Báo cáo khoa học: Role of Tyr84 in controlling the reactivity of Cys34 of human albumin potx

Role of Tyr84 in controlling the reactivity of Cys34 of human albumin Alan J. Stewart 1 , Claudia A. Blindauer 1 , Stephen Berezenko 2 , Darrell Sleep 2 , David Tooth 2 and Peter J. Sadler 1 1 School of Chemistry, University of Edinburgh, Edinburgh, UK 2 Delta Biotechnology Ltd, Nottingham, UK Human albumin (66.5 kDa), a single-chain of 585 amino acids, is the most abundant protein in blood plasma, typically present at concentrations of  0.6 mm [1]. It consists of three structurally homolog- ous, largely helical (67%) domains (I, II and III), each consisting of two subdomains, A and B [2–4]. Like other mammalian albumins, human albumin contains 17 disulfide bridges and a free thiol at Cys34, which provides the largest fraction of free thiol in blood serum. Cys34 is completely conserved within mamma- lian albumins. In plasma, about 30% of the Cys34 thiol is blocked by disulfide bond formation with cys- teine, homocysteine, or glutathione. Moreover, prepa- rations of albumin usually contain 5–10% of dimeric species, with Cys34 being a possible site of dimeriza- tion [1]. Thus, the state of Cys34 is an important ori- gin for heterogeneity in albumin. There has been much interest in the Cys34 site, because not only does it act as a physiological antioxidant [5,6], but also as a bind- ing site for a wide variety of biologically and clinically important small molecules, such as gold(I) antiarthritic drugs [7,8], platinum(II) anticancer drugs [9,10], mer- curials [11], as well as a variety of drugs which bind as mixed disulfides, including captopril (an antihyperten- sive) [12] and disulfiram (an alcohol-abuse drug) [13]. Importantly, 82% of nitric oxide in blood ( 7 lm)is transported as an S-nitrosothiol at Cys34 [14]. Modifi- cations of Cys34 are known to have allosteric effects upon the reactivity and binding properties of other sites on albumin. For example, nitrosylation of Cys34 decreases the binding affinity of albumin toward Cu 2+ ions, phenolsulfophthalein and palmitic acid [15,16], whilst oxidation of Cys34 with cystine results in faster N-homocysteinylation at Lys525 [17]. In order to maintain Cys34 in a reduced state in the majority of albumin molecules in blood, and yet Keywords Cys34; disulfide interchange; human albumin; NMR; thiol Correspondence P. J. Sadler, School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK Fax: +44 131650 6453 Tel: +44 131650 4729 E-mail: p.j.sadler@ed.ac.uk (Received 21 September 2004, revised 5 November 2004, accepted 10 November 2004) doi:10.1111/j.1742-4658.2004.04474.x Cys34 in domain I of the three-domain serum protein albumin is the bind- ing site for a wide variety of biologically and clinically important small molecules, provides antioxidant activity, and constitutes the largest portion of free thiol in blood. Analysis of X-ray structures of albumin reveals that the loop containing Tyr84 occurs in multiple conformations. In structures where the loop is well defined, there appears to be an H-bond between the OH of Tyr84 and the sulfur of Cys34. We show that the reaction of 5,5¢-di- thiobis(2-nitrobenzoic acid) (DTNB) with Tyr84Phe mutant albumin is approximately four times faster than with the wild-type protein between pH 6 and pH 8. In contrast, the His39Leu mutant reacts with DTNB more slowly than the wild-type protein at pH < 8, but at a similar rate at pH 8. Above pH 8 there is a dramatic increase in reactivity for the Tyr84Phe mutant. We also report 1 H NMR studies of disulfide interchange reac- tions with cysteine. The tethering of the two loops containing Tyr84 and Cys34 not only appears to control the redox potential and accessibility of Cys34, but also triggers the transmission of information about the state of Cys34 throughout domain I, and to the domainI ⁄ II interface. Abbreviations DTNB, 5,5¢-dithiobis(2-nitrobenzoic acid); rHA, recombinant human albumin; HSA, human serum albumin; TNB, 5-thio(2-nitrobenzoic acid); amu, atomic mass units. FEBS Journal 272 (2005) 353–362 ª 2004 FEBS 353 simultaneously allow albumin to act as an antioxidant, the reactivity of Cys34 must be finely controlled by the protein environment. Crystallographic studies of albu- min show that Cys34 is