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Glucose oxidase is a flavin-dependent enzyme which catalyses the oxidation of beta-D-glucose by molecular oxygen to delta-gluconolactone and hydrogen peroxide. The structure of the enzyme from Aspergillus niger, previously refined at 2.3 A resolution, has been refined at 1.9 A resolution to an R value of 19.0%, and the structure of the enzyme from Penicillium amagasakiense, which has 65% sequence identity, has been determined by molecular replacement and refined at 1.8 A resolution to an R value of 16.4%. The structures of the partially deglycosylated enzymes have an r.m.s. deviation of 0.7 A for main-chain atoms and show four N-glycosylation sites, with an extended carbohydrate moiety at Asn89. Substrate complexes of the enzyme from A. niger were modelled by force-field methods. The resulting model is consistent with results from site-directed mutagenesis experiments and shows the beta-D-glucose molecule in the active site of glucose oxidase, stabilized by 12 hydrogen bonds and by hydrophobic contacts to three neighbouring aromatic residues and to flavin adenine dinucleotide. Other hexoses, such as alpha-D-glucose, mannose and galactose, which are poor substrates for the enzyme, and 2-deoxy-D-glucose, form either fewer bonds or unfavourable contacts with neighbouring amino acids. Simulation of the complex between the reduced enzyme and the product, delta-gluconolactone, has provided an explanation for the lack of product inhibition by the lactone.

The thioredoxin-related protein tryparedoxin I from Crithidia fasciculata has been crystallized using PEG 4000 as a precipitant. The enzyme forms long needle-shaped crystals which diffract to at least 1.7 A. A native data set has been collected at the DESY synchrotron from a flash-frozen crystal at 90 K to 1.7 A resolution. The data set shows that the crystals belong to the orthorhombic space group P212121 and have unit-cell parameters a = 37.94, b = 51. 39, c = 71.46 A. Tryparedoxin I is involved in a trypanothione-dependent peroxide metabolic pathway specific for trypanosomatids and may therefore be a suitable candidate for the design of drugs for the specific treatment of a variety of important tropical diseases caused by these parasites.

Tyrosine aminotransferase from Trypanosoma cruzi has been crystallized from PEG 4000 at pH 6.8. The crystals belong to the monoclinic space group P21 and have lattice constants of a = 59.1, b = 103.0, c = 77.8 A, beta = 113.1 degrees for a data set measured at 138 K. The presence of a non-crystallographic twofold axis together with a Matthews parameter Vm of 2.5 A3 Da-1 indicates that the asymmetric unit contains one dimeric molecule. The crystals diffract to at least 2.7 A and are stable in the X-ray beam in a shock-frozen state. Native data sets have been collected at temperatures of 285 and 138 K using a Siemens X1000 detector on a rotating-anode generator.

Peroxidase from the brown alga Ascophyllum nodosum, a non-heme vanadium-dependent haloperoxidase, has been purified to homogeneity and crystallized from ammonium sulfate solutions in a form suitable for X-ray diffraction analysis. The crystals have been grown by the vapour-diffusion technique using the sitting-drop method. X-ray diffraction studies show that the crystals belong to the tetragonal space group P4(1)2(1)2 or P4(3)2(1)2 with a = b = 114.3 and c = 276.0 A. The crystals diffract to at least 2.4 A resolution.

The procedure described in the papers I-V of this series [Giacovazzo, Siliqi & Ralph (1994). Acta Cryst. A50, 503-505; Giacovazzo, Siliqi & Spagna (1994). Acta Cryst. A50, 609-621; Giacovazzo, Siliqi & Zanotti (1995). Acta Cryst. A51, 177-188; Giacovazzo & Gonzalez Platas (1995). Acta Cryst. A51, 398-404; Giacovazzo, Siliqi & Gonzalez Platas (1995). Acta Cryst. A51, 811-820], aiming at estimating protein phases via a single heavy-atom derivative, has been improved so as to extend phase determination to all the reflections up to derivative resolution. The quality of the resulting electron-density maps is checked for a number of test strutures. Some of the maps are immediately interpretable, and some can be interpreted after some cycles of solvent flattening and/or histogram matching. The correlation with classical SIR techniques is also discussed.