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Protein Science (2003), 12:185-191.
Copyright © 2003 The Protein Society

FOR THE RECORD

Partial NMR assignments and secondary structure mapping of the isolated {alpha} subunit of Escherichia coli tryptophan synthase, a 29-kD TIM barrel protein

Ramakrishna Vadrevu1, Christopher J. Falzone2 and C. Robert Matthews1

1 Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA
2 Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA

Reprint requests to: C. Robert Matthews, Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01005, USA; e-mail: c.robert.matthews{at}umassmed.edu; fax: (508) 856-8358.

(RECEIVED June 27, 2002; FINAL REVISION September 27, 2002; ACCEPTED October 3, 2002)

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.0221103.


   Abstract
TOP
 Abstract
Introduction
 Results and Discussion
 Materials and methods
 References
 
The {alpha} subunit of tryptophan synthase ({alpha}TS) from S. typhimurium belongs to the triosephosphate isomerase (TIM) or the (ß/{alpha})8 barrel fold, one of the most common structures in biology. To test the conservation of the global fold in the isolated Escherichia coli homolog, we have obtained a majority of the backbone assignments for the 29-kD {alpha}TS by using standard heteronuclear multidimensional NMR methods on uniformly 15N- and 15N/13C-labeled protein and on protein selectively 15N-labeled at key hydrophobic residues. The secondary structure mapped by chemical shift index, nuclear Overhauser enhancements (NOEs), and hydrogen-deuterium (H-D) exchange, and several abnormal chemical shifts are consistent with the conservation of the global TIM barrel fold of the isolated E. coli {alpha}TS. Because most of the amide protons that are slow to exchange with solvent correspond to the ß-sheet residues, the ß-barrel is likely to play an important role in stabilizing the previously detected folding intermediates for E. coli {alpha}TS. A similar combination of uniform and selective labeling can be extended to other TIM barrel proteins to obtain insight into the role of the motif in stabilizing what appear to be common partially folded forms.

Keywords: 3D NMR; nuclear magnetic resonance; 15N- selective labeling; H-D exchange; folding intermediates

Abbreviations: {alpha}TS, the {alpha} subunit of tryptophan synthase from Escherichia coli • CSI, Chemical shift index • DTE, dithioerythritol • H-D, hydrogen-deuterium • HSQC, heteronuclear single quantum coherence • I1, equilibrium intermediate of {alpha}TS maximally populated at ~3 M urea • I2, equilibrium intermediate of {alpha}TS maximally populated at ~5 M urea • K2EDTA, ethylenediaminetetraacetic acid, dipotassium salt • NOE, nuclear Overhauser enhancement • NOESY, nuclear Overhauser enhancement spectroscopy • ppm, parts per million • TIM, triosephosphate isomerase


   Introduction
TOP
 Abstract
 Introduction
Results and Discussion
 Materials and methods
 References
 
The (ß/{alpha})8, or triosephosphate isomerase (TIM) barrel structure, is one of the most common motifs in proteins, comprising ~10% of the open reading frames in all three super kingdoms (Farber and Petsko 1990; Orenga et al. 1997; Nagano et al. 2001). The ubiquitous nature of the TIM barrel is probably related to its utility as a platform for enzyme catalysis; representatives are found in five of the six primary functional classes (Pujadas and Palau 1999; Nagano et al. 2002). A defining feature of the canonical TIM barrel is the presence of eight parallel ß-strands, forming a closed cylinder secured by hydrogen bonds between consecutive strands; hydrogen bonds between the N- and C-terminal strands seal the barrel. The amphipathic {alpha}-helices, which alternate in sequence with the strands, form an amphipathic shell covering the hydrophobic ß-barrel and, thereby, enhance its solubility. Loops between the C termini of the strands and the N termini of the helices comprise the active site.