buried in a shallow crevice,  9.5 A ˚ deep (Fig. 1A,B). The sulfur atom of Cys34 is close to three ionisable groups: the carboxylate group of Asp38, the imidazole ring from His39, and the hydroxyl group of Tyr84. This environment is likely to be respon- sible for the unusual properties of Cys34. In order to elucidate the factors that control the reactivity of Cys34, we compared the redox activity of native recombinant human albumin with that of two mutants, His39Leu and Tyr84Phe. The latter mutation in particular has a dramatic effect on the reactivity of Cys34. We also dem- onstrate that disulfide bond formation with cysteine at Cys34 leads to structural changes around distant residues. Results and Discussion Reaction of recombinant human albumin with DTNB Disulfide interchange reactions are of importance for the physiological function of albumin and can be conveniently monitored by studies of reactions with AB C D Fig. 1. Structural features of the Cys34 site in human serum albumin. (A) Domain structure of human serum albumin and location of Cys34. The coordinates used are those of pdb 1AO6 (albumin isolated from blood plasma). (B) Cys34 and Tyr84 are located in juxtaposed loops. The yellow ball is the S of Cys34, a potential H-bond from Tyr84 is shown in green. Further residues mentioned in the text are also shown. (C) Superposition of the Cys34 region in 21 published X-ray crystal structures of human albumin, showing Cys34, Asp38, His39 and Tyr84. The overlay has been generated by aligning the backbone atoms of residues Cys34 and His39 in each of the structures, using Swiss pdb viewer (v. 3.7). Structures in blue are fatty acid-free, and structures in green contain bound fatty acid. The structure in magenta (pdb 1UOR; [2]) refers to wild-type fatty-acid-free albumin, but differs substantially from all other structures with respect to the position of the loop con- taining Tyr84, which is more than 12 A ˚ away from Cys34. In three fatty acid-free structures (1E78 (no ligands), 1E7A (with bound propofol), 1E7B (with bound halothane)), the side chain of Tyr84 is not resolved, and in three further fatty acid-free structures (1HK1, 1HK2, 3HK3, with bound thyroxine), the entire loop from Val77 to Ala88 is not resolved. (D) Schematic showing secondary structure elements and disulfide connectivities around Cys34 and Tyr84. The location of His residues likely to be perturbed by reactions at Cys34 is also shown. Residues are labelled in one-letter code, and lower-case ‘h’ signifies helices. Redox activity of albumin Cys34 A. J. Stewart et al. 354 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS 5,5¢-dithiobis(2-nitrobenzoic acid) (DTNB) or 2,2¢- dithiodipyridine, which are known to react specifically with free sulfhydryl groups [18]. We determined appar- ent rate constants for the reactions of wild-type recom- binant human albumin (rHA) and His39Leu and Tyr84Phe rHA mutants with DTNB and their depen- dences on pH. Mass spectrometric analysis of the product from the reaction of rHA [experimentally determined mass, 66 438 atomic mass units (amu); the- oretical mass, 66 438 amu] with DTNB gave a peak corresponding to 66 643 amu. This is consistent with the formation of a 1 : 1 albumin ⁄ TNB mixed disulfide, which has a theoretical mass of 66 645 amu. In order to allow the determination of rates using conventional absorption spectroscopy, the kinetic studies were car- ried out at 283 K, and over the pH range 6.0–10.2, under pseudo-first order conditions. As expected, the rates are pH-dependent (Fig. 2). The reactivity of Cys34 appears to rise sharply between pH 6 and 8 for all these albumins. The variation of rate constant with pH for wild-type albumin follows a similar trend to that reported by Pedersen and Jacobsen [19], who studied reactions of human serum albumin (HSA) with 2,2¢-dithiodipyridine over the pH range 3–9. Between pH 6 and pH 8, the reaction is approxi- mately four times faster for the Tyr84Phe mutant than for the wild-type protein. In contrast, the His39Leu mutant reacts with DTNB much more slowly than the wild-type protein below pH 8, but at a similar rate at pH 8. Between pH 8 and 8.5 there is a pronounced increase in reactivity for each of the albumins; how- ever, the effect is much more dramatic for the