A partially folded intermediate species of the beta-sheet protein apo-pseudoazurin is trapped during proline-limited folding

Proline Peptide Bond Isomerization in Ubiquitin Under Folding and Denaturing Conditions by Pressure-Jump NMR

Proline isomerization is widely recognized as a kinetic bottleneck in protein folding, amplified for proteins rich in Pro residues. We introduced repeated hydrostatic pressure jumps between native and pressure-denaturing conditions inside an NMR sample cell to study proline isomerization in the pressure-sensitized L50A ubiquitin mutant. Whereas in two unfolded heptapeptides, X-Pro peptide bonds isomerized ca 1.6-fold faster at 1 bar than at 2.5 kbar, for ubiquitin ca eight-fold faster isomerization was observed for Pro-38 and ca two-fold for Pro-19 and Pro-37 relative to rates measured in the pressure-denatured state. Activation energies for isomerization in pressure-denatured ubiquitin were close to literature values of 20 kcal/mole for denatured polypeptides but showed a substantial drop to 12.7 kcal/mole for Pro-38 at atmospheric pressure. For ubiquitin isomers with a cis E18-P19 peptide bond, the 1-bar NMR spectrum showed sharp resonances with near random coil chemical shifts for the C-terminal half of the protein, characteristic of an unfolded chain, while most of the N-terminal residues were invisible due to exchange broadening, pointing to a metastable partially folded state for this previously recognized 'folding nucleus'. For cis-P37 isomers, a drop in pressure resulted in the rapid loss of nearly all unfolded-state NMR resonances, while the recovery of native state intensity revealed a slow component attributed to cis → trans isomerization of P37. This result implies that the NMR-invisible cis-P37 isomer adopts a molten globule state that encompasses the entire length of the ubiquitin chain, suggestive of a structure that mostly resembles the folded state.

Revealing the Underestimated Anticancer Effect of Azurin by Mechanical Unfolding

As a potential anticancer agent, azurin has attracted extensive attraction among chemists, physicists, and material scientists. Its structural and unfolding/folding information has been partially understood, but some detailed information, such as the difference in the unfolding processes between apo-azurin and holo-azurin, the mechanical stability, and the role of the copper cluster in its stability, has not been addressed adequately, especially at the single-molecule level. Here, we employed AFM-based single-molecule force spectroscopy to investigate the unfolding process of azurin in the apo and holo forms under an external force. The results indicated that the unfolding processes of apo-azurin and holo-azurin are different, and holo-azurin requires a stronger force to unfold than does apo-azurin. The copper cluster exhibited a more significant impact on the stability and the folding process of holo-azurin: the copper cluster was completely broken, and the copper ion left the unfolded azurin during the unfolding process of azurin. We suspected that the presence of the disulfide bond in azurin made the unfolding of the copper cluster different from that in pseudoazurin, which is also a type I copper protein like azurin. Rarely reported in previous studies, the mechanical strength of the Cu-N(His) bond of the copper cluster was obtained in this study, which is weaker than that of most metal-S(Cys) bonds but higher than that of the Fe-N(His) bond. Altogether, our results offer a possible new scenario for azurin to widely extend its anticancer activity.

Role of the copper ion in pseudoazurin during the mechanical unfolding process

Metalloproteins require the corresponding metal cofactors to exert their proper function. The presence of metal cofactors in the metalloprotein makes it more difficult to investigate its folding and unfolding process. In this study, we employed atomic-force-microscopy-based single-molecule force spectroscopy to reveal the unfolding process of pseudoazurin (PAZ) that belongs to blue copper proteins. Our study shows that holo-PAZ requires a higher rupture force for mechanical unfolding comparing with the apo-PAZ. This result demonstrates that the copper atom not only enables PAZ access to transfer electron, but should also have an influence on its stability. The results also suggest that the electronic configuration of the metal cofactors has a striking effect on the strength of the organometallic bonds. Moreover, the results also reveal that there is an intermediate state during the unfolding process of PAZ. This study provides insight into the characteristics of metalloproteins and leads to a better knowledge of their interaction at the individual molecule level.