The {alpha} subunit of the tetrameric bifunctional enzyme tryptophan synthase from Salmonella typhimurium adopts a TIM barrel fold in the {alpha}2ß2 complex (Hyde et al. 1988). The hydrophobic core of the protein is largely defined by side-chains from the eight ß-strands; the 11 {alpha}-helices, including one that caps the N terminus of the barrel, are interspersed with the ß-strands and are solvent exposed. Several residues in the {alpha} subunit are involved in side-chain/side-chain and side-chain/main-chain hydrogen bond interactions with ß subunit residues in the {alpha}2ß2 complex. The {alpha} subunits from S. typhimurium and Escherichia coli are 85% identical in sequence, and both {alpha} subunits are stable and stimulate the activity of both ß subunits (Nichols and Yanofsky 1979). Although these properties imply that the structures are highly conserved, high-resolution structural information that can test this presumption is not available. We have used standard multidimensional NMR methods at pH 7.8 and 25°C on uniformly 15N- and 13C/15N-labeled protein and selective 15N-isotopic labeling of a few key hydrophobic residues to obtain assignments for a significant majority of the backbone 1H-15N heteronuclear single quantum coherence (HSQC) cross-peaks. The secondary structure predicted by chemical shift indices and by several abnormal chemical shifts is consistent with the placement of helices and strands expected for the TIM barrel structure and the conservation of the global fold in the isolated E. coli homolog. To the best of our knowledge, this effort represents the first example of sequence specific backbone assignments for a TIM barrel protein.


   Results and Discussion
TOP
 Abstract
 Introduction
 Results and Discussion
Materials and methods
 References
 
Chemical shift assignments
A 1H-15N HSQC spectrum of the {alpha} subunit of tryptophan synthase ({alpha}TS) at 25°C and pH 7.8 is shown in Figure 1Go. Of the expected 249 (268 residues less 19 prolines) backbone cross-peaks, ~215 are observed in the 1H-15N HSQC spectrum. For a few very weak cross-peaks, no correlations are observed in several of the triple-resonance experiments used in this study. The inability to detect the remaining backbone peaks may reflect exchange broadening owing to conformational dynamics inherent to the TIM barrel and dynamics, resulting from the absence of the ß subunit interactions for the isolated {alpha} subunit in solution. The side-chain 1H-15N resonances of asparagine and glutamine have not been assigned; however, they can be identified from the 1H-15N HSQC and three-dimensional (3D) 1H-15N nuclear Overhauser enhancement spectroscopy (NOESY)-HSQC spectra (Fig. 1Go).



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  		Figure 1. 1H-15N heteronuclear single quantum coherence (HSQC) spectrum of the {alpha} subunit of tryptophan synthase from Escherichia coli ({alpha}TS), at 25°C (pH 7.8), in 50 mM phosphate, 0.2 mM K2EDTA, and 2 mM dithioerythritol. The assignments are indicated by the single-letter amino acid code and the residue number. The horizontal lines connect the side-chain NH2 resonances of asparagine and glutamine. The protein concentration was ~0.6 mM.

 
Three problems served to complicate the assignments of the cross-peaks:

  1. The crowded region between 7.5 and 8.3 parts per million (ppm) and between ~117 and 119 ppm along the 1H and 15N dimensions and the chemical shift degeneracy hindered the standard sequential assignments procedure.
  2. The limited thermal stability of {alpha}TS (Matthews et al. 1980) precluded measurements at >=40°C; key experiments like HNCACB, CBCA(CO)NH, which provide the characteristic Cß information, were ineffective and less efficient, respectively, at 25°C in fully protonated {alpha}TS.
  3. The 19 prolines introduced numerous breaks in the sequential linking of the backbone atoms.

To resolve these problems, we explored the application of selective 15N- labeling of key hydrophobic residues to reduce the spectral complexity and to obtain unambiguous residue-specific starting points. An auxotrophic bacterial strain was used to label individually the backbone nitrogens of the 27 leucine, 17 valine, 20 isoleucine, or 12 phenylalanine residues of {alpha}TS, which together constitute ~30% of the total residues. The number of cross-peaks observed is in very close agreement with the number of residues expected from the sequence: 27 of 27 leucines, 19 of 20 isoleucines, 17 of 17 valines, and 11 of 12 phenylalanines are observed in the 1H-15N HSQC spectra (Fig. 2Go). With this simplification and various double- and triple-resonance experiments performed at 25°C (pH 7.8) on uniformly 15N- and 13C/15N-labeled {alpha}TS, ~80% of the observed 1H-15N cross-peaks have been assigned (Fig. 1Go). The difficulty in obtaining unambiguous assignments of the remaining peaks reflects, in part, the lack of dispersion in a protein with significant {alpha}-helix content (~50%).



