1H NMR spectroscopic studies of calcium-binding proteins. 2. Histidine microenvironments in alpha- and beta-parvalbumins as determined by protonation and laser photochemically induced dynamic nuclear polarization effects

Parvalbumin as a metal-dependent antioxidant

Parvalbumin (PA) is a Ca(2+)-binding protein of vertebrates massively expressed in tissues with high oxygen uptake and respectively elevated level of reactive oxygen species (ROS). To characterize antioxidant properties of PA, antioxidant capacity (AOC) of intact rat α-PA has been explored. ORAC, TEAC and hydrogen peroxide AOC assays evidence conformation-dependent oxidation of the PA. AOC value for the apo-PA 4-11-fold exceeds that for the Ca(2+)-loaded protein. Despite folded conformation of apo-PA, it has AOC equivalent to that of the proteolized protein. The most populated under resting conditions PA form, Mg(2+)-bound PA, has AOC similar to that of apo-PA. ROS-induced changes in absorption spectrum of PA evidence an oxidation of PA's phenylalanines in the ORAC assay. Sensitivity of PA oxidation to its conformation enabled characterization of its metal affinity and pH-dependent behavior: a transition with pKa of 7.6 has been revealed for the Ca(2+)-loaded PA. Since total AOC of PA under in vivo conditions may reach the level of reduced glutathione, we propose that PA might modulate intracellular redox equilibria and/or signaling in a calcium-dependent manner. We speculate that the oxidation-mediated damage of some of PA-GABAergic interneurons observed in schizophrenia is due to a decline in total AOC of the reduced glutathione-PA pair.

Photo-CIDNP NMR spectroscopy of amino acids and proteins

Photo-chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance (NMR) phenomenon which, among other things, is exploited to extract information on biomolecular structure via probing solvent-accessibilities of tryptophan (Trp), tyrosine (Tyr), and histidine (His) amino acid side chains both in polypeptides and proteins in solution. The effect, normally triggered by a (laser) light-induced photochemical reaction in situ, yields both positive and/or negative signal enhancements in the resulting NMR spectra which reflect the solvent exposure of these residues both in equilibrium and during structural transformations in "real time". As such, the method can offer - qualitatively and, to a certain extent, quantitatively - residue-specific structural and kinetic information on both the native and, in particular, the non-native states of proteins which, often, is not readily available from more routine NMR techniques. In this review, basic experimental procedures of the photo-CIDNP technique as applied to amino acids and proteins are discussed, recent improvements to the method highlighted, and future perspectives presented. First, the basic principles of the phenomenon based on the theory of the radical pair mechanism (RPM) are outlined. Second, a description of standard photo-CIDNP applications is given and it is shown how the effect can be exploited to extract residue-specific structural information on the conformational space sampled by unfolded or partially folded proteins on their "path" to the natively folded form. Last, recent methodological advances in the field are highlighted, modern applications of photo-CIDNP in the context of biological NMR evaluated, and an outlook into future perspectives of the method is given.

Apo-parvalbumin as an intrinsically disordered protein

Recently defined family of intrinsically disordered proteins (IDP) includes proteins lacking rigid tertiary structure meanwhile fulfilling essential biological functions. Here we show that apo-state of pike parvalbumin (alpha- and beta-isoforms, pI 5.0 and 4.2, respectively) belongs to the family of IDP, which is in accord with theoretical predictions. Parvalbumin (PA) is a 12-kDa calcium-binding protein involved into regulation of relaxation of fast muscles. Differential scanning calorimetry measurements of metal-depleted form of PA revealed the absence of any thermally induced transitions with measurable denaturation enthalpy along with elevated specific heat capacity, implying the lack of rigid tertiary structure and exposure of hydrophobic protein groups to the solvent. Calcium removal from the PAs causes more than 10-fold increase in fluorescence intensity of hydrophobic probe bis-ANS and is accompanied by a decrease in alpha-helical content and a marked increase in mobility of aromatic residues environment, as judged by circular dichroism spectroscopy (CD). Guanidinium chloride-induced unfolding of the apo-parvalbumins monitored by CD showed the lack of fixed tertiary structure. Theoretical estimation of energetics of the charge-charge interactions in the PAs indicated their pronounced destabilization upon calcium removal, which is in line with sequence-based predictions of disordered protein chain regions. Far-UV CD studies of apo-alpha-PA revealed hallmarks of cold denaturation of the protein at temperatures below 20 degrees C. Moreover, a cooperative thermal denaturation transition with mid-temperature at 10-15 degrees C is revealed by near-UV CD for both PAs. The absence of detectable enthalpy change in this temperature region suggests continuous nature of the transition. Overall, the theoretical and experimental data obtained show that PA in apo-state is essentially disordered nevertheless demonstrates complex denaturation behavior. The native rigid tertiary structure of PA is attained upon association of one (alpha-PA) or two (beta-PA) calcium ions per protein molecule, as follows from calorimetric and calcium titration data.

