Copper(II) Affects the Biochemical Behavior of Proinsulin C-peptide by Forming Ternary Complexes with Serum Albumin

Peptide hormones are essential signaling molecules with therapeutic importance. Identifying regulatory factors that drive their activity gives important insight into their mode of action and clinical development. In this work, we demonstrate the combined impact of Cu(II) and the serum protein albumin on the activity of C-peptide, a 31-mer peptide derived from the same prohormone as insulin. C-peptide exhibits beneficial effects, particularly in diabetic patients, but its clinical use has been hampered by a lack of mechanistic understanding. We show that Cu(II) mediates the formation of ternary complexes between albumin and C-peptide and that the resulting species depend on the order of addition. These ternary complexes notably alter peptide activity, showing differences from the peptide or Cu(II)/peptide complexes alone in redox protection as well as in cellular internalization of the peptide. In standard clinical immunoassays for measuring C-peptide levels, the complexes inflate the quantitation of the peptide, suggesting that such adducts may affect biomarker quantitation. Altogether, our work points to the potential relevance of Cu(II)-linked C-peptide/albumin complexes in the peptide's mechanism of action and application as a biomarker.

Investigation of metal modulation of oxytocin structure receptor-mediated signaling

Oxytocin is a 9-amino acid peptide hormone. Since its discovery in 1954, it has most commonly been studied in relation to its role in stimulating parturition and lactation. However, it is now known that oxytocin has a widely diverse set of functions throughout the body including neuromodulation, bone growth, and inflammation. Previous research has suggested that divalent metal ions may be required for oxytocin activity, but the exact metal species and specific pathways have yet to be fully elucidated. In this work, we focus on characterizing copper and zinc bound forms of oxytocin and related analogs through far-UV circular dichroism. We report that Cu(ii) and Zn(ii) bind uniquely to oxytocin and all analogs investigated. Furthermore, we investigate how these metal bound forms may affect downstream signaling of MAPK activation upon receptor binding. We find that both Cu(ii) and Zn(ii) bound oxytocin attenuates the activation of the MAPK pathway upon receptor binding relative to oxytocin alone. Interestingly, we observed that Zn(ii) bound forms of linear oxytocin facilitate increased MAPK signaling. This study lays the foundation for future work on elucidating the metal effects on oxytocin's diverse bioactivity.

Quantifying the Binding Interactions Between Cu(II) and Peptide Residues in the Presence and Absence of Chromophores

Copper(II) is an essential metal in biological systems, conferring unique chemical properties to the biomolecules with which it interacts. It has been reported to directly bind to a variety of peptides and play both necessary and pathological roles ranging from mediating structure to electron transfer properties to imparting catalytic function. Quantifying the binding affinity and thermodynamics of these Cu(II)-peptide complexes in vitro provides insight into the thermodynamic driving force of binding, potential competitions between different metal ions for the peptide or between different peptides for Cu(II), and the prevalence of the Cu(II)-peptide complex in vivo. However, quantifying the binding thermodynamics can be challenging due to a myriad of factors, including accounting for all competing equilibria within a titration experiment, especially in cases where there are a lack of discrete spectroscopic handles representing the peptide, the d-block metal ion, and their interactions. Here, a robust set of experiments is provided for the accurate quantification of Cu(II)-peptide thermodynamics. This article focuses on the use of electronic absorption spectroscopy in the presence and absence of chromophoric ligands to provide the needed spectroscopic handle on Cu(II) and the use of label-free isothermal titration calorimetry. In both experimental techniques, a process is described to account for all competing equilibria. While the focus of this article is on Cu(II), the described set of experiments can apply beyond Cu(II)-peptide interactions, and provide a framework for accurate quantification of other metal-peptide systems under physiologically relevant conditions.