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  		Figure 2. 1H-15N heteronuclear single quantum coherence (HSQC) spectra of the subunit of tryptophan synthase from Escherichia coli ({alpha}TS), selectively 15N-labeled at phenylalanine (A), valine (B), isoleucine (C), and leucine (D) residues at 25°C (pH 7.8). The unassigned resonances are indicated by an asterisk. The protein concentration was ~0.6 mM, and the buffer used is described in the caption for Figure 1Go.

 
Secondary structure in E. coli {alpha} TS
Chemical shift index (CSI) for the 13C{alpha} and 1H{alpha} resonances and hydrogen-deuterium (H-D) exchange data are summarized in Figure 3Go. The residues in the ß-strands in general showed the expected downfield- and upfield-shifted 1H{alpha} and 13C{alpha} deviations from random coil values, respectively (Wishart et al. 1991, 1995). Many of these same residues displayed relatively strong sequential d{alpha}n (i, i + 1) and weaker d{alpha}n (i, i) nuclear Overhauser enhancements (NOEs) in the 3D 1H-15N NOESY spectrum, leading to the positive identification of the eight strands of the ß-barrel. Expected upfield- and downfield-shifted 1H{alpha} and 13C{alpha} chemical shifts are observed for most of residues in the {alpha}-helical regions (Fig. 3Go), and the same regions showed the dnn (i, i + 1) NOEs expected for helices. The ß-strands and {alpha}-helices thus identified by using the combined information from NOEs, CSI, and H-D exchange compare well with the location of their counterparts in the {alpha}TS component of the {alpha}2ß2 complex from S. typhimurium (Fig. 3Go).



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  		Figure 3. Summary of the chemical shift index data of the 1H{alpha} and 13C{alpha} nuclei and the hydrogen-deuterium (H-D) exchange data for slowly exchanging amide protons. The amino acid sequences of the {alpha} subunits of tryptophan synthase from Salmonella typhimurium ({alpha}TSSt) and Escherichia coli ({alpha}TSEc) are shown. The secondary structure for the {alpha}TSSt derived from the crystal structure (Protein Data Bank code 1BKS) by using the program DSSP (Kabsh and Sander 1983) is indicated; helices and strands are shown in red and blue, respectively. The filled circles indicate amide protons observed in the 1H-15N heteronuclear single quantum coherence (HSQC) spectrum recorded 22 h after dissolving the protein in 2H2O at 25°C and pH 7.8. Residues of the {alpha} subunit involved in hydrogen bond interactions with the ß subunit residues in the tryptophan synthase complex are indicated by open circles, and residues 178–189 (boxed) do not show electron density in the crystal structure complex. The gaps in chemical shift information are owing to either lack of assignments or cross-peaks in the 1H-15N HSQC spectrum.

 
The absence of assignments for most of helix {alpha}2' and the preceding loop may reflect exchange broadening. This region, located at the interface between the two subunits, is dynamic even in the {alpha}2ß2 complex and only becomes ordered in the presence of ligands (Hyde et al. 1988). The dynamic behavior is thought to play a role in catalysis and in allosteric communication between the {alpha} and ß subunits (Miles et al. 1999). The region between ß6 and {alpha}6 (residues 178–189) is highly disordered in the native enzyme complex, as evidenced by the poor electron density. The NMR data imply that these regions remain flexible in the isolated {alpha}TS in solution.

Tertiary interactions in E. coli {alpha}TS
Abnormal chemical shifts often imply unique features in the tertiary structure, that is, the global fold. For example, the NH proton of Ala103 in {alpha}3, shows an abnormal downfield shift (Fig. 1Go) that is consistent with the hydrogen bond interaction with the O{delta}1 of Asp130 observed in the crystal structure. Similarly, the downfield shifts for Phe19 and Ile97 NHs of ß2 and ß3, respectively, are consistent with hydrogen bonds to the O{delta}2s of Asp46 and Asp124, respectively. The observed upfield 1H{alpha} chemical shift of 2.85 ppm for Val 106 of {alpha}3 is consistent with its proximity to the aromatic ring of Phe114 and in close agreement with the value predicted (Williamson and Asakura 1993) from the crystal structure coordinates. These abnormal chemical shifts imply that the tertiary structures of the isolated {alpha}TS from E. coli and {alpha}TS in the {alpha}2ß2 complex from S. typhimurium are similar, consistent with the presumption of a conserved global fold.

