The catalytic activity of horse spleen apoferritin. Preliminary kinetic studies and the effect of chemical modification

1. Horse spleen apoferritin catalyses the oxidation of Fe(2+) to Fe(3+) with molecular O(2) as electron acceptor under conditions where a number of other proteins have no such effect. The product is similar to ferritin by a number of criteria. 2. The progress curve is hyperbolic and the increase in initial velocity is linear with increasing apoferritin concentration. With respect to Fe(2+) the reaction follows Michaelis-Menten kinetics. The pH-dependence of the reaction was determined between pH4.3 and 6.0. 3. Modification of both tryptophan residues/apoferritin subunit with 2-nitrophenylsulphenyl chloride does not affect either k(cat.) or K(m) for the oxidation. Neither does the guanidination of seven out of nine lysine residues/subunit, the modification of nine out of ten arginine residues/subunit with cyclohexanedione, or the nitration of one out of five tyrosine residues/subunit with tetranitromethane. 4. The carboxymethylation of two out of three cysteine residues/subunit and of one out of six histidine residues/subunit can be achieved with iodoacetic acid. This carboxymethylated apoferritin is completely inactive in Fe(2+) oxidation. 5. Apoferritin does not take up Fe(3+). It appears from these results that Fe(2+) is the form in which iron is taken up by ferritin in a reaction where the protein acts as an enzyme which traps the product in the interior of the protein shell.     

Mechanism of the self-assembly of apoferritin from horse spleen

Apoferritin from horse spleen can be reversibly dissociated at pH 2 or in 7.2 M G-HCl (pH 3.5). Reconstitution of the native icositetramer in 0.1 M TEA buffer (pH 7.9) in the presence of 1 mM EDTA and 3 mM dithioerythritol leads to yields higher than 80%. To monitor the kinetic mechanism, intrinsic fluorescence, far-UV circular dichroism, and covalent cross-linking with glutaraldehyde were applied.The overall mechanism of assembly is characterized by a sequence of concentration-dependent association reactions involving structured monomers and a dimeric intermediate as the most prominent species, apart from trimers and dodecamers. The parallel decrease in monomers, dimers and trimers indicates that association equilibria precede the formation of the final assembly product.The assembly reaction is accompanied by characteristic changes in fluorescence emission and dichroic absorption. To a first approximation, renaturation and reassociation may be quantitatively described by one single rate-determining second-order process, subsequent to fast folding steps at the monomer level.Abbreviations CD circular dichroism - DTE dithioerythritol - G-HCl guanidinium chloride - SDS sodium dodecylsulfate - TEA triethanolamine - Tris tris hydroxymethylamino methane Dedicated to Professor Harold A. Scheraga on the occasion of his 65^th birthday     

Ferroxidase kinetics of horse spleen apoferritin.

Protein ferroxidase site(s), which catalyze the reaction between ferrous ion and dioxygen, have long been thought to play a role in core formation in ferritin; however, the mechanism of the reaction has never been studied in detail. In the present work, the enzymatic activity of ferritin was examined using oximetry, the net Fe2+ oxidation reaction being as follows. [formula: see text] The reaction exhibits saturation kinetics with respect to both Fe2+ and O2 (apparent Michaelis constants: Km,Fe = 0.35 +/- 0.01 mM and Km,O2 = 0.14 +/- 0.03 mM). The enzyme has a turnover number kcat = 80 +/- 3 min-1 at 20 degrees C with maximal activity at pH 7. The kinetics are discussed in terms of two mechanisms, one involving monomeric and the other dimeric iron protein complexes. In both instances Fe(II) oxidation occurs in 1-electron steps. Zinc(II) is a competitive inhibitor of iron(II) oxidation at Zn2+/apoprotein ratios > or = 6 (inhibitor constant KI,Zn = 0.067 +/- 0.011 mM) but appears to be a noncompetitive inhibitor at lower ratios (< or = 2), indicating the presence of more than one type of zinc binding site on the protein. At increments of 50 Fe2+/protein or less, all of the iron is oxidized via the protein ferroxidase site(s), independent of the amount of core already present. However, when larger increments are employed, some iron oxidation appears to occur on the surface of the mineral core. The results of these studies emphasize the role of the protein shell in all phases of core growth and confirm the presence of a functionally important catalytic site in ferritin in addition to other binding sites on the protein for iron.     