Elucidation of a Copper Binding Site in Proinsulin C-peptide and Its Implications for Metal-Modulated Activity

The connecting peptide (C-peptide) is a hormone with promising health benefits in ameliorating diabetes-related complications, yet mechanisms remain elusive. Emerging studies point to a possible dependence of peptide activity on bioavailable metals, particularly Cu(II) and Zn(II). However, little is known about the chemical nature of the interactions, hindering advances in its therapeutic applications. This work uncovers the Cu(II)-binding site in C-peptide that may be key to understanding its metal-dependent function. A combination of spectroscopic studies reveal that Cu(II) and Zn(II) bind to C-peptide at specific residues in the N-terminal region of the peptide and that Cu(II) is able to displace Zn(II) for C-peptide binding. The data point to a Cu(II)-binding site consisting of 1N3O square-planar coordination that is entropically driven. Furthermore, the entire random coil peptide sequence is needed for specific metal binding as mutations and truncations reshuffle the coordinating residues. These results expand our understanding of how metals influence hormone activity and facilitate the discovery and validation of both new and established paradigms in peptide biology.

Analysis of Metal Effects on C-Peptide Structure and Internalization

The connecting peptide (C-peptide) has received increased attention for its potential therapeutic effects in ameliorating illnesses such as kidney disease and diabetes. Although the mechanism of C-peptide signaling remains elusive, evidence supports its internalization and intracellular function. Emerging research is uncovering the diverse biological roles metals play in controlling and affecting the function of bioactive peptides. The work presented herein investigates interactions between C-peptide and first-row d-block transition metals, as well as their effects on C-peptide internalization into cells. Through spectroscopic techniques, it is demonstrated that Cr^III , Cu^II , and Zn^II bind to C-peptide with differing stoichiometries and biologically relevant affinities. In addition, metal binding elicits both subtle changes in secondary structure and inhibits adoption of an α-helical character in environments where the dielectric constants are reduced. This study shows how metal ions can modulate peptide hormone activity through subtle structural changes to disrupt cellular uptake.

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.

A photoactive semisynthetic metalloenzyme exhibits complete selectivity for CO2 reduction in water

A series of artificial metalloenzymes containing a ruthenium chromophore and [NiII(cyclam)]2+, both incorporated site-selectively, have been constructed within an azurin protein scaffold. These light-driven, semisynthetic enzymes do not evolve hydrogen, thus displaying complete selectivity for CO2 reduction to CO. Electrostatic effects rather than direct excited-state electron transfer dominate the ruthenium photophysics, suggesting that intramolecular electron transfer from photogenerated RuI to [NiII(cyclam)]2+ represents the first step in catalysis. Stern-Volmer analyses rationalize the observation that ascorbate is the only sacrificial electron donor that supports turnover. Collectively, these results highlight the important interplay of elements that must be considered when developing and characterizing molecular catalysts.

Light-Driven Hydrogen Evolution by Nickel-Substituted Rubredoxin

An enzymatic system for light-driven hydrogen generation has been developed through covalent attachment of a ruthenium chromophore to nickel-substituted rubredoxin (NiRd). The photoinduced activity of the hybrid enzyme is significantly greater than that of a two-component system and is strongly dependent on the position of the ruthenium phototrigger relative to the active site, indicating a role for intramolecular electron transfer in catalysis. Steady-state and time-resolved emission spectra reveal a pathway for rapid, direct quenching of the ruthenium excited state by nickel, but low overall turnover numbers suggest initial electron transfer is not the rate-limiting step. This approach is ideally suited for detailed mechanistic investigations of catalysis by NiRd and other molecular systems, with implications for generation of solar fuels.

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

Structural and Biochemical Characterization of Organotin and Organolead Compounds Binding to the Organomercurial Lyase MerB Provide New Insights into Its Mechanism of Carbon-Metal Bond Cleavage