Tyr84Phe mutant. Cys34 reactivity for wild-type albu- min appears to plateau at around pH 8.4, whilst signi- ficant increases in Cys34 reactivity were observed for the His39Leu and Tyr84Phe mutants until the pH was increased to > 9. At pH 10.2, the reaction with DTNB was approximately three times faster for the His39Leu mutant, and 170 times faster for the Tyr84Phe mutant than for wild-type rHA. It is generally thought that disulfide interchange reactions proceed via a nucleophilic attack of the deprotonated thiolate on the disulfide [20]. Therefore, an increase in reaction rate with increasing pH is expected. In simple cases, the pH dependence of the reaction rate represents the titration curve for the pro- tein thiol, but in the case of albumin other factors, most notably conformational transitions, also play a role. Nevertheless, from the point at which the plateau in the pH profile is reached, an upper limit for the thiol pK a can be estimated by subtracting 1.7 pH units, based on the assumption that only deprotonated thio- late is present in the plateau region. This suggests that the thiol of Cys34 has a pK a lower than 6.7 for the wild-type protein, a finding consistent with previously suggested literature values of  7 [20]. Thus, the pK a of Cys34 in recombinant HSA is substantially lower than the average value of 8–9, which is typical for a free cysteine side chain. Abnormally low pK a values have been reported for a number of proteins. In most cases, electrostatic interactions with positively charged side chains or the dipole of helices have been identified as the basis for low thiol pK a values. In several cases (e.g. papain [20], protein tyrosine phosphatases [21], O 6 -alkylguanine-DNA alkyltransferases [22] and some proteins of the thioredoxin family [23]) the low pK a is due to the formation of an ion pair with protonated histidine. The proximity to a His residue can lower the pK a by up to 5 units. Our data show that the lowering of the pK a of Cys34 can be ascribed partly to its proxi- mity to His39 and Tyr84, as both mutants exhibit pK a values that are  0.5 pH units higher. Both Tyr84Phe and His39Leu mutations involve replacement of a polar residue by a nonpolar one. This is expected to Fig. 2. Pseudo-first order rate constants for the reaction of DTNB with rHA and mutant albumins over the pH range 6.0–10.2. The acid dissociation constants of the carboxyl groups in DTNB are 1.57 and 2.15 [39] and the pK a of the thiol of TNB is 4.50 [36]. Therefore the carboxyl groups of DTNB will be deprotonated and only the S – form of the TNB product is present over the pH range studied (pH 6.0–10.2). Note that at pH 6.0 and 6.2 no detectable activity was observed with the His39Leu mutant, these points are there- fore not present on the graph due to the logarithmic scale. A. J. Stewart et al. Redox activity of albumin Cys34 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS 355 lead to an increase in thiol pK a , as charged species are stabilized by a polar environment, and destabilized by nonpolar surroundings. Albumin is known to undergo a structural transition above pH 8 from the N (neutral) to B (basic) form of the protein [24]. It is thought that the N-to-B-trans- ition involves a ‘loosening-up’ of the entire protein structure [24], and that this transition might have a role in the delivery of metabolites, e.g. to the liver. The formation of the B-conformer is likely to account for the steep rise in reactivity between pH 8 and 8.5. The enhanced antioxidant activity of HSA at alkaline pH has previously been attributed to the B conforma- tion [25], and Cornell et al. [26] concluded that the Cys34 crevice opens in the B conformation. Our results regarding the Tyr84Phe mutant provide a clue as to how this can be achieved. Apparently, the removal of the hydroxyl group dramatically enhances the redox activity of Cys34. A survey of the environ- ment of Cys34 in all published X-ray crystal structures of albumin (Fig. 1C) reveals that there is a consider- able variability with respect to the position of Tyr84. Remarkably, in a number of crystal structures of fatty acid-free albumin (pdb codes 1E78, 1E7A, 1E7B, 1HK1, 1HK2, 1HK3), Tyr84 does not seem to be resolved. Tyr84 is situated in a loop, and in the former three