An analysis of side-chain conformation in proteins

18th Sir Hans Krebs lecture. Knowledge-based protein modelling and design

A systematic technique for protein modelling that is applicable to the design of drugs, peptide vaccines and novel proteins is described. Our approach is knowledge-based, depending on the structures of homologous or analogous proteins and more generally on a relational data base of protein three-dimensional structures. The procedure simultaneously aligns the known tertiary structures, selects fragments from the structurally conserved regions on the basis of sequence homology, aligns these with the 'average structure' or 'framework', builds on the loops selected from homologous proteins or a wider database, substitutes sidechains and energy minimises the resultant model. Applications to modelling an homologous structure, tissue plasminogen activator on the basis of another serine proteinase, and to modelling an analogous protein, HIV viral proteinase on the basis of aspartic proteinases, are described. The converse problem of ab initio design is also addressed: this involves the selection of an amino acid sequence to give a particular tertiary structure, in this case a symmetrical domain of two Greek-key motifs.

Criteria that discriminate between native proteins and incorrectly folded models

Various theoretical concepts, such as free energy potentials, electrostatic interaction potentials, atomic packing, solvent-exposed surface, and surface charge distribution, were tested for their ability to discriminate between native proteins and misfolded protein models. Misfolded models were constructed by introducing incorrect side chains onto polypeptide backbones: side chains of the alpha-helical hemerythrin were modeled on the beta-sheeted backbone of immunoglobulin VL domain, whereas those of the VL domain were similarly modeled on the hemerythrin backbone. CONGEN, a conformational space sampling program, was used to construct the side chains, in contrast to the previous work, where incorrect side chains were modeled in all trans conformations. Capability of the conformational search procedure to reproduce native conformations was gauged first by rebuilding (the correct) side chains in hemerythrin and the VL domain: constructs with r.m.s. differences from the x-ray side chains 2.2-2.4 A were produced, and many calculated conformations matched the native ones quite well. Incorrectly folded models were then constructed by the same conformational protocol applied to incorrect amino acid sequences. All CONGEN constructs, both correctly and incorrectly folded, were characterized by exceptionally small molecular surfaces and low potential energies. Surface charge density, atomic packing, and Coulomb formula-based electrostatic interactions of the misfolded structures and the correctly folded proteins were similar, and therefore of little interest for diagnosing incorrect folds. The following criteria clearly favored the native structures over the misfolded ones: 1) solvent-exposed side-chain nonpolar surface, 2) number of buried ionizable groups, and 3) empirical free energy functions that incorporate solvent effects.

Comparative molecular model building of two serine proteinases from cytotoxic T lymphocytes

Two genes that are expressed when precursor cytotoxic T lymphocytes are transformed to T killer cells have been cloned and sequenced. The derived amino acid sequences, coding for cytotoxic cell protease 1 (CCP1) and Hannuka factor (HF) are highly homologous to members of the serine proteinase family. Comparative molecular model building using the known three-dimensional structures and the derived amino acid sequences of the lymphocyte enzymes has provided useful structural information, especially in predicting the conformations of the substrate binding sites. In applying this modelling procedure, we used the X-ray structures of four serine proteinases to provide a structurally based sequence alignment: alpha-chymotrypsin (CHT), bovine trypsin (BT), Streptomyces griseus trypsin (SGT), and rat mast cell protease 2 (RMCP2). The root mean square differences in alpha-carbon atom positions among these four structures when compared in a pairwise fashion range from 0.79 to 0.97 A for structurally equivalent residues. The sequences of the two lymphocyte enzymes were then aligned to these proteinases using chemical criteria and the superimposed X-ray structures as guides. The alignment showed that the sequence of CCP1 was most similar to RMCP2, whereas HF has regions of homology with both RMCP2 and BT. With RMCP2 as a template for CCP1 and the two enzymes RMCP2 and BT as templates for HF, the molecular models were constructed. Intramolecular steric clashes that resulted from the replacement of amino acid side chains of the templates by the aligned residues of CCP1 and HF were relieved by adjustment of the side chain conformational angles in an interactive computer graphics device. This process was followed by energy minimization of the enzyme model to optimize the stereochemical geometry and to relieve any remaining unacceptably close nonbonded contacts. The resulting model of CCP1 has an arginine residue at position 226 in the specificity pocket, thereby predicting a substrate preference for P1 aspartate or glutamate residues. The model also predicts favorable binding for a small hydrophobic residue at the P2 position of the substrate. The primary specificity pocket of HF resembles that of BT and therefore predicts a lysine or arginine preference for the P1 residue. The arginine at position 99 in the model of HF suggests a preference for aspartate or glutamate side chains in the P2 position of the substrate. Both CCP1 and HF have a free cysteine in the segment of polypeptide 88 to 93.(ABSTRACT TRUNCATED AT 400 WORDS)

