Recombinant expression and initial characterization of the putative human enteric ferric reductase Dcytb

Recent studies of mutant mice with compromised ability to absorb dietary iron have identified involvement of two integral membrane proteins in the intestinal epithelial lining in iron uptake, a divalent metal ion transporter and a ferric reductase. The current study concerns the recombinant expression, purification, and initial spectroscopic characterization of a recombinant form of the human ferric reductase that was expressed and purified as the apoprotein from Escherichia coli. Reconstitution of the recombinant protein with ferriprotoporphyrin IX produced a red product with Soret (Fe3+, lambdamax 413.5 nm; Fe2+, lambdamax = 426 nm) and visible absorption maxima indicative of bisimidazole axial coordination. This observation was confirmed by electron paramagnetic resonance and magnetic circular dichroism spectroscopy. Titration of apo-Dcytb with ferriprotoporphyrin IX was consistent with the binding of two heme groups to the protein as predicted by the phylogenetic relationship of this protein to the cytochrome b561 family. Similar titrations and spectroscopic studies of two double variants of Dcytb, each lacking a pair of histidyl residues (H50 and H120 or H86 and H159) proposed on the basis of sequence alignment with other members of the cytochrome b561 family to provide axial ligands to bound heme, indicated that these variants were able to bind just one heme group each.     

Phospholipid membranes affect tertiary structure of the soluble cytochrome b5 heme-binding domain

The influence of charged phospholipid membranes on the conformational state of the water-soluble fragment of cytochrome b5 has been investigated by a variety of techniques at neutral pH. The results of this work provide the first evidence that aqueous solutions with high phospholipid/protein molar ratios (pH 7.2) induce the cytochrome to undergo a structural transition from the native conformation to an intermediate state with molten-globule like properties that occur in the presence of an artificial membrane surface and that leads to binding of the protein to the membrane. At other phospholipid/protein ratios, equilibrium was observed between cytochrome free in solution and cytochrome bound to the surface of vesicles. Inhibition of protein binding to the vesicles with increasing ionic strength indicated for the most part an electrostatic contribution to the stability of cytochrome b5-vesicle interactions at pH 7.2. The possible physiological role of membrane-induced conformational change in the structure of cytochrome b5 upon the interaction with its redox partners is discussed.     

Changes in the mineral composition of soybean xylem sap during monocarpic senescence and alterations by depodding

Evidence from earlier studies with explants (stem cutting with a leaf and a pod) indicates that a decline in the supply of mineral nutrients from the roots may prepare the leaves for induction of monocarpic senescence in soybean [ Glycine max (L.) Merrill cv. Anoka). In order to assess the changes in mineral flux from the root system, xylem sap was collected from a decapitated plant under 100 kPa pressure over 50 min. The sap volume yield declines after flowering starts, but increases during pod extension and then decreases again during podfill. The concentrations of K, Ca, Mg, P, S, Zn, Fe, Mn, Cu, Mo and Si rise and then fall during reproductive development, but the exact timing differs among the elements. In contrast, B, Al and Na concentrations show a slow rise initially with a large increase in late pod development. Depodding, which prevents the early death of the plant, inhibits the changes of some elements (K, Mg) but not others (Ca. Mg, P, S, Zn. Fe. Mn, B, Cu, Al), and it does not prevent the decrease in sap volume delivered. Inasmuch as the mineral concentration of xylem sap quantitatively reflects upward mineral flux, the supply of most minerals to the shoot declines, and this decrease seems to be an important factor in the preparatory phase of monocarpic senescence. The different minerals show different patterns of change, which indicate differences in the transport mechanisms and their regulation.     