Secondary structure in a stable folding intermediate
Thermodynamic analysis of the urea-induced unfolding of {alpha}TS from E. coli revealed the presence of two stable intermediates, I1 and I2, maximally populated at ~3.0 and 5.0 M urea, respectively, at pH 7.8 and 25°C (Gualfetti et al. 1999). On the basis of a far-UV CD spectrum reconstructed from singular value decomposition analysis, it has been proposed that the ß-barrel remains folded and is largely responsible for stabilizing the I1 intermediate. The I2 species lacks secondary and tertiary structure and is thought to be stabilized by long-range hydrophobic interactions between strings of nonpolar side-chains found in ß-strands (Saab-Rincon et al. 1996). Stable folding intermediates similar to I1 have also been observed for other TIM barrel proteins (Jasanoff et al. 1994; Andreotti et al. 1997; Sanchez del Pino and Fersht 1997; Forsyth and Matthews 2002), indicating that the ß-barrel motif may play a significant role in defining the equilibrium and, perhaps, the kinetic properties of the folding reactions.

This conjecture is supported by the observation that the majority of the ~50 cross-peaks observed 22 h after dissolving the protein in D2O at pH 7.8 and 25°C correspond to almost all the amide protons of the eight strands that stabilize the ß-barrel; very few of the amide protons in helices are protected (Fig. 3Go). Hydrogen exchange experiments (Mayo and Baldwin 1993; Bai et al. 1995; Bhuyan and Udgaonkar 1998; Fuentes and Wand 1998; Li and Woodward 1999 [and references cited therein]; Parker and Marqusee 2001) are required to identify the cooperative networks of hydrogen bonds that stabilize the partially folded forms.

Perspective
By using a combination of 3D double- and triple-resonance NMR experiments at pH 7.8 and 25°C on uniformly 15N- and 13C/15N- labeled and 15N-selective labeled proteins, we have assigned most of the backbone cross-peaks in a 29-kD TIM barrel protein and identified probes for monitoring the secondary structure in a stable folding intermediate. This approach can readily be extended to other TIM barrel proteins and, thereby, enable a test of the role played by the ß-barrel motif in stabilizing what appear to be similar folding intermediates for one of the most common folds in biology.


   Materials and methods
TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and methods
References
 
Uniformly 15N- and 13C/15N-labeled {alpha}TS was expressed in E. coli BL21/DE3 cells transformed with the plasmid pT7.WS1 grown on M9 minimal medium with 1 g/L 15NH4Cl and 2 g/L 13C6 glucose. Cells were grown at 37°C to an OD of 1.0 at 600 nm, induced with IPTG, and harvested after 3 h. After sonication, unfractionated cell extracts were incubated for a few minutes in 100 mM potassium phosphate (pH 7.8), 5 mM K2EDTA, 2 mM dithioerythritol (DTE), 10 mM MgCl2, 0.1 M NaCl, and 0.01% Triton X-100, containing 10 µg/mL RNase and DNase.

The protein was found in both the soluble and insoluble fractions of the cell lysate after sonication. The protein from the soluble fraction was purified by using a modified protocol of Kirschner et al. (1975). After dialysis in 10 mM potassium phosphate (pH 7.8), 2 mM K2EDTA, and 2 mM DTE, the protein was loaded on a preequilibrated DE52 resin column and eluted by increasing the K2EDTA concentration to 4 mM (Gualfetti et al. 1999). The insoluble fraction of the cell lysate was washed three times; solubilized in 6 M urea, 10 mM potassium phosphate, 1 mM K2EDTA, and 1 mM DTE; and subsequently dialyzed to remove the urea. The refolded protein was then loaded onto a DE52 resin column preequilibrated with 10 mM potassium phosphate (pH 7.8), 1 mM K2EDTA, and 1 mM DTE. After washing with 10 mM potassium phosphate, the protein was eluted using a nonlinear 10 to 300 mM potassium phosphate (pH 7.8) gradient (Yee et al. 1996). The structural integrity of the protein obtained from both the fractions was verified by the comparison of the 1H-15N HSQC spectra. Identical spectra ensured the complete recovery of the secondary and tertiary structure in the protein recovered from the insoluble fraction of the lysate.

