Hilary Muirhead

Three-dimensional Fourier Synthesis of Horse Oxyhaemoglobin at 2.8 Å Resolution: The Atomic Model

The secondary structure of the haemoglobin chains is similar to that of myoglobin, but some of the helical segments are more irregular and some parts of the non-helical segments have different conformations. The structure of the contacts between unlike subunits suggests that the tetramer, rather than the αβ dimer, is the functional unit of haemoglobin.

Crystal structure of cat muscle pyruvate kinase at a resolution of 2.6 Å

The structure of pyruvate kinase (EC 2.7.1.40) has been determined from a 2.6 A resolution electron density map. This map shows more detail than the previous 3.1 A map (Stammers & Muirhead, 1977) and has enabled a detailed chain folding to be established for two out of the three domains which make up each of the four identical subunits. A provisional chain folding has been established for the third domain. The results have been briefly reported in a previous paper (Levine et al., 1978). Details of the structure determination and a further discussion of the results are presented in this paper. Domain A (the three domains of pyruvate kinase are referred to as A, B and C) can be described in terms of a cylindrical eight-stranded parallel β sheet and an outer coaxial cylinder of eight α helices. The α helices connect adjacent strands of the β sheet. Domain B is made up of a closed anti-parallel β sheet structure. Domain C is a five-stranded β sheet of which the fourth strand is anti-parallel and the rest parallel. These strands are also interconnected by α helices. Domain A can be dissected into eight consecutive β strand—α helix units starting from the N-terminus. The arrangement of these relative to each other can be most simply described by relating them to eight planes, each at 40 ° to the cylinder axis and symmetrically placed around the cylinder. When unit 2 is aligned with one of these planes then units 1, 3, 4, 5 and 8 are also closely aligned with a plane. This analysis is also applied to triosephosphate isomerase and a strikingly similar arrangement is found. A detailed comparison of the two structures is presented. Although the lack of a chemical sequence makes it difficult to identify the amino acid residues of pyruvate kinase, side-chains are clearly visible in the map and this information is correlated with the results of previous 6 A substrate soaking experiments and with the structure of triosephosphate isomerase. The similarities and differences are discussed in terms of similarities and differences in the reactions catalysed and also of different subunit packing.

The atomic structure of crystalline porcine pancreatic elastase at 2.5 A resolution: comparisons with the structure of alpha-chymotrypsin.

Abstract Three isomorphous heavy-atom derivatives have been used to calculate a 2.5 A resolution electron density map of tosyl-elastase at pH 5.0, from which an accurate atomic model has been constructed. Atomic co-ordinates measured from this model have been refined using model building, real-space refinement and energy minimization programs. The three-dimensional conformation of the polypeptide chain is described in terms of conformational angles, hydrogen-bonding networks and the environment of different types of amino acid side-chain. Difference Fourier calculation of the high resolution structure of native elastase at pH 5.0 shows it to be virtually identical to that of the tosyl derivative, except near the tosyl group. The conformation of the catalytically important residues in native elastase is very similar to that of native α-chymotrypsin, except for the orientation of the active centre serine oxygen. The significance of important structural similarities and differences between these two enzymes is discussed. Elastase contains 25 internal water molecules which play an important role in stabilizing the active conformation of the enzyme. Many of these water molecules are in identical positions to those found in the interior of α-chymotrypsin

XXXVII. Nuclear transmutations produced by cosmic-ray particles of great energy.—Part IV. The distribution in energy, and the secondary interactions of the particles emitted from stars

Summary The measurements of “multiple scattering” and “grain density” discussed in Parts II. and III. of the present series of papers, have now been extended to include a total of 1000 tracks of particles associated with “stars”. The method of measurement and the criteria for the selection of tracks were similar to those described previously (III.). The new material provides additional evidence in support of the view that most of the mesons ejected from nuclear explosions with kinetic energy less than 150 MeV. are π-particles.

CXXIV. Nuclear transmutations produced by cosmic-ray particles of great energy.—Part VI. Experimental results on meson production

Summary An analysis has been made of the secondary particles ejected from nuclear disintegrations observed in electron sensitive emulsions exposed to the cosmic radiation at 68,000 ft. Scattering and grain density measure-ments have been carried out on the tracks of 2000 particles associated with these stars. In addition, the grain density and angular distribution of 3070 shower particles and 1508 “grey” tracks have been measured. For 200 stars, the energy of the primary particle which produced the disintegration was measured. A detailed analysis was made of such events. Single fast τ-mesons of kinetic energy less than 1 BeV. are found to interact strongly with nuclear matter. An estimate of the frequency of occurrence of neutral mesons was made from a consideration of the energy balance in stars of low multiplicity, n s .

Observations With Electron-Sensitive Plates Exposed to Cosmic Radiation

WE have recently made observations with the new Kodak nuclearresearch emulsions, described by Dr. R. W. Berriman1, which record the tracks of particles of charge e, even at minimum ionization. The new plate represent a technical advance of great importance ; they allow us to obtain a much deeper insight into nuclear processes than was possible withytfhe older emulsions, and they greatly extend the field of application of the photographic method to physical and biological problems.

Crystallographic structure analysis of glucose 6-phosphate isomerase at 3·5 Å resolution

X-ray diffraction data sets have been measured to a nominal resolution of 3·5 A from crystals of porcine skeletal muscle glucose 6-phosphate isomerase using a four-circle diffractometer, and phase angles have been estimated by the method of isomorphous replacement with anomalous scattering measurements. In order that the results should not be too seriously affected by the radiation sensitivity of these crystals, certain precautions and corrections were used in the data measurement and processing. A Fourier map has been calculated and has been interpreted in terms of the folding of the polypeptide chain. The subunit of glucose phosphate isomerase contains two domains of protein, each with a parallel β-sheet at its core, and the β-strands are interconnected by α-helices.

Molecular basis of neurological dysfunction coupled with haemolytic anaemia in human glucose-6-phosphate isomerase (GPI) deficiency

Glucose-6-phosphate isomerase (GPI) deficiency, an autosomal recessive genetic disorder with the typical manifestation of nonspherocytic haemolytic anaemia, can be associated in some cases with neurological impairment. GPI has been found to be identical to neuroleukin (NLK), which has neurotrophic and lymphokine properties. To focus on the possible effects of GPI mutations on the central nervous system through an effect on neuroleukin activity, we analysed DNA isolated from two patients with severe GPI deficiency, one of them with additional neurological deficits, and their families. The neurologically affected patient (GPI Homburg) is compound heterozygous for a 59 A→C (H20P) and a 1016 T→C (L339P) exchange. Owing to the insertion of proline, the H20P and L339P mutations are likely to affect the folding and activity of the enzyme. In the second family studied, the two affected siblings showed no neurological symptoms. The identified mutations are 1166 A→G (H389R) and 1549 C→G (L517V), which are located at the subunit interface. We propose that mutations that lead to incorrect folding destroy both catalytic (GPI) and neurotrophic (NLK) activities, thereby leading to the observed clinical symptoms (GPI Homburg). Those alterations at the active site, however, that allow correct folding retain the neurotrophic properties of the molecule (GPI Calden).

The engineering of a more thermally stable lactate dehydrogenase by reduction of the area of a water-accessible hydrophobic surface

A site-directed mutant of Bacillus stearothermophilus lactate dehydrogenase (lactate:NAD+ oxidoreductase, EC 1.1.1.27) has been engineered in which the conserved hydrophobic residue isoleucine-250 has been replaced by the more hydrophilic residue asparagine. This isoleucine forms a large part of a water-accessible, hydrophobic surface in the active site of the apo-enzyme which is covered by the B-face of the nicotinamide ring when coenzymes are bound. Reduction in the area of this hydrophobic surface results in the mutant tetramer being more thermally stable than the wild-type enzyme.

Triose phosphate isomerase, pyruvate kinase and other α/β-barrel enzymes

Abstract The presence of a highly symmetrical, eight-stranded α/β-barrel structure in several unrelated enzymes seems to have significance for substrate binding. Further information from gene studies may indicate whether these homologies reflect convergent or divergent evolution.