothionein II (MT-II) (Ac-Ile-Cys-Lys-Gly-Ala-Ser-Asp-Lys-Cys-Ser-Cys-Cys-Ala-COOH),
which includes the only sequential four cysteines bound to the same metal ion in Cd7MT, forms a stable, monomeric Cd-peptide complex with 1 : 1 stoichiometry (Cd:peptide) via Cd-thiolate interactions. This
represents the first synthesis of a single metal-binding site of MT independent of the domains. The 111Cd NMR chemical shift at 716 ppm indicates that the 111Cd2+ in the metal site is terminally coordinated to four side-chain thiolates of the cysteine residues. The pH of half dissociation
for this Cd-peptide derivative, ∼3.3, demonstrates an affinity similar to that for Cd7MT. Molecular mechanics calculations show that the thermodynamically most stable folding for this isolated Cd2+ center has the same counterclockwise chirality (Λ or S) observed in the native holo-protein. These properties are consistent with its proposed role as a nucleation center for cadmium-induced
protein folding. However, the kinetic reactivity of the CdS4 structure toward 5,5′-dithiobis(5-nitrobenzoate) (DTNB) and EDTA is greatly increased compared to the complete cluster (α-domain
or holo-protein). The rate law for the reaction with DTNB is rate=(k
uf +k
1,f +k
2,f [DTNB])[peptide], where k
uf=0.15 s–1, k
1,f=2.59×10–3 s–1, and k
2,f=0.88 M–1 s–1. The ultrafast step (uf), observable only by stopped-flow measurement, is unprecedented for mammalian (M7MT) and crustacean (M6MT) holo-proteins or the isolated domains. The accommodation of other metal ions by the peptide indicates a rich coordination
chemistry, including stoichiometries of M-peptide for Hg2+, Cd2+, and Zn2+, M2-peptide for Hg2+ and Au+, and (Et3PAu)2-peptide.
Received: 9 December 1998 / Accepted: 20 May 1999 相似文献
16.
Transpiration and whole-tree conductance in ponderosa pine trees of different heights 总被引:17,自引:0,他引:17
M. G. Ryan B. J. Bond B. E. Law R. M. Hubbard D. Woodruff E. Cienciala J. Kucera 《Oecologia》2000,124(4):553-560
Changes in leaf physiology with tree age and size could alter forest growth, water yield, and carbon fluxes. We measured tree
water flux (Q) for 14 ponderosa pine trees in two size classes (12 m tall and ∼40 years old, and 36 m tall and ∼ 290 years old) to determine
if transpiration (E) and whole-tree conductance (g
t) differed between the two sizes of trees. For both size classes, E was approximately equal to Q measured 2 m above the ground: Q was most highly correlated with current, not lagged, water vapor pressure deficit, and night Q was <12% of total daily flux. E for days 165–195 and 240–260 averaged 0.97 mmol m–2 (leaf area, projected) s–1 for the 12-m trees and 0.57 mmol m–2 (leaf area) s–1 for the 36-m trees. When photosynthetically active radiation (I
P) exceeded the light saturation for photosynthesis in ponderosa pine (900 μmol m–2 (ground) s–1), differences in E were more pronounced: 2.4 mmol m–2 (leaf area) s–1 for the 12-m trees and 1.2 mmol m–2 s–1 for the 36-m trees, yielding g
t of 140 mmol m–2 (leaf area) s–1 for the 12-m trees and 72 mmol m–2 s–1 for the 36-m trees. Extrapolated to forests with leaf area index =1, the 36-m trees would transpire 117 mm between 1 June
and 31 August compared to 170 mm for the 12-m trees, a difference of 15% of average annual precipitation. Lower g
t in the taller trees also likely lowers photosynthesis during the growing season.
Received: 19 April 1999 / Accepted: 23 March 2000 相似文献
17.