structures, no electron density for the side chain of Tyr84 is resolved, whereas in the latter three, the entire loop from residue 77 to residue 88 is absent, indicating conformational flexibility in this region. In all but two of the structures in which Tyr84 is resolved, however, the hydroxyl oxygen is within hydrogen-bonding distance to the Cys34 sulfur (O–S distances vary between 2.7 and 3.4 A ˚ ). Tyr84 is thus located on a flexible loop between two short helices, and is tethered to the Cys34 site by a hydrogen bond (Fig. 1B,D). This appears to be the only H-bond that stabilizes the relative positions of the Tyr84 loop and the Cys34 loop, although the Tyr84 loop itself is con- nected via two disulfide bonds (C75 ⁄ 91 and C90 ⁄ 101) to helices 4 and 5 (Fig. 1D). Such hydrogen bonding is expected to stabilize an anionic thiolate side chain of Cys34, similar to the way in which glutathione S-transferases acidify the SH group of glutathione by the formation of a hydrogen bond with an active site Tyr OH group [27]. Also such an environment is likely to stabilize the reduced thiol form [28]. The tethering of the loop makes Cys34 less accessible than it would otherwise be. Thus, the role of Tyr84 is threefold: it enhances the nucleophilicity of the sulfur of Cys34, contributes to its relatively high redox potential, and simultaneously restricts access. The stabilizing action of the Tyr84-OHÆÆÆS-Cys34 H-bond appears to be especially important when other stabilizing H-bonds are cleaved, as is thought to occur during the N-to-B transition. Tyr84 is known to be the cleavage site for chymase, a serine peptidase enzyme secreted by mast cells [29]. It is likely that chymase-cleaved albumin molecules in the blood have altered Cys34 reactivity in vivo. At pre- sent, the extent and consequences of chymase-cleavage on albumin in the body is unknown. Several studies suggest that chymase is also expressed in the liver, where albumin is synthesized. Chymase expression in the liver is increased in patients who suffer from auto- immune disease [30]. The lowered reactivity of DTNB toward the His39Leu mutant compared to wild-type rHA at pH values < 8 indicates that His39 also has a role in con- trolling the reactivity of Cys34. At low pH, His39 might increase reactivity by formation of an ion pair, as noted for other proteins with reactive Cys residues [28]. Reaction of recombinant serum albumins with cystine Previous 1 H NMR studies have shown that the His He1 resonance of His3 is sensitive to reactions at Cys34 [31]. In most albumin preparations, whether isolated from plasma or produced by recombinant techniques, two sets of peaks are observed for His3, with the lower inten- sity set ( 30%) being assignable to albumin containing oxidized (or blocked) Cys34 (Fig. 3, peak 11b; Fig. 4A, His3¢). The nature of the conformational change which occurs at His3 on oxidizing or blocking Cys34 is not clear, but in the case of reactions with antiarthritic gold compounds or disulfide-bond forming drugs, it has been suggested that Cys34 becomes more exposed [31]. We acquired 1D and 2D NMR spectra of the His39Leu, Tyr84Phe, and Cys34Ala mutants, to enable identification of His39 and assess the folding behaviour of the mutant proteins. Despite recent advances in pro- tein NMR, the size of albumin and its dynamic prop- erties do not allow a complete sequential assignment of its resonances. However, by employing resolution enhancement during data processing, it is possible to resolve sharp resonances for slowly relaxing protons [31]. We focus here on the He1 resonances of His side chains, which can be conveniently observed in the low- field region of spectra for samples in which backbone NH protons have been exchanged with deuterium. The low-field region of resolution-enhanced 1D 1 H and 2D[ 1 H, 1 H] TOCSY spectra of wild-type rHA, and Redox activity of albumin Cys34 A. J. Stewart et al. 356 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS the mutants Cys34Ala rHA, His39Leu rHA, and Tyr84Phe rHA are shown in supplementary Figs S1– S4. Overall, the spectra of the wild-type and mutant proteins are similar. Small differences in chemical shifts are due mainly to small variations (< 0.1 pH unit) in the pH* of the solutions. As the chosen pH* is close to the pK