Anion and ionic strength effects upon the oxidation of cytochrome c by cytochrome c oxidase

Citrate and other polyanion binding to ferricytochrome c partially blocks reduction by ascorbate, but at constant ionic strength the citrate-cytochrome c complex remains reducible; reduction by TMPD is unaffected. At a constant high ionic strength citrate inhibits the cytochrome c oxidase reaction competitively with respect to cytochrome c, indicating that ferrocytochrome c also binds citrate, and that the citrateferrocytochrome c complex is rejected by the binding site at high ionic strength. At lower ionic strengths, citrate and other polyanions change the kinetic pattern of ferrocytochrome c oxidation from first-order towards zero-order, indicating preferential binding of the ferric species, followed by its exclusion from the binding site. The turnover at low cytochrome c concentrations is diminished by citrate but not the K_m (apparent non-competitive inhibition) or the rate of cytochrome a reduction by bound cytochrome c. Small effects of anions are seen in direct measurements of binding to the primary site on the enzyme, and larger effects upon secondary site binding. It is concluded that anion-cytochrome c complexes may be catalytically competent but that the redox potentials and/or intramolecular behaviour of such complexes may be affected when enzyme-bound. Increasing ionic strength diminishes cytochrome c binding not only by decreasing the ‘association’ rate but also by increasing the ‘dissociation’ rate for bound cytochrome c converting the ‘primary’ (T) site at high salt concentrations into a site similar kinetically to the ‘secondary’ (L) site at low ionic strength. A finite K_m of 170 μM at very high ionic strength indicates a ratio of $\text{K}{}^{\mathrm{\infty }}{}_{\mathrm{<mtext>M</mtext>}}\text{K}{}^0{}_{\mathrm{<mtext>M</mtext>}}$ of about 5000. It is proposed that anions either modify the E¹₀ of cytochrome c bound at the primary (T) site or that they perturb an equilibrium between two forms of bound c in favour of a less active form.     

Influence of root diameter on the penetration of seminal roots into a compacted subsoil

A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg^-1, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress.Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage.The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.     

Year-class strength regulation in plaice (Pleuronectes platessa L.) on the Swedish west coast

Year-class strength of plaice (Pleuronectes platessa) has been studied over periods of 10 and 35 years, respectively, in two shallow sandy areas on the Swedish west coast. In one area, Gullmar Bay (N 58 ° 19 – W 11 ° 33), 0-group plaice were sampled quantitatively with a drop trap at 0–0.7 m depth and densities between 0.2 and 3.8 ind./m² were recorded in early summer. In the other area, Laholm Bay (N 56 ° 30 – W 12 ° 55), 0-group plaice were sampled semi-quantitatively in August with young-fish trawl at 1.5 m depths. Densities between 0.001 and 0.28 ind./m² were assessed.The effects of temperature and wind (in the winter and early spring) and predation (in early summer), on the recruitment of 0-group plaice were investigated. In Gullmar Bay high recruitment occurred after severe winters and in years when on-shore winds dominated during spring. No such correlations were found in Laholm Bay. In Gullmar Bay a significant inverse relationship was found between the density of O-group plaice in early summer and the biomass of brown shrimp (Crangon crangon) and the shore crab (Carcinus maenas), the main predators in the nursery area.The importance of physical and biological factors regulating recruitment in plaice are discussed.     

Studies on the yield and gel strength of agar from Gracilaria domingensis Sonder ex Kuetzing (Gracilariales,Rhodophyta) following the addition of calcium

Studies were carried out on the seasonal variation in yield and gel strength of agar from Gacilaria domingensis with and without the addition of calcium chloride. Extraction was done with and without treatment with 1% hydrochloric acid. The results showed an increase in yield and gel strength when an alkaline solution of calcium was used, but the gel strength was low. For commercial use, Gracilaria domingensis should be mixed with better quality Gracilaria species because of its low gel strength.     

Ionic strength effects on cytochrome aa3 kinetics

1. The occurrence of an optimal ionic strength for the steady-state activity of isolated cytochrome aa₃ can be attributed to two opposite effects: upon lowering of the ionic strength the affinity between cytochrome c and cytochrome aa₃ increases, whereas in the lower ionic strength region the formation of a less active cytochrome c-aa₃ complex limits the ferrocytochrome c association to the low affinity site.2. At low ionic strength, the reduction of cytochrome c-aa₃ complex by ferrocytochrome c₁ proceeds via non-complex-bound cytochrome c. Under these conditions the positively charged cytochrome c provides the electron transfer between the negatively charged cytochromes c₁ and aa₃.3. Polylysine is found to stimulate the release of tightly bound cytochrome c from the cytochrome c-aa₃ complex. This property points to the existence of negative cooperativity between the two binding sites. We suggest that the stimulation is not restricted to polylysine, but also occurs with cytochrome c.4. Dissociation rates of both high and low affinity sites on cytochrome aa₃ were determined indirectly. The dissociation constants, calculated on the basis of pre-steady-state reaction rates at an ionic strength of 8.8 mM, were estimated to be 0.6 nM and 20 μM for the high and low affinity site, respectively.     