Role of tyrosine-103 in myoglobin peroxidase activity: kinetic and steady-state studies on the reaction of wild-type and variant recombinant human myoglobins with H(2)O(2)

Myoglobin (Mb) catalyzes a range of oxidation reactions in the presence of hydrogen peroxide (H(2)O(2)) through a peroxidase-like cycle. C110A and Y103F variants of human Mb have been constructed to assess the effects of removing electron-rich oxidizable amino acids from the protein on the peroxidase activity of Mb: a point mutation at W14 failed to yield a viable protein. Point mutations at C110 and Y103 did not result in significant changes to structural elements of the heme pocket, as judged by low-temperature electron paramagnetic spectroscopy (EPR) studies on the ground-state ferric proteins. However, compared to the native protein, the yield of globin radical (globin*) was significantly decreased for the Y103F but not the C110A variant Mb upon reaction of the respective proteins with H(2)O(2). In contrast with our expectation that inhibiting pathways of intramolecular electron transfer may lead to enhanced Mb peroxidase activity, mutation of Y103 marginally decreased the rate constant for reaction of Mb with H(2)O(2) (1.4-fold) as judged by stopped-flow kinetic analyses. Consistent with this decrease in rate constant, steady-state analyses of Y103F Mb-derived thioanisole sulfoxidation indicated decreased V(max) and increased K(m) relative to the wild-type control. Additionally, thioanisole sulfoxidation proceeded with lower stereoselectivity, suggesting that Y103 plays a significant role in substrate binding and orientation in the heme pocket of Mb. Together, these results show that electron transfer within the globin portion of the protein is an important modulator of its stability and catalytic activity. Furthermore, the hydrogen-bonding network involving the residues that line the heme pocket of Mb is crucial to both efficient peroxidase activity and stereospecificity.     

The structural and functional role of lysine residues in the binding domain of cytochrome c in the electron transfer to cytochrome c oxidase.

The interactions of yeast iso-1 cytochrome c with bovine cytochrome c oxidase were studied using cytochrome c variants in which lysines of the binding domain were substituted by alanines. Resonance Raman spectra of the fully oxidized complexes of both proteins reveal structural changes of both the heme c and the hemes a and a3. The structural changes in cytochrome c are the same as those observed upon binding to phospholipid vesicles where the bound protein exists in two conformers, B1 and B2. Whereas the structure of B1 is the same as that of the unbound cytochrome c, the formation of B2 is associated with substantial alterations of the heme pocket. In cytochrome c oxidase, the structural changes in both hemes refer to more subtle perturbations of the immediate protein environment and may be a result of a conformational equilibrium involving two states. These changes are qualitatively different to those observed for cytochrome c oxidase upon poly-l-lysine binding. The resonance Raman spectra of the various cytochrome c/cytochrome c oxidase complexes were analyzed quantitatively. The spectroscopic studies were paralleled by steady-state kinetic measurements of the same protein combinations. The results of the spectra analysis and the kinetic studies were used to determine the stability of the complexes and the conformational equilibria B2/B1 for all cytochrome c variants. The complex stability decreases in the order: wild-type WT > J72K > K79A > K73A > K87A > J72A > K86A > K73A/K79A (where J is the natural trimethyl lysine). This order is not exhibited by the conformational equilibria. The electrostatic control of state B2 formation does not depend on individual intermolecular salt bridges, but on the charge distribution in a specific region of the front surface of cytochrome c that is defined by the lysyl residues at positions 72, 73 and 79. On the other hand, the conformational changes in cytochrome c oxidase were found to be independent of the identity of the bound cytochrome c variant. The maximum rate constants determined from steady-state kinetic measurements could be related to the conformational equilibria of the bound cytochrome c using a simple model that assumes that the conformational transitions are faster than product formation. Within this model, the data analysis leads to the conclusion that the interprotein electron transfer rate constant is around two times higher in state B2 than in B1. These results can be interpreted in terms of an increase of the driving force in state B2 as a result of the large negative shift of the reduction potential.     

