Ab initio determination of the crystal structure of cytochrome c6 and comparison with plastocyanin

Dynamic interaction of plastocyanin with the cytochrome bf complex

The interaction between plastocyanin and the intact cytochrome bf complex, both from spinach, has been studied by stopped-flow kinetics with mutant plastocyanin to elucidate the site of electron transfer and the docking regions of the molecule. Mutation of Tyr-83 to Arg or Leu provides no evidence for a second electron transfer path via Tyr-83 of plastocyanin, which has been proposed to be the site of electron transfer from cytochrome f. The data found with mutations of acidic residues indicate that both conserved negative patches are essential for the binding of plastocyanin to the intact cytochrome bf complex. Replacing Ala-90 and Gly-10 at the flat hydrophobic surface of plastocyanin by larger residues slowed down and accelerated, respectively, the rate of electron transfer as compared with wild-type plastocyanin. These opposing effects reveal that the hydrophobic region around the electron transfer site at His-87 is divided up into two regions, of which only that with Ala-90 contributes to the attachment to the cytochrome bf complex. These binding sites of plastocyanin are substantially different from those interacting with photosystem I. It appears that each of the two binding regions of plastocyanin is split into halves, which are used in different combinations in the molecular recognition at the two membrane complexes.

Site-directed mutagenesis of cytochrome c(6) from Anabaena species PCC 7119. Identification of surface residues of the hemeprotein involved in photosystem I reduction

A number of surface residues of cytochrome c(6) from the cyanobacterium Anabaena sp. PCC 7119 have been modified by site-directed mutagenesis. Changes were made in six amino acids, two near the heme group (Val-25 and Lys-29) and four in the positively charged patch (Lys-62, Arg-64, Lys-66, and Asp-72). The reactivity of mutants toward the membrane-anchored complex photosystem I was analyzed by laser flash absorption spectroscopy. The experimental results indicate that cytochrome c(6) possesses two areas involved in the redox interaction with photosystem I: 1) a positively charged patch that may drive its electrostatic attractive movement toward photosystem I to form a transient complex and 2) a hydrophobic region at the edge of the heme pocket that may provide the contact surface for the transfer of electrons to P(700). The isofunctionality of these two areas with those found in plastocyanin (which acts as an alternative electron carrier playing the same role as cytochrome c(6)) are evident.

Extinction coefficients and midpoint potentials of cytochrome c(6) from the cyanobacteria Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803

Cytochrome c(6) is a soluble heme protein that serves as a photosynthetic electron transport component in cyanobacteria and algae, carrying electrons from the cytochrome bf complex to photosystem I. The rapid accumulation of cytochrome c(6) sequence data from a wide range of species, combined with significant advances in determining high resolution three-dimensional structures, provides a powerful database for investigating the relationship between structure and function. The fact that the gene encoding cytochrome c(6) can be readily modified in a number of species adds to the usefulness of cytochrome c(6) as a tool for comparative analysis. Efforts to relate cytochrome c(6) sequence information to structure, and structural information to function depend on knowledge of the physical and thermodynamic properties of the cytochrome from different species. To this end we have determined the optical extinction coefficient, the oxidation/reduction midpoint potential, and the pH dependence of the midpoint potential of cytochrome c(6) isolated from three cyanobacteria, Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803.

Crystal structures of myoglobin-ligand complexes at near-atomic resolution

We have used x-ray crystallography to determine the structures of sperm whale myoglobin (Mb) in four different ligation states (unligated, ferric aquomet, oxygenated, and carbonmonoxygenated) to a resolution of better than 1.2 A. Data collection and analysis were performed in as much the same way as possible to reduce model bias in differences between structures. The structural differences among the ligation states are much smaller than previously estimated, with differences of <0.25 A root-mean-square deviation among all atoms. One structural parameter previously thought to vary among the ligation states, the proximal histidine (His-93) azimuthal angle, is nearly identical in all the ferrous complexes, although the tilt of the proximal histidine is different in the unligated form. There are significant differences, however, in the heme geometry, in the position of the heme in the pocket, and in the distal histidine (His-64) conformations. In the CO complex the majority conformation of ligand is at an angle of 18 +/- 3 degrees with respect to the heme plane, with a geometry similar to that seen in encumbered model compounds; this angle is significantly smaller than reported previously by crystallographic studies on monoclinic Mb crystals, but still significantly larger than observed by photoselection. The distal histidine in unligated Mb and in the dioxygenated complex is best described as having two conformations. Two similar conformations are observed in MbCO, in addition to another conformation that has been seen previously in low-pH structures where His-64 is doubly protonated. We suggest that these conformations of the distal histidine correspond to the different conformational substates of MbCO and MbO(2) seen in vibrational spectra. Full-matrix refinement provides uncertainty estimates of important structural parameters. Anisotropic refinement yields information about correlated disorder of atoms; we find that the proximal (F) helix and heme move approximately as rigid bodies, but that the distal (E) helix does not.