An Effective Cumulative Torsion Angles Model for Prediction of Protein Folding Rates

BACKGROUND

Protein folding rate is mainly determined by the size of the conformational space to search, which in turn is dictated by factors such as size, structure and amino-acid sequence in a protein. It is important to integrate these factors effectively to form a more precisely description of conformation space. But there is no general paradigm to answer this question except some intuitions and empirical rules. Therefore, at the present stage, predictions of the folding rate can be improved through finding new factors, and some insights are given to the above question.

OBJECTIVE

Its purpose is to propose a new parameter that can describe the size of the conformational space to improve the prediction accuracy of protein folding rate.

METHODS

Based on the optimal set of amino acids in a protein, an effective cumulative backbone torsion angles (CBTAeff) was proposed to describe the size of the conformational space. Linear regression model was used to predict protein folding rate with CBTAeff as a parameter. The degree of correlation was described by the coefficient of determination and the mean absolute error MAE between the predicted folding rates and experimental observations.

RESULTS

It achieved a high correlation (with the coefficient of determination of 0.70 and MAE of 1.88) between the logarithm of folding rates and the (CBTAeff)0.5 with experimental over 112 twoand multi-state folding proteins.

CONCLUSION

The remarkable performance of our simplistic model demonstrates that CBTA based on optimal set was the major determinants of the conformation space of natural proteins.

Identification and Rationalization of Kinetic Folding Intermediates for a Low-Density Lipoprotein Receptor Ligand-Binding Module

Many mutations that cause familial hypercholesterolemia localize to ligand-binding domain 5 (LA5) of the low-density lipoprotein receptor, motivating investigation of the folding and misfolding of this small, disulfide-rich, calcium-binding domain. LA5 folding is known to involve non-native disulfide isomers, yet these folding intermediates have not been structurally characterized. To provide insight into these intermediates, we used nuclear magnetic resonance (NMR) to follow LA5 folding in real time. We demonstrate that misfolded or partially folded disulfide intermediates are indistinguishable from the unfolded state when focusing on the backbone NMR signals, which provide information on the formation of only the final, native state. However, ¹³C labeling of cysteine side chains differentiated transient intermediates from the unfolded and native states and reported on disulfide bond formation in real time. The cysteine pairings in a dominant intermediate were identified using ¹³C-edited three-dimensional NMR, and coarse-grained molecular dynamics simulations were used to investigate the preference of this disulfide set over other non-native arrangements. The transient population of LA5 species with particular non-native cysteine connectitivies during folding supports the conclusion that cysteine pairing is not random and that there is a bias toward certain structural ensembles during the folding process, even prior to the binding of calcium.

Statistical analyses of protein folding rates from the view of quantum transition

Understanding protein folding rate is the primary key to unlock the fundamental physics underlying protein structure and its folding mechanism. Especially, the temperature dependence of the folding rate remains unsolved in the literature. Starting from the assumption that protein folding is an event of quantum transition between molecular conformations, we calculated the folding rate for all two-state proteins in a database and studied their temperature dependencies. The non-Arrhenius temperature relation for 16 proteins, whose experimental data had previously been available, was successfully interpreted by comparing the Arrhenius plot with the first-principle calculation. A statistical formula for the prediction of two-state protein folding rate was proposed based on quantum folding theory. The statistical comparisons of the folding rates for 65 two-state proteins were carried out, and the theoretical vs. experimental correlation coefficient was 0.73. Moreover, the maximum and the minimum folding rates given by the theory were consistent with the experimental results.