Proton NMR study of peptides from myelin basic protein: evidence for Lys74-His77 interaction revealed from histidine line broadening

Residues 69-84 of guinea pig myelin basic protein contain the encephalitogenic determinant for the Lewis rat. Insertion of histidine and glycine at positions 77 and 78 in bovine MBP greatly reduces the encephalitogenicity of the protein. Synthetic peptides analogous to this region of MBP containing glycine and histidine are encephalitogenic if they lack the N-terminal half, residues 69-74. However, if they contain both histidine plus the N-terminal half, encephalitogenicity is abolished, suggesting that an interaction of histidine with an amino acid in the N-terminal half changes the conformation or the properties of the peptide. This was investigated by measuring the 1H-NMR spectra of synthetic peptides analogous to this region of MBP, both containing histidine but with and without the N-terminal half. The major difference in the spectra of the two peptides was the pH dependence of line broadening of the histidine resonances. The histidine C2H and C4H resonances were broadened at intermediate pH values in both peptides. However, sharpening of the lines at high pH showed a different pH dependence in the two peptides. For the longer peptide containing the N-terminal half, the lines did not sharpen until the pH was increased above 10.2, coinciding with the pKa of Lys-74. Acetylation of this peptide caused the pH at which the lines began to sharpen to drop to 8.8. In the shorter peptide, lacking the N-terminal half and Lys-74, the lines also sharpened at pH 8.8. The greater broadening which persisted up above pH 10 for the longer peptide suggests slow exchange between two different conformations or environments of the histidine. One of these could be a conformation in which the deprotonated histidine hydrogen bonds with Lys-74. The Lys side-chain resonances indicated a decrease in rotational freedom above the pKa of histidine, consistent with this conclusion. Although this putative interaction between His and Lys-74 did not appear to have a significant effect on the overall conformation of the peptide, it could result in a reduction in encephalitogenicity by altering the properties of the peptide. This could affect processing and presentation of this determinant by antigen presenting cells.

Comparative studies of the interaction of human and bovine platelet factor 4 with heparin using histidine NMR resonances as spectroscopic probes

The pKa values of His-38 and His-50 of the heparin-binding protein, bovine platelet factor 4, are 5.6 and 6.5, respectively, as determined by 1H NMR spectroscopy. The 1H NMR resonance of His-38 of bovine platelet factor 4 which exhibits the lower pKa value is perturbed upon heparin binding to a greater degree than the resonance of His-50. Human platelet factor 4 contains the homologous residues His-23 and His-35. The pKa values of the two histidine residues of human platelet factor 4 are 5.3 and 6.4. The 1H NMR resonance of the histidine of human platelet factor 4 exhibiting the lower pKa value also is perturbed upon heparin binding to a greater degree than the histidine resonance exhibiting the higher pKa, thereby suggesting comparable heparin-protein interactions in bovine and human platelet factor 4.

Proton nuclear magnetic resonance study of the histidine residues of pituitary bovine growth hormone

The pKa value of histidines 20, 22 and 170 of pituitary bovine growth hormone (pbGH) and their C2H deuterium exchange rates have been determined by high-resolution 1H-NMR spectroscopy. Partial assignment of these parameters indicate that His-170 is a surface residue with a half-life for deuterium exchange at the C2H position of approx. 36 h and a pKa value of 5.68 +/- 0.11. A reversible conformational change of pbGH has also been characterized in terms of the involvement of either histidines 20 or 22. At or near physiological pH values, one of these residues is expelled from a buried, hydrophobic position to become fully solvent-exposed. The disparity in pKa values between histidines 20 and 22 (4.67 +/- 0.20 and 5.94 +/- 0.10, in some combination) has been shown by computer modeling, to be compatible with these residues residing in an alpha-helical region of the protein.

1H NMR studies of mastoparan binding by calmodulin

Association with the cytoactive tetradecapeptide mastoparan perturbs the downfield 1H NMR spectrum of the calmodulin-Ca42+ complex. Changes occur in the resonances assigned to His-107 and Tyr-138. Composite peaks assigned to Phe-16 and Phe-89 and to Phe-68 and Tyr-99 are also affected. Both the upfield and downfield 1H NMR spectra contain evidence for spectroscopically distinct intermediates in Ca2+ binding by the calmodulin-mastoparan complex.