Amino acid sequence, crystallization and structure determination of reduced and oxidized cytochrome c6 from the green alga Scenedesmus obliquus

Cytochrome c6from the unicellular green alga Scenedesmus obliquus was sequenced, crystallized in its reduced and oxidized state and the three-dimensional structure of the protein in both redox states was determined by X-ray crystallography. Reduced cytochrome c6crystallized as a monomer in the space group P 21212, whereas the oxidized protein crystallized as a dimer in the space group P 3121. The structures were solved by molecular replacement and refined to 1. 9 and 2.0 A, respectively. Comparison of the structures of both redox states revealed only slight differences on the protein surface, whereas a distortion along the axis between the heme iron and its coordinating Met61 residue was observed. No redox-dependent movement of internal water molecules could be detected. The high degree of similarity of the surfaces and charge distributions of both redox states, as well as the dimerization of cytochrome c6as observed in the oxidized crystal, is discussed with respect to its biological relevance and its implications for the reaction mechanisms between cytochrome c6and its redox partners. The dimer of oxidized cytochrome c6may represent a molecular structure occurring in a binary complex with cytochrome b6f. This assembly might be required for the correct orientation of cytochrome c6with respect to its redox partner cytochrome b6f, facilitating the electron transfer within the complex. If the dimerization is not redox-dependent in vivo, the almost identical surfaces of both redox states do not support a long range differentiation between reduced and oxidized cyt c6, i.e. a random collision model for the formation of an electron transfer complex must be assumed.

Oxidizing side of the cyanobacterial photosystem I. Evidence for interaction between the electron donor proteins and a luminal surface helix of the PsaB subunit

Photosystem I (PSI) interacts with plastocyanin or cytochrome c6 on the luminal side. To identify sites of interaction between plastocyanin/cytochrome c6 and the PSI core, site-directed mutations were generated in the luminal J loop of the PsaB protein from Synechocystis sp. PCC 6803. The eight mutant strains differed in their photoautotrophic growth. Western blotting with subunit-specific antibodies indicated that the mutations affected the PSI level in the thylakoid membranes. PSI proteins could not be detected in the S600R/G601C/N602I, N609K/S610C/T611I, and M614I/G615C/W616A mutant membranes. The other mutant strains contained different levels of PSI proteins. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-mediated electron transfer activity when either cytochrome c6 or an artificial electron donor was used. In contrast, cytochrome c6 could not function as an electron donor to the W622C/A623R mutant, even though the PSI activity mediated by an artificial electron donor was detected in this mutant. Thus, the W622C/A623R mutation affected the interaction of the PSI complex with cytochrome c6. Biotin-maleimide modification of the mutant PSI complexes indicated that His-595, Trp-622, Leu-628, Tyr-632, and Asn-638 in wild-type PsaB may be exposed on the surface of the PSI complex. The results presented here demonstrate the role of an extramembrane loop of a PSI core protein in the interaction with soluble electron donor proteins.

Site-directed mutagenesis of cytochrome c6 from Synechocystis sp. PCC 6803. The heme protein possesses a negatively charged area that may be isofunctional with the acidic patch of plastocyanin

This paper reports the first site-directed mutagenesis analysis of any cytochrome c6, a heme protein that performs the same function as the copper-protein plastocyanin in the electron transport chain of photosynthetic organisms. Photosystem I reduction by the mutants of cytochrome c6 from the cyanobacterium Synechocystis sp. PCC 6803 has been studied by laser flash absorption spectroscopy. Their kinetic efficiency and thermodynamic properties have been compared with those of plastocyanin mutants from the same organism. Such a comparative study reveals that aspartates at positions 70 and 72 in cytochrome c6 are located in an acidic patch that may be isofunctional with the well known "south-east" patch of plastocyanin. Calculations of surface electrostatic potential distribution in the mutants of cytochrome c6 and plastocyanin indicate that the changes in protein reactivity depend on the surface electrostatic potential pattern rather than on the net charge modification induced by mutagenesis. Phe-64, which is close to the heme group and may be the counterpart of Tyr-83 in plastocyanin, does not appear to be involved in the electron transfer to photosystem I. In contrast, Arg-67, which is at the edge of the cytochrome c6 acidic area, seems to be crucial for the interaction with the reaction center.