NMR analysis of a kinetically trapped intermediate of a disulfide-deficient mutant of the starch-binding domain of glucoamylase

A thermally unfolded disulfide-deficient mutant of the starch-binding domain of glucoamylase refolds into a kinetically trapped metastable intermediate when subjected to a rapid lowering of temperature. We attempted to characterise this intermediate using multidimensional NMR spectroscopy. The (1)H-(15)N heteronuclear single quantum coherence spectrum after a rapid temperature decrease (the spectrum of the intermediate) showed good chemical shift dispersion but was significantly different from that of the native state, suggesting that the intermediate adopts a nonnative but well-structured conformation. Large chemical shift changes for the backbone amide protons between the native and the intermediate states were observed for residues in the β-sheet consisting of strands 2, 3, 5, 6, and 7 as well as in the C-terminal region. These residues were found to be in close proximity to aromatic residues, suggesting that the chemical shift changes are mainly due to ring current shifts caused by the aromatic residues. The two-dimensional nuclear Overhauser enhancement (NOE) spectroscopy experiments showed that the intermediate contained substantial, native-like NOE connectivities, although there were fewer cross peaks in the spectrum of the intermediate compared with that of the native state. It was also shown that there were native-like interresidue NOEs for residues buried in the protein, whereas many of the NOE cross peaks were lost for the residues involved in a surface-exposed aromatic cluster. These results suggest that, in the intermediate, the aromatic cluster at the surface is structurally less organised, whereas the interior of the protein has relatively rigid, native-like side-chain packing.

Identification and comparative analysis of the peptidyl-prolyl cis/trans isomerase repertoires of H. sapiens, D. melanogaster, C. elegans, S. cerevisiae and Sz. pombe

The peptidyl-prolyl cis/trans isomerase (PPIase) class of proteins comprises three member families that are found throughout nature and are present in all the major compartments of the cell. Their numbers appear to be linked to the number of genes in their respective genomes, although we have found the human repertoire to be smaller than expected due to a reduced cyclophilin repertoire. We show here that whilst the members of the cyclophilin family (which are predominantly found in the nucleus and cytoplasm) and the parvulin family (which are predominantly nuclear) are largely conserved between different repertoires, the FKBPs (which are predominantly found in the cytoplasm and endoplasmic reticulum) are not. It therefore appears that the cyclophilins and parvulins have evolved to perform conserved functions, while the FKBPs have evolved to fill ever-changing niches within the constantly evolving organisms. Many orthologous subgroups within the different PPIase families appear to have evolved from a distinct common ancestor, whereas others, such as the mitochondrial cyclophilins, appear to have evolved independently of one another. We have also identified a novel parvulin within Drosophila melanogaster that is unique to the fruit fly, indicating a recent evolutionary emergence. Interestingly, the fission yeast repertoire, which contains no unique cyclophilins and parvulins, shares no PPIases solely with the budding yeast but it does share a majority with the higher eukaryotes in this study, unlike the budding yeast. It therefore appears that, in comparison with Schizosaccharomyces pombe, Saccharomyces cerevisiae is a poor representation of the higher eukaryotes for the study of PPIases.

pH (low) insertion peptide (pHLIP) inserts across a lipid bilayer as a helix and exits by a different path

What are the molecular events that occur when a peptide inserts across a membrane or exits from it? Using the pH-triggered insertion of the pH low insertion peptide to enable kinetic analysis, we show that insertion occurs in several steps, with rapid (0.1 sec) interfacial helix formation, followed by a much slower (100 sec) insertion pathway to give a transmembrane helix. The reverse process of unfolding and peptide exit from the bilayer core, which can be induced by a rapid rise of the pH from acidic to basic, proceeds approximately 400 times faster than folding/insertion and through different intermediate states. In the exit pathway, the helix-coil transition is initiated while the polypeptide is still inside the membrane. The peptide starts to exit when about 30% of the helix is unfolded, and continues a rapid exit as it unfolds inside the membrane. These insights may guide understanding of membrane protein folding/unfolding and the design of medically useful peptides for imaging and drug delivery.

