A proton nuclear magnetic resonance study of human serum albumin in the neutral pH region

Interaction of Cm(III) with human serum albumin studied by time-resolved laser fluorescence spectroscopy and NMR

The complexation of Cm(III) with human serum albumin (HSA) was investigated using time-resolved laser fluorescence spectroscopy (TRLFS). The Cm(III) HSA species is dominating the speciation between pH 7.0 and 9.3. The first coordination sphere is composed by three to four H₂O molecules and five to six coordinating ligands from the protein. For the complex formation at pH 8.0 a conditional stability constant of logK = 6.16 ± 0.50 was determined. Furthermore, information on the Cm(III) HSA binding site were obtained. With increasing Cu(II) concentration the Cm(III) HSA complexation is suppressed whereas the addition of Zn(II) has no effect. This points to the complexation of Cm(III) at the N-terminal binding site (NTS) which is the primary Cu(II) binding site. NMR experiments with Cu(II), Eu(III) and Am(III) HSA show a decrease of the peak assigned to the His C2 proton of His 3, which is part of the NTS, with increasing metal ion concentration. This confirms the complexation of Eu(III) and Am(III) at the Cu(II) binding site NTS. The results presented in this study contribute to a better understanding of relevant biochemical reactions of incorporated actinides.

Structure, properties, and engineering of the major zinc binding site on human albumin

Most blood plasma zinc is bound to albumin, but the structure of the binding site has not been determined. Zn K-edge extended x-ray absorption fine structure spectroscopy and modeling studies show that the major Zn(2+) site on albumin is a 5-coordinate site with average Zn-O/N distances of 1.98 A and a weak sixth O/N bond of 2.48 A, consistent with coordination to His(67) and Asn(99) from domain I, His(247) and Asp(249) from domain II (residues conserved in all sequenced mammalian albumins), plus a water ligand. The dynamics of the domain I/II interface, thought to be important to biological function, are affected by Zn(2+) binding, which induces cooperative allosteric effects related to those of the pH-dependent neutral-to-base transition. N99D and N99H mutations enhance Zn(2+) binding but alter protein stability, whereas mutation of His(67) to alanine removes an interdomain H-bond and weakens Zn(2+) binding. Both wild-type and mutant albumins promote the safe management of high micromolar zinc concentrations for cells in cultures.

Site-directed mutagenesis study of the role of histidine residues in the neutral-to-basic transition of human serum albumin

Site-directed mutagenesis was used to study the role of histidine residues located in domain I in the neutral-to-basic (N-B) transition of human serum albumin (HSA). Based on a previous study of the N-B transition by means of proton NMR, the following recombinant HSA species were synthesized in the yeast species, Pichia pastoris: H9F, H9S, H39F, H39S, H67F, H67S, H105F, H105S, H128F, H128S, H146F, H146S, and wild type HSA. By monitoring the fluorescent intensity of warfarin bound to the above recombinant human serum albumin species as a function of pH, the mutational effect of individual histidine residues on the N-B transition was examined. While H9, H67, H105, H128 and H146 contribute to the transition significantly, H39 appears to have virtually no contribution to the transition. Based on the X-ray crystallographic structure, it is suggested that electrostatic interactions are the principal factor in determining the histidine pK shifts.

Probing the structure of the warfarin-binding site on human serum albumin using site-directed mutagenesis

The binding of warfarin to the following human serum albumin (HSA) mutants was examined: K195M, K199M, F211V, W214L, R218M, R222M, H242V, and R257M. Warfarin bound to human serum albumin (HSA) exhibits an intrinsic fluorescence that is approximately 10-fold greater than the corresponding signal for warfarin in aqueous solution. This property of the warfarin/HSA complex has been widely used to determine the dissociation constant for the interaction. In the present study, such a technique was used to show that specific substitutions in subdomain 2A altered the affinity of HSA for warfarin. The fluorescence of warfarin/mutant HSA complexes varied widely from the fluorescence of the warfarin/wild-type HSA complex at pH = 7.4, suggesting changes in the structure of the complex resulting from specific substitutions. The fluorescence of the warfarin/wild-type HSA complex increases about twofold as the pH is increased from 6.0 to 9.0 due to the neutral-to-base (N-B) transition, a conformational change that occurs in HSA as a function of pH. Changes in the fluorescence of warfarin/mutant HSA complexes as a function of pH suggests novel behavior for most HSA species examined. For the HSA mutants F211V and H242V, the midpoint of the N-B transition shifts from a wild-type pH of 7.8 to a pH value of 7.1-7.2.

Species differences of serum albumins: III. Analysis of structural characteristics and ligand binding properties during N-B transitions

PURPOSE

The aim of this study was to investigate the characteristics of the structural transitions and changes in ligand binding properties of different albumins during the pH-dependent structural transition, often referred to as the N-B transition.

METHODS

Structural transitions were evaluated by means of spectrometry, differential scanning calorimetry and chemical modification. In addition, ligand binding properties were investigated using typical site-specific bound drugs (warfarin, phenylbutazone, ibuprofen and diazepam).

RESULTS

Conformational changes, including N-B transition, clearly occurred in albumins from all species used in this study. The conformational stabilities of all the albumins were clearly lost in the weakly alkaline pH range. This was probably the result of the destruction of salt bridges between domain I and domain III in the albumin molecule. In addition, the profiles of the ANS-induced fluorescence were different and could be classified into two patterns, suggesting that hydrophobic pockets in the albumin molecules were different for the different species. The data suggest that the amino acid residues responsible for the transitions were some of the His residues located in domain I. Further, the ligand binding properties of the albumins were slightly different but statistically significant.