Propensity for spontaneous succinimide formation from aspartyl and asparaginyl residues in cellular proteins

One mechanism for the spontaneous degradation of polypeptides is the intramolecular attack of the peptide bond nitrogen on the side chain carbonyl carbon atom of aspartic acid and asparagine residues. This reaction results in the formation of succinimide derivatives and has been shown to be largely responsible for the racemization, isomerization, and deamidation of these residues in several peptides under physiological conditions (Geiger, T. & Clarke, S. J. Biol. Chem. 262, 785-794 (1987]. To determine if similar reactions might occur in proteins, I examined the sequence and conformation about aspartic acid and asparagine residues in a sample of stable, well-characterized proteins. There did not appear to be any large bias against dipeptide sequences that readily form succinimides in small peptides. However, it was found that aspartyl and asparaginyl residues generally exist in native proteins in conformations where the peptide bond nitrogen atom cannot approach the side chain carbonyl carbon to form a succinimide ring. These orientations also represent energy minimum states, and it appears that this factor may account for a low rate of spontaneous damage to proteins by succinimide-linked reactions. The presence of aspartic acid and asparagine residues in other conformations, such as those in partially denatured, conformationally flexible regions, may lead to more rapid succinimide formation and contribute to the degradation of the molecule. The possible role of isoimide intermediates, formed by the attack of the peptide oxygen atom on the side chain carboxyl group, in protein racemization, isomerization, and deamidation is also considered.

Analysis of the relationship between side-chain conformation and secondary structure in globular proteins

The relationship between the preferred side-chain dihedral angles and the secondary structure of a residue was examined. The structures of 61 proteins solved to a resolution of 2.0 A (1 A = 0.1 nm) or better were analysed using a relational database to store the information. The strongest feature observed was that the chi 1 distribution for most side-chains in an alpha-helix showed an absence of the g- conformation and a shift towards the t conformation when compared to the non-alpha/beta structures. The exceptions to this tendency were for short polar side-chains that form hydrogen bonds with the main-chain which prefer g+. Shifts in the chi 1 preferences for residues in the beta-sheet were observed. Other side-chain dihedral angles (chi 2, chi 3, chi 4) were found to be influenced by the main-chain. This paper presents more accurate distributions for the side-chain dihedral angles which were obtained from the increased number of proteins determined to high resolution. The means and standard deviations for chi 1 and chi 2 angles are presented for all residues according to the secondary structure of the main-chain. The means and standard deviations are given for the most popular conformations for side-chains in which chi 3 and chi 4 rotations affect the position of C atoms.

Structure and refinement at 1.8 A resolution of the aspartic proteinase from Rhizopus chinensis

The structure of rhizopuspepsin (EC 3.4.23.6), the aspartic proteinase from Rhizopus chinensis, has been refined to a crystallographic R-factor of 0.143 at 1.8 A resolution. The positions of 2417 protein atoms have been determined with a root-mean-square (r.m.s.) error of 0.12 A. In the final model, the r.m.s. deviation from ideality for bond distances is 0.010 A, and for angle distances it is 0.034 A. During the course of the refinement, a calcium ion and 373 water molecules, of which 17 are internal, have been located. The active aspartate residues, Asp35 and Asp218, are involved in similar hydrogen-bonding interactions with neighboring residues and with several water molecules. One water molecule is located between the two carboxyl groups of the catalytic aspartate residues in a tightly hydrogen-bonded position. The refinement resulted in an unambiguous interpretation of the highly mobile "flap", a beta-hairpin loop region that projects over the binding pocket. Large solvent channels are formed when the molecules pack in the crystal, exposing the binding pocket and making it easily accessible. Intermolecular contacts involve mainly solvent molecules and a few protein atoms. The three-dimensional structure of rhizopuspepsin closely resembles other aspartic proteinase structures. A detailed comparison with the structure of penicillopepsin showed striking similarities as well as subtle differences in the active site geometry and molecular packing.

