Chemical Modification of Aspartic Acid and Arginine Residues in Horseradish Peroxidase Role of ASP 43 and ARG 38 in the Catalytic Cycle

In an attempt to alter the catalytic properties of horseradish peroxidase (HRP, EC 1.11.1.7), aspartic, glutamic and arginine residues were modified using ethanedithiol and diacetyl. Modification of Asp and Glu led to a marked increase in V_max along with denaturation of the protein. The thiol groups introduced were thought to be responsible, despite being situated on the periphery of the molecule as shown by the modification of the apoenzyme. The role of Arg 38 in the activation of hydrogen peroxide was indicated by the modifications of both enzyme and apoenzyme. An amino acid residue close to Arg 38 was thought to take over its function after blocking the group.     

Photochemical reactivity of the homologous proteinsα-lactalbumin and lysozyme

Summary The fluorescent behaviour and the photodynamic effect was studied in native and structurally modified lysozyme and-lactalbumin.The Tyr residues in lysozyme and-lactalbumin show different sensitivities to the photodynamic effect. The effect is zero in the case of Tyr from native lysozyme. In contrast, the Tyr residues in-lactalbumin are susceptible to photooxidation, which indicates a greater degree of exposure to the solvent. The three His residues of-lactalbumin have different degrees of exposure and show two different kinetics of photooxidation whereas the His residue of lysozyme is photooxidized with a single kinetic.Two photooxidation kinetics were obtained for the Trp residues of both native proteins, an indication that in both cases there are Trp residues that are differently exposed to the solvent. The wavelengths of maximum fluorescent emission of the Trp residues were different for the two proteins, an effect which can also be explained in terms of a difference in the environment of these residues. The modified form of these proteins emit at wavelengths longer than those of the native forms. When modified the proteins photooxidize with noticeably greater quantum yields.     

Differential accessibility to two insect neurones doe not account for differences in sensitivity to α-bungarotoxin

The nicotinic acetylcholine receptor probe α-bungarotoxin (1.0 × 10^?7 M) blocks the depolarising response to ionophoretic application of acetylcholine onto the cell body membrane of the fast coxal depressor motoneurone (D_f) of desheathed cockroach (Periplaneta americana) metathoracic ganglia, but at the same concentration is completely ineffective in blocking the depolarising action of acetylcholine on dorsal unpaired median (DUM) neurones in the same ganglion. The possibility that this is due to differences in accessibility of the toxin to the neurones has been tested by a combination of ionophoretic injection of horseradish peroxidase into single neurones with a study of the distribution of the exogenous tracer lanthanum, which is of similar effective size to α-bungarotoxin. The peripherally located cell body membranes and the fine axonal processes of D_f and DUM neurones of desheathed metathoracic ganglia are equally accessible to lanthanum. Differential accessibility to the two cell types does not account therefore for the differences in sensitivity to α-bungarotoxin.     

The porphyrin-sensitized photooxidation of horseradish apoperoxidase.

Horseradish apoperoxidase (apoHRP) was reconstituted with various porphyrin derivatives, e.g., ferric, cupric, manganese, and zinc protoporphyrin IX, metal-free protoporphyrin IX, hematoporphyrin IX and deuteroporphyrin IX. The visible absorption spectra of these porphyrin-apoHRP complexes were examined. The time required for maximum development of the new Soret peak after reconstitution was used to measure the rate of porphyrin-apoHRP reconstitution. All of the four metal-protoporphyrins reconstituted with apoHRP at the same rate as metal-free protoporphyrin IX, whereas, for the metal-free porphyrins, the rates of reconstitution were in the order of deuteroporphyrin IX > hematoporphyrin IX > protoporphyrin IX. The porphyrins on the reconstituted porphyrin-apoHRP complexes were used as localized photosensitizers for photodynamic studies. No amino acid residues were oxidized on illumination of the ferric, cupric and manganese protoporphyrin IX-apoHRP complexes due to the paramagnetic properties of these metal ions. With diamagnetic zinc ion, two histidine and one methionine residues were oxidized which was the same as in the protoporphyrin IX- and hematoporphyrin IX-apoHRP complexes. However, only one histidine was destroyed on illumination of the deuteroporphyrin IX-apoHRP complex. The results confirmed the resistance of horseradish peroxidase to photodynamic action and suggested the involvement of at least one histidine residue in the heme environment of horseradish peroxidase.     