3S-fluoroproline as a probe to monitor proline isomerization during protein folding by 19F-NMR

Variable-temperature inversion transfer NMR is used to determine the kinetic and thermodynamic parameters of cis-trans isomerization of N-Ac-(3R) and (3S)-fluoroproline-OMe.

Kinetically trapped metastable intermediate of a disulfide-deficient mutant of the starch-binding domain of glucoamylase

Refolding of a thermally unfolded disulfide-deficient mutant of the starch-binding domain of glucoamylase was investigated using differential scanning calorimetry, isothermal titration calorimetry, CD, and (1)H NMR. When the protein solution was rapidly cooled from a higher temperature, a kinetic intermediate was formed during refolding. The intermediate was unexpectedly stable compared with typical folding intermediates that have short half-lives. It was shown that this intermediate contained substantial secondary structure and tertiary packing and had the same binding ability with beta-cyclodextrin as the native state, suggesting that the intermediate is highly-ordered and native-like on the whole. These characteristics differ from those of partially folded intermediates such as molten globule states. Far-UV CD spectra showed that the secondary structure was once disrupted during the transition from the intermediate to the native state. These results suggest that the intermediate could be an off-pathway type, possibly a misfolded state, that has to undergo unfolding on its way to the native state.

Alkaline transition of pseudoazurin Met16X mutant proteins: protein stability influenced by the substitution of Met16 in the second sphere coordination

Several blue copper proteins are known to change the active site structure at alkaline pH (alkaline transition). Spectroscopic studies of Met16Phe, Met16Tyr, Met16Trp, and Met16Val pseudoazurin variants were performed to investigate the second sphere role through alkaline transition. The visible electronic absorption and resonance Raman spectra of Met16Phe, Met16Tyr, and Met16Trp variants showed the increasing of axial component at pH approximately 11 like wild-type PAz. The visible electronic absorption and far-UV CD spectra of Met16Val demonstrated that the destabilization of the protein structure was triggered at pH>11. Resonance Raman (RR) spectra of PAz showed that the intensity-weighted averaged Cu-S(Cys) stretching frequency was shifted to higher frequency region at pH approximately 11. The higher frequency shift of Cu-S(Cys) bond is implied the stronger Cu-S(Cys) bond at alkaline transition pH approximately 11. The visible electronic absorption and far-UV CD spectra of Met16X PAz revealed that the Met16Val variant is denatured at pH>11, but Met16Phe, Met16Tyr, and Met16Trp mutant proteins are not denatured even at pH>11. These observations suggest that Met16 is important to maintain the protein structure through the possible weak interaction between methionine -SCH3 part and coordinated histidine imidazole moiety. The introduction of pi-pi interaction in the second coordination sphere may be contributed to the enhancement of protein structure stability.

Protein folding: then and now

Over the past three decades the protein folding field has undergone monumental changes. Originally a purely academic question, how a protein folds has now become vital in understanding diseases and our abilities to rationally manipulate cellular life by engineering protein folding pathways. We review and contrast past and recent developments in the protein folding field. Specifically, we discuss the progress in our understanding of protein folding thermodynamics and kinetics, the properties of evasive intermediates, and unfolded states. We also discuss how some abnormalities in protein folding lead to protein aggregation and human diseases.

Role of copper in folding and stability of cupredoxin-like copper-carrier protein CopC

CopC is a periplasmic copper carrier that, in contrast to cytoplasmic copper chaperones, has a beta-barrel fold and two metal-binding sites distinct for Cu(II) and Cu(I). The copper sites are located in each end of the molecule: the Cu(I) site involves His and Met coordination whereas the Cu(II) site consists of charged residues. To reveal biophysical properties of this protein, we have explored the effects of the cofactors on CopC unfolding in vitro. We demonstrate that Cu(II) coordination affects both protein stability and unfolding pathway, whereas Cu(I) has only a small effect on stability. Apo-CopC unfolds in a two-state reaction between pH 4 and 7.5 with maximal stability at pH 6. In contrast, Cu(II)-CopC unfolds in a three-state reaction at pH6 that involves a partly folded intermediate that retains Cu(II). This intermediate exhibits high thermal and chemical stability. Unique energetic and structural properties of different metalated CopC forms may help facilitate metal transport to many partners in vivo.