Structure of the C-terminal domain of the ribosomal protein L7/L12 from Escherichia coli at 1.7 A

The structure of a C-terminal fragment of the ribosomal protein L7/L12 from Escherichia coli has been refined using crystallographic data to 1.7 A resolution. The R-value is 17.4%. Six residues at the N terminus are too disordered in the structure to be localized. These residues are probably part of a hinge in the complete L7/L12 molecule. The possibility that a 2-fold crystallographic axis is a molecular 2-fold axis is discussed. A patch of invariant residues on the surface of the dimer is probably involved in functional interactions with elongation factors.

Refined structure of glutathione reductase at 1.54 A resolution

The crystal structure of human glutathione reductase has been established at 1.54 A resolution using a restrained least-squares refinement method. Based on 77,690 independent reflections of better than 10 A resolution, a final R-factor of 18.6% was obtained with a model obeying standard geometry within 0.025 A in bond lengths and 2.4 degrees in bond angles. The final 2Fo-Fc electron density map allows for the distinction of carbon, nitrogen and oxygen atoms with temperature factors below about 25 A2. Apart from 461 amino acid residues and the prosthetic group FAD, the model contains 524 solvent molecules, about 118 of which can be considered an integral part of the enzyme. The largest solvent cluster is at the dimer interface and contains 104 interconnected solvent molecules, part of which are organized in a warped sheet-like structure. The main-chain dihedral angles are well-concentrated in the allowed regions of the Ramachandran plot. The spread of dihedral angles in beta-pleated sheets is much larger than in alpha-helices and especially in alpha-helix cores, indicating the higher plasticity of beta-structures. The analysis revealed a large amount of 3(10)-helix. The side-chain conformations cluster at the staggered positions, and show well-defined preferences. Also, a mobility gradient is observed for side-chains. Non-polar and polar side-chains show average temperature factor increases per bond of 10% and 25%, respectively. A number of alternative conformations of internal side-chains, in particular serines and methionines, have been detected. The extended FAD molecule also shows a mobility gradient between the very rigid flavin (mean value of B) = 8.7 A2) and the more mobile adenine (mean value of B = 16.2 A2). The entire active center is particularly well ordered, with temperature factors around 10 A2. The dimer interface consists of a rigid contact area, which is well conserved in the Escherichia coli enzyme, and a flexible area that is not. Altogether, the buried surfaces at the crystal contacts are half as large as at the dimer interface, but less specific. The refined structure shows clearly that there are no buried cations compensating the charge of the pyrophosphate moiety of FAD. The flavin deviates slightly from standard geometry, which is possibly caused by the polypeptide environment. In contrast to an earlier interpretation, atom N5 of the flavin can accommodate a proton, and it is conceivable that this proton proceeds to the redox-active disulfide.(ABSTRACT TRUNCATED AT 400 WORDS)

Tertiary templates for proteins. Use of packing criteria in the enumeration of allowed sequences for different structural classes

We assume that each class of protein has a core structure that is defined by internal residues, and that the external, solvent-contacting residues contribute to the stability of the structure, are of primary importance to function, but do not determine the architecture of the core portions of the polypeptide chain. An algorithm has been developed to supply a list of permitted sequences of internal residues compatible with a known core structure. This list is referred to as the tertiary template for that structure. In general the positions in the template are not sequentially adjacent and are distributed throughout the polypeptide chain. The template is derived using the fixed positions for the main-chain and beta-carbon atoms in the test structure and selected stereochemical rules. The focus of this paper is on the use of two packing criteria: avoidance of steric overlap and complete filling of available space. The program also notes potential polar group interactions and disulfide bonds as well as possible burial of formal charges. Central to the algorithm is the side-chain rotamer library. In an update of earlier studies by others, we show that 17 of the 20 amino acids (omitting Met, Lys and Arg) can be represented adequately by 67 side-chain rotamers. A list of chi angles and their standard deviations is given. The newer, high-resolution, refined structures in the Brookhaven Protein Data Bank show similar mean chi values, but have much smaller deviations than those of earlier studies. This suggests that a rotamer library may be a better structural approximation than was previously thought. In using packing constraints, it has been found essential to include all hydrogen atoms specifically. The "unified atom" representation is not adequate. The permitted rotamer sequences are severely restricted by the main-chain plus beta-carbon atoms of the test structure. Further restriction is introduced if the full set of atoms of the external residues are held fixed, the full-chain model. The space-filling requirement has a major role in restricting the template lists. The preliminary tests reported here make it appear likely that templates prepared from the currently known core structures will be able to discriminate between these structures. The templates should thus be useful in deciding whether a sequence of unknown tertiary structure fits any of the known core classes and, if a fit is found, how the sequence should be aligned in three dimensions to fit the core of that class.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of bacteriophage T4 lysozyme refined at 1.7 A resolution