Aromatic Amino Acid Residues in Japanese-radish Peroxidase a and Apoenzyme†

The nature of aromatic amino acid residues in Japanese-radish peroxidase a and the apoprotein was investigated by means of spectrophotometry and fluorospectrophotometry. The tyrosine residues in the holoenzyme were masked in the alkali-titration, giving an abnormally high value of 12.6, while they were exposed in the apoenzyme, exhibiting a value of 10.8. The difference spectra in the ultraviolet region between the holo-and apo-enzyme showed characteristic bands of tryptophan and phenylalanine as well as tyrosine. The perturbation of the aromatic amino acid residues by 50% ethyleneglycol was observed in the apoenzyme but not in the holoenzyme. The fluorophotometric experiments also revealed that the aromatic amino acid residues were in different environments in the holo- and apoenzyme. The difference between the conformation of peroxidase and that of the apoprotein was discussed.     

Protoporphyrin-induced photodynamic effects on band 3 protein of human erythrocyte membranes

In previous studies it has been shown that protoporphyrin-induced photodynamic effects on red blood cells are caused by photooxidation of amino acid residues in membrane proteins and by the subsequent covalent cross-linking of these proteins. Band 3, the anion transport protein of the red blood cell membrane, has a relatively low sensitivity to photodynamic cross-linking. This cannot be attributed to sterical factors inherent in the specific localization of band 3 in the membrane structure. Solubilized band 3, for instance, showed a similar low sensitivity to cross-linking. By extracellular chymotrypsin cleavage of band 3 into fragments of 60 000 and 35 000 daltons it could be shown that both fragments were about equally sensitive to photodynamic cross-linking. The 17 000 dalton transmembrane segment, on the other hand, was completely insensitive. Inhibition of band 3-mediated sulfate transport proceeded much faster than band 3 interpeptide cross-linking, presumably indicating that the inhibition of transport is caused by photooxidation of essential amino acid residues or intrapeptide cross-linking. A close parallel was observed between photodynamic inhibition of anion transport and decreased binding of 4,4′-diisothiocyanodihydrostilbene-2,2′-disulfonate (H₂DIDS), suggesting that a photooxidation in the immediate vicinity of the H₂DIDS binding site may be responsible for transport inhibition.     

The gene pool of the Belgorod oblast population: II. “Family name portraits” in groups of districts with different degrees of subdivision and the role of migrations in their formation

The frequencies and spectra of surnames have been analyzed in groups of raions (districts) of the Belgorod oblast (region) with different degrees of population subdivision. The “family name portraits” of districts with low (0.00003 < < f*_r < 0.00022, \(\overline {f_r^ * } \) = 0.00015) and moderate (0.00023 < f*_r < 0.00042, \(\overline {f_r^ * } \) = 0.00029) inbreeding levels are similar both to each other and to the “family name portrait” of the Belgorod oblast as a whole. Districts with high subdivision levels (0.00043 < f*_r < 0.00125, \(\overline {f_r^ * } \) = 0.00072) had very distinctive surname spectra and the highest surname frequencies. Intense immigration to the Belgorod oblast significantly affects its population genetic structure, decreasing the population subdivision.     