a values of histidine residues, small changes in pH have significant effects on chemical shifts of histidine protons. Human albumin contains 16 histidine residues. In the spectra of the wild-type and Cys34Ala mutant pro- tein, 11–12 major resonances from histidine He1 pro- tons can be distinguished between about 7.8 and 8.5 p.p.m. With the aid of 2D TOCSY spectra (supple- mentary Figs S1–S4), most of the corresponding Hd2 protons can be identified as well. Comparison of the 1 H NMR spectra of the wild- type and His39Leu rHA show that none of the sharp He1 resonances has disappeared. However, the relat- ively broad resonance at 8.115 p.p.m. (Fig. 3, 7) dis- appears upon mutation of His39. Therefore resonance 7 can be assigned to the He1 proton of His39. Due to relaxation phenomena, it is expected for a protein the size of albumin that only flexible side chains give rise to sharp resonances; protons of buried residues gener- ally have broad lines. The appearance of the His39 peak suggests that it is buried to some extent, and indeed, in the published X-ray crystal structures of albumin [3,4] His39 lies slightly deeper in the crevice that contains Cys34. The 1D 1 H spectra of Tyr84Phe rHA contain broader lines than those of wild-type rHA and the other mutants, suggesting that this mutation changes the dynamic behaviour of the protein. In contrast, the His39 resonance in the spectrum of the Tyr84Phe mutant appears slightly sharper than for wild-type rHA, which is consistent with an increased mobility of His39 in the mutant protein in the crevice holding Cys34 and His39. This observation supports the idea that the hydroxyl group of Tyr84 is essential for main- taining a closed crevice. The pair of peaks 11 and 11b (Fig. 3) has previously been assigned to His3 [31]. In this context it is import- ant to note that 1 H spectra of the Cys34Ala mutant show only a single peak for He1 of His3 (see 2D NMR data in supplementary Figs S1–S4). The reaction of albumin with cystine, leading to disulfide formation at Cys34, is of physiological importance in blood. We monitored the reaction of wild-type rHA with a 2.5-fold molar excess of l-cystine by 1D 1 H NMR in 100 mm potassium phosphate buf- fer (Fig. 4) at pH 7.0. Due to the use of resolution enhancement, analysis of the area of His peaks cannot be made in a quantita- tive fashion, but qualitative observations have import- ant implications. It appears that small local alterations have a substantial effect on the entire domain I, and even on the interface between domains I and II. Our data reveal that not only the He1 resonance of His3, but up to six other His He1 resonances, are affected by the disulfide interchange reaction at Cys34. Fig. 4A shows that the intensity of His3¢ resonance increases during the reaction (any decrease in intensity of peak H3 cannot directly be observed as it is over- lapped by peak 12). The effects on the pair of peaks 6 ⁄ 6b are more readily seen: peak 6 decreases, and peak 6b increases in intensity. As for His3, there is only a single He1 resonance (peak 6) for this histidine residue in the Cys34Ala mutant (Fig. 3). Notably, the low-field He1 peaks 1, 2, 3 and 4 are also perturbed during the reaction. Peak 1 gradu- ally decreased in intensity over time, with no new resonance being detectable. Peak 4 also decreased, and a new resonance appeared (4b) next to it. A minor Fig. 3. 1D 1 H NMR spectra of recombinant albumins. Solutions of respective albumins (1 m M) were prepared in 50 mM Tris ⁄ DCl, 50 m M NaCl, pH* 7.33–7.35. He1 proton resonances are labelled with numbers from 1 to 12, f denotes formate, added as a chem- ical shift reference. Peaks 7 and 11 are assigned to His39 and His3, respectively. The arrows indicate resonances that are absent in the respective spectra, e.g. the resonance 6b is not observed in the Cys34Ala mutant, suggesting that for the wild-type protein it pertains to a second conformation of His proton 6 in a species with blocked or oxidized Cys34. A. J. Stewart et al. Redox activity of albumin Cys34 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS 357 decrease in intensity was also noticeable for peaks 2 and 3. Finally, the resonance for His39 He1 became somewhat sharper during the reaction. Similar obser- vations were made for