CONCLUSIONS

The overall mechanisms of the N-B transition may be similar for all the albumins, but its impact is considerably different among the species in terms of both structural characteristics and ligand binding properties. Furthermore, the transitions appear to be multi-step transitions.

pH-induced structural transitions of bovine serum albumin. Histidine pKa values and unfolding of the N-terminus during the N to F transition

1. Titration curves have been constructed for all the resolvable resonances of defatted bovine serum albumin [pH meter readings in 2H2O solution (pH*) 2-10], including complete curves for six His residues and partial curves for a further four. Three His residues have unusually low pKa values (less than 5.5). 2. Abrupt discontinuities in the titration curves of nearly all resonances were observed near pH 4.3 (N to F transition), together with a marked loss of chemical shift dispersion at low pH*. No such loss of dispersion occurs in the high pH region previously associated with the N to B transition (approximately pH 8). The NMR data provide strong evidence that the N to F transition is not localised in domain III as had been previously suggested. 3. A localised unfolding of the N-terminal region occurs below pH* 4.5.

Proton NMR studies of bovine serum albumin. Assignment of spin systems

A variety of one- and two-dimensional 1H-NMR methods have been applied to the study of defatted 66.5-kDa bovine serum albumin in solution. 1. The majority of the protons gave rise to broad unresolved resonances and spectral enhancement methods for one-dimensional spectra were investigated in detail. A combination of exponential and sine-bell functions was particularly effective. 2. The presence of contaminating glycoproteins in some commercial samples of bovine serum albumin was readily detectable from their N-acetyl resonances at about 2.1 ppm. 3. The release of bound Cys (from mixed disulphide at Cys34) was observed after addition of dithiothreitol. 4. Through the use of two-dimensional shift-correlated spectroscopy, assignments of some 80 spin systems to amino acid type were made. 5. The pKa of the N-terminal Asp was measured as 7.8 (0.1 M phosphate buffer, 310 K). 6. 1H NMR spectra of bovine, human, porcine and rat serum albumins have been compared. Using sequence comparisons, specific assignments have been made for the N-terminal residues of bovine (Asp-Thr-His), human (Asp-Ala-His), porcine (Asp-Thr-Tyr) and rat (Glu-Ala-His) albumins, and for Thr189, Tyr155 and His59/377 of bovine albumin. 7. These NMR data suggest that certain local regions of bovine serum albumin are highly mobile yet structured in solution, and demonstrate that the application of both one- and and two-dimensional NMR methods will allow more detailed investigations of structural transitions in serum albumins induced by, for example, pH, drug and metal binding.

Ionization of tyrosine residues in human serum albumin and in its complexes with bilirubin and laurate

Spectrophotometric titration of human serum albumin indicates that ionization of the 18 tyrosine residues takes place between pH 9 and 12.7. A Hill plot indicates that protons dissociate co-operatively from tyrosine residues, in pure albumin between pH 11.0 and 11.4 with a Hill coefficient 1.7, and in the bilirubin-albumin complex between pH 11.2 and 11.7 with a Hill coefficient 1.6. With a stopped-flow technique it is shown that about seven of the tyrosines ionize fast, with rate constants well above 10(2) s-1, when pH is suddenly changed from near neutral to pH 11.76. Further residues ionize slowly, with rate constants around 10(2) s-1 or less. The N-form of albumin (pH 6) contains one more fast ionizing tyrosine than the B-form of albumin (pH 10). Binding of bilirubin or laurate to the albumin molecule (molar ratio 1:1) transforms one to three of the fast ionizing tyrosines to slowly ionizing.

Structural transition of bovine plasma albumin in the alkaline region--the N-B transition

Bovine plasma albumin (BPA) has approximately one SH group (Cys-34) which catalyzes the intramolecular SH, S-S exchange reaction in the alkaline region at low ionic strength, resulting in the formation of the aged form. So, the N-B transition at ionic strength above 0.20 and below 0.10 was studied using BPA and iodoacetamide-blocked BPA (IA-BPA), respectively. (1) pH profiles of [theta]262 and [theta]268 of BPA in 0.20 M KCl showed the characteristic changes in the pH region 7.0-9.0, corresponding to the N-B transition. On going from pH 7.0 to 9.0 in 0.10 M KCl or NaCl, IA-BPA did not show significant changes in rotational relaxation times of tryptophyl fluorophors, CD-resolved secondary structures, spin-echo 1H-n.m.r. spectra and cross-relaxation times (TIS) between irradiated and observed protein protons, which might reflect the rigidity of the domains and/or subdomains. On the other hand, rotational relaxation times of 1-anilino-8-naphthalenesulfonate-IA-BPA complex (IA-BPA-ANS0.9, molar ratio of ANS to IA-BPA = 0.9/1) showed significant decreases from 131 to 114 ns on going from the N- to the B-forms in 0.10 M KCl. The above results and reported experimental evidence might indicate that on going from the N- to the B-forms in 0.10 M KCl or NaCl, the mutual movement of subdomains, connected with a flexible hinge region (Brown & Shockley (1982)) might increase without loss in the helicity and the rigidity of subdomains. (2) The N-B transition of IA-BPA in the absence of salt was quite different from those in 0.10 M KCl or NaCl. Decreases in the helicity and the intramolecular rigidity, as monitored by TIS-measurements, were observed on going from the N- to the B-forms.