A Mathematical Model of the Cerebellar-Olivary System II: Motor Adaptation Through Systematic Disruption of Climbing Fiber Equilibrium

The implications for motor learning of the model developed in the previous article are analyzed using idealized Pavlovian eyelid conditioning trials, a simple example of cerebellar motor learning. Results suggest that changes in grPkj synapses produced by a training trial disrupt equilibrium and lead to subsequent changes in the opposite direction that restore equilibrium. We show that these opposing phases would make the net plasticity at each grPkj synapse proportional to the change in its activity during the training trial, as influenced by a factor that precludes plasticity when changes in activity are inconsistent. This yields an expression for the component of granule cell activity that supports learning, the across-trials consistency vector, the square of which determines the expected rate of learning. These results suggest that the equilibrium maintained by the cerebellar-olivary system must be disrupted in a specific and systematic manner to promote cerebellar-mediated motor learning.     

Small substrates and cytochrome c are oxidized at different sites of cytochrome c peroxidase.

Modeling studies suggest that electrons are transferred from cytochrome c to cytochrome c peroxidase (CcP) with cytochrome c predominantly bound at a site facing the gamma-meso edge of the CcP prosthetic heme group (Poulos, T.L., and Kraut, J. (1980) J. Biol. Chem. 255, 10322-10330). As shown here, guaiacol and ferrocyanide are oxidized at a different site of CcP. Thus, the oxidations of cytochrome c and guaiacol are differentially inactivated by phenylhydrazine and sodium azide. The loss of guaiacol oxidation activity correlates with covalent binding of 1 equivalent of [14C]phenylhydrazine to the protein, whereas the slower loss of cytochrome c activity correlates with the appearance of a 428-nm absorbance maximum attributed to the formation of a sigma-phenyl-iron heme complex. The delta-meso-phenyl and 8-hydroxymethyl derivatives of heme are formed as minor products. Catalytic oxidation of azide to the azidyl radical results in inactivation of CcP and formation of delta-meso-azidoheme. Reconstitution of apo-CcP with delta-meso-azido-, -ethyl-, and -(2-phenylethyl)heme yields holoproteins that give compound I species with H2O2 and exhibit 80, 59, and 31%, respectively, of the control kcat value for cytochrome c oxidation but little or no guaiacol or ferrocyanide oxidizing activity. Conversely, CcP reconstituted with gamma-meso-ethylheme is fully active in the oxidation of guaiacol and ferrocyanide but only retains 27% of the cytochrome c oxidizing activity. These results indicate that guaiacol and ferrocyanide are primarily oxidized near the delta-meso-heme edge rather than, like cytochrome c, at a surface site facing the gamma-meso edge.     

Influence of method of application of paclobutrazol on soil residues and growth retardation in a ‘Starkrimson-Delicious’ apple orchard

A 3-year field study was conducted to determine the influence of mode of application of the gibberellin-inhibitor paclobutrazol (PP333), [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4 triazol-1-yl)pentan-3-ol], on PP333 soil residue levels and vegetative growth retardation of 10-year-old Starkrimson Delicious (Malus Domestica Borkh.) spur-type apple trees. Treatments were applied in March, 1986 and consisted of foliar or soil sprays (200 ppm, 7 × applications at petal fall (+) 2, 4, 6, 8, 10 and 12 weeks) or a single soil drench (8.2 g A.I./tree) applied to the collar at petal fall. Foliar sprays were applied with and without a plastic ground cover to evaluate the influence of foliar runoff on the degree of soil absorption and its subsequent effect on vegetative growth. PP333 was extracted over a 3 year period (1986–88) from 400 cm² soil patches located at the drip line of each tree, with the exception of soil drenches which were sampled near the collar. PP333 soil extracts were purified and quantitatively analyzed by HPLC. PP333 soil residue levels following foliar sprays were comparable to the soil spray treatment for each year and decreased at a rate of 50% per year from 1986–1988. Foliar sprays retarded terminal growth in the year of application, whereas the soil spray did not inhibit growth until the following year. PP333 residue levels were highest in the soil drench where growth retardation was evident in 1987 and 1988. The greatest carry-over effect occurred in the soil treatments, especially the soil drench application which resulted in the highest soil residue rates throughout the 3 year period.