The reaction between cytochrome C1 and cytochrome C

The kinetics of electron transfer between the isolated enzymes of cytochrome c₁ and cytochrome c have been investigated using the stopped-flow technique. The reaction between ferrocytochrome c₁ and ferricytochrome c is fast; the second-order rate constant (k₁) is 3.0 · 10⁷ M^?1 · s^?1 at low ionic strength (I = 223 mM, 10°C). The value of this rate constant decreases to 1.8 · 10⁵ M^?1 · s^?1 upon increasing the ionic strength to 1.13 M. The ionic strength dependence of the electron transfer between cytochrome c₁ and cytochrome c implies the involvement of electrostatic interactions in the reaction between both cytochromes. In addition to a general influence of ionic strength, specific anion effects are found for phosphate, chloride and morpholinosulphonate. These anions appear to inhibit the reaction between cytochrome c₁ and cytochrome c by binding of these anions to the cytochrome c molecule. Such a phenomenon is not observed for cacodylate. At an ionic strength of 1.02 M, the second-order rate constants for the reaction between ferrocytochrome c₁ and ferricytochrome c and the reverse reaction are k₁ = 2.4 · 10⁵ M^?1 · s^?1 and k_?1 = 3.3 · 10⁵ M^?1 · s^?1, respectively (450 mM potassium phosphate, pH 7.0, 1% Tween 20, 10°C). The ‘equilibrium’ constant calculated from the rate constants (0.73) is equal to the constant determined from equilibrium studies. Moreover, it is shown that at this ionic strength, the concentrations of intermediary complexes are very low and that the value of the equilibrium constant is independent of ionic strength. These data can be fitted into the following simple reaction scheme: cytochrome c²⁺₁ + cytochrome c³⁺ai cytochrome c³⁺₁ + cytochrome c²⁺.     

Fucose in the surface deposits of axenic and field grown roots ofZea mays L.

Summary The location of materials containing terminal fucose residues on the surface of axenic and field grown roots of corn has been determined.Binding patterns of FITC-labelled,Lotus purpureus Moench lectin indicate the presence of the fucose residues in the cell walls and mucilage of the peripheral region of the root cap. During development, fucose residues also appear in the outer periclinal walls and overlying mucilage of columnar epidermal cells. Surface material rich in these residues persists between the mature root hairs but is not found on their surface. Fucose-rich mucilage is present on the exposed surface of aerial roots and at the point where they enter the soil. No lectin binding residues are indicated elsewhere in the roots.     

Effects of ion-pairing on calculations of ionic activities of major ions in freshwater

Ion-pairing has little effect on ionic activity calculations for weakly mineralized natural water (ionic strength < 2 mM). However, for more strongly mineralized freshwaters, corrections for ion-pairing are necessary if highly accurate ionic activities are required.     

Can the problem of hybridization in threatened species be evaluated using a fieldwork research? A case study in snapdragons

Hybridization, natural or artificial, is considered disadvantageous for species biodiversity when it threatens the population integrity of endangered species. Frequently, studies investigating whether hybridization poses a legitimate risk to rare species are based on genetic data obtained in molecular biology laboratories. In this study, we used field research to approach the problem that hybridization could cause for the viability of a population of a rare species and to be able to propose the most appropriate initial conservation strategy. Specifically, using the model genus Antirrhinum, the reproductive barriers between the rare A. pulverulentum and its common congener A. litigiosum have been analysed under the reproductive isolation index (RI). A. pulverulentum had a high value of total RI, indicating that there are barriers to gene flow from A. litigiosum towards this species, and also had a high value for the intrinsic RI, reflecting a low inherent capacity for production of hybrid plants; in addition, the possibility of successful backcrosses between this species with hybrids produced from A. litigiosum ovules were low, given the high intrinsic RI of A. litigiosum. These data indicate the current existence of strong and permanent barriers to hybridization between the two species, suggesting that hybridization does not seem to be a serious problem for the conservation of A. pulverulentum in the studied population, nor for the near future. This study shows how the RI index can provide useful information for conservation purposes and proposes different management recommendations.