Brownian dynamics study of the interaction between plastocyanin and cytochrome f

The electrostatic interaction between plastocyanin (PC) and cytochrome f (cyt f), electron transfer partners in photosynthesis was studied using Brownian dynamics (BD) simulations. By using the software package MacroDox, which implements the BD algorithm of Northrup et al. (Northrup, S. H., J. O. Boles, and J. C. L. Reynolds. 1987. J. Phys. Chem. 91:5991-5998), we have modeled the interaction of the two proteins based on crystal structures of poplar PC and turnip cyt f at pH 7 and a variety of ionic strengths. We find that the electrostatic attraction between positively charged residues (K58, K65, K187, and R209, among others) on cyt f and negatively charged residues (E43, D44, E59, and E60, among others) on PC steers PC into a single dominant orientation with respect to cyt f, and furthermore, that the single dominant orientation that we observe is one that we had predicted in our previous work (Pearson, D. C., E. L. Gross, and E. S. David. 1996. Biophys. J. 71:64-76). This dominant orientation permits the formation of hydrophobic interactions, which are not implemented in the MacroDox algorithm. This proposed complex between PC and cyt f implicates H87, a copper ligand on PC, as the residue that accepts electrons from the heme on cyt f (and possibly through Y1 as we proposed previously). We argue for the existence of this single dominant complex on the basis of observations that the most favorable orientations of the interaction between PC and cyt f, as determined by grouping successful BD trajectories on the basis of closest contacts of charged residues, tend to overlap one another and have very close distances between the metal centers on the two proteins (copper on PC, iron on cyt f). We use this knowledge to develop a model for PC/cyt f interaction that places a reaction between the two proteins occurring when the copper-to-iron distance is between 16 and 17 A. This reaction distance gives a good estimate of the experimentally observed rate constant for PC-cyt f interaction. Analysis of BD results as a function of ionic strength predicts an interaction that happens less frequently and becomes less specific as ionic strength increases.

The 1.25 A resolution refinement of the cholera toxin B-pentamer: evidence of peptide backbone strain at the receptor-binding site

Crystals of the 61 kDa complex of the cholera toxin B-pentamer with the ganglioside GM1 receptor pentasaccharide diffract to near-atomic resolution. We have refined the crystallographic model for this complex using anisotropic displacement parameters for all atoms to a conventional crystallographic residual R=0.129 for all observed Bragg reflections in the resolution range 22 A to 1.25 A. Remarkably few residues show evidence of discrete conformational disorder. A notable exception is a minority conformation found for the Cys9 side-chain, which implies that the Cys9-Cys86 disulfide linkage is incompletely formed. In all five crystallographically independent instances, the peptide backbone in the region of the receptor-binding site shows evidence of strain, including unusual bond lengths and angles, and a highly non-planar (omega=153.7(7) degrees) peptide group between residues Gln49 and Val50. The location of well-ordered water molecules at the protein surface is notable reproduced among the five crystallographically independent copies of the peptide chain, both at the receptor-binding site and elsewhere. The 5-fold non-crystallographic symmetry of this complex allows an evaluation of the accuracy, reproducibility, and derived error estimates from refinement of large structures at near-atomic resolution. We find that blocked-matrix treatment of parameter covariance underestimates the uncertainty of atomic positions in the final model by approximately 10% relative to estimates based either on full-matrix inversion or on the 5-fold non-crystallographic symmetry.