Emile Bol Nicolette J. Broers Wilfred R. Hagen 《Journal of biological inorganic chemistry》2008,13(1):75-84
Formaldehyde ferredoxin oxidoreductase from Pyrococcus furiosus is a homotetrameric protein with one tungstodipterin and one [4Fe–4S] cubane per 69-kDa subunit. The enzyme kinetics have
been studied under steady-state conditions at 80 °C and pre-steady state conditions at 50 °C, in the latter case via monitoring
of the relatively weak (ε ≈ 2 mM−1 cm−1) optical spectrum of the tungsten cofactor. The steady-state data are consistent with a substrate substituted-enzyme mechanism
for three substrates (formaldehyde plus two ferredoxin molecules). The K
M value for free formaldehyde (21 μM) with ferredoxin as an electron acceptor is approximately 3 times lower than the value
measured when benzyl viologen is used as an acceptor. The K
M of ferredoxin (14 μM) is an order of magnitude less than previously reported values. An explanation for this discrepancy
may be the fact that high concentrations of substrate are inhibitory and denaturing to the enzyme. Pre-steady-state difference
spectra reveal peak shifts and a lack of isosbestic points, an indication that several processes happen in the first seconds
of the reaction. Two fast processes (k
obs1 = 4.7 s−1, k
obs2 = 1.9 s−1) are interpreted as oxidation of the substrate followed by rearrangement of the active site. Alternatively, these processes
could be the entry/binding of the substrate followed by its oxidation. The release of the product and the electron shuffling
over the tungsten and iron–sulfur center in the absence of an external electron acceptor are slower (k
obs3 = 6.10 × 10−2 s−1, k
obs4 = 2.18 × 10−2 s−1). On the basis of these results in combination with results from previous electron paramagnetic resonance studies an activation
route plus catalytic redox cycle is proposed. 相似文献
18.
The tolerances of 20 Beauveria bassiana isolates derived from host insects worldwide to UV-B irradiation were assessed quantitatively in multi-dose bioassays. Conidial
suspensions of the isolates smeared on glass slides were exposed to the gradient UV-B doses of 0.1–1.6 J cm−2 (D), which generated from 0.75 to 10.17 min irradiation of weighted 312-nm wavelength at 2.0–2.61 mW cm−2. Irradiated conidia were then incubated for 24 h at 25°C under saturated humidity. The ratio of germination at each dose
over that in the blank control was defined as survival index (I
s). For all isolates, the I
s − D observations fit well with the survival model I
s = 1/[1 + exp(a + bD)] (0.94 ≤ r
2 ≤ 0.99) generated widely spanned lethal doses of 0.154–0.928, 0.240–1.139, and 0.383–1.493 J cm−2 for their losses of 50%, 75%, and 95% viabilities, respectively. These were far below the solar UV-B dose of 2.439 J cm−2 measured in a sunny day during the summer. The large variation of UV-B tolerance among the isolates indicates a necessity
to select UV-tolerant candidates for formulations applied to insect control during summer. The highly efficient bioassay method
was developed to measure accurately the UV-B tolerances of fungal biocontrol agents as lethal doses. 相似文献
19.
Photosynthesis of lichen symbiotic alga Trebouxia erici as affected by irradiance and osmotic stress
The relation between oxygen evolution rate (OER) and quantum yield of photochemical reactions in photosystem 2 (ΦPS2) was examined in lichen symbiotic alga Trebouxia erici Ahmadjian (strain UTEX 911) exposed to different irradiances and osmotic stress (2 M sucrose for 60 h). Linear relationship
was found between OER and ΦPS2 in control cell suspension within irradiance range of 0 – 500 μmol m−2 s−1. Under osmotic stress, OER and ΦPS2 were significantly reduced. Relation between OER and ΦPS2 was curvilinear due to strong osmotically-induced inhibition of OER at high irradiance. The highest used irradiance (500
μmol m−2 s−1) was photoinhibitory for osmotically-stressed T. erici because non-photochemical quenching (NPQ) increased substantially. Energy-dependent quenching represented major part of NPQ
increase. Osmotic stress led also to the reduction of capacity of photochemical processes in PS 2 (FV/FM) and increase in F0/FM. These changes indicated negative effects of osmoticum on structure and function of photosynthetic apparatus. 相似文献
20.
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