The organomercurial lyase MerB has the unique ability to cleave carbon-Hg bonds, and structural studies indicate that three residues in the active site (C96, D99, and C159 in E. coli MerB) play important roles in the carbon-Hg bond cleavage. However, the role of each residue in carbon-metal bond cleavage has not been well-defined. To do so, we have structurally and biophysically characterized the interaction of MerB with a series of organotin and organolead compounds. Studies with two known inhibitors of MerB, dimethyltin (DMT) and triethyltin (TET), reveal that they inhibit by different mechanisms. In both cases the initial binding is to D99, but DMT subsequently binds to C96, which induces a conformation change in the active site. In contrast, diethyltin (DET) is a substrate for MerB and the Sn^IV product remains bound in the active site in a coordination similar to that of Hg^II following cleavage of organomercurial compounds. The results with analogous organolead compounds are similar in that trimethyllead (TML) is not cleaved and binds only to D99, whereas diethyllead (DEL) is a substrate and the Pb^IV product remains bound in the active site. Binding and cleavage is an exothermic reaction, while binding to D99 has negligible net heat flow. These results show that initial binding of organometallic compounds to MerB occurs at D99 followed, in some cases, by cleavage and loss of the organic moieties and binding of the metal ion product to C96, D99, and C159. The N-terminus of MerA is able to extract the bound Pb^VI but not the bound Sn^IV. These results suggest that MerB could be utilized for bioremediation applications, but certain organolead and organotin compounds may present an obstacle by inhibiting the enzyme.

Stabilization of Cu(I) for binding and calorimetric measurements in aqueous solution

Conditions have been developed for the comproportionation reaction of Cu(2+) and copper metal to prepare aqueous solutions of Cu(+) that are stabilized from disproportionation by MeCN and other Cu(+)-stabilizing ligands. These solutions were then used in ITC measurements to quantify the thermodynamics of formation of a set of Cu(+) complexes (Cu(I)(MeCN)3(+), Cu(I)Me6Trien(+), Cu(I)(BCA)2(3-), Cu(I)(BCS)2(3-)), which have stabilities ranging over 15 orders of magnitude, for their use in binding and calorimetric measurements of Cu(+) interaction with proteins and other biological macromolecules. These complexes were then used to determine the stability and thermodynamics of formation of a 1 : 1 complex of Cu(+) with the biologically important tri-peptide glutathione, GSH. These results identify Me6Trien as an attractive Cu(+)-stabilizing ligand for calorimetric experiments, and suggest that caution should be used with MeCN to stabilize Cu(+) due to its potential for participating in unquantifiable ternary interactions.

Effect of supplementing rumen-protected methionine on production and nitrogen excretion in lactating dairy cows

Two 4 x 4 Latin square trials (4-wk periods; 16 wk total) were conducted to see whether supplementing rumen-protected Met (RPM; fed as Mepron) would allow feeding less crude protein (CP), thereby reducing urinary N excretion, but without losing production. In trial 1, 24 Holsteins were fed 4 diets as total mixed rations containing [dry matter (DM) basis]: 18.6% CP and 0 g of RPM/d; 17.3% CP and 5 g of RPM/d; 16.1% CP and 10 g of RPM/d; or 14.8% CP and 15 g of RPM/d. Dietary CP was reduced by replacing soybean meal with high-moisture shelled corn. All diets contained 21% alfalfa silage, 28% corn silage, 4.5% roasted soybeans, 5.8% soyhulls, 0.6% sodium bicarbonate, 0.5% vitamins and minerals, and 27% neutral detergent fiber. There was no effect of diet on intake, weight gain, or yields of protein, lactose, and solids-not-fat. However, production was greater at 17.3% CP plus RPM and 16.1% CP plus RPM than on the other 2 diets. Apparent N efficiency (milk N:N intake) was greatest on the lowest CP diet containing the most RPM. Linear reductions in milk urea N and urinary N excretion were observed with lower dietary CP. In trial 2, 32 Holsteins were fed 4 diets as total mixed rations, formulated from ingredients used in trial 1 and containing 16.1 or 17.3% CP with 0 or 10 g of RPM/d. On average, cows were calculated to be in negative N balance on all diets because of lower than expected DM intake. There was no effect of RPM supplementation on any production trait. However, higher CP gave small increases in yields of milk, protein, and solids-not-fat and tended to increase DM intake and lactose yield. Apparent N efficiency was greater, and milk urea nitrogen was lower, on 16.1% CP. In trial 1, feeding lower CP diets supplemented with RPM resulted in improved N efficiency and reduced urinary N excretion. However, in trial 2, reducing dietary CP from 17.3 to 16.1% reduced milk secretion, an effect that was not reversed by RPM supplementation at low DM intakes when cows were apparently mobilizing body protein.