Fluorescence and circular dichroism spectroscopic studies on bovine lactoperoxidase*

Intrinsic steady-state fluorescence of lactoperoxidase (LPO) and its ligand-bound complexes has been characterized as a structural probe of its structure in solution. On excitation at 295 nm, a broad emission maximum is observed around 338 nm for LPO and for its ligand-bound complexes. The quantum yield is 0.0185±0.0005 for LPO and indicates tryptophan heme energy transfer. Tryptophan residues are located away from heme and are approximately equally distributed among hydrophobic and hydrophilic environments. From Förster resonance energy transfer equations, the average distance between tryptophans and heme within the enzyme is computed to be 25.1±0.2 Å. These fluorescence properties are consistent with the recent theoretical three-dimensional model for LPO and reveal that Trp337 and Trp404 dominate the intrinsic fluorescence, and together contribute 64% of the observed intensity. The effects of the denaturing agents guanidine hydrochloride and urea on the intrinsic fluorescence of LPO and CD of the backbone amide chromophores have been examined. The considerably red shifted emission maximum at 356 nm indicates that tryptophans, buried in the hydrophobic environment, are exposed to the solvent on denaturation. A simple two-state transition between the native and denatured forms of the protein has been used to explain the results. [Denaturant]_1/2 5.5 M, determined from both these experiments, indicates that LPO is relatively stable toward the denaturing agents. Quenching studies using. I^–, Cs⁺ and polar neutral acrylamide are consistent with this picture. Acrylamide can penetrate the protein matrix. It is an efficient quencher and the quenching process is essentially homogeneous with all the tryptophans being accessible. Cs⁺ ion is a very inefficient quencher but the iodide ion shows the quenching process to be predominantly heterogeneous with widely differing tryptophan accessibility. The Stern–Volmer constants deduced are K ^sv =8.4±1.4 M^–1 and K ^sv =4.05±0.65 M_–1 for acrylamide and iodide quenching, respectively. The fractional accessibility, f _a , deduced is f _a =0.52±0.03 for iodide quenching.     

Colicin M is inactivated during import by its immunity protein

Colicin M (Cma) displays a unique activity that interferes with murein and O-antigen biosynthesis through inhibition of lipid-carrier regeneration. Immunity is conferred by a specific immunity protein (Cmi) that inhibits the action of colicin M in the periplasm. The subcellular location of Cmi was determined by constructing hybrid proteins between Cmi and the TEM--lactamase (BlaM), which confers resistance to ampicillin only when it is translocated across the cytoplasmic membrane with the aid of Cmi. The smallest Cmi'-BlaM hybrid that conferred resistance to 50 g/ml ampicillin contained 19 amino acid residues of Cmi; cells expressing Cmi'-BlaM with only five N-terminal Cmi residues were ampicillin sensitive. These results support a model in which the hydrophobic sequence of Cmi comprising residues 3–23 serves to translocate residues 24–117 of Cmi into the periplasm and anchors Cmi to the cytoplasmic membrane. Residues 8–23 are integrated in the cytoplasmic membrane and are not involved in Cma recognition. This model was further tested by replacing residues 1–23 of Cmi by the hydrophobic amino acid sequence 1–42 of the penicillin binding protein 3 (PBP3). In vivo, PBP3'-'Cmi was as active as Cmi, demonstrating that translocation and anchoring of Cmi is not sequence-specific. Substitution of the 23 N-terminal residues of Cmi by the cleavable signal peptide of BlaM resulted in an active BlaM'-'Cmi hybrid protein. The immunity conferred by BlaM'-'Cmi was high, but not as high as that associated with Cmi and PBP3'-'Cmi, demonstrating that soluble Cmi lacking its membrane anchor is still active, but immobilization in the cytoplasmic membrane, the target site of Cma, increases its efficiency. Cmi1-23 remained in the cytoplasm and conferred no immunity. We propose that the immunity protein inactivates colicin M in the periplasm before Cma can reach its target in the cytoplasmic membrane.     