the Tyr84Phe mutant (Fig. 4B). Two of these peaks (1 and 4) can be assigned to His67 and His247 (CA Blindauer, KE Bunyan, AJ Stewart, D Sleep, S Berezenko & PJ Sadler, unpublished results). In a typical crystal structure of fatty acid-free albu- min (pdb: 1AO6), the His39 He1 proton is within 4 A ˚ of the Cys34 sulfur; thus it is not surprising that a reaction at Cys34 affects this proton. However, the dis- tances between Cys34 and the next nearest six His resi- dues in albumin are between 16 A ˚ (His67 and His105) and 29 A ˚ (His9). His146 and His247 are about 20 and 22 A ˚ away, and His242 is 26 A ˚ from Cys34. The dis- tance between His3 and Cys34 in (fatty acid loaded) albumin (pdb: 1BJ5; His3 is not resolved in structures of fatty acid-free albumin) is over 30 A ˚ . Clearly, reac- tions at Cys34 have far-reaching allosteric effects, but how are these effects transmitted throughout domain I and beyond? The largely helical structure of albumin makes it an extremely flexible molecule; the interhelix contacts mainly being stabilized by disulfide bonds and weak interactions. We can speculate that the formation of a Cys disulfide in the Cys34-containing crevice will have a major effect on the Tyr84 containing loop, and is likely to lead to the loss of the H-bond between Cys34 and Tyr84. From Fig. 2D it is evident that movement of the Tyr84 loop will affect the two helices (4 and 5) to which it is connected. His67 is at the start of helix 4, and Asn99 at the start of helix 5. His247 is in close proximity to His67, as both residues are connected via hydrogen bonds to Asp249; these three residues form, together with Asn99, the interdomain high-affinity zinc site on albumin [32]. In this way, the interface between domains I and II is affected by reac- tions at Cys34. His105, which is situated in the loop following helix 5, is also likely to be influenced by movements of the Tyr84 loop. The three domains of mammalian albumins are structurally homologous, including the arrangement of disulfide pairs. Interestingly however, the disulfide bonding pattern for the first 50 residues in domain I differs from that of the other two domains. The first two helices in the albumin sequence in domain I are not tethered by disulfide bridges. Cys34 is located in the loop after helix 2, thus a structural change in this loop is also likely to alter the orientation of these two helices with respect to the entire domain. This assump- tion could account for the effects observed for His3, but also implies that His9 might be affected in a sim- ilar manner. In this context it is noteworthy that resi- due 35 is proline; and it has previously been suggested that the structural change occurring upon reactions at Cys34 might involve a cis –trans isomerization of this residue [31]. In conclusion, we have shown that both His39 and Tyr84 influence the reactivity of Cys34, with Tyr84 Fig. 4. 1 H NMR studies of the reaction of wild-type and Tyr84Phe mutant rHA with L-cystine. Conditions: 1 mM respective albumin, pH* 7.0, 100 m M potassium phosphate buffer, 2.5 molar excess of L-cystine, 310 K. (A) Recombinant wild-type albumin. (B) Tyr84Phe mutant rHA. Redox activity of albumin Cys34 A. J. Stewart et al. 358 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS playing a pivotal role in both lowering the pK a of the thiol and contributing to its high redox potential. In addition, the loop containing Tyr84 appears to be vital in controlling accessibility to Cys34, and is tethered to the Cys34-containing loop by a Tyr84-OHÆÆÆS-Cys34 hydrogen bond. We also find that physiologically rele- vant reactions at Cys34 such as disulfide bond forma- tion with half cystine have an impact on the conformation and dynamics of the entire domain I, and on the domain I ⁄ II interface, with the Tyr84 loop again being involved in some of the observed struc- tural changes. This has a number of implications. Allosteric effects on Tyr84 caused by ligand binding elsewhere on the molecule, or post-translational modi- fications (such as cleavage by chymase) are likely to alter Cys34 reactivity in vivo. Consequently, a signifi- cant reduction or increase in Cys34 reactivity is likely to affect circulatory processes such as the transport of nitric oxide, removal of reactive oxygen species, or