A tetragonal crystal form of horse spleen apoferritin and its relationship to the cubic modification

Horse spleen apoferritin has been crystallized as tetragonal plates and needles with a unit cell with $\text{a = b = 147 ± 0.5}\text{A}\text{?}\text{and}\text{c = 154.4 ± 0.5}\text{A}\text{?}$. The space group is P42₁2 and the unit cell contains two molecules in a pseudo-body-centred arrangement. The intensity distributions and calculated rotation functions of tetragonal and cubic crystals have been compared. The symmetry of the diffraction patterns from cubic crystals indicates that the molecules have 432 symmetry with their 4-fold axes lying along the cube axes. In the tetragonal crystals one molecular 4-fold axis lies parallel to c, the unique axis, while the rest of the molecular point symmetry is not used by the lattice. Instead the remaining 4-fold axes of the two molecules, which lie in planes perpendicular to c, are rotated ± 17.5 ° with respect to the tetragonal a axis. The finding that apoferritin reassembled from subunits can be crystallized in both tetragonal and cubic forms confirms its conformational similarity to native molecules.     

Reversible modification of Escherichia coli ribosomes with 2,3-dimethylmaleic anhydride. A new method to obtain protein-deficient ribosomal particles.

Treatment of Escherichia coli ribosomes with the protein reagent 2,3-dimethylmaleic anhydride is accompanied by inactivation of polypeptide polymerization and by dissociation of ribosomal proteins. Regeneration of the modified amino groups at pH 6.0 is followed by reactivation and reconstitution of the ribosomes. Prior to regeneration of the amino groups, ribosomal particles and split proteins can be separated by centrifugation, which allows the preparation of new protein-deficient particles. The ribosomal particles obtained by three successive treatments with 2,3-dimethyl-maleic anhydride at a molar ratio of reagent to ribosome equal to 16,000 lack proteins S1, S2, S3, S5, S10, S13, S14, L7, L8, L10, L11, L12, and L20 and have lost part of proteins S4, L1, L6, L16, and L25. This new procedure to obtain protein-deficient ribosomal particles is mild and might be useful to dissociate other protein-containing structures in addition to ribosomes.     

Hydrogen ion interactions of horse spleen ferritin and apoferritin.

The interactions of horse spleen ferritin and its derivative apoferritin with H+ ions were studied by potentiometric and spectrophotometric titration; to aid in data analysis, heats of ionization over a limited pH range and amide content were also determined. Per apoferritin subunit, all tyrosine and cysteine side chains, two of the nine lysine side chains and at least three of the six histidine side chains were found not to titrate; a preliminary but self-consistent analysis of the titration data is proposed. The titration curve of ferritin was identical with that of apoferritin in the pH range 5.5 to 3. In addition, under the conditions used, the reactivities of ferritin histidines to bromoacetate and of ferritin lysines to formaldehyde were identical with those in apoferritin. Above pH 8, a time-dependent titration of the ferritin core occurs which prevents comparison of the titration curves of the two proteins in this region. However, in the pH regions 5.5 to 7.5, two extra groups per subunit titrate reversibly in ferritin relative to apoferritin. Moreover, although the isoionic points of ferritin and apoferritin are identical in water, the isoionic point of ferritin is 0.5 pH unit lower than that of apoferritin in 0.16 to 1 M KCl. The different effects of KCl and NaCl on the two proteins indicate the presence of cation binding sites in ferritin that are absent in apoferritin and possibly also the presence of anion binding sites in apoferritin that are occupied in ferritin by anions of the core. The difference between the isoionic points of the two proteins in KCl has been interpreted to indicate the presence of approximately 2 phosphate residues per ferritin subunit which serve as cation binding sites and which are negatively charged at the isoionic point in KCl. These phosphates may also represent the additional residues that titrate in ferritin between pH 5.5 and 7.5, or may interact with positively charged residues on the inner surface of the ferritin shell, or both.     

