Tyrosine modification enhances metal-ion binding

Calcium-binding properties, stability, and osteogenic ability of phosphorylated soy peptide-calcium chelate

Introduction

Bioactive peptides based on foodstuffs are of particular interest as carriers for calcium delivery due to their safety and high activity. The phosphorylated peptide has been shown to enhance calcium absorption and bone formation.

Method

A novel complex of peptide phosphorylation modification derived from soybean protein was introduced, and the mechanism, stability, and osteogenic differentiation bioactivity of the peptide with or without calcium were studied.

Result

The calcium-binding capacity of phosphorylated soy peptide (SPP) reached 50.24 ± 0.20 mg/g. The result of computer stimulation and vibration spectrum showed that SPP could chelate with calcium by the phosphoric acid group, carboxyl oxygen of C-terminal Glu, Asp, and Arg, and phosphoric acid group of Ser on the SPP at a stoichiometric ratio of 1:1, resulting in the formation of the complex of ligand and peptide. Thermal stability showed that chelation enhanced peptide stability compared with SPP alone. Additionally, in vitro results showed that SPP-Ca could facilitate osteogenic proliferation and differentiation ability.

Discussion

SPP may function as a promising alternative to current therapeutic agents for bone loss.

Histidine phosphorylation in metalloprotein binding sites

Post-translational modifications (PTMs) are invaluable regulatory tools for the control of catalytic functionality, protein-protein interactions, and signaling pathways. Historically, the study of phosphorylation as a PTM has been focused on serine, threonine, and tyrosine residues. In contrast, the significance of mammalian histidine phosphorylation remains largely unexplored. This gap in knowledge regarding the molecular basis for histidine phosphorylation as a regulatory agent exists in part because of the relative instability of phosphorylated histidine as compared with phosphorylated serine, threonine and tyrosine. However, the unique metal binding abilities of histidine make it one of the most common metal coordinating ligands in nature, and it is interesting to consider how phosphorylation would change the metal coordinating ability of histidine, and consequently, the properties of the phosphorylated metalloprotein. In this review, we examine eleven metalloproteins that have been shown to undergo reversible histidine phosphorylation at or near their metal binding sites. These proteins are described with respect to their biological activity and structure, with a particular emphasis on how phosphohistidine may tune the primary coordination sphere and protein conformation. Furthermore, several common methods, challenges, and limitations of studying sensitive, high affinity metalloproteins are discussed.

Towards robust cell culture processes - Unraveling the impact of media preparation by spectroscopic online monitoring

Biopharmaceutical manufacturing processes can be affected by variability in cell culture media, e.g. caused by raw material impurities. Although efforts have been made in industry and academia to characterize cell culture media and raw materials with advanced analytics, the process of industrial cell culture media preparation itself has not been reported so far. Within this publication, we first compare mid-infrared and two-dimensional fluorescence spectroscopy with respect to their suitability as online monitoring tools during cell culture media preparation, followed by a thorough assessment of the impact of preparation parameters on media quality. Through the application of spectroscopic methods, we can show that media variability and its corresponding root cause can be detected online during the preparation process. This methodology is a powerful tool to avoid batch failure and is a valuable technology for media troubleshooting activities. Moreover, in a design of experiments approach, including additional liquid chromatography-mass spectrometry analytics, it is shown that variable preparation parameters such as temperature, power input and preparation time can have a strong impact on the physico-chemical composition of the media. The effect on cell culture process performance and product quality in subsequent fed-batch processes was also investigated. The presented results reveal the need for online spectroscopic methods during the preparation process and show that media variability can already be introduced by variation in media preparation parameters, with a potential impact on scale-up to a commercial manufacturing process.

Metal-dependent hormone function: the emerging interdisciplinary field of metalloendocrinology

For over 100 years, there has been an incredible amount of knowledge amassed concerning hormones in the endocrine system and their central role in human health. Hormones represent a diverse group of biomolecules that are released by glands, communicate signals to their target tissue, and are regulated by feedback loops to maintain organism health. Many disease states, such as diabetes and reproductive disorders, stem from misregulation or dysfunction of hormones. Increasing research is illuminating the intricate roles of metal ions in the endocrine system where they may act advantageously in concert with hormones or deleteriously catalyze hormone-associated disease states. As the critical role of metal ions in the endocrine system becomes more apparent, it is increasingly important to untangle the complex mechanisms underlying the connections between inorganic biochemistry and hormone function to understand and control endocrinological phenomena. This tutorial review harmonizes the interdisciplinary fields of endocrinology and inorganic chemistry in the newly-termed field of "metalloendocrinology". We describe examples linking metals to both normal and aberrant hormone function with a focus on highlighting insight to molecular mechanisms. Hormone activities related to both essential metal micronutrients, such as copper, iron, zinc, and calcium, and disruptive nonessential metals, such as lead and cadmium are discussed.