indeed, drug binding and transport. Mutant albumins with altered reactivity at this site could prove useful for scavenging reactive oxygen species or could be administered to increase the efficacy of Cys34-specific therapeutics. Experimental procedures Mutagenesis, protein expression and purification Oligonucleotide-directed mutagenesis was used to prepare cDNAs encoding the mutated albumins. Mutagenesis was performed by the method of Kunkel [33]. A clone contain- ing the desired mutation was identified by nucleotide sequence analysis across the mutation site by the dideoxy chain termination sequencing. The mutated cDNA was inserted into a pAYE316 based yeast expression plasmid [34] and Saccharomyces cerevisiae strain DXY1 [35] was transformed to leucine prototrophy by electroporation. Albumin was expressed in S. cerevisiae DXY1 cells and purified by cation-exchange chromatography on SP-Seph- arose (Amersham Bioscience, Buckinghamshire, UK; final elution 85 mm sodium acetate containing 5 mm octanoic acid, pH 5.5), anion-exchange chromatography on DEAE- Sepharose (Amersham Biosciences; elution with 110 mm borate, pH 9.4) and by affinity chromatography on Delta Blue Agarose (ProMetic Biosciences; elution with 50 mm phosphate buffer containing 2 m NaCl, pH 6.9). Purity of the proteins by SDS ⁄ PAGE was > 99%, and ESI-MS of native and mutant proteins gave peaks within 4 amu of the calculated masses. CD was carried out as described in [32]. CD spectra of the Cys34Ala, His39Leu and Tyr84Phe mutants were similar to the native protein showing that the mutations did not cause any significant changes in secon- dary structure. Aliquots of the mutant proteins (10 mL in 50 mm phosphate buffer ⁄ 2 m NaCl) and native rHA (3.8 mm, 145 mm NaCl, containing 15 mgÆL )1 Tween-80 and 40 mm octanoic acid) were routinely dialysed twice in 100 mm ammonium carbonate (277 K, 5 L, 24 h) before use. Dialysis reduced the amount of octanoate bound to rHA (from  8to<4molÆmol protein )1 ). Mass spectrometry rHA (15 lm) was incubated with 40 molar equivalents of DTNB in 100 mm potassium phosphate pH 8.0 for 1 h at 298 K. All solutions were then desalted ⁄ concentrated using reversed phase solid phase extraction with recovered protein at concentrations of  20 lm. The solid phase extraction protocol used mobile phase flow-through cartridges (Inter- national Sorbent Technology Ltd, Hengoed, UK) under vacuum. Protein samples were loaded in aqueous solutions and eluted using 70% acetonitrile in 0.2% formic acid (v ⁄ v). The protein solution was introduced into a triple quadru- pole mass spectrometer (Micromass Quattro, Elstree, Hertfordshire, UK), equipped with a conventional geometry AP-ESI source in positive ion mode, using flow injection analysis and 20 scans were typically averaged. For protein analysis, the mass spectrometer was calibrated against the protonated molecular ions of horse heart myoglobin (Sigma, Poole, Dorset, UK) with resolution set similar to 2000. Peptide samples were introduced using custom nanoelectro- spray ion sources (both continuous flow ⁄ on-line and off-line nanovial configurations) in positive ion mode. MS and tan- dem MS product ion spectra were acquired with significant scan averaging and the analysers were calibrated against protonated and sodiated ions from a mixture of polyethy- lene glycol. Disulfide interchange reaction with DTNB Reactions mixtures were set up as follows: 1.35 mL of respective buffer, 1.35 mL of 50 mm KCl, 150 l Lof15mm DTNB (dissolved in methanol). The following buffers were used: 0.2 m potassium phosphate (pH range 6.0–8.0), 0.2 m Tris ⁄ HCl (pH range 8.1–9.0) and 0.2 m, CAPS (pH range 9.1–10.2). It has been noted previously for the reaction of BSA with DTNB that the rates are sensitive to ionic strength but apparently independent of the buffer used [36]. The ionic strengths of the buffers were therefore kept con- stant and were adjusted to that of 0.2 m phosphate (0.466 m) with KCl when necessary. The reaction mixture was equilibrated in the cuvette at 283 K whilst mixing. Following equilibration, the reaction was initiated by addition of 150 lL of 150 lm albumin, at 283 K. The reactions were performed at 283 K so that the rates were slow enough to measure by conventional UV-visible spectroscopy. All reactions were carried out in 1-cm pathlength cells with stirring and were followed by the A. J. Stewart et al. Redox activity of albumin Cys34 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS 359 change in absorbance at 412 nm corresponding to the formation of the TNB product (e 412 ¼ 13.9 mm )1 Æcm )1 [37]) on a Cary 300 Scan spectrophotometer (Varian Ltd, Walton-on-Thames, UK) fitted with a Peltier dual cell tem- perature controller. The thiol contents, as determined from the absorbance of TNB at 412 nm after completion of the reaction, of rHA, His39Leu and Tyr84Phe albumins were 0.68, 0.63 and 0.63 molÆmol )1 , respectively. 