The structure of the lysozyme from bacteriophage T4 has been refined at 1.7 A resolution to a crystallographic residual of 19.3%. The final model has bond lengths and bond angles that differ from "ideal" values by 0.019 A and 2.7 degrees, respectively. The crystals are grown from electron-dense phosphate solutions and the use of an appropriate solvent continuum substantially improved the agreement between the observed and calculated structure factors at low resolution. Apart from changes in the conformations of some side-chains, the refinement confirms the structure of the molecule as initially derived from a 2.4 A resolution electron density map. There are 118 well-ordered solvent molecules that are associated with the T4 lysozyme molecule in the crystal. Four of these are more-or-less buried. There is a clustering of water molecules within the active site cleft but, other than this, the solvent molecules are dispersed around the surface of the molecule and do not aggregate into ice-like structures or pentagonal or hexagonal clusters. The apparent motion of T4 lysozyme in the crystal can be interpreted in terms of significant interdomain motion corresponding to an opening and closing of the active site cleft. For the amino-terminal domain the motion can be described equally well (correlation coefficients approx. 0.87) as quasi-rigid-body motion either about a point or about an axis of rotation. The motion in the crystals of the carboxy-terminal domain is best described as rotation about an axis (correlation coefficient 0.80) although in this case the apparent motion seems to be influenced in part by crystal contacts and may be of questionable relevance to dynamics in solution.

Conformational properties of trans Ac-Asn-Pro-Tyr-NHMe and trans Ac-Tyr-Pro-Asn-NHMe in dimethylsulfoxide and in water determined by multinuclear n.m.r. spectroscopy

Vicinal coupling constants between various nuclei provide backbone and side-chain conformational information for a series of asparagine- and tyrosine-containing peptides in DMSO and in H2O. By enriching Tyr of Ac-Asn-Pro-Tyr-NHMe with 15N, it has been possible to distinguish between the resonances of the two side-chain beta protons of Tyr. Analysis of the coupling constants in terms of the distributions of side-chain conformations in these peptides indicates that the addition of Asn to the Pro-Tyr sequence leads to a less random conformational distribution. When compared to the side-chain rotamer distribution of Ac-Asn-NHMe and Ac-Tyr-NHMe, particular Asn and Tyr side-chain conformations of Ac-Asn-Pro-Tyr-NHMe are stabilized in dimethylsulfoxide solution. The interaction(s) which stabilize a unique Tyr side-chain conformation of Ac-Asn-Pro-Tyr-NHMe in dimethylsulfoxide are not present in Ac-Ala-Pro-Tyr-NHMe and are unaffected by the addition of Val-Pro to the C-terminus of Asn-Pro-Tyr. In water, a preferential stabilization of one Asn side-chain conformation of Ac-Asn-Pro-Tyr-NHMe is also observed, while the Tyr side-chain rotamer distribution is similar to that of Ac-Tyr-NHMe. An interaction between the Asn side chain and the Pro-Tyr-NHMe backbone was previously shown to stabilize a beta-bend conformation at Pro-Tyr in water. Data are also presented for Ac-Tyr-Pro-Asn-NHMe, for which local interactions do not stabilize particular backbone conformations in dimethylsulfoxide or in water. The conformations of the peptides studied here are relatively insensitive to temperatures between 27 degrees and 62 degrees, both in dimethylsulfoxide and in water. The sequences Asn-Pro-Tyr and Tyr-Pro-Asn occur in ribonuclease A, and these tripeptides serve as models for the interactions involved in the folding of this protein.

