A model for the three-dimensional structure of insulin-like growth factor (IGF) is proposed based on the close sequence homology of IGF with insulin, the tertiary structure of which is known. The IGF molecule is postulated to have an insulin-like main chain conformation for residues equivalent to B6-B27 and A1-A21 and a hydrophobic core nearly identical to that of insulin. A short connecting peptide of twelve residues and an extension at the COOH-terminus are easily accommodated on the molecular surface. MATERIALS AND METHODS The model was built in several stages. The proposed structure was first constructed using Lapquip model parts at a scale of 1 cm = 1 A. This was examined for unfavorable intramolecular contracts and readjusted before the coordinates of each atom were read off using a mechanical device. The approximate coordinates were then regularized using the "modelfit" computer program of Isaacs et al (12), and bond lengths and angles and other intramolecular distances were calculated using an IBM 360/65 computer. The model was then displayed on a computer graphics system (Digital Equipment Corp. graphics with a PDPll computer) using programs written by D. Richardson, P. Pauling, and C. Chothia, and adjustments were made using the interactive facilities of the graphics system to optimize the intramolecular distances. Finally, the model was regeometricized using "modelfit" and stereo pairs of the model were generated in hard copy. RESULTS AND DISCUSSIONThree-dimensional model for IGF I Table 1 shows the sequences of human IGF I, IGF II, and porcine insulin aligned so that the maximum homology is obtained. The numbering for the insulin A and B chains is indicated, as is the numbering for the IGF I polypeptide. The sequence of the IGF connecting peptide of 12 residues is also included, but because this shows no homology with the proinsulin connecting peptide, the latter is omitted. IGF I has an extension of eight residues at the COOH terminus of the A chain. Table 2 gives the differences in numbers of amino acids between the 51 amino acids of porcine insulin and the equivalent residues of other insulins, the protein hormone relaxin (13)(14)(15), which also appears to be homologous with insulin, and IGF.The sequence comparison shows a close homology for residues 5-25 of IGF (B6-B26 of insulin) and 42-61 (A1-A20). The arrangement of cystines is identical in IGF and insulin, and glycines 7 (B8), 19 (B20), and 22 (B23), which have dihedral angles that are disallowed for residues with side chains, are conserved, so that the polypeptide backbone can assume the same three-dimensional structure as insulin. We began by building the sequence 5-26 and 42-61 into an insulin-like structure. This conformation is shown schematically in Fig. 1. Residues 8-18 (B9-B19) and [43][44][45][46][47][48] (A2-A7) are right-handed a-helices and residues 54-60 (A13-A19) formed a less organized right-handed helix. Cys 47 and 52 (A6 and All) and Cys 61 and 18 (A20 and B19) have their disulfides placed in the core, while the Cys ...

The structures of [Met5]enkephalin (Tyr-GlyGly-Phe-Met) and [Leu5]enkephalin (Tyr-Gly-Gly-Phe-Leu) have been determined from single crystal x-ray diffraction data and refined to residuals of 0.100 and 0.092, respectively. The [Met5]enkephalin structure consists of dimers forming antiparallel a-sheets extending in the monoclinic ac plane with 10.6 water molecules per dimer. The two molecules, related by pseudo two-fold axes, have similar backbone conformations and similar tyrosine and phenylalanine side-chain conformations. Both methionine residues are disordered and the disorder is different in the two independent molecules. Additional hydrogen bonds connect adjacent dimers to form infinite sheets normal to the b axis. The water molecules are found mainly in the interstices between the sheets. [Leu5]Enkephalin crystallizes as a monohydrate that is isomorphous with the [Met5]enkephalin structure with respect to the a-sheet but different with respect to the tyrosine and phenylalanine side-chain conformations and water content. The peptide chains in both structures are fully extended and more nearly planar than pleated. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

The subunit structure of human macrophage migration inhibitory factor (MIF) has been studied by preliminary X-ray analysis of wild-type and selenomethionine-MIF and dynamic light scattering. Crystal form I of MIF belongs to space group P2(1)2(1)2(1) and is grown from 2 M ammonium sulfate at pH 8.5. A native data set has been collected to 2.4 A resolution. Self-rotation studies and Van values indicate that three molecules per asymmetric unit are present. A data set to 2.8 A resolution has been collected for crystal form II, which belongs to space group P3(1)21 or P3(2)21 and grows from 2 M ammonium sulfate, 2% polyethylene glycol (average molecular mass 400) 0.1 M HEPES, pH 7.5. Three, four, five or six monomers in the asymmetric unit are consistent with Van values for this crystal form. Analysis of crystal form II containing selenomethionine-MIF indicates nine selenium sites are present per asymmetric unit. Dynamic light scattering of MIF suggests that the major form of the protein in solution is a trimer. The results of these studies are in contrast to previous reports indicating that MIF is a monomer or dimer. The subunit arrangement of MIF is similar to that of tumor necrosis factor and suggests that signal transduction might require trimerization of receptor subunits.

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