Comparative study of procedures for histological detection of lectin binding by use of Griffonia simplicifolia agglutinin I and gastrointestinal mucosa of the rat

Summary The histological localisation of -D-galactopyranosyl residues in glycoconjugates of rat stomach and duodenal mucosae was studied by use of Griffonia simplicifolia agglutinin I, i.e. the isolectin mixture (A+B) and the isolectin B₄ (B₄). Cryostat sections which were either unfixed or acetone fixed and paraffin sections from both ethanolacetic acid and formaldehyde fixed tissue blocks were compared. Cellular details were better preserved in paraffin than in cryostat sections. Reactivity of cells binding GS I was less sensitive after formaldehyde than after ethanol-acetic acid fixation inasmuch as higher concentrations of lectins were needed. This drawback could be overcome by trypsinisation of the sections. The binding pattern of GS I (A+B) corresponded with that of GS I (B₄) in either cryostat or paraffin sections. GS I was detected in the cytoplasm of parietal cells and in Brunner's gland cells. In duodenal crypts and villi, lectin was bound to supranuclear regions in the cytoplasm of columnar and goblet cells. The staining efficiency of fluorescein (FITC), horseradish peroxidase (HRP) and colloidal gold particle (CGP) labels in both direct and indirect lectin stainings was compared. Under all experimental conditions, indirect methods required lower concentrations of lectins than direct ones; indirect procedures increased sensitivity about 5–10 fold. CGP labels were always of highest sensitivity when gold particles were further developed by a silver precipitation method. HRP was not as efficient in lectin localisation as CGP, but cytochemical staining was more convenient in routine work. Direct FITC labellings proved to be of lowest sensitivity.     

Structural Basis of Efficient Electron Transport between Photosynthetic Membrane Proteins and Plastocyanin in Spinach Revealed Using Nuclear Magnetic Resonance

In the photosynthetic light reactions of plants and cyanobacteria, plastocyanin (Pc) plays a crucial role as an electron carrier and shuttle protein between two membrane protein complexes: cytochrome b₆f (cyt b₆f) and photosystem I (PSI). The rapid turnover of Pc between cyt b₆f and PSI enables the efficient use of light energy. In the Pc-cyt b₆f and Pc-PSI electron transfer complexes, the electron transfer reactions are accomplished within <10⁻⁴ s. However, the mechanisms enabling the rapid association and dissociation of Pc are still unclear because of the lack of an appropriate method to study huge complexes with short lifetimes. Here, using the transferred cross-saturation method, we investigated the residues of spinach (Spinacia oleracea) Pc in close proximity to spinach PSI and cyt b₆f, in both the thylakoid vesicle–embedded and solubilized states. We demonstrated that the hydrophobic patch residues of Pc are in close proximity to PSI and cyt b₆f, whereas the acidic patch residues of Pc do not form stable salt bridges with either PSI or cyt b₆f, in the electron transfer complexes. The transient characteristics of the interactions on the acidic patch facilitate the rapid association and dissociation of Pc.     

The disposition of the LZCC protein residues in wenxiang diagram provides new insights into the protein-protein interaction mechanism

Wenxiang diagram is a new two-dimensional representation that characterizes the disposition of hydrophobic and hydrophilic residues in α-helices. In this research, the hydrophobic and hydrophilic residues of two leucine zipper coiled-coil (LZCC) structural proteins, cGKIα¹⁻⁵⁹ and MBS_CT35 are dispositioned on the wenxiang diagrams according to heptad repeat pattern (abcdefg)_n, respectively. Their wenxiang diagrams clearly demonstrate that the residues with same repeat letters are laid on same side of the spiral diagrams, where most hydrophobic residues are positioned at a and d, and most hydrophilic residues are localized on b, c, e, f and g polar position regions. The wenxiang diagrams of a dimetric LZCC can be represented by the combination of two monomeric wenxiang diagrams, and the wenxiang diagrams of the two LZCC (tetramer) complex structures can also be assembled by using two pairs of their wenxiang diagrams. Furthermore, by comparing the wenxiang diagrams of cGKIα¹⁻⁵⁹ and MBS_CT35, the interaction between cGKIα¹⁻⁵⁹ and MBS_CT35 is suggested to be weaker. By analyzing the wenxiang diagram of the cGKIα^1−59.·MBS_CT42 complex structure, most affected residues of cGKIα¹⁻⁵⁹ by the interaction with MBS_CT42 are proposed at positions d, a, e and g of the LZCC structure. These findings are consistent with our previous NMR results. Incorporating NMR spectroscopy, the wenxiang diagrams of LZCC structures may provide novel insights into the interaction mechanisms between dimeric, trimeric, tetrameric coiled-coil structures.     