Computer aided prediction and evaluation of the tertiary structure for rat elastase II

Predictive methods have been extended to generate a proposed tertiary structure of rat elastase II on the basis of primary amino acid sequence and structural homologies within the family of mammalian serine proteinases. Force field refinement calculations were used to relax the structure. Structurally conserved molecules of solvation were introduced and the structure was again refined by means of force-field calculations. The resulting structure suggests probable substrate cleavage preferences. An independent statistical analysis of the crystallographically refined structures of serine proteinases (0.01-0.13 A, RMS) shows a close similarity to the final predicted model of rat pancreatic elastase II (0.03-0.14 A, RMS).

Structure of ferricytochrome c' from Rhodospirillum molischianum at 1.67 A resolution

The structure of ferricytochrome c' from Rhodospirillum molischianum has been crystallographically refined to 1.67 A resolution using a combination of reciprocal space and restrained least-squares refinement methods. The final crystallographic R-factor for 30,533 reflections measured with I greater than sigma (I) between infinity and 1.67 A is 0.188. The final model incorporates 1944 unique protein atoms (of a total of 1972) together with 194 bound solvent molecules. The structure has been analysed with respect to its detailed conformational properties, secondary structural features, temperature factor behavior, bound solvent sites, and heme geometry. The asymmetric unit of the cytochrome c' crystal contains a dimer composed of chemically identical 128-residue polypeptide chains. Although the refined structure shows the monomers to be very similar, examination of the differences that do occur allows an evaluation of how different lattice contacts affect protein conformation and solvent binding. In particular, comparison of solvent binding sites in the two subunits allows identification of a common set that are not altered by lattice interactions. The preservation of these solvent interactions in different lattice environments suggests that they play a structural role in protein stabilization in solution. The refined structure additionally reveals some new features that relate to the ligand binding properties and unusual mixed-spin state character of cytochrome c'. Finally, comparison of the heme binding geometry in cytochrome c' and other structurally unrelated c-type cytochromes shows that two alternative, but sterically favorable, conformational variants occur among the seven examples examined.

Effects of pH on the structure and function of carboxypeptidase A: crystallographic studies

High-resolution crystal structures are described for carboxypeptidase A (EC 3.4.17.1) in crystals grown at pH 8.5, 9.0, and 9.5 and compared with the structure at pH 7.5. The comparison shows that in the pH range of 7.5-9.5 the enzyme structure is practically unchanged, and, most importantly, that the flexible side chain of Tyr-248 remains exclusively in the "up" position, away from the Zn atom, throughout the pH range. There is no evidence for binding of Tyr-248 to Zn at any of these pH values. We conclude that the interaction of Tyr-248 with Zn is not an essential part of the mechanism of carboxypeptidase A and that its occurrence is an artifact of chemical modification of Tyr-248. It is also suggested that Tyr-248 is not uniquely associated with the observed high pK of the enzymatic hydrolysis.

Intrahelical hydrogen bonding of serine, threonine and cysteine residues within alpha-helices and its relevance to membrane-bound proteins

A survey of known protein structures reveals that approximately 70% of serine residues and at least 85% (potentially 100%) of threonine residues in helices make hydrogen bonds to carbonyl oxygen atoms in the preceding turn of the helix. The high frequency of intrahelical hydrogen bonding is of particular significance for intrinsic membrane-bound proteins that form transmembrane helices. Hydrogen bonding within a helix provides a way for serine, threonine and cysteine residues to satisfy their hydrogen-bonding potential permitting such residues to occur in helices buried within a hydrophobic milieu.

X-ray diffraction studies of enkephalins. Crystal structure of [(4'-bromo) Phe4,Leu5]enkephalin

In order to investigate the structure-activity relationship of [Leu5]- and [Met5]enkephalins, [(4'-bromo)Phe4, Leu5]-, [(4'-bromo)Phe4, Met5]- and [Met5] enkephalins were synthesized and crystallized. The crystal structure of [(4'-bromo) Phe4, Leu5]- enkephalin was determined by X-ray diffraction method using the heavy atom method and refined to R = 0.092 by the least-squares method. The molecule in this crystal took essentially the same type I' beta-turn conformation found in [Leu5]enkephalin [Smith & Griffin (1978) Science 199, 1214-1216). On the other hand, the preliminary three-dimensional Patterson analyses showed that the most probable conformations of [(4'-bromo)Phe4,Met5]- and [Met5]enkephalins are both the dimeric extended forms. Based on these insights, the biologically active conformation of enkephalin was discussed in relation to the mu- and delta-receptors.

