The structure of diferric duck ovotransferrin (DOT) has been determined and refined at a resolution of 2.35 A. The DOT structure, which contains two iron binding sites, is similar to the known transferrin and lactoferrin structures. The two iron-binding sites, one in the Nterminal lobe and one in the C-terminal lobe of the molecule, are similar but not identical. The main differences between the three known structures lie in the relative orientations of the N-and C-lobes with respect to each other. In the DOT structure the large aromatic side chain of Phe322 in the N-lobe packs against the conserved residue Gly387 in the C-lobe. This interaction is at the centre of the interface between the two lobes and could play a crucial role in determining their relative orientation. Other differences between the structures occur in the surface loops and in the peptide connecting the two lobes. The final crystallographic model consists of 5309 protein atoms (686 residues), two Fe 3+ ions, two (bi)carbonate ions and three carbohydrate moities. 318 water molecules have been added to the model. The final R factor is 0.22 for 25400 observed reflections between 10 and 2.35A resolution. I. AbbreviationsDOT, duck ovotransferrin; HOT, hen ovotransferrin; HST, human serum transferrin; HMT, human melanotransferrin; RST, rabbit serum transferrin; HLT, human lactoferrin; MLT, mouse lactoferrin; MST, Manduca sexta transferrin; r.m.s., root-mean-square; NAG, N-acetylglucosamine; FUC, fucose.

The Escherichia coli enzyme transketolase, a dimeric protein of 2 x 70 kDa (662 amino acids) has been prepared from an overexpression system in E. coli. The purified enzyme has been crystallized from PIPES buffer pH 6.4 and ammonium sulfate. The crystals which grow as large plates diffract to greater than 1.9 A, resolution and are of the space group P2(1)2(1)2(1) with unit-cell dimensions of a = 74.6, b = 125.6 and c = 151.0 A, (Z = 8 with one transketolase dimer in the asymmetric unit). The structure has been solved by molecular replacement using the yeast transketolase enzyme structure as a search model. The enzyme is being used for large-scale biotransformations using various aldehydes and hydroxypyruvate as substrates.

The structure of apo duck ovotransfemn (APODOT) has been determined at a resolution of 4.0A by the molecular replacement method using the structure of duck ovotransfemn (DOT) as the search model. The DOT structure contains two iron binding sites; one in the N-terminal lobe lying between domains N1 and N2 and one in the C-terminal lobe between domains C 1 and C2. Both lobes have a closed structure. Models of various forms of both the N and C lobes were used in the search. The final model was refined to give an R factor of 0.22. The comparison of the structure of APODOT with that of DOT shows that both the N and the C lobes are in an open form, where the N2 and C2 domains undergo large rigid-body rotations of 51.6 and 49.9 ° relative to the N1 and C1 domains, respectively. The interface between the N and C lobes, which is formed by the N1--C 1 contact in the core of the molecule does not change significantly. The DOT molecule may be described in terms of three rigid bodies; the N1 and C1 domains as one rigid body forming the static core of the molecule and the N2 and C2 domains as two other rigid bodies which, on the release of iron, move away from the static core of the molecule to form the open structure of APODOT.

Abstract-A newly fallen Sudanese meteorite named Al Zarnkh was investigated using room and liquid nitrogen temperature Mössbauer measurements, X-ray diffraction (XRD), and electron probe microanalysis (EPMA) in conjunction with energy dispersive X-ray microscopy. The Mössbauer spectra exhibited strong paramagnetic doublets with magnetic sextets. The doublets are assigned to olivine and pyroxene, while the magnetic sextets are assigned to troilite and kamacite. Based on microprobe analyses and textural studies, olivine is the most abundant phase and occurs as fine to medium grained laths both in the groundmass and in barred olivine chondrules. Both orthopyroxenes and clinopyroxenes are present and these tend to be granular. Plagioclase is an abundant interstitial groundmass phase. Chromites were detected in some groundmass olivine and are highly chromiumand iron-rich with no Fe 3+ detected. The kamacite contains small amounts of Co. The mole fraction of the Fe end-member of olivine (fayalite) and orthopyroxene (ferrosilite) are found to be about 28% and 23%, respectively. These values are compared with that obtained from two chondritic meteorites. Based on these results, the studied meteorite is classified as an ordinary LL5 chondrite.

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