{alpha}TS selectively labeled with 15N at the amide nitrogen of phenylalanine, valine, isoleucine, and leucine residues, respectively, was obtained using the auxotrophic strain DL39avtA/DE3 and purified following the protocol applied to the soluble fraction of the uniformly labeled protein as described above. The cells were grown on M9 minimal medium supplemented with nucleic acid bases and all the amino acids in unlabeled form except those at which 15N- labeling was desired. Protein expression was induced with IPTG.

NMR spectroscopy
The protein samples were prepared by dialyzing the protein in 50 mM potassium phosphate (pH 7.8), 0.2 mM K2EDTA, and 2 mM DTE. The protein concentration was ~0.6 to 0.8 mM in 95% 1H20 and 5% 2H20. All the NMR experiments were conducted at 25°C by using a Bruker 600 MHz spectrometer equipped with a triple-resonance probe including Z-axis pulse field gradients. 1H-15N HSQC, 15N TOCSY-HSQC, and 15N NOESY-HSQC experiments were performed on uniformly labeled {alpha}TS. 1H-15N HSQC and 15N NOESY-HSQC experiments were performed on selective 15N-labeled proteins. 3D triple-resonance HNCA, HN(CO)CA, HNCO, CBCA(CO)NH, HNCACB, HCCH COSY, and HCCH TOCSY experiments were performed on uniformly 13C/15 N-labeled protein. Slowly exchanging amides were identified by recording a series of 1H-15N HSQC experiments on the lyophilized protein dissolved in 2H2O. All the data were processed and analyzed using FELIX 2000 (Accelyrs).


   Acknowledgments
 
We thank Prof. Charles Yanofsky (Stanford University) for providing the plasmid for {alpha}TS and Dr. David Waugh (National Cancer Institute) for providing the axuotrophic strain DL39/DE3. We are grateful to Dr. Juliette Lecomte for helpful discussions and Dr. Jill Zitzewitz for her critical reading of the manuscript and suggestions. This work was supported by the National Institutes of Health (GM 23303) to C.R.M.

The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.


   References
TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and methods
 References
 
Andreotti, G., Cubellis, M.V., Di Palo, M., Fessas, D., Sannia, G., and Marino G. 1997. Stability of a thermophile TIM-barrel enzyme: Indole-3-glycerol phosphate synthase from the thermophilic archaeon Sulfolobus solfataricus. Biochem. J. 323: 259–264.

Bai, Y., Sosnick, T.R., Mayne, L., and Englander, S.W. 1995. Protein folding intermediates: Native-state hydrogen exchange. Science 269: 192–197.[Abstract/Free Full Text]

Bhuyan, A.K. and Udangoankar, J.B. 1998. Two structural subdomains of barstar detected by rapid mixing NMR measurement of amide hydrogen exchange. Proteins 30: 295–308.[CrossRef][Medline]

Farber, G.K. and Petsko, G.A. 1990. The evolution of {alpha}/ß barrel enzymes. TIBS 15: 228–234.

Forsyth, W.R. and Matthews, C.R. 2002. Folding mechanism of indole-3-glyceraldehyde synthase from Sulfolobus solfataricus: A test of the conservation of folding mechanisms hypothesis in (ß{alpha})8 barrels. J. Mol. Biol. 320: 1119–1133.[CrossRef][Medline]

Fuentes, E.J. and Wand, A.J. 1998. Local dynamics and stability of apocytochrome b562 examined by hydrogen exchange. Biochemistry 37: 3687–3698.[CrossRef][Medline]

Gualfetti, P.J., Bilsel, O., and Matthews, C.R. 1999. The progressive development of structure and stability during the equilibrium folding of the {alpha} subunit of tryptophan synthase from Escherichia coli. Protein Sci. 8: 1623–1635.[Abstract]

Hyde, C.C., Ahmed, S.A., Padlan, E.A., Miles, E.W., and Davies, D.R. 1988. Three-dimensional structure of the tryptophan synthase {alpha}2ß2 multienzyme complex from Salmonella typhimurium. J. Biol. Chem. 263: 17857–17871.[Abstract/Free Full Text]