X-ray studies on crystalline complexes involving amino acids and peptides. X. Head-to-tail sequences in the crystal structure of L-lysine acetate

L-Lysine acetate crystallises in the monoclinic space group P21 with a = 5.411 (1), b = 7.562(1), c = 12.635(2) A and beta = 91.7(1) degrees. The crystal structure was solved by direct methods and refined to an R value of 0.049 using the full matrix least squares method. The conformation and the aggregation of lysine molecules in the structure are similar to those found in the crystal structure of L-lysine L-aspartate. A conspicuous similarity between the crystal structures of L-arginine acetate and L-lysine acetate is that in both cases the strongly basic side chain, although having the largest pK value, interacts with the weakly acidic acetate group leaving the alpha-amino and the alpha-carboxylate groups to take part in head-to-tail sequences. These structures thus indicate that electrostatic effects are strongly modulated by other factors so as to give rise to head-to-tail sequences which have earlier been shown to be an almost universal feature of amino acid aggregation in the solid state.

X-ray studies on crystalline complexes involving amino acids and peptides. IX. Crystal structure of L-ornithine L-aspartate hemihydrate

L-Ornithine L-aspartate hemihydrate crystallizes in the space group C2 with a = 21.858(2), b = 4.718(1), c = 18.046(2) A and beta = 137.4(1) degrees. The crystal structure, solved by direct methods, has been refined to an R value of 0.041 for 1270 observed reflections. The conformation of the two amino acid molecules in the structure are somewhat different from those observed in other crystal structures which contain them. The crystal structure is stabilized by ionic interactions accompanied by hydrogen bonds. The unlike molecules aggregate into separate two-fold helices; each helix of one type is surrounded by, and is in hydrogen bonded contact with, four helices of the other type. The arrangement of the molecules in the structure is such that it can be described as consisting of alternating hydrophilic and hydrophobic regions. The hydrophilic regions contain hydrogen bonded loops, each made up of two amino groups and two carboxylate groups. The structure also provides the first example of a head-to-tail sequence involving two types of amino acids.

Statistical and energetic analysis of side-chain conformations in oligopeptides

The distributions of side-chain conformations in 258 crystal structures of oligopeptides have been analyzed. The sample contains 321 residues having side chains that extend beyond the C beta atom. Statistically observed preferences of side-chain dihedral angles are summarized and correlated with stereochemical and energetic constraints. The distributions are compared with observed distributions in proteins of known X-ray structures and with computed minimum-energy conformations of amino acid derivatives. The distributions are similar in all three sets of data, and they appear to be governed primarily by intraresidue interactions. In side chains with no beta-branching, the most important interactions that determine chi 1 are those between the C gamma H2 group and atoms of the neighboring peptide groups. As a result, the g- conformation (chi 1 congruent to -60 degrees) occurs most frequently for rotation around the C alpha-C beta bond in oligopeptides, followed by the t conformation (chi 1 congruent to 180 degrees), while the g+ conformation (chi 1 congruent to 60 degrees) is least favored. In residues with beta-branching, steric repulsions between the C gamma H2 or C gamma H3 groups and backbone atoms govern the distribution of chi 1. The extended (t) conformation is highly favored for rotation around the C beta-C gamma and C gamma-C delta bonds in unbranched side chains, because the t conformer has a lower energy than the g+ and g- conformers in hydrocarbon chains. This study of the observed side-chain conformations has led to a refinement of one of the energy parameters used in empirical conformational energy computations.

Comparative theoretical conformational analysis of the insulin A6-A11 peptides

The insulins of pig, ox, horse and goat differ only by the cyclic A6-A11 peptide which is thought to be an antigenic determinant of the molecule. The structure of the four peptides is investigated by conformational energy calculations in order to verify whether a common backbone conformation can be found for all four species, the antigenic difference being consequently due only to the difference in the side chains exposed to the solvent, or whether each sequence gives rise to a preferential backbone conformation, which would lead to the conclusion that the antigenic difference is conveyed by a more pronounced difference in the molecular structure. From the results of the study on the isolated peptides, it appears that an energetically favourable backbone conformation common to all four species can be found. This conformation is compared with the structure deduced from the X-ray diffraction data available for pig insulin.