1 H NMR spectroscopy To eliminate NH resonances, lyophilized samples of albu- min were dissolved in D 2 O (99.9% isotopic purity; Aldrich, Gillingham, Dorset, UK) at  50 mgÆmL )1 , kept at 277 K for 48 h, lyophilized, and then dissolved to give a 1 mm protein solution in D 2 O containing either 50 mm NaCl, 50 mm Tris, or 100 mm potassium phosphate. Sodium for- mate was added at a concentration of 1 mm as internal cal- ibration standard [8.48 p.p.m. at pH* 7 relative to sodium 3-(trimethylsilyl) propionate]. The pH meter reading for D 2 O solutions (pH*, calibrated with H 2 O buffers) is not corrected for the effect of deuterium on the glass electrode; the corresponding pD value can be obtained by adding 0.4 units to pH* [38]. pH* values were adjusted with 1 m DCl or 2 m NaOD. 1D and 2D 1 H NMR experiments were car- ried out at 310 K on a Bruker Avance 600 spectrometer (Coventry, UK) operating at 599.82 MHz using a Z-gradi- ent triple-resonance ( 1 H, 13 C, 15 N) probe head. Typically, 512 transients were acquired for the 1D spectra (90° excita- tion pulse, 9 kHz sweepwidth, 8 k time domain data points) using a presaturation pulse sequence for residual water suppression (1.5 s relaxation delay). The data were zero- filled to 32 k, apodized with an optimized combination of squared-sinebell and Gaussian functions for resolution enhancement, and Fourier transformed. For 2D TOCSY experiments (90° excitation pulse, 8.4 kHz frequency width, mixing time 65 ms, 1.3 s relaxation delay), 48 or 56 transients for each of 2 · 512 t 1 increments (hypercomplex acquisition using time-proportional phase incrementation) were acquired into 4 k complex data points, using a sensitivity-enhanced, double-pulsed field-gradient spin-echo sequence for residual water suppression. The data were apodized using squared-sinebell functions, and the real Fourier transform was carried out on 2 k · 2 k data points. Reaction with cystine followed by 1D 1 H NMR l-Cystine (Sigma) was dissolved in 2 m NaOD in D 2 Otoa final concentration of 100 mm. After the recording of an initial spectrum of 1 mm rHA (wild-type or Tyr84Phe) in 100 mm potassium phosphate (in D 2 O; pH* 7.0), an aliquot corresponding to a 2.5-fold molar excess of cystine was added directly into the NMR tube. The pH* was readjusted to 7.0 with 1 m DCl, and 1D spectra were recorded at 20-min intervals. At this pH* the reaction was slow enough to be followed by NMR. As the observed reaction involves (de)protonation steps, we used potassium phosphate in these experiments instead of Tris, since it is a more effective buffer in this pH* range. During these studies we noted that the chemical shifts of the histidine He1 protons are also dependent on the nature of the buffer. Generally, sig- nals were shifted upfield in potassium phosphate buffer compared to spectra taken at the same pH* in Tris buffer. 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Biochem J 133, 67–80. Redox activity of albumin Cys34 A. J. Stewart et al. 362 FEBS Journal 272 (2005) 353–362 ª 2004 FEBS . Structural features of the Cys34 site in human serum albumin. (A) Domain structure of human serum albumin and location of Cys34. The coordinates used are those of pdb 1AO6 (albumin isolated from. dramatic increase in reactivity for the Tyr84Phe mutant. We also report 1 H NMR studies of disulfide interchange reac- tions with cysteine. The tethering of the two loops containing Tyr84 and Cys34. disulfide bonding pattern for the first 50 residues in domain I differs from that of the other two domains. The first two helices in the albumin sequence in domain I are not tethered by disulfide bridges. Cys34