PHOTOSYNTHETIC CYTOCHROMES c IN CYANOBACTERIA, ALGAE, AND PLANTS

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Molecular dynamics simulation of cytochrome c3: studying the reduction processes using free energy calculations

The tetraheme cytochrome c3 from Desulfovibrio vulgaris Hildenborough is studied using molecular dynamics simulation studies in explicit solvent. The high heme content of the protein, which has its core almost entirely made up of c-type heme, presents specific problems in the simulation. Instability in the structure is observed in long simulations above 1 ns, something that does not occur in a monoheme cytochrome, suggesting problems in heme parametrization. Given these stability problems, a partially restrained model, which avoids destruction of the structure, was created with the objective of performing free energy calculations of heme reduction, studies that require long simulations. With this model, the free energy of reduction of each individual heme was calculated. A correction in the long-range electrostatic interactions of charge groups belonging to the redox centers had to be made in order to make the system physically meaningful. Correlation is obtained between the calculated free energies and the experimental data for three of four hemes. However, the relative scale of the calculated energies is different from the scale of the experimental free energies. Reasons for this are discussed. In addition to the free energy calculations, this model allows the study of conformational changes upon reduction. Even if the precise details of the structural changes that take place in this system upon individual heme reduction are probably out of the reach of this study, it appears that these structural changes are small, similarly to what is observed for other redox proteins. This does not mean that their effect is minor, and one example is the conformational change observed in propionate D from heme I when heme II becomes reduced. A motion of this kind could be the basis of the experimentally observed cooperativity effects between heme reduction, namely positive cooperativity.

Who checks the checkers? Four validation tools applied to eight atomic resolution structures. EU 3-D Validation Network

Eight protein crystal structures, which have been refined against X-ray diffraction data extending to atomic resolution, 1.2 A or better, were inspected using four different validation tools, PROCHECK, PROVE, SQUID and WHATCHECK. Two general questions were addressed. (1) Do the structures imply changes in "expected" stereochemical properties and are the target values used for restraints in the validation programs and the refinement protocol appropriate? (2) Can errors in models be detected and how reliable are the coordinates after refinement? Preliminary analysis by members of the network led to modifications both to the validation programs and to the refinement protocols. The results of the final analyses are reported here. Apparent discrepancies in cell dimensions were identified. Most stereochemical properties are shown to be more tightly clustered than for lower resolution analyses. In contrast the omega angle has a wider distribution. The validation software is generally available and can be accessed at servers listed at the end of the paper.

Cloning and correct expression in Escherichia coli of the petE and petJ genes respectively encoding plastocyanin and cytochrome c6 from the cyanobacterium Anabaena sp. PCC 7119

The genes coding for plastocyanin (petE) and cytochrome c6 (petJ) from Anabaena sp. PCC 7119 have been cloned and properly expressed in Escherichia coli. The recombinant proteins are identical to those purified from the cyanobacterial cells. The products of both the petE and petJ genes are correctly processed in E. coli, as deduced from their identical N-terminal amino acid sequences as compared with those of the metalloproteins isolated from the cyanobacterium. Physicochemical and functional properties of the native and recombinant protein preparations are also identical, thereby confirming that expression of petE and petJ genes in E. coli is an adequate tool to address the study of the structure/function relationships in plastocyanin and cytochrome c6 from Anabaena by site-directed mutagenesis.

Solution structure of cytochrome c6 from the thermophilic cyanobacterium Synechococcus elongatus

Cytochrome c6 is a small, soluble electron carrier between the two membrane-bound complexes cytochrome b6f and photosystem I (PSI) in oxygenic photosynthesis. We determined the solution structure of cytochrome c6 from the thermophilic cyanobacterium Synechococcus elongatus by NMR spectroscopy and molecular dynamics calculations based on 1586 interresidual distance and 28 dihedral angle restraints. The overall fold exhibits four alpha-helices and a small antiparallel beta-sheet in the vicinity of Met58, one of the axial heme ligands. The flat hydrophobic area in this cytochrome c6 is conserved in other c6 cytochromes and even in plastocyanin of higher plants. This docking region includes the site of electron transfer to PSI and possibly to the cytochrome b6f complex. The binding of cytochrome c6 to PSI in green algae involves interaction of a negative patch with a positive domain of PSI. This positive domain has not been inserted at the evolutionary level of cyanobacteria, but the negatively charged surface region is already present in S. elongatus cytochrome c6 and may thus have been optimized during evolution to improve the interaction with the positively charged cytochrome f. As the structure of PSI is known in S.elongatus, the reported cytochrome c6 structure can provide a basis for mutagenesis studies to delineate the mechanism of electron transfer between both.

Charges, hydrogen bonds, and correlated motions in the 1 A resolution refined structure of the mating pheromone Er-1 from Euplotes raikovi

A detailed description is given of the structure of the small protein mating pheromone Er-1 at atomic resolution. Emphasis is placed on the locations of charges and hydrogen bonds. The model includes all the protein atoms, anisotropic displacement parameters, four disordered side chains, 22 water molecules, a disordered ethanol, and "riding" hydrogen atoms. Analysis of the model revealed that this dense crystal is perfused by hydrogen-bonding networks of solvent and protein atoms. The termini of helices are capped by hydrogen bonding to solvent and protein atoms, and to symmetry-related molecules. An examination of the valencies and charges of the hydrogen-bonding groups suggests that three of the "water" molecules capping the C termini of two helices, and one other, may instead be NH4 ions. Water molecules mediate all but one of the interhelical hydrogen bonds, and many of the lattice interactions. Regions of the molecule where the atomic vibrations deviate from isotropy are identified. There is almost no overall libration of the molecule allowed by the packing, but the side-chains vibrate relative to the backbone. Four side-chains display alternate conformations. Indirect evidence is presented that the switches between their conformations are correlated and driven by protonation of acidic side-chains. These structural features are discussed in the context of function and stability. Equipped with the analysis of the model, we review the course and results of the refinement of the model against 1 A X-ray diffraction data to a crystallographic R-factor of 12.92%.

The benefits of atomic resolution

After a long gestation, the elucidation of the crystal structures of proteins at atomic resolution is now maturing. The use of such data for both refinement and structure solution is advancing space. The necessary technology is generally available, in terms of data collection, computing hardware and software. The structures appearing in the literature mainly relate to demonstration projects on native proteins. The importance of these alone is already obvious. Biologically significant results, in terms of ligand complexes and prosthetic groups, are just starting to emerge.

Thermus thermophilus cytochrome-c552: A new highly thermostable cytochrome-c structure obtained by MAD phasing

The three-dimensional structure of cytochrome-c552 from Thermus thermophilus has been determined by the multiple anomalous dispersion technique using synchrotron radiation and refined to a resolution of 1.28 A. Data collection at 90 K and the recording of three data sets (f'-minimum: 7125 eV, f"-maximum: 7138 eV and reference for scaling: 10,077 eV) resulted in an initial electron density of very high quality at 2.1 A, which was readily interpretable for model building. The model was refined to an R value of 19.1% (Rfree=22.4%) at 1.28 A resolution using a fourth data set collected at a photon energy of 11,810 eV. Comparison of this thermophilic cytochrome with its mesophilic mitochondrial or bacterial counterparts reveals significant structural differences which are discussed with respect to their importance for thermostability and binding between this cytochrome and its corresponding ba3-oxidase. Amino acid sequence similarities to other class I cytochromes are very weak and entirely limited to the region around the CXXCH motif close to the N terminus. The N-terminal two-thirds of cytochrome-c552 cover spatial regions around the heme prosthetic group that are similar to those observed for other cytochromes. The actual secondary structural elements that are responsible for that shielding do not, however, correlate well to other structures. Only the N-terminal helix (containing the heme binding cysteine residues) aligns reasonably well with other class I cytochromes. The most striking differences that distinguish the present structure from all other class I cytochromes is the C-terminal one-third of the molecule that wraps around the remainder of the structure as a stabilizing clamp, the existence of an extended beta-sheet covering one edge of the heme and the lack of any internal water molecule.

Atomic resolution (1.0 A) crystal structure of Fusarium solani cutinase: stereochemical analysis

X-ray data have been recorded to 1.0 A resolution from a crystal of Fusarium solani cutinase using synchrotron radiation and an imaging-plate scanner. The anisotropic treatment of thermal motion led to a fivefold increase in accuracy and to a considerable quality improvement in the electron density maps with respect to an intermediate isotropic model. The final model has an R-factor of 9.4%, with a mean coordinate error of 0.021 A, as estimated from inversion of the least-squares matrix. The availability of an accurate structure at atomic resolution and of meaningful estimates of the errors in its atomic parameters, allowed an extensive analysis of several stereochemical parameters, such as peptide planarity, main-chain and some side-chain bond distances. The hydrogen atoms could be clearly identified in the electron density, thus providing unambiguous evidence on the protonation state of the catalytic histidine residue. The atomic resolution revealed an appreciable extent of flexibility in the cutinase active site, which might be correlated with a possible adaptation to different substrates. The anisotropic treatment of thermal factors provided insights into the anisotropic nature of motions. The analysis of these motions in the two loops delimiting the catalytic crevice pointed out a "breath-like" movement in the substrate binding region of cutinase.

