Effect of exogenous phytase on degradation of inositol phosphate in dairy cows.

The effect of exogenous phytase on inositol phosphate degradation in the rumen of dairy cows was investigated in a 4 × 4 Latin square design. Four lactating Danish Holstein cows fitted with ruminal, duodenal, and ileal cannulas were offered a total mixed ration (TMR) with a high content of inositol phosphate and supplemented with 1 of 4 concentrations of phytase [none, low, medium, or high, corresponding to 23, 2,023, 3,982, and 6,015 phytase units/kg of dry matter (DM)]. Exogenous phytase lead to a higher rumen pool of phytase. Inositol phosphate content in digesta samples from rumen, duodenum, ileum, and feces was almost entirely composed of myo-inositol hexakisphosphate (InsP(6)), indicating that degradation of this compound is the rate-limiting step in inositol phosphate degradation in the digestive tract. Ruminal and total-tract degradations of InsP(6) were higher when exogenous phytase was added to the TMR. Degradation of InsP(6) occurred mainly before the duodenum. The ruminal degradability of InsP(6) was increased with increasing dietary concentrations of phytase: 86.4, 93.7, 94.5, and 96.3% for none, low, medium, or high, respectively. A comparison of the InsP(6) content in individual feedstuffs and in samples of the TMR revealed that the exogenous phytase started degrading the inositol phosphate when feeds and phytase were mixed, and thus the InsP(6) phosphorus (InsP(6)-P) content in the TMR was found to decrease with higher doses of phytase (1.69, 1.51, 1.39, and 1.25 g/kg of DM for the none, low, medium, and high phytase doses, respectively). It was not possible to distinguish between the degradation of inositol phosphate occurring in the TMR and in the rumen. Exogenous phytase had no effect on total P intake or flow of total P to the duodenum and ileum, whereas exogenous phytase increased flow of microbial P to the duodenum and total fecal P excretion. None of the investigated rumen variables (pH, degradability of neutral detergent fiber, and rumen kinetics for neutral detergent fiber) were affected by treatment. Rumen and total-tract degradations of inositol phosphate were increased when exogenous phytase was added to the TMR, which offers the potential for reducing P excretion through reduced dietary P.

Chemical and thermal unfolding of calreticulin.

Calreticulin is a soluble endoplasmic reticulum chaperone, which has a relatively low melting point due to its remarkable structure with a relatively high content of flexible structural elements. Using far ultraviolet circular dichroism (CD) spectroscopy and a fluorescent dye binding thermal shift assay, we have investigated the chemical and thermal stability of calreticulin. When the chemical stability of calreticulin was assessed, a midpoint for calreticulin unfolding was calculated to 3.0M urea using CD data at 222 nm. Using the fluorescent dye binding thermal shift assay, calreticulin was found to obtain a molten structure in urea concentrations between 1-1.5 M urea, and to unfold/aggregate at high and low pH values. The results demonstrated that the fluorescent dye binding assay could measure the thermal stability of calreticulin in aqueous buffers with results comparable to melting points obtained by other techniques.

Toxicological studies on a novel phytase expressed from synthetic genes in Aspergillus oryzae.

Phytases are widely used as feed additives for monogastric animals, which cannot easily utilise the phosphorus bound in phytate (myo-inositol hexakisphosphate). The current study presents a safety evaluation of a 6-phytase produced by an Aspergillus oryzae strain expressing two synthetic genes, both mimicking a phytase gene from a Citrobacter braakii strain. Oral administration of the phytase preparation to rats at a dose level of 0.86 g total organic solids/kg body weight/day for 13 weeks did not cause any adverse effect. The phytase preparation did not exhibit irritative potential when applied locally to the eyes of rabbits or when applied to the skin using the in vitro three-dimensional epidermis model of adult human-derived epidermal keratinocytes. Furthermore, the phytase preparation was found not to represent mutagenic or clastogenic potential in the bacterial reverse mutation assay and in the in vitro micronucleus assays. Based on the toxicological data, the large safety factors calculated under common recommended dose assumptions for broiler chickens and weaned piglets, and the fact that Aspergillus oryzae is considered a safe strain lineage, it is concluded that there are no reasons for safety concerns when using this phytase as a feed additive.

