Structure of the two-subsite beta-d-xylosidase from Selenomonas ruminantium in complex with 1,3-bis[tris(hydroxymethyl)methylamino]propane

The three-dimensional structure of the catalytically efficient β-xylosidase from Selenomonas ruminantium in complex with competitive inhibitor 1,3-bis[tris(hydroxymethyl)methylamino]propane (BTP) was determined by using X-ray crystallography (1.3 Å resolution). Most H bonds between inhibitor and protein occur within subsite −1, including one between the carboxyl group of E186 and an N group of BTP. The other N of BTP occupies subsite +1 near K99. E186 (pK_a 7.2) serves as catalytic acid. The pH (6-10) profile for is bell-shaped with pK_a’s 6.8 and 7.8 on the acidic limb assigned to E186 and inhibitor groups and 9.9 on the basic limb assigned to inhibitor. Mutation K99A eliminates pK_a 7.8, strongly suggesting that the BTP monocation binds to the dianionic enzyme D14⁻E186⁻. A sedimentation equilibrium experiment estimates a K_d ([dimer]²/[tetramer]) of 7 × 10⁻⁹ M. Similar k_cat and k_cat/K_m values were determined when the tetramer/dimer ratio changes from 0.0028 to 26 suggesting that dimers and tetramers are equally active forms.     

Interaction of human α-lactalbumin with fatty acids: Determination of binding parameters

The interaction of holo- and apo-forms of human alpha-lactalbumin with fatty acids was studied by a partition equilibrium method. Apo-alpha-lactalbumin, obtained by treatment with EDTA, displays one binding site for fatty acids, the association constants for oleic and palmitic acids being 1.9.10(6) and 4.2.10(5) M(-1), respectively. However, holo-alpha-lactalbumin was unable to bind fatty acids as measured by this technique. Likewise, no fatty acids bound to holo-alpha-lactalbumin, isolated using nondenaturing conditions, were detected by gas chromatography. These results demonstrate that the conformational change induced in alpha-lactalbumin by the removal of calcium enables the protein to interact with fatty acids.     

Must There Be a Balance of Nature?

The balance of nature concept is an old idea that manifests itself in anumber of forms in population and community ecology. This paper focuseson population ecology, where controversy surrounding the balance ofnature takes the form of perennial debates over the significance ofdensity dependence, population regulation, and species interactions suchas competition. One of the most striking features of these debates, overthe course of the previous century in ecology, is the tendency to arguethe case on largely conceptual grounds. This paper explores twoquestions. Why this tendency to settle on conceptual grounds what is soobviously an empirical issue? Are there any good conceptual arguments tobe had in this area?     

Photogravitropic equilibrium in <Emphasis Type="Italic">Avena</Emphasis> coleoptiles: fluence rate–response relationships and dependence on dark adaptation and clinostating

Phototropism of Avena coleoptiles was measured in response to blue-light irradiation lasting between 2 and 24 h. During this time the coleoptiles established a bending angle of photogravitropic equilibrium that was dependent on the time of irradiation and also on the pretreatment in light or darkness prior to stimulation. The absolute threshold for the photogravitropic equilibrium in response to blue light was 10⁻⁸ μmol m⁻² s⁻¹. Photon fluence rate–response curves, which were generated after several hours of dark adaptation, had a characteristic shape with a prominent optimum in the middle of the dynamic range. Curves which were generated without prior dark adaptation displayed no such optimum. Clinostating dark-adapted coleoptiles caused an increase of sensitivity and responsiveness during a 2-h period of unilateral irradiation. The advantages and the drawbacks of long-term irradiation experiments for the investigation of phototropism and the generation of action spectra are discussed. Received: May 14, 2001 / Accepted: December 7, 2001     

Carbon dioxide fixation by reversible pyrrole-2-carboxylate decarboxylase and its application

Inducible pyrrole-2-carboxylate decarboxylase from Bacillus megaterium PYR2910 catalyzes the decarboxylation of pyrrole-2-carboxylate to stoichiometric amounts of pyrrole and CO₂. A unique feature of the homodimeric enzyme is its requirement for an organic acid such as acetate, propionate, butyrate or pimelate. A catalytic mechanism including a cofactor function of the organic acid was proposed. Due to an equilibrium constant of 0.3–0.4 M, the enzyme also catalyzes the reverse carboxylation of pyrrole after the addition of bicarbonate. For the synthesis of pyrrole-2-carboxylate, the reverse reaction was optimized and the equilibrium shifted towards the carboxylate. The product yield was 230 mM (25.5 g/l) pyrrole-2-carboxylate from 300 mM pyrrole in a batch reaction and 325 mM (36.1 g/l) from 400 mM pyrrole in a fed batch reaction, using both whole cells and the purified enzyme in a pH 8.0 reaction mixture with bicarbonate saturation of 1.9 M.     

Desertification,no‐change or alternative states: Can we trust simple models on livestock impact in dry rangelands?

Abstract. It is remarkable that after many thousands of years of pastoralism in arid and semi‐arid savannas, there is still no clear answer to the basic question: Does livestock grazing have more than a marginal effect on the dynamics of arid and semi‐arid land vegetation? A small study of semi‐arid savanna vegetation along a spatial gradient in grazing pressure, repeated three times over 19 years is used as a basis for discussing the behaviour of dry land vegetation under heavy grazing. Three basic theories are compared: (1) the theory that heavy grazing causes desertification; (2) the theory that heavy grazing causes no directed change in the vegetation of arid and semi‐arid rangelands; and (3) the theory that heavy grazing leads to a switch between alternative states of vegetation. On the basis of the current data the first two theories are rejected, but the conclusion is that there is much evidence for a more complex behaviour of dry rangelands under grazing than what is accommodated in any of the three theories. Probably, site‐specific properties including interactive and indirect effects of herbivory in the system are important for vegetation development under grazing in arid and semi‐arid rangelands.     

一个具暂时免疫且总人数可变的传染病动力学模型

建立了一个具常恢复率和接触率依赖于总人数的SIRS传染病动力学模型，讨论了系统平衡点的存在性和稳定性，对双线性传染率的特殊情形，给出了传染病平衡点的全局稳定性结论，推广和改进了已有的相应结果。     

Picosecond fluorescence decay studies on water-stressed pea leaves: energy transfer and quenching processes in photosystem 2

Detached leaves of pea (Pisum sativum) were submitted to water stress at different relative air humidities. The photosynthetic activity of photosystem 2 (PS2) was monitored by time-resolved picosecond chlorophyll (Chl) fluorescence spectroscopy. In the first days the well-known fast Chl fluorescence decay was observed which indicated high PS2 activity. After a few days the average fluorescence decay time τm reached a maximum, depending on the wilting conditions, but always at a relative loss of leaf mass of 80%. After this maximum, τm decreased within a few hours, the fluorescence decay became similar to that one of an intact leaf, but an additional fluorescence decay component with a lifetime of 3.6 ns appeared. At first the primary quinone QA was reduced due to inhibition of the electron transfer to the secondary quinone QB. Simultaneously, water deficiency caused an electron lack at the oxidizing site of PS2. This disabled the primary electron donor of PS2, tyrosine Z, from reducing the oxidized reaction centre of PS2 (P680+). Thus a recombination of P680+-pheophytin-QA- took place, and the energy was lost as heat. With further water stress, QA was decoupled from PS2. The new fluorescence decay component could therefore be assigned to energetically decoupled antenna complexes. This revised version was published online in June 2006 with corrections to the Cover Date.