Jasanoff, A., Davis, B., and Fersht, A.R. 1994. Detection of an intermediate in the folding of the (ß {alpha})8-barrel N-(5'-phosphoribosyl) anthranilate isomerase from Escherichia coli. Biochemistry 33: 6350–6972.[CrossRef][Medline]

Kabsh, W. and Sander, C. 1983. Dictionary of protein secondary structure. Biopolymers 22: 2577–2637.[CrossRef][Medline]

Kirschner, K., Wiskocil, R.L., Foehn, M., and Rezeau, L. 1975. The tryptophan synthase from Escherichia coli: An improved purification procedure for the {alpha} subunit and binding studies with substrate analogues. Eur. J. Biochem. 60: 513–523.[Medline]

Li, R. and Woodword, C. 1999. The hydrogen exchange core and protein folding. Protein Sci. 8: 1571–1591.[Abstract]

Matthews, C. R., Crisanti, M.M., Gepner, G. L., Velicelebi, G., and Sturtevant, J.M. 1980. Effect of single amino acid substitutions on the thermal stability of the {alpha} subunit of tryptophan synthase. Biochemistry 19: 1290–1293.[CrossRef][Medline]

Mayo, S.L. and Baldwin, R.L. 1993. Guanidinium chloride induction of partial unfolding in amide proton exchange in RNase A. Science 262: 873–876.[Abstract/Free Full Text]

Miles, E.W., Rhee, S., and Davies, D.R. 1999. The molecular basis of substrate channeling. J. Biol. Chem. 274: 12193–12196.[Free Full Text]

Nagano, N., Porter, C.T., and Thornton, J.M. 2001. The (ß?/{alpha})8 glycosidases: sequence and structure analyses suggest distant evolutionary relationships. Protein Eng. 14: 845–855.[Abstract/Free Full Text]

Nagano, N., Orenga, C.A., and Thornton, J.M. 2002. One fold with many functions: The evolutionary relationships between TIM barrel families based on their sequences, structures and functions. J. Mol. Biol. 321: 741–765.[CrossRef][Medline]

Nichols, B.P. and Yanofsky, C. 1979. Nucleotide sequences of trpA of Salmonella typhimurium and Escherichia coli: An evolutionary comparison. Proc. Natl. Acad. Sci. 76: 5244–5248.[Abstract/Free Full Text]

Orenga, C.A., Michie, A.D., Jones, S., Jones, D.T., Swindells, M.B., and Thornton. J.M. 1997. CATH: A hierarchic classification of protein domain structures. Structure 5: 1093–1108.[Medline]

Parker, M.J. and Marqusee, S. 2001. A kinetic folding intermediate probed by native state hydrogen exchange. J. Mol. Biol. 305: 593–602.[CrossRef][Medline]

Pujadas, G. and Palau, J. 1999. TIM barrel fold: Structural, functional and evolutionary characteristics in natural and designed molecules. Biologia, Bratislavia 54: 231–254.

Saab-Rincon, G., Gualfetti, P.J., and Matthews, C.R. 1996. Mutagenic and thermodynamic analyses of residual structure in the {alpha} subunit of trytophan synthase. Biochemistry 35: 1988–1994.[CrossRef][Medline]

Sanchez del Pino, M.M. and Fersht, A.R. 1997. Nonsequential unfolding of the {alpha}/ß barrel protein indole-3-glycerol-phosphate synthase. Biochemistry 36: 5560–5565.[CrossRef][Medline]

Williamson, M.P. and Asakura, T. 1993. Empirical comparisions of models for chemical shift calculation in proteins. J. Magn. Reson. Ser. B 101: 63–71.[CrossRef]

Wishart, D.S., Sykes, B.D., and Richards, F.M. 1991. Relationship between the nuclear magnetic resonance chemical shift and protein secondary structure. J. Mol. Biol. 222: 311–333.[CrossRef][Medline]

Wishart, D.S., Bigam, C.G., Holm, A., Hodges, R.S., and Sykes, B.D. 1995. 1H, 13C and 15N random coil NMR chemical shifts of the common amino acids, I: Investigations of nearest-neighbor effects. J. Biol. NMR 5: 67–81.

Yee, M.C., Horn, V., and Yanofsky, C. 1996. On the role of helix 0 of the tryptophan synthetase {alpha} chain of Escherichia coli. J. Biol. Chem. 271: 14754–14763.[Abstract/Free Full Text]


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