Effect of exogenous phytase on feed inositol phosphate hydrolysis in an in vitro rumen fluid buffer system.

Three in vitro experiments using a rumen fluid buffer system were performed to investigate the effect of addition of 4 experimental phytases (Phy1, Phy2, Phy3, and Phy4) compared with no addition of phytase on feed inositol phosphate hydrolysis in wheat and rapeseed cake to determine which of the 4 phytases was most suitable under rumen-like conditions. The feedstuffs were incubated with a mixture of physiological buffer, ruminal fluid, and exogenous phytase at pH 6.2, after which the samples were incubated for different periods. Incubations were stopped using HCl, and the samples were analyzed for inositol phosphates via high performance ion chromatography. Addition of phytase (Phy1) resulted in enhanced degradation of myo-inositol hexakisphosphate (InsP(6)) in rapeseed cake, whereas addition of exogenous phytase did not improve the degradation of InsP(6) in wheat. Only rapeseed cake was therefore used subsequently. All 4 phytases increased degradation of InsP(6) in rapeseed cake in the in vitro system, and degradability of InsP(6) increased with higher incubation time and higher phytase dosages, independent of phytase. Addition of 2 units of phytase per gram of substrate of the phytases Phy1, Phy2, Phy3, and Phy4 led to an undegraded InsP(6) content of 56, 49, 70, and 18%, respectively, when incubated with rapeseed cake for 6h, indicating that Phy2 and Phy4 were the most effective phytases. However, Phy2 had a higher specific activity than Phy4, as 60% of the original InsP(6) content was remaining after 3h when 5mg of enzyme protein per gram of substrate of Phy2 was added to rapeseed cake, whereas 150 mg of enzyme protein per gram of substrate of Phy4 was necessary to achieve a similar result. Therefore, Phy2 appeared to be most applicable under rumen-like conditions.

Interrelationship of steric stabilization and self-crowding of a glycosylated protein.

In the eukaryotic cell, protein glycosylation takes place in the crowded environment of the endoplasmatic reticulum. With the purpose of elucidating the impact of high concentration on the interactions of glycoproteins, we have conducted a series of small-angle x-ray scattering experiments on the heavily glycosylated enzyme Peniophora lycii phytase (Phy) and its deglycosylated counterpart (dgPhy). The small-angle x-ray scattering data were analyzed using an individual numerical form factor for each of the two glycoforms combined with two structure factors, a hard sphere and a screened coulomb potential structure factor, respectively, as determined by ab initio analysis. Based on this data analysis, three main conclusions could be drawn. First, at comparable protein concentrations (mg/ml), the relative excluded volume of Phy was approximately 75% higher than that of dgPhy, showing that the glycans significantly increase excluded-volume interactions. Second, the relative excluded volume of dgPhy increased with concentration, as expected; however, the opposite effect was observed for Phy, where the relative excluded volume decreased in response to increasing protein concentration. Third, a clear difference in the effect of salinity on the excluded-volume interactions was observed between the two glycol forms. Although the relative excluded volume of dgPhy decreased with increasing ionic strength, the relative excluded volume of Phy was basically insensitive to increased salinity. We suggest that protrusion forces from the glycans contribute to steric stabilization of the protein, and that glycosylation helps to sustain repulsive electrostatic interactions under crowded conditions. In combination, this aids in stabilizing high concentrations of glycosylated proteins.

Crystal structures of amylosucrase from Neisseria polysaccharea in complex with D-glucose and the active site mutant Glu328Gln in complex with the natural substrate sucrose.

The structure of amylosucrase from Neisseria polysaccharea in complex with beta-D-glucose has been determined by X-ray crystallography at a resolution of 1.66 A. Additionally, the structure of the inactive active site mutant Glu328Gln in complex with sucrose has been determined to a resolution of 2.0 A. The D-glucose complex shows two well-defined D-glucose molecules, one that binds very strongly in the bottom of a pocket that contains the proposed catalytic residues (at the subsite -1), in a nonstrained (4)C(1) conformation, and one that binds in the packing interface to a symmetry-related molecule. A third weaker D-glucose-binding site is located at the surface near the active site pocket entrance. The orientation of the D-glucose in the active site emphasizes the Glu328 role as the general acid/base. The binary sucrose complex shows one molecule bound in the active site, where the glucosyl moiety is located at the alpha-amylase -1 position and the fructosyl ring occupies subsite +1. Sucrose effectively blocks the only visible access channel to the active site. From analysis of the complex it appears that sucrose binding is primarily obtained through enzyme interactions with the glucosyl ring and that an important part of the enzyme function is a precise alignment of a lone pair of the linking O1 oxygen for hydrogen bond interaction with Glu328. The sucrose specificity appears to be determined primarily by residues Asp144, Asp394, Arg446, and Arg509. Both Asp394 and Arg446 are located in an insert connecting beta-strand 7 and alpha-helix 7 that is much longer in amylosucrase compared to other enzymes from the alpha-amylase family (family 13 of the glycoside hydrolases).

