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X-ray studies on human insulins prepared by semisynthetic and biosynthetic methods have recently been undertaken. Human insulin differs from porcine insulin only at the COOH terminus of the B-chain. The present study reports the crystal structure of 4-zinc human insulin, which is used clinically as a slow-acting preparation. The structure has been refined, using 1.85-A resolution data, to a residual of 0.173. The unit cell is rhombohedral, space group R3, with hexagonal cell constants a = 80.953 and c = 37.636 A, and it is nearly isomorphous with that of 4-zinc porcine insulin. As a result of a conformational change of the first eight residues of the Bchain of molecule 1 from an extended conformation observed in the 2-zinc structure to an a-helical one, the coordination around one of the zinc ions on the 3-fold axis has changed, an additional zinc ion in a general position is bound by the hexamer, and additional hydrogen-bonded interactions help stabilize dimer and hexamer formation. Unlike the surface of the 2-zinc insulin hexamer, which possesses a shallow depression containing a zinc ion and its coordinating water molecules, the 4-zinc human insulin hexamer contains a zinc and chloride ion at the bottom of an 8-A tunnel produced by three parallel ahelices. These a-helices shield the zinc ion from the environment, decreasing the rate of dissociation of the hexamer, and provide an explanation for the slow-acting aspect of the 4-zinc crystalline form.In mammals, insulin is synthesized in the B cells of the pancreas and stored as a zinc-containing hexameric aggregate. However, this hormone, consisting of an A and a B chain, is a monomer as it circulates in the blood stream and when it interacts with its receptor. Binding and activity data on several insulin analogues suggest that the insulin molecule must undergo a substantial conformational change when it binds to its receptor (1, 2).Crystallographic investigations have been carried out on insulin derived from various species (3), chemically modified insulins (2, 4), and different crystalline modifications (5-7). The most extensive studies have been carried out on porcine insulin, which differs from human insulin by the change of alanine to threonine at B30. As a result of these studies, the nature of the interactions that hold the hexamer together, the interactions between monomers to produce dimers, and finally, the identity and spatial relationship of the residues that are thought to bind to the receptor are known.Comparison between the human and porcine 2-zinc insulin crystal structures revealed alterations in the hormone's conformation only at B29 and B30 (8). While other studies have shown that the insulin molecule is flexible, the largest changes in conformation have been observed in the 4-zinc insulin structure. This is of particular interest, since a 4-zinc crystalline preparation is used clinically as a "slow-acting" insulin because it is slow to dissolve and enter the bloodstream. The present study was undertaken to determine the effect of the...

The refinement of the crystal structure of two-Zn pig insulin using 1.5-A (1A = 0.1 nm) resolution data by Fourier and fast Fourier least-squares methods allows us to make detailed comparisons between the two independent molecules present in the two-Zn insulin dimer and to describe their interactions in the monomer, dimer, and hexamer. The main chain structures for the two molecules agree well except at the N terminus of the A chain and the C terminus of the B chain. The residues along the line of the local two-fold axes, apart from the B25 side chain, conform extremely closely to the two-fold symmetry, although the discrepancies are much more apparent away from this axis. The ability of the insulin molecule to adopt different conformations may be an important factor in the expression of its biological activity.

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The factors that control the relative and absolute stereochemistry of 1.3-disubstituted and 1,2,3trisubstituted tetrahydro-P-carbolines formed via the Pictet-Spengler reaction are discussed. in particular, the stereochemical factors that lead to the predominance of cis-1.3-disubstituted products under conditions of kinetic control are presented, with the aid of X-ray crystallographic data on a number of compounds; methods for assigning relative stereochemistry on the basis of NMR data are given; the mechanism by which racemisation can occur during the Pictet-Spengler reaction has also been studied, and procedures for eliminating this problem are given.The Pictet-Spengler reaction' is the most direct method of forming the tetrahydro-P-carboline system 1, which is the commonest structural unit of indole alkaloids. For members 1 7 3 Paper 2/05 1691