Transition Metal Ion-Mediated Tyrosine-Based Short-Peptide Amphiphile Nanostructures Inhibit Bacterial Growth

We report the design and synthesis of a biocompatible small-peptide-based compound for the controlled and targeted delivery of encapsulated bioactive metal ions through transformation of the internal nanostructures of its complexes. A tyrosine-based short-peptide amphiphile (sPA) was synthesized and observed to self-assemble into β-sheet-like secondary structures. The self-assembly of the designed sPA was modulated by application of different bioactive transition-metal ions, as was confirmed by spectroscopic and microscopic techniques. These bioactive metal-ion-conjugated sPA hybrid structures were further used to develop antibacterial materials. As a result of the excellent antibacterial activity of zinc ions the growth of clinically relevant bacteria such as Escherichia coli was inhibited in the presence of zinc⋅sPA conjugate. Bacterial testing demonstrated that, due to high biocompatibility with bacterial cells, the designed sPA acted as a metal ion delivery agent and might therefore show great potential in locally addressing bacterial infections.

Tyrosine modification increases the affinity of gastrin for ferric ions

The peptide hormone gastrin₁₇, which occurs naturally in both tyrosine sulphated and unsulphated forms, binds two ferric ions with pM affinities. The aim of this study was to investigate the hypothesis that sulphation or phosphorylation of gastrin₁₇ altered ferric ion binding, and/or affinity for the CCK1 or CCK2 receptor. To investigate the effect of tyrosine modification on ferric ion binding, the changes in absorbance of gastrin₁₇, gastrin₁₇SO₄ and gastrin₁₇PO₄ on addition of Fe³⁺ ions were monitored. Binding of gastrin₁₇, gastrin₁₇SO₄ and gastrin₁₇PO₄ to the human CCK1 and CCK2 receptors was assessed by competition with [¹²⁵I]-Bolton and Hunter-labelled cholecystokinin₈ in transiently transfected COS cells. Tyrosine sulphation or phosphorylation increased the affinity of gastrin₁₇ for the first ferric ion bound from 267 to 83 pM and 14 pM, respectively, but had no effect on the stoichiometry of ferric ion binding. In contrast the affinity of gastrin₁₇ for the second ferric ion bound was reduced from 94 pM to 7.32 µM and 671 nM, respectively. While sulphation of gastrin₁₇ increased its affinity for the CCK2 receptor approximately 50 fold, phosphorylation had no effect on receptor binding. These results demonstrate that tyrosine modification may have profound effects on the interaction of gastrins with ferric ions and with the CCK2 receptor.

Iron depletion results in Src kinase inhibition with associated cell cycle arrest in neuroblastoma cells

Iron is required for cellular proliferation. Recently, using systematic time studies of neuroblastoma cell growth, we better defined the G1 arrest caused by iron chelation to a point in mid-G1, where cyclin E protein is present, but the cyclin E/CDK2 complex kinase activity is inhibited. In this study, we again used the neuroblastoma SKNSH cells lines to pinpoint the mechanism responsible for this G1 block. Initial studies showed in the presence of DFO, these cells have high levels of p27 and after reversal of iron chelation p27 is degraded allowing for CDK2 kinase activity. The initial activation of CDK2 kinase allows cells to exit G1 and enter S phase. Furthermore, we found that inhibition of p27 degradation by DFO is directly associated with inhibition of Src kinase activity measured by lack of phosphorylation of Src at the 416 residue. Activation of Src kinase occurs very early after reversal from the DFO G1 block and is temporally associated with initiation of cellular proliferation associated with entry into S phase. For the first time therefore we show that iron chelation inhibits Src kinase activity and this activity is a requirement for cellular proliferation.

Activation of Src family tyrosine kinases by ferric ions

The Src-family tyrosine kinases (SFKs) are oncogenic enzymes that contribute to the initiation and progression of many types of cancer. In normal cells, SFKs are kept in an inactive state mainly by phosphorylation of a consensus regulatory tyrosine near the C-terminus (Tyr(530) in the SFK c-Src). As recent data indicate that tyrosine modification enhances binding of metal ions, the hypothesis that SFKs might be regulated by metal ions was investigated. The c-Src C-terminal peptide bound two Fe(3+) ions with affinities at pH4.0 of 33 and 252μM, and phosphorylation increased the affinities at least 10-fold to 1.4 and 23μM, as measured by absorbance spectroscopy. The corresponding phosphorylated peptide from the SFK Lyn bound two Fe(3+) ions with much higher affinities (1.2pM and 160nM) than the Src C-terminal peptide. Furthermore, when Lyn or Hck kinases, which had been stabilised in the inactive state by phosphorylation of the C-terminal regulatory tyrosine, were incubated with Fe(3+) ions, a significant enhancement of kinase activity was observed. In contrast Lyn or Hck kinases in the unphosphorylated active state were significantly inhibited by Fe(3+) ions. These results suggest that Fe(3+) ions can regulate SFK activity by binding to the phosphorylated C-terminal regulatory tyrosine.

Increased cholecystokinin labeling in the hippocampus of a mouse model of epilepsy maps to spines and glutamatergic terminals

The neuropeptide cholecystokinin (CCK) is abundant in the CNS and is expressed in a subset of inhibitory interneurons, particularly in their axon terminals. The expression profile of CCK undergoes numerous changes in several models of temporal lobe epilepsy. Previous studies in the pilocarpine model of epilepsy have shown that CCK immunohistochemical labeling is substantially reduced in several regions of the hippocampal formation, consistent with decreased CCK expression as well as selective neuronal degeneration. However, in a mouse pilocarpine model of recurrent seizures, increases in CCK-labeling also occur and are especially striking in the hippocampal dendritic layers of strata oriens and radiatum. Characterizing these changes and determining the cellular basis of the increased labeling were the major goals of the current study. One possibility was that the enhanced CCK labeling could be associated with an increase in GABAergic terminals within these regions. However, in contrast to the marked increase in CCK-labeled structures, labeling of GABAergic axon terminals was decreased in the dendritic layers. Likewise, cannabinoid receptor 1-labeled axon terminals, many of which are CCK-containing GABAergic terminals, were also decreased. These findings suggested that the enhanced CCK labeling was not due to an increase in GABAergic axon terminals. The subcellular localization of CCK immunoreactivity was then examined using electron microscopy, and the identities of the structures that formed synaptic contacts were determined. In pilocarpine-treated mice, CCK was observed in dendritic spines and these were proportionally increased relative to controls, whereas the proportion of CCK-labeled terminals forming symmetric synapses was decreased. In addition, CCK-positive axon terminals forming asymmetric synapses were readily observed in these mice. Double labeling with vesicular glutamate transporter 1 and CCK revealed colocalization in numerous terminals forming asymmetric synapses, confirming the glutamatergic identity of these terminals. These data raise the possibility that expression of CCK is increased in hippocampal pyramidal cells in mice with recurrent, spontaneous seizures.

Ferric ions inhibit proteolytic processing of progastrin

The gastrointestinal hormone gastrin is generated from an 80 amino acid precursor (progastrin) by cleavage after dibasic residues by prohormone convertase 1. Phosphorylation of Ser(75) has previously been suggested, on the basis of indirect evidence, to inhibit cleavage of progastrin after Arg(73)Arg(74). Gastrins bind two ferric ions with high affinity, and iron binding is essential for the biological activity of non-amidated gastrins in vitro and in vivo. This study directly investigated the effect of iron binding and of serine phosphorylation on the cleavage of synthetic progastrin-derived peptides. The affinity of synthetic progastrin(55-80) for ferric ions, and the rate of cleavage by prohormone convertase 1, were not affected by phosphorylation of Ser(75). In contrast, in the presence of ferric ions the rate of cleavage of both progastrin(55-80) and phosphoSer(75)progastrin(55-80) by prohormone convertase 1 was significantly reduced. Hence iron binding to progastrin may regulate processing and secretion in vivo, and regulation may be particularly important in diseases with altered iron homeostasis.

Phosphorylation of cyclin-dependent kinase 2 peptides enhances metal binding

The cyclin-dependent kinase CDK2 is inactivated by phosphorylation on either of the two neighbouring residues Thr14 or Tyr15. The effect of phosphorylation on metal ion binding has been investigated with peptides incorporating residues 6-20 of CDK2. The stoichiometry of Ca(2+) binding increased from 1 in the un- and singly-phosphorylated peptides to 2 in the doubly phosphorylated peptide, without large changes in the affinity (75-250 microM). In contrast although binding of ferric ions to the un-phosphorylated peptide was not detected, both singly- and doubly-phosphorylated peptides bound two Fe(3+) ions. Binding of Ca(2+) or Zn(2+) ions to the doubly phosphorylated CDK2 peptide did not cause any change in absorbance, but increased the affinity of the peptide for Fe(3+) ions. These results demonstrate that double phosphorylation of CDK2 peptides increases the stoichiometry of metal ion binding, and hence may contribute to the previously observed regulation of CDK2 activity by metal ions.