Root biomass fraction as a function of growth degree days in wheat

Root, underground and above-ground biomass were measured on various wheat cultivars from 1986 to 1988 in the south-east of France. The results are expressed as root: total (f _r) or underground: total (f _u) biomass fractions. Observed f _r and f _u values are in good agreement with previous results. f _r and f _u decrease steadily from emergence to maturity, with an exponential tendency. When using cumulative growth degree days since emergence relative to cumulative growth degree days until ear emergence () as time scale, f _r and f _u can be expressed as simple functions of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWGMb% addaWgaaqaaiaadkhaaeqaamaabmaabaGccqaH4oqCdaahaaWcbeqa% aiaacQcaaaaamiaawIcacaGLPaaakiabg2da9iaaicdacaGGUaGaaG% imaiaaiwdacqGHRaWkcaaIWaGaaiOlaiaaiwdacaaI4aGaamyzamaa% CaaaleqabaGaeyOeI0IaaGymaiaac6cacaaI0aGaaGioaiabeI7aXn% aaCaaameqabaGaaiOkaaaaaaaakeaacaWGMbaddaWgaaqaaiaadwha% aeqaamaabmaabaGccqaH4oqCdaahaaWcbeqaaiaacQcaaaaamiaawI% cacaGLPaaakiabg2da9iaaicdacaGGUaGaaGymaiaaikdacqGHRaWk% caaIWaGaaiOlaiaaiIdacaaI4aGaamyzamaaCaaaleqabaGaeyOeI0% IaaGOmaiaac6cacaaIYaGaaGioaiabeI7aXnaaCaaameqabaGaaiOk% aaaaaaaaaaa!610D!\[\begin{gathered} f_r \left( {\theta ^* } \right) = 0.05 + 0.58e^{ - 1.48\theta ^* } \hfill \\ f_u \left( {\theta ^* } \right) = 0.12 + 0.88e^{ - 2.28\theta ^* } \hfill \\ \end{gathered} \]The incremental root biomass partitioning coefficient, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqySde2aaS% baaSqaaiaadkhaaeqaaOGaeyypa0JaaiikaiaadsgacaWGxbWaaSba% aSqaaiaadkhaaeqaaOGaai4laiaadsgacaWG0bGaaiykaiaac+caca% GGOaGaamizaiaadEfadaWgaaWcbaGaamiDaaqabaGccaGGVaGaamiz% aiaadshacaGGPaaaaa!4834!\[\alpha _r = (dW_r /dt)/(dW_t /dt)\], which describes the net increase in root biomass dW _r over time dt relative to the increase in total biomass (dW _r) over the same time period, has been derived from f and the relative growth rate. Its time course is accurately represented by% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqySdegdda% WgaaqaaiaadkhaaeqaamaabmaabaGccqaH4oqCdaahaaWcbeqaaiaa% cQcaaaaamiaawIcacaGLPaaakiabg2da9iabgkHiTiaaicdacaGGUa% GaaGymaiaaiwdacqGHRaWkcaaIWaGaaiOlaiaaiAdacaaIZaGaamyz% amaaCaaaleqabaGaeyOeI0IaaGimaiaac6cacaaI5aGaaGioaiabeI% 7aXnaaCaaameqabaGaaiOkaaaaaaaaaa!4D15!\[\alpha _r \left( {\theta ^* } \right) = - 0.15 + 0.63e^{ - 0.98\theta ^* } \]Under our experimental conditions, with no severe water stresses or nutrient deficiencies, and for our sampling frequency, around 2 weeks, the development scale , is the main factor governing the time courses of f _r, f _u and _r.     

