CD studies of peptides that mimic the four helicoidal sequences of the uteroglobin monomer

Summary Short peptides spanning the helicoidal sequences of the uteroglobin monomer (crystal forms P2₁ and C222₁) were synthesized and studied by circular dichroism spectroscopy. None of them showed any secondary structure in the absence of HFIP. However, most peptides achieved a helical conformation when this structuring agent was used, with the exception of the analogue corresponding to the helicoidal fragment 19–24 (helix II, crystal P2₁). These results indicate that other factors, such as interchain interactions, have to contribute to helix stabilization in the molecule. On the other hand, while peptides corresponding to N- and C-terminal fragments that contain the first and fourth helices of the monomer, respectively (1–14 and 48–70) achieved a -like structure when 10–15% of HFIP was used, this behaviour was not observed when TFE was used. Moreover, substitution of cysteine by -aminobutyric acid at position 3 increased both the helicity of fragment 1–14 and its ability to adopt a -like structure, but the opposite effect was observed for fragment 48–70 when -aminobutyric acid was introduced at position 69. These results indicate that this part of the protein might be sensitive to the chemical environment it is exposed to and that the two cysteine residues at positions 3 and 69 of the monomer could play a different role in the folding process.     

Identification and cellular localization of the actin-binding protein ABP-120 from Entamoeba histolytica

Several actin-binding proteins participate in the morphological changes that occur during amoeboid movement. The gene encoding one of these proteins, the gelation factor ABP-120, was identified and characterized from trophozoites of Entamoeba histolytica . The sequence contains 2574 nucleotides, with an open reading frame of 858 amino acids, giving a protein of 93 kDa belonging to the spectrin family. The N-terminal domain of ABP-120 from E. histolytica revealed a consensus site for actin binding homologous to the actin-binding sites of ABP-120 of Dictyostelium discoideum , α-actinin and spectrin. Analysis of the central domain revealed the presence of four repeats of a 73-amino-acid motif constituting 31% of the protein. In addition, a stretch of 105 amino acids was highly divergent when compared with the C-terminal domain of D. discoideum ABP-120. This sequence showed short motifs that are homologous to microtubule-binding domains. We found that ABP-120 from E. histolytica binds to F-actin. In addition, upon motility of the parasite, this protein localized in the pseudopod and the uroid region, implying a role for ABP-120 in movement and capping of surface receptors in E. histolytica .     

A new method to determine the aw range in which immobilized lipases display optimum activity in organic media

Summary We describe a qualitative method to predict the pre-equilibration a_w, system value in which, covalent immobilized lipase B from Candida antarctica to sepharose and silica, displayed best synthetic activity. The methodology is based in the analysis of the water adsorption isotherms of the biocatalyst in air and in the organic solvent. The biocatalyst is active at pre-equilibration a_w values higher than the divergence point between both isotherms. In addition, native and immobilized lipase display highest activity if the biocatalyst is pre-equilibrated at a_w=P point. For preparative purposes, the validity of the method was proved in the esterification of racemic 2-(4-isobutyl phenyl) propionic acid with 1-propanol in isooctane at long reaction time.     

Ehrlich cell plasma membrane redox system is modulated through signal transduction pathways involvingcGMP and Ca2+ as second messengers

Ehrlich cell plasma membrane ferricyanide reductase activity increased in the presence of mastoparan, a generic activator of G proteins, using either whole cells or isolated plasma membrane fractions. Agents that increase intracellularcAMP also increased the rate of ferricyanide reduction by Ehrlich cells. For the first time, evidence is shown on a modulation of plasma membrane redox system bycGMP. In fact, permeant analogs ofcGMP, dibutyrylcGMP, and 8-bromo-cGMP increased the rate of ferricyanide reduction by the Ehrlich cell plasma membrane redox system. Furthermore, specific inhibition ofcGMP-phosphodiesterases by dipyridamole was also accompanied by an enhancement in the rate of ferricyanide reduction. On the other hand, treatments expected to increase cytoplasmic Ca²⁺ concentrations were accompanied by a remarkable stimulation of the reductase activity. Taking all these data together, it seems that the Ehrlich cell plasma membrane redox system is under a multiple and complex regulation by different signal transduction pathways involving G proteins, cyclic nucleotides, and Ca²⁺ ions.     

The structural role of the carotenoid in the bacterial light-harvesting protein 2 (LH2) of Rhodonbacter capsulatus. A Fourier transform Raman spectroscopy and circular dichroism study

In previous work (Zurdo J, Fernández-Cabrera C and Ramírez JM (1993) Biochem J 290: 531–537), it had been shown that selective extraction of the carotenoid from the light-harvesting protein 2 (LH2) of Rhodobacter capsulatus induced the dissociation of 800-nm absorbing bacteriochlorophyll (Bchl), a 10-nm red shift of 854-nm Bchl, and a decrease of the stability of the protein in detergent solution. In the present study, the Fourier transform Raman and near-infrared circular dichroism spectra of native and carotenoid-depleted LH2 membrane preparations were compared. It was found that while the coupled carbonyls of 854-nm Bchl remained specifically H-bonded to the peptides after carotenoid extraction, the optical activity of the near-infrared electronic transition was significantly altered. Given the excitonic origin of such optical activity, our data suggest that carotenoid extraction elicits a rearrengement of the chromophore cluster and of the associated polypeptide subunits. This implies a significant role of the carotenoid in maintaining the native quaternary structure of the protein, which would be consistent with the observed dissociation of 800-nm Bchl and the loss of solubilized LH2 stability that result from carotenoid removal. There is no evidence for a similar role of the carotenoid in the LH1 protein.Abbreviations Bchl bacteriochlorophyll - FT Fourier transform - CD circular dichroism - LH1 and LH2 the bacterial light-harvesting proteins 1 and 2 In memoriam of Daniel I. Arnon.