Amylosucrase, a glucan-synthesizing enzyme from the alpha-amylase family.

Amylosucrase (E.C. 2.4.1.4) is a member of Family 13 of the glycoside hydrolases (the alpha-amylases), although its biological function is the synthesis of amylose-like polymers from sucrose. The structure of amylosucrase from Neisseria polysaccharea is divided into five domains: an all helical N-terminal domain that is not similar to any known fold, a (beta/alpha)(8)-barrel A-domain, B- and B'-domains displaying alpha/beta-structure, and a C-terminal eight-stranded beta-sheet domain. In contrast to other Family 13 hydrolases that have the active site in the bottom of a large cleft, the active site of amylosucrase is at the bottom of a pocket at the molecular surface. A substrate binding site resembling the amylase 2 subsite is not found in amylosucrase. The site is blocked by a salt bridge between residues in the second and eight loops of the (beta/alpha)(8)-barrel. The result is an exo-acting enzyme. Loop 7 in the amylosucrase barrel is prolonged compared with the loop structure found in other hydrolases, and this insertion (forming domain B') is suggested to be important for the polymer synthase activity of the enzyme. The topology of the B'-domain creates an active site entrance with several ravines in the molecular surface that could be used specifically by the substrates/products (sucrose, glucan polymer, and fructose) that have to get in and out of the active site pocket.

Crystallization and preliminary X-ray studies of recombinant amylosucrase from Neisseria polysaccharea.

Recombinant amylosucrase from Neisseria polysaccharea was crystallized by the vapour-diffusion procedure in the presence of polyethylene glycol 6000. The crystals belong to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 95.7, b = 117.2, c = 62.1 A, and diffract to 1.6 A resolution. A p-chloromercuribenzene sulfonate (pcmbs) derivative has been identified and a selenomethionine-substituted protein has been produced and crystallized.

Photoinduced electron transfer in singly labeled thiouredopyrenetrisulfonate azurin derivatives.

A novel method for the initiation of intramolecular electron transfer reactions in azurin is reported. The method is based on laser photoexcitation of covalently attached thiouredopyrenetrisulfonate (TUPS), the reaction that generates the low potential triplet state of the dye with high quantum efficiency. TUPS derivatives of azurin, singly labeled at specific lysine residues, were prepared and purified to homogeneity by ion exchange HPLC. Transient absorption spectroscopy was used to directly monitor the rates of the electron transfer reaction from the photoexcited triplet state of TUPS to Cu(II) and the back reaction from Cu(I) to the oxidized dye. For all singly labeled derivatives, the rate constants of copper ion reduction were one or two orders of magnitude larger than for its reoxidation, consistent with the larger thermodynamic driving force for the former process. Using 3-D coordinates of the crystal structure of Pseudomonas aeruginosa azurin and molecular structure calculation of the TUPS modified proteins, electron transfer pathways were calculated. Analysis of the results revealed a good correlation between separation distance from donor to Cu ligating atom (His-N or Cys-S) and the observed rate constants of Cu(II) reduction.

Human ceruloplasmin. Intramolecular electron transfer kinetics and equilibration.