Variations in the cytoplasmic region account for the heterogeneity of the chicken MHC class I (B-F) molecules

Molecular variation among major histocompatibility complex (MHC) class I (B-F) proteins from B-homozygous chickens is apparently caused by C-terminal variation. Analysis of the total B-F protein pool revealed substantial heterogeneity with two or three molecular mass constituents, each being comprised by several isoelectric focusing variants. This heterogeneity could not be reduced by enzymatic deglycosylation. By contrast, proteolytic removal of a small (M _r 1000–4000) fragment from the chain resulted in the generation of a M _r 36 000 fragment, common to all the molecular mass variants. Unlike the parent proteins, the M _r 36 000 fragment derived from isolated variants yielded identical, simple patterns in two-dimensional gel electrophoresis and identical finger prints in peptide mapping. This, together with N-terminal amino acid sequencing, as well as comparison of hydrophobicity properties of fragments obtained by gradual proteolytic digestion, indicated that the small peptide responsible for the major B-F heterogeneity was situated in the intracellular, C-terminal part.     

Sensitized photooxidation of low spin horseradish peroxidase.

Horseradish peroxidase differs from most enzymes in that it is almost completely resistant to photodynamic action due to the paramagnetic ferric ion in the prosthetic group, heme. Chelation of horseradish peroxidase at the sixth coordination position of the iron with a cyanide or hydroxyl group converts it to a low spin diamagnetic state. Upon illumination with visible light with eosin Y, flavin mononucleotide or methylene blue as sensitizer, the low spin enzyme lost both peroxidative and oxidative activities with the same quantum yields. Several amino acid residues, including one histidine and one tyrosine were destroyed in the low spin enzyme after 60 min of illumination with eosin Y as sensitizer.     

Primary structure of yeast cytochrome c peroxidase. I. Chemical characterization of the polypeptide chain and of tryptic and chymotryptic peptides

The amino acid composition of yeast cytochrome c peroxidase was determined and calculated assuming that the enzyme contained one protoheme per molecule. On the basis of the amino acid composition and heme content the minimum M_r was calculated to be 35,235. Gel electrophoresis in the presence of sodium dodecyl sulfate indicated the presence of a single polypeptide chain with a M_r of approximately 33,000. A single cysteinyl residue present in the molecule was shown to be resistant against reaction with iodoacetic acid in the native form of both holo- and apo-enzymes, but readily modified with the reagent in the denatured form. Automated Edman degradation yielded an aminoterminal sequence of 11 residues beginning with threonine. Twenty-eight tryptic and 47 chymotryptic peptides were isolated from the carboxymethylated apoprotein and subjected to the sequence analysis by the dansyl-Edman method. The results with these peptides confirmed and extended the amino- and carboxyl-terminal sequences and in addition provided a partial sequence covering approximately 90% of the polypeptide chain.     

The control of protein synthesis by hemin in rabbit reticulocytes

Summary The control of protein synthesis by hemin in rabbit reticulocytes or lysates is mediated by the formation of a high molecular weight protein inhibitor of polypeptide chain initiation termed the hemin-controlled translational repressor (HCR). HCR becomes activated in the absence of hemin from a presynthesized precursor (prorepressor) in a manner that is still unclear but appears to involve a series of discrete conformational changes in a single protein. At a very early stage of activation, HCR (reversible) can be inactivated by hemin, at a somewhat later stage (intermediate HCR) it can still be inactivated in a GTP-dependent reaction by a soluble lysate protein termed the supernatant factor, and after more than several hours of warming, HCR (irreversible) can no longer be inactivated. Formation of HCR involves no detectable change in molecular size but may involve, directly or indirectly, disulfide bond formation or interchange, since activation occurs very rapidly in the presence of such sulfhydryl reagents as N-ethylmaleimide. Once activated, HCR (all three forms) acts by phosphorylating the 35,000 M_r () subunit of eIF-2, the initiation factor that mediates binding of Met-tRNA_f to 40 s ribosomal subunits. The protein kinase action of HCR is relatively specific for eIF-2, although HCR also autophosphorylates a 90–100,000 M_r component of itself. While most of the protein synthsized by rabbit reticulocytes is globin, the synthesis, at low levels, of other reticulocyte proteins is also reduced by HCR, consistent with its action on eIF-2, a factor that acts in initiation before mRNA is bound. At present, the mechanism by which phosphorylation of eIF-2 by HCR causes inhibition of polypeptide chain initiation is only partially understood. There is general agreement that the binding of Met-tRNA_f to 40 s ribosomal subunits is reduced, perhaps due to impaired interaction of eIF-2-P with other ribosomal protein components. There is also evidence that HCR causes the accumulation of 48 s intermediate initiation complexes, containing a 40 s ribosomal subunit, mRNA, and tRNA_f ^met that is largely deacylated. This suggests that the joining of 48 s complexes with 60 s subunits to form 80 s initiation complexes is also blocked and results in the deacylation of subunit-bound Met-tRNA_f. Additional work will be required to delineate the precise molecular mechanisms by which HCR becomes activated in the absence of hemin and how the phosphorylation of eIF-2 interrupts the process of polypeptide chain initiation.Abbreviations HCR hemin-controlled translational repressor - eIF eukaryotic initiation factor     