Folding and unfolding of a non-fluorescent mutant of green fluorescent protein

Green fluorescent protein (GFP), from the Pacific jellyfish A. victoria, has numerous uses in biotechnology and cell and molecular biology as a protein marker because of its specific chromophore, which is spontaneously created after proper protein folding. After formation, the chromophore is very stable and remains intact during protein unfolding, meaning that the GFP unfolding process is not the reverse of the original folding reaction; i.e., the principles of microscopic reversibility do not apply. We have generated the mutant S65T/G67A-GFP, which is unable to form the cyclic chromophore, with the goal of investigating the folding, unfolding and competing aggregation of GFP under fully reversible conditions. Our studies have been performed in the presence of GdnHCl. The GFP conformation was monitored using intrinsic tryptophan fluorescence, and fluorescence of bis-ANS. Light scattering was used to follow GFP aggregation. We conclude from these fluorescence measurements, that S65T/G67A-GFP folding is largely reversible. During equilibrium folding, the first step is formation of molten globule, prone to aggregation.

Substitutions of prolines examine their role in kinetic trap formation of the caspase recruitment domain (CARD) of RICK

Caspase recruitment domains (CARDs) are small helical protein domains that adopt the Greek key fold. For the two CARDs studied to date, RICK-CARD and caspase-1-CARD (CP1-CARD), the proteins unfold by an apparent two-state process at equilibrium. However, the folding kinetics are complex for both proteins and may contain kinetically trapped species on the folding pathway. In the case of RICK-CARD, the time constants of the slow refolding phases are consistent with proline isomerism. RICK-CARD contains three prolines, P47 in turn 3, and P85 and P87. The latter two prolines constitute a nonconserved PxP motif in helix 6. To examine the role of the prolines in the complex folding kinetics of RICK-CARD, we generated seven proline-to-alanine mutants, including three single mutants, three double mutants, and one triple mutant. We examined the spectroscopic properties, equilibrium folding, binding to CP1-CARD, and folding kinetics. The results show that P85 is critical for maintaining the function of the protein and that all mutations decrease the stability. Results from single mixing and sequential mixing stopped-flow studies strongly suggest the presence of parallel folding pathways consisting of at least two unfolded populations. The mutations affect the distribution of the two unfolded species, thereby affecting the population that folds through each channel. The two conformations also are present in the triple mutant, demonstrating that interconversion between them is not due to prolyl isomerism. Overall, the data show that the complex folding pathway, especially formation of kinetically trapped species, is not due to prolyl isomerism.

The cyclophilin repertoire of the fission yeast Schizosaccharomyces pombe

The cyclophilin repertoire of the fission yeast Schizosaccharomyces pombe is comprised of nine members that are distributed over all three of its chromosomes and range from small single-domain to large multi-domain proteins. Each cyclophilin possesses only a single prolyl-isomerase domain, and these vary in their degree of consensus, including at positions that are likely to affect their drug-binding ability and catalytic activity. The additional identified motifs are involved in putative protein or RNA interactions, while a novel domain that is specific to SpCyp7 and its orthologues may have functions that include an interaction with hnRNPs. The Sz. pombe cyclophilins are found throughout the cell but appear to be absent from the mitochondria, which is unique among the characterized eukaryotic repertoires. SpCyp5, SpCyp6 and SpCyp8 have exhibited significant upregulation of their expression during the meiotic cycle and SpCyp5 has exhibited significant upregulation of its expression during heat stress. All nine have identified members in the repertoires of H. sapiens, D. melanogaster and A. thaliana. However, only three identified members in the cyclophilin repertoire of S. cerevisiae with SpCyp7 identifying a fourth protein that is not a member of the recognized repertoire due to its possession of a degenerate prolyl-isomerase domain. The cyclophilin repertoire of Sz. pombe therefore represents a better model group for the study of cyclophilin function in the higher eukaryotes.