Structural investigation of the complexation properties between horse spleen apoferritin and metalloporphyrins

Crystallographic studies of L-chain horse spleen apoferritin (HSF) co-crystallized with Pt-hematoporphyrin IX and Sn-protoporphyrin IX have brought significant new insights into structure-function relationships in ferritins. Interactions of HSF with porphyrins are discussed. Structural results show that the nestling properties into HSF are dependent on the porphyrin moiety. (Only protoporphyrin IX significantly interacts with the protein, whereas hematoporphyrin IX does not.) These studies additionally point out the L-chain HSF ability to demetalate metalloporphyrins, a result which is of importance in looking at the iron storage properties of ferritins. In both compound investigated (whether the porphyrin reaches the binding site or not), the complexation appears to be concomitant with the extraction of the metal from the porphyrin. To analyze further the previous results, a three-dimensional alignment of ferritin sequences based on available crystallographic coordinates, including the present structures, is given. It confirms a high degree of homology between these members of the ferritin family and thus allows us to emphasize observed structural differences: 1) unlike L-chain HSF, H-chain human ferritin presents no preformed binding site; and 2) despite the absence of axial ligands, and due to the demetalation, L-chain HSF is able to host protoporphyrin at a similar location to that naturally found in bacterioferritin.     

Subunit interactions in horse spleen apoferritin. Dissociation by extremes of pH

1. The dissociation of horse spleen apoferritin as a function of pH was analysed by sedimentation-velocity techniques. The oligomer is stable in the range pH2.8-10.6. Between pH2.8 and 1.6 and 10.6 and 13.0 both oligomer and subunits can be detected. At pH values between 1.6 and 1.0 the subunit is the only species observed, although below pH1.0 aggregation of the subunits to a particle sedimenting much faster than the oligomer occurs. 2. When apoferritin is first dissociated into subunits at low pH values and then dialysed into buffers of pH1.5-5.0, the subunit reassociates to oligomer in the pH range 3.1-4.3. 3. U.v.-difference spectroscopy was used to study conformational changes occurring during the dissociation process. The difference spectrum in acid can be accounted for by the transfer of four to five tyrosine residues/subunit from the interior of the protein into the solvent. This process is reversed on reassociation, but shows the same hysteresis as found by sedimentation techniques. The difference spectrum in alkali is more complex, but is consistent with the deprotonation of tyrosine residues, which appear to have rather high pK values. 4. In addition to the involvement of tyrosine residues in the conformational change at low pH values, spectral evidence is presented that one tryptophan residue/subunit also changes its environment before dissociation and subsequent to reassociation. 5. Analysis of the dissociation and reassociation of apoferritin at low pH values suggests that this is a co-operative process involving protonation and deprotonation of at least two carboxyl functions of rather low intrinsic pK. The dissociation at alkaline pH values does not appear to be co-operative. 6. Of the five tyrosine residues/subunit only one can be nitrated with tetranitromethane. Guanidination of lysine residues results in the modification of seven out of a total of nine residues/subunit. Nine out of the ten arginine residues/subunit react with cyclohexanedione.     

Characterization of horse spleen apoferritin reactive lysines by MALDI-TOF mass spectrometry combined with enzymatic digestion

Ferritins are a class of iron storage protein spheres found mainly in the liver and spleen, which have attracted many research interests due to their unique structural features and biological properties. Recently, ferritin and apoferritin (ferritin devoid of the iron core), have been employed as chemically addressable nanoscale building blocks for functional materials development. However, the reactive residues of apoferritin or ferritin have never been specified and it is still unclear about the chemoselectivity of apoferritin towards different kinds of bioconjugation reagents. In this work, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry combined with enzymatic digestion analysis was used to identify the reactive lysine residues of horse spleen apoferritin when conjugated with N-hydroxysuccinimide reagents. The result demonstrated that among all the lysine residues, K97, K83, K104, K67 and K143 are the reactive ones that can be addressed.     

Protein-deficient ribosomal particles obtained by reversible modification with dimethylmaleic anhydride

Preparation of protein-deficient ribosomal particles from Escherichia coli ribosomes by reversible modification of protein amino groups with dimethylmaleic anhydride (J. A. Pintor-Toro, D. Vázquez, and E. Palacián, 1979, Biochemistry18, 3219) is accompanied by degradation of r-RNA and reagent-independent inactivation. Alternative conditions to regenerate the modified amino groups have been found, which reduce the time needed to prepare the ribosomal “cores” from 9 to 3 days, and prevent RNA degradation and inactivation. The ribosomal particles obtained from 70 S ribosomes and 50 S subunits by this modified procedure show no extensive degradation of RNA and very little reagent-independent inactivation, which allow good recovery of the polypeptide synthesizing activity when incubated with the corresponding split proteins.     