Pulse radiolytic reduction of disulfide bridges in ceruloplasmin yielding RSSR(-) radicals induces a cascade of intramolecular electron transfer (ET) processes. Based on the three-dimensional structure of ceruloplasmin identification of individual kinetically active disulfide groups and type 1 (T1) copper centers, the following is proposed. The first T1 copper(II) ion to be reduced in ceruloplasmin is the blue copper center of domain 6 (T1A) by ET from RSSR(-) of domain 5. The rate constant is 28 +/- 2 s(-1) at 279 K and pH 7.0. T1A is in close covalent contact with the type 3 copper pair and indeed electron equilibration between T1A and the trinuclear copper center in the domain 1-6 interface takes place with a rate constant of 2.9 +/- 0.6 s(-1). The equilibrium constant is 0.17. Following reduction of T1A Cu(II), another ET process takes place between RSSR(-) and T1B copper(II) of domain 4 with a rate constant of 3.9 +/- 0.8. No reoxidation of T1B Cu(I) could be resolved. It appears that the third T1 center (T1C of domain 2) is not participating in intramolecular ET, as it seems to be in a reduced state in the resting enzyme.

Electron tunneling in azurin: the coupling across a beta-sheet.

BACKGROUND: We would like to understand how electron flow is controlled in biological molecules. Standard theories calculate the rate for long distance electron transfer (ET) as the product of electronic coupling (the square of the electron tunneling matrix element) and nuclear (Franck-Condon) factors. Much attention has been directed to the role of protein secondary and tertiary structure in the tunneling coupling, focusing on the interplay between different types of chemical bonds. Here we have evaluated the relative contributions of covalent bonds, hydrogen bonds and through-space jumps in coupling through a beta-strand or across a beta-sheet section of a blue copper protein, azurin. RESULTS: We have analyzed four distant electronic couplings in azurin. Each coupling is between the copper atom and a Ru(bpy)2(im) complex attached to a histidine on the protein surface. In three experiments the intervening medium was a simple beta-strand, while in the fourth experiment it was a section of beta-sheet. CONCLUSIONS: We have shown that electron tunneling in a protein can be broken down into ET 'tubes' of pathways through specific covalent and hydrogen bonds. These ET tubes encapsulate trivial interference effects and can expose crucial inter-tube interference effects. In coupling through a beta-sheet, hydrogen bonds are as important as covalent links, and are the primary source for tube interference.

Electron transfer in ruthenium-modified proteins.

Photochemical techniques have been used to measure the kinetics of intramolecular electron transfer in Ru(bpy)2(im)(His)2(+)-modified (bpy = 2,2'-bipyridine; im = imidazole) cytochrome c and azurin. A driving-force study with the His33 derivatives of cytochrome c indicates that the reorganization energy (lambda) for Fe2+-->Ru3+ ET reactions is 0.8 eV. Reductions of the ferriheme by either an excited complex, *Ru2+, or a reduced complex, Ru+, are anomalously fast and may involve formation of an electronically excited ferroheme. The distance dependence of Fe2+-->Ru3+ and Cu+-->Ru3+ electron transfer in 12 different Ru-modified cytochromes and azurins has been analyzed using a tunneling-pathway model. The ET rates in 10 of the 12 systems exhibit an exponential dependence on metal-metal separation (decay constant of 1.06 A-1) that is consistent with prediction of the pathway model.

Intramolecular electron transfer in single-site-mutated azurins.

Single-site mutants of the blue, single-copper protein, azurin, from Pseudomonas aeruginosa were reduced by CO2- radicals in pulse radiolysis experiments. The single disulfide group was reduced directly by CO2- with rates similar to those of the native protein [Farver, O., & Pecht, I. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6968-6972]. The RSSR- radical produced in the above reaction was reoxidized in a slower intramolecular electron-transfer process (30-70 s-1 at 298 K) concomitant with a further reduction of the Cu(II) ion. The temperature dependence of the latter rates was determined and used to derive information on the possible effects of the mutations. The substitution of residue Phe114, situated on the opposite side of Cu relative to the disulfide, by Ala resulted in a rate increase by a factor of almost 2. By assuming that this effect is only due to an increase in driving force, lambda = 135 kJ mol-1 for the reorganization energy was derived. When Trp48, situated midway between the donor and the acceptor, was replaced by Leu or Met, only a small change in the rate of intramolecular electron transfer was observed, indicating that the aromatic residue in this position is apparently only marginally involved in electron transfer in wild-type azurin. Pathway calculations also suggest that a longer, through-backbone path is more efficient than the shorter one involving Trp48. The former pathway yields an exponential decay factor, beta, of 6.6 nm-1. Another mutation, raising the electron-transfer driving force, was produced by changing the Cu ligand Met121 to Leu, which increases the reduction potential by 100 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron self-exchange in azurin: calculation of the superexchange electron tunneling rate.