Elucidation of the Molecular Basis for Arabinoxylan-Debranching Activity of a Thermostable Family GH62 α-l-Arabinofuranosidase from Streptomyces thermoviolaceus

Xylan-debranching enzymes facilitate the complete hydrolysis of xylan and can be used to alter xylan chemistry. Here, the family GH62 α-l-arabinofuranosidase from Streptomyces thermoviolaceus (SthAbf62A) was shown to have a half-life of 60 min at 60°C and the ability to cleave α-1,3 l-arabinofuranose (l-Araf) from singly substituted xylopyranosyl (Xylp) backbone residues in wheat arabinoxylan; low levels of activity on arabinan as well as 4-nitrophenyl α-l-arabinofuranoside were also detected. After selective removal of α-1,3 l-Araf substituents from disubstituted Xylp residues present in wheat arabinoxylan, SthAbf62A could also cleave the remaining α-1,2 l-Araf substituents, confirming the ability of SthAbf62A to remove α-l-Araf residues that are (1→2) and (1→3) linked to monosubstituted β-d-Xylp sugars. Three-dimensional structures of SthAbf62A and its complex with xylotetraose and l-arabinose confirmed a five-bladed β-propeller fold and revealed a molecular Velcro in blade V between the β1 and β21 strands, a disulfide bond between Cys27 and Cys297, and a calcium ion coordinated in the central channel of the fold. The enzyme-arabinose complex structure further revealed a narrow and seemingly rigid l-arabinose binding pocket situated at the center of one side of the β propeller, which stabilized the arabinofuranosyl substituent through several hydrogen-bonding and hydrophobic interactions. The predicted catalytic amino acids were oriented toward this binding pocket, and the catalytic essentiality of Asp53 and Glu213 was confirmed by site-specific mutagenesis. Complex structures with xylotetraose revealed a shallow cleft for xylan backbone binding that is open at both ends and comprises multiple binding subsites above and flanking the l-arabinose binding pocket.     

Ultrastructural localization of the maltose-binding protein within the cell envelope of Escherichia coli

Logarithmically growing cells of Escherichia coli were fixed with glutaraldehyde and incubated with antimaltose-binding protein F_ab coupled to horseradish peroxidase (molecular weight of the complex 80,000). The position of this complex within the cell envelope was determined by reacting with diaminobenzidine-H₂O₂, staining with osmium tetroxide and processing for thin section electron microscopy. The following observations were made: (i) induction of the maltose-binding protein resulted in swelling and staining of the outer membrane; (ii) the swelling and staining was more prominent in short cells, less prominent or absent in long cells; (iii) rare examples exhibited granular staining in the space between the plasma membrane and the peptidoglycan layer. These stainings were observable mainly in pole caps; (iv) a mutant lacking the receptor for phage showed altered staining pattern. Treatment of glutaraldehyde-fixed cells with EDTA-lysozyme prevented the specific labelling of the maltose-binding protein.Lists of Non Common Abbreviations MBP maltose-binding protein - MBP-F_ab)-HRPO F_ab fragments against maltose-binding, protein coupled to horseradish peroxidase - IgG immunoglobulin - PBS pnosphate buffered saline