Assembly of apoferritin from horse spleen: comparison of the protein in its native and reassembled state

The in vitro selfassembly of apoferritin after previous dissociation and unfolding in 7.2M guanidinium chloride, pH 3.5, yields up to 80% of a protein complex exhibiting the molecular mass of the native icositetramer of greater than or equal to 450 kDa. After removal of high molecular mass byproducts, the final reassembly product proves to be indistinguishable from native apoferritin with respect to its functional and conformational properties. These refer to the intrinsic fluorescence and to the far and near UV circular dichroism. The unfolding transitions of the native and reassembled protein in aqueous guanidinium chloride or at acid pH coincide within the range of error. The reassembled protein is also able to catalyze the oxidation of Fe(II). Higher polymers of the apoferritin complex represent most of the residual 20% of the reconstituted protein. They are stabilized by non-covalent (preferentially hydrophobic) interactions, and may be disassembled to the icositetramer by preferential solvation of the protein in the presence of less than or equal to 50% (v/v) ethylene glycol. The change in fluorescence emission accompanying polymerization reflects altered surface properties of the apoferritin subunits compatible with those reported for the ferritin----hemosiderin transition.     

On the mechanism of horse spleen apoferritin assembly: a sedimentation velocity and circular dichroism study

The apoferritin shell is known to assemble spontaneously from its subunits obtained at acid pH upon neutralization. The reassembly of apoferritin from horse spleen has been followed by means of sedimentation velocity and circular dichroism experiments as a function of the pH and the nature of the assembly buffer in order to obtain information on the assembly pathway. In all the buffer systems tested the subunits sediment as a single peak of varying sedimentation and diffusion coefficients, and shell assembly starts at pH values around 3.5. In dilute glycine-acetate buffers the subunits are essentially dimeric up to this pH value. Therefore, the dimeric building blocks of the apoferritin shell that are apparent in the X-ray structure represent the first assembly intermediates. When the pH is increased to 4.0-4.3, the weight-average sedimentation velocity of the subunits increases to 3.6-4.7 S, respectively, and the subunit population becomes heterogeneous. Concomitantly, significant changes in the circular dichroism properties of the aromatic residues take place. On the basis of the X-ray structure, where aromatic residues appear to be located at or near the fourfold symmetry axes, these data suggest that assembly proceeds from dimers through tetramers and octamers. In the pH range 4.5-6.5 the reassembly process cannot be followed due to reversible precipitation of the subunits near their isoelectric point; at neutral pH values essentially quantitative reassembly is obtained.(ABSTRACT TRUNCATED AT 250 WORDS)     

Properties of the abnormal human plasma lipoprotein (LP-X) characteristic of cholestasis after chemical modification with succinic anhydride

The abnormal human low-density lipoprotein class characteristic of biliary obstruction (LP-X) was reacted with [¹⁴C]succinic anhydride to an extent of 70–80 moles of succinyl groups incorporated per 10⁵ g of LP-X protein. The modified lipoprotein retained the typical morphology and ultracentrifugal flotation and sedimentation properties of LP-X but failed to react with antiserum to the native lipoprotein. On agar and agarose gel electrophoresis the succinylated lipoprotein had an increased mobility toward the anode relative to LP-X, as a result of the increased negative charge on the protein component.Partial delipidation of succinylated LP-X and ultracentrifugal fractionation of the protein into a fraction containing phospholipids plus at least three relatively small proteins (Apo-X) and an essentially lipid-free protein, chemically similar and immunologically identical with albumin, permitted us to evaluate the extent of reaction of these two protein classes with succinic anhydride in intact LP-X. On the average, the Apo-X fraction had 72 moles of succinyl groups incorporated per 10⁵ g of protein, whereas the albumin fraction incorporated 55 moles per 10⁵ g of protein.Extensive reaction of susceptible amino acid residues (mostly lysines) with succinic anhydride, without disruption of the lipoprotein structure, indicates that these protein groups are accessible to the reagent and are not involved in critical protein-lipid interactions. Elimination of immunoreactivity upon succinylation of LP-X implies that, at least for the Apo-X component, lysine residues participate in the interaction with LP-X antibodies. Also, the present results strongly support the view that albumin is not merely adsorbed to LP-X, and suggest, furthermore, that protein-protein interactions are not directly responsible for the characteristic stacking of LP-X discs as seen in the electron microscope.