Electronic coupling between the copper atoms in an azurin dimer has been calculated in this conformationally well-defined system by using many-electronic wave functions. When one of the two water molecules forming intermolecular hydrogen bonds between the copper-ligating His-117 of the two azurins is removed, the calculated coupling element is reduced from 2.5 x 10(-6) to 1.1 x 10(-7) eV (1 eV = 1.602 x 10(-19) J). Also, the effects of the relative orientations of the two water molecules have been analyzed. The results show that water molecules may play an important role as switches for biological electron transfer. The rate of electron self-exchange between two azurins has been calculated, and the result is in very good agreement with the rate found experimentally.

The binding of a complex cobalt ruthenium polyamine by deoxyribonucleic acid and a lipopolysaccharide: a model for a novel class of drugs.

In this paper we discuss the following: 1. Synthesis of [Co(H3CsarNHCH2pyRu(NH3)5)] (PF6)5, (CoRu). 2. Interaction of CoRu with calf thymus DNA and with lipopolysaccharide from Escherichia coli C (LPS) has been estimated using the absorption of the complex at 242 and 420 nm. 3. DNA and LPS increase the rate of fall of absorption at 420 nm due to autooxidation of the complex. 4. The fall in absorption of CoRu(II) at 420 nm can be used to give an approximate measure of binding to DNA and to LPS. 5. Both macromolecules are aggregated by CoRu at high concentrations and the cation and macromolecule complex can be removed by low speed centrifugation. 6. The DNA-CoRu complex can also be removed by high speed centrifugation when the cation concentration is too low to cause aggregation (20 microM CoRu/155 microM DNA-P). Absorption of redissolved complex at 420 nm is restored by reduction with ascorbic acid. 7. At saturation the ratio of mole CoRu bound/mole DNA-P is 0.16.

The effect of driving force on intramolecular electron transfer in proteins. Studies on single-site mutated azurins.

An intramolecular electron-transfer process has previously been shown to take place between the Cys3--Cys26 radical-ion (RSSR-) produced pulse radiolytically and the Cu(II) ion in the blue single-copper protein, azurin [Farver, O. & Pecht, I. (1989) Proc. Natl Acad. Sci. USA 86, 6868-6972]. To further investigate the nature of this long-range electron transfer (LRET) proceeding within the protein matrix, we have now investigated it in two azurins where amino acids have been substituted by single-site mutation of the wild-type Pseudomonas aeruginosa azurin. In one mutated protein, a methionine residue (Met44) that is proximal to the copper coordination sphere has been replaced by a positively charged lysyl residue ([M44K]azurin), while in the second mutant, another residue neighbouring the Cu-coordination site (His35) has been replaced by a glutamine ([H35Q]azurin). Though both these substitutions are not in the microenvironment separating the electron donor and acceptor, they were expected to affect the LRET rate because of their effect on the redox potential of the copper site and thus on the driving force of the reaction, as well as on the reorganization energies of the copper site. The rate of intramolecular electron transfer from RSSR- to Cu(II) in the wild-type P. aeruginosa azurin (delta G degrees = -68.9 kJ/mol) has previously been determined to be 44 +/- 7 s-1 at 298 K, pH 7.0. The [M44K]azurin mutant (delta G degrees = -75.3 kJ/mol) was now found to react considerably faster (k = 134 +/- 12 s-1 at 298 K, pH 7.0) while the [H35Q]azurin mutant (delta G degrees = -65.4 kJ/mol) exhibits, within experimental error, the same specific rate (k = 52 +/- 11 s-1, 298 K, pH 7.0) as that of the wild-type azurin. From the temperature dependence of these LRET rates the following activation parameters were calculated: delta H++ = 37.9 +/- 1.3 kJ/mol and 47.2 +/- 0.7 kJ/mol and delta S++ = -86.5 +/- 5.8 J/mol.K and -46.4 +/- 4.4 J/mol.K for [H35Q]azurin and [M44K]azurin, respectively. Using the Marcus relation for intramolecular electron transfer and the above parameters we have determined the reorganization energy, lambda and electronic coupling factor, beta. The calculated values fit very well with a through-bond LRET mechanism.