Crystal structure of the dihaem cytochrome c4 from Pseudomonas stutzeri determined at 2.2A resolution

BACKGROUND

. Cytochromes c4 are dihaem cytochromes c found in a variety of bacteria. They are assumed to take part in the electron-transport systems associated with both aerobic and anaerobic respiration. The cytochrome c4 proteins are located in the periplasm, predominantly bound to the inner membrane, and are able to transfer electrons between membrane-bound reduction systems and terminal oxidases. Alignment of cytochrome c4 sequences from three bacteria, Pseudomonas aeruginosa, Pseudomonas stutzeri and Azotobacter vinelandii, suggests that these dihaem proteins are composed of two similar domains. Two distinctly different redox potentials have been measured for the Ps. stutzeri cytochrome c4, however.

RESULTS

. The crystal structure of the dihaem cytochrome c4 from Ps. stutzeri has been determined to 2.2A resolution by isomorphous replacement. The model, consisting of two entire cytochrome c4 molecules and 138 water molecules in the asymmetric unit, was refined to an R value of 20.1% for all observations in the resolution range 8-2.2A. The molecule is organized in two cytochrome c-like domains that are related by a pseudo-twofold axis. The symmetry is virtually perfectly close to the twofold axis, which passes through a short hydrogen bond between the two haem propionic acid groups, connecting the redox centre of each domain. This haem-haem interaction is further stabilized by an extensive symmetrical hydrogen-bond network. The twofold symmetry is not present further away from the axis, however, and the cytochrome c4 molecule can be considered to be a dipole with charged residues unevenly distributed between the two domains. The haem environment in the two domains show pronounced differences, mainly on the methionine side of the haem group.

CONCLUSIONS

. The structure, in conjunction with sequence alignment, suggests that the cytochrome protein has evolved by duplication of a cytochrome c gene. The difference in charge distribution around each haem group in the two domains allows the haem group in the N-terminal domain to be associated with the lower redox potential of 241 mV and the C-terminal haem group with the higher potential of 328 mV. The molecular dipole characteristic of cytochrome c4 is important for its interaction with, and recognition of, its redox partners. In cytochrome c4, the hydrogen-bond network (between residues that are conserved in all known cytochrome c4 subspecies) seems to provide an efficient pathway for an intramolecular electron transfer that can ensure cooperativity between the two redox centres. The C-pyrrole corners of the haem edges are potential sites for external electron exchange.

Crystal structures of reduced, oxidized, and mutated human thioredoxins: evidence for a regulatory homodimer

BACKGROUND

Human thioredoxin reduces the disulfide bonds of numerous proteins in vitro, and can activate transcription factors such as NFkB in vivo. Thioredoxin can also act as a growth factor, and is overexpressed and secreted in certain tumor cells.

RESULTS

Crystal structures were determined for reduced and oxidized wild type human thioredoxin (at 1.7 and 2.1 A nominal resolution, respectively), and for reduced mutant proteins Cys73-->Ser and Cys32-->Ser/Cys35-->Ser (at 1.65 and 1.8 A, respectively). Surprisingly, thioredoxin is dimeric in all four structures; the dimer is linked through a disulfide bond between Cys73 of each monomer, except in Cys73-->Ser where a hydrogen bond occurs. The thioredoxin active site is blocked by dimer formation. Conformational changes in the active site and dimer interface accompany oxidation of the active-site cysteines, Cys32 and Cys35.

CONCLUSIONS

It has been suggested that a reduced pKa in the first cysteine (Cys32 in human thioredoxin) of the active-site sequence is important for modulation of the redox potential in thioredoxin. A hydrogen bond between the sulfhydryls of Cys32 and Cys35 may reduce the pKa of Cys32 and this pKa depression probably results in increased nucleophilicity of the Cys32 thiolate group. This nucleophilicity, in tum, is thought to be necessary for the role of thioredoxin in disulfide-bond reduction. The physiological role, if any, of thioredoxin dimer formation remains unknown. It is possible that dimerization may provide a mechanism for regulation of the protein, or a means of sensing oxidative stress.

Proteins at atomic resolution

Experimental advances in data collection, including bright sources, cryogenic cooling and two-dimensional detectors, have made it tractable to record data to beyond 1.2 A for several proteins, yielding high-accuracy models and fine details of structure. For small metalloproteins, atomic-resolution data have enabled ab initio solution of the phase problem.