Apo-azurin folds via an intermediate that resembles the molten-globule

The folding of Pseudomonas aeruginosa apo-azurin was investigated with the intent of identifying putative intermediates. Two apo-mutants were constructed by replacing the main metal-binding ligand C112 with a serine (C112S) and an alanine (C112A). The guanidinium-induced unfolding free energies (DeltaG(U-N)(H2O)) of the C112S and C112A mutants were measured to 36.8 +/- 1 kJ mole(-1) and 26.1 +/- 1 kJ mole(-1), respectively, and the m-value of the transition to 23.5 +/- 0.7 kJ mole(-1) M(-1). The difference in folding free energy (DeltaDeltaG(U-N)(H2O)) is largely attributed to the intramolecular hydrogen bonding properties of the serine Ogamma in the C112S mutant, which is lacking in the C112A structure. Furthermore, only the unfolding rates differ between the two mutants, thus pointing to the energy of the native state as the source of the observed Delta DeltaG(U-N)(H2O). This also indicates that the formation of the hydrogen bonds present in C112S but absent in C112A is a late event in the folding of the apo-protein, thus suggesting that formation of the metal-binding site occurs after the rate-limiting formation of the transition state. In both mutants we also noted a burst-phase intermediate. Because this intermediate was capable of binding 1-anilinonaphtalene-8-sulfonate (ANS), as were an acid-induced species at pH 2.6, we ascribe it molten globule-like status. However, despite the presence of an intermediate, the folding of apo-azurin C112S is well approximated by a two-state kinetic mechanism.

Unfolding process of rusticyanin: evidence of protein aggregation

The unfolding process of the Blue Copper Protein (BCP) rusticyanin (Rc) has been studied using a wide variety of biochemical techniques. Fluorescence and CD spectroscopies reveal that the copper ion plays an essential role in stabilizing the protein and that the oxidized form is more efficient than the reduced species in this respect. The addition of guanidinium chloride to Rc samples produces aggregation of the protein. Gel filtration chromatography and glutaraldehyde cross-linking experiments confirm the formation of such aggregates. Among the BCPs, this feature is exclusive to Rc. The aggregation could be related to the large molecular mass and large number of hydrophobic residues of this protein compared with those of other BCPs.

Kinetic traps in the folding/unfolding of procaspase-1 CARD domain

We have examined the folding and unfolding of the caspase recruitment domain of procaspase-1 (CP1-CARD), a member of the alpha-helical Greek key protein family. The equilibrium folding/unfolding of CP1-CARD is described by a two-state mechanism, and the results show CP1-CARD is marginally stable with a DeltaG(H2O) of 1.1 +/- 0.2 kcal/mole and an m-value of 0.65 +/- 0.06 kcal/mole/M (10 mM Tris-HCl at pH 8.0, 1 mM DTT, 25 degrees C). Consistent with the equilibrium folding data, CP1-CARD is a monomer in solution when examined by size exclusion chromatography. Single-mixing stopped-flow refolding and unfolding studies show that CP1-CARD folds and unfolds rapidly, with no detectable slow phases, and the reactions appear to reach equilibrium within 10 msec. However, double jump kinetic experiments demonstrate the presence of an unfolded-like intermediate during unfolding. The intermediate converts to the fully unfolded conformation with a half-time of 10 sec. Interrupted refolding studies demonstrate the presence of one or more nativelike intermediates during refolding, which convert to the native conformation with a half-time of about 60 sec. Overall, the data show that both unfolding and refolding processes are slow, and the pathways contain kinetically trapped species.

Folding of a beta-sheet protein monitored by real-time NMR spectroscopy

At low ionic strength, apoplastocyanin forms an unfolded state under non-denaturing conditions. The refolding of this state is sufficiently slow to allow real-time NMR experiments to be performed. Folding of apoplastocyanin, initiated by the addition of salt and followed by real-time 2D 1H-15N heteronuclear single quantum coherence (HSQC) spectroscopy, is highly cooperative. A concomitant increase in the intensity of both sequential and long-range nuclear Overhauser effects (NOEs) between backbone amide protons in successive acquisitions of 1H-15N HSQC-NOESY-HSQC spectra provides the first direct observation of the development of structure-specific NOEs as a protein folds. Our results show that the local and long-range interactions in the native apoplastocyanin are formed simultaneously, consistent with highly cooperative formation of the native structure.

In vitro unfolding, refolding, and polymerization of human gammaD crystallin, a protein involved in cataract formation

Human gammaD crystallin (HgammaD-Crys), a major protein of the human eye lens, is a primary component of cataracts. This 174-residue primarily beta-sheet protein is made up of four Greek keys separated into two domains. Mutations in the human gene sequence encoding HgammaD-Crys are implicated in early-onset cataracts in children, and the mutant protein expressed in Escherichia coli exhibits properties that reflect the in vivo pathology. We have characterized the unfolding, refolding, and competing aggregation of human wild-type HgammaD-Crys as a function of guanidinium hydrochloride (GuHCl) concentration at neutral pH and 37 degrees C, using intrinsic tryptophan fluorescence to monitor in vitro folding. Wild-type HgammaD-Crys exhibited reversible refolding above 1.0 M GuHCl. The GuHCl unfolded protein was more fluorescent than its native counterpart despite the absence of metal or ion-tryptophan interactions. Aggregation of refolding intermediates of HgammaD-Crys was observed in both equilibrium and kinetic refolding processes. The aggregation pathway competed with productive refolding at denaturant concentrations below 1.0 M GuHCl, beyond the major conformational transition region. Atomic force microscopy of samples under aggregating conditions revealed the sequential appearance of small nuclei, thin protofibrils, and fiber bundles. The HgammaD-Crys fibrous aggregate species bound bisANS appreciably, indicating the presence of exposed hydrophobic pockets. The mechanism of HgammaD-Crys aggregation may provide clues to understanding age-onset cataract formation in vivo.

Metal-ligand interplay in blue copper proteins studied by 1H NMR spectroscopy: Cu(II)-pseudoazurin and Cu(II)-rusticyanin

The blue copper proteins (BCPs), pseudoazurin from Achromobacter cycloclastes and rusticyanin from Thiobacillus ferrooxidans, have been investigated by (1)H NMR at a magnetic field of 18.8 T. Hyperfine shifts of the protons belonging to the coordinated ligands have been identified by exchange spectroscopy, including the indirect detection for those resonances that cannot be directly observed (the beta-CH(2) of the Cys ligand, and the NH amide hydrogen bonded to the S(gamma)(Cys) atom). These data reveal that the Cu(II)-Cys interaction in pseudoazurin and rusticyanin is weakened compared to that in classic blue sites (plastocyanin and azurin). This weakening is not induced by a stronger interaction with the axial ligand, as found in stellacyanin, but might be determined by the protein folding around the metal site. The average chemical shift of the beta-CH(2) Cys ligand in all BCPs can be correlated to geometric factors of the metal site (the Cu-S(gamma)(Cys) distance and the angle between the CuN(His)N(His) plane and the Cu-S(gamma)(Cys) vector). It is concluded that the degree of tetragonal distortion is not necessarily related to the strength of the Cu(II)-S(gamma)(Cys) bond. The copper-His interaction is similar in all BCPs, even for the solvent-exposed His ligand. It is proposed that the copper xy magnetic axes in blue sites are determined by subtle geometrical differences, particularly the orientation of the His ligands. Finally, the observed chemical shifts for beta-CH(2) Cys and Ser NH protons in rusticyanin suggest that a less negative charge at the sulfur atom could contribute to the high redox potential (680 mV) of this protein.