Three-dimensional structure of acylphosphatase. Refinement and structure analysis.

We report here the complete determination of the solution structure of acylphosphatase, a small enzyme that catalyses the hydrolysis of organic acylphosphates, as determined by distance geometry methods based on nuclear magnetic resonance information. A non-standard strategy for the distance geometry calculations was used and is described here some detail. The five best structures were then refined by restrained energy minimization and molecular dynamics in order to explore the conformational space consistent with the experimental data. We address the question of whether the solution structure of acylphosphatase follows the general principles of protein structure, i.e. those learned from analysing crystal structures. Static and dynamic features are discussed in detail. An uncommon beta-alpha-beta motif, so far found only in procarboxypeptidase B and in an RNA-binding protein, is present in acylphosphatase.     

Fine structure of the secretory and non-secretory ameloblasts in the frog. II. Fine structure of the non-secretory ameloblast.

The non-secretory ameloblasts present at the enamel-free surfaces of maxillary teeth in the frog Rana pipiens were examined by electron microscopy at different stages of tooth development. Their main fine structural features seem to reflect a transport function. During early tooth development, the non-secretory ameloblasts adjacent to odontoblasts and predentin exhibit extensive lateral surface specializations and numerous cytoplasmic vesicles. During late tooth development, the non-secretory ameloblasts adjacent to mineralizing dentin show numerous cellular junctions, well-developed intercellular channels with numerous interdigitating processes and labyrinthine configurations at their distal surfaces. An intact basal lamina is present between the non-secretory ameloblasts and the dentin surface until the dentin becomes fully mineralized. At this stage the adjacent cells no longer exhibit surface specializations. It is suggested that the non-secretory ameloblasts may participate in the mineralization of adjacent dentin at the enamel-free surfaces. This surface dentin becomes fully mineralized at a later stage of development than the underlying dentin.     

Biologic significance of the structure of hydrocarbons. I. Chain structure

1. The biologic experiments with the links of the methane series—n-pentane, n-hexane, n-heptane, n-octane, i-octane, and pentene—gave these qualitative results: (a) The higher the number of CH₂ groups, the longer the chain, the longer the average lifetime of the animal. (b) The ramified chain does not appear to act differently from the saturated straight chain with the same number of C atoms. (c) One double bond within the chain shortens the lifetime to a considerable degree. 2. The quantitative discussion shows that the lifetimes depend exponentially on the molecular weight. 3. Qualitatively the hypothesis is supported that with rising molecular weight the concentration of CH₂ groups within the animal diminishes according to the vapor pressure or the thermodynamic potential. However, lifetime and these physical properties obey different functions. 4. These physical properties are of high biologic importance. But they are not sufficient to explain the biologic effects quantitatively.     

Fine structure of the secretory and nonsecretory ameloblasts in the frog. I. Fine structure of the secretory ameloblasts.

Amelogenesis in the tooth germs of the frog Rana pipiens was examined by electron microscopy at different stages of tooth development. Cellular changes in secretory ameloblasts during this process showed many basic similarities to those in mammalian amelogenesis. Amelogenesis can be divided into three stages based on histological criteria such as thickness of enamel and the relative position of the tooth germ within the continuous succession of teeth. These stages are early, transitional and late. The fine structure of the enamel-secreting cells reflects the functional role of these ameloblasts as primarily secretory in the early stage, possibly transporting in the late stage and reorganizing between the two functions in the transitional stage. In early amelogenesis the cell exhibits well-developed granular endoplasmic reticulum, Golgi complex, microtubules, dense granules, smooth and coated vesicles, lysosome-like bodies in supranuclear and distal portions of the cell and mitochondria initially concentrated in the basal part of the cell. Numerous autophagic vacuoles are observed concomitant with the loss of some cell organelles at the transitional stage. During late amelogenesis the ameloblasts exhibit numerous vesicles, granules, convoluted cell membranes, junctional complexes and widely distributed mitochondria. Toward the end of amelogenesis, cells become oriented parallel to the enamel surface and the number of organelles is reduced. Amelogenesis in the frog is an extracellular process and mineralization seems to occur simultaneously with matrix formation.     

Stability and mobility of the collagen structure.

The thermodynamic analysis of microcalorimetric and hydrogen-exchange data on the stability and mobility of collagen structures from different species with different physiological temperatures has shown that not only the thermostability but the enthalpy and entropy of disruption of the native collagen structure are increasing functions of the total prolyl and hydroxyprolyl content. At the same time the number of stable hydrogen bonds maintaining the native structure is constant and consists of one stable and a less stable (0.7) bond per triplet for all the collagens studied, in agreement with Ramachandran's model (Ramachandran &; Kartha, 1955). Thus the observed difference between the enthalpies of disruption of collagens with a different imino acid content cannot be explained by the difference in the amount of stable hydrogen bonds involved in maintaining the native collagen structure. With an increase in the imino acid content of collagen, the Gibbs energy of micro-unfolding, which determines the mobility of a structure, also increases. It seems probable that the rigidity and order of the core of a native macromolecule influence the order of the surrounding water and that they are the main source of the observed large values for the enthalpy and entropy of collagen unfolding. The observed link between the stability and mobility properties of collagens explains the correlation that exists between the thermostabilities of collagens and the physiological temperatures of different species if it is assumed that some definite level of mobility of collagen structure is required for its efficient functioning in living systems. The experimental data presented here support the validity of this assumption.     

Consciousness and the structure of neuronal representations.

The hypothesis is defended that brains expressing phenomenal awareness are capable of generating metarepresentations of their cognitive processes, these metarepresentations resulting from an iteration of self-similar cortical operations. Search for the neuronal substrate of awareness therefore converges with the search for the nature of neuronal representations. It is proposed that evolved brains use two complementary representational strategies. One consists of the generation of neurons responding selectively to a particular constellation of features and is based on selective recombination of inputs in hierarchically structured feedforward architectures. The other relies on the dynamic association of feature-specific cells into functionally coherent cell assemblies that, as a whole, represent the constellation of features defining a particular perceptual object. Arguments are presented that favour the notion that the metarepresentations supporting awareness are established in accordance with the second strategy. Experimental data are reviewed that are compatible with the hypothesis that evolved brains use assembly codes for the representation of contents and that these assemblies become organized through transient synchronization of the discharges of associated neurons. It is argued that central states favouring the formation of assembly-based representations are similar to those favouring awareness.     

Comparative studies on the structure and aggregative properties of the myosin molecule. I. The structure of the lobster myosin molecule.

Myosin purified from the abdominal flexor muscle of the lobster, Homarus americanus, has a number average length of 1559 +/- 218 A, a rod like tail 1335 A long and a globular head 225 X 45 A as determined from electron microscopic observations on platinum shadowed preparations. The mass of the molecule was determined to be ca. 486,000 daltons from high speed equilibrium centrifugation studies at neutral and alkaline pH, and by SDS-acrylamide gel electrophoresis. Both sedimentation equilibrium centrifuge studies at alkaline pH and SDS-acrylamide gel electrophoresis experiments, indicate that the molecule contains a heavy chain core (two polypeptide chains weighing ca. 210,000 daltons each) and ca. four light chains of two weight classes (ca. 16,000 and 20,000 daltons). The amino acid composition of the myosin was determined. The specific activities of the Mg2+ -activated, K+/EDTA-activated, and Ca2+ -activated ATPases of the myosin were determined. Kinetic analysis of the digestion of lobster myosin with trypsin suggests that lobster myosin contains three classes of lysine and arginine residues; slowly split (k = 2.07 +/- 0.31 X 10(-2) moles/min2), rapidly split (k = 11.0 +/- 1.83 X 10(-2) moles/min2) and trypsin insensitive. There are 187 +/- 22 slowly split residues, 280 +/- 35 rapidly split residues, and 144 +/- 41 trypsin insensitive bonds per molecule. Comparison of these molecular parameters with those for the vertebrate skeletal muscle myosin indicates that the two myosins are similar in terms of mass, shape and overall polypeptide chain composition but may be considerably different in terms of local polypeptide chain conformation or composition.     

Bacteriophage SPO1 structure and morphogenesis. I. Tail structure and length regulation.

Bacteriophage SPO1, a structually complex phage with hydroxymethyl uracil replacing thymine, has been studied by structural and chemical methods with the aim of defining the virion organization. The contractile tail of SPO1 consists of a complex baseplate, a tail tube, and a 140-nm-long sheath composed of stacked disks (4.1 nm repeat), each containing six subunits of molecular weight 60,300. The subunits are arranged in six parallel helices, each with a helical screw angle (omega 0) of 22.5 degrees. The baseplate was shown to undergo a structural rearrangement during tail contraction into a hexameric pinwheel. A mutation in gene 8 which produced unattached heads and tails also produced tails of different lengths. The tail length distribution suggests that the smallest integral length increment is a single disk of subunits. The structural arrangement of subunits in long tails is identical to that of normal tails, and the tails can contract. Many of the long tails showed partial stain penetration within the tail tube to a point which coincides with the top of a unit-length tail. The implications of these findings with respect to tail length regulation are discussed.     

Anisotropic flexibility of DNA and the nucleosomal structure.

Potential energy calculations of the DNA duplex dimeric subunit show that the double helix may be bent in the direction of minor and major grooves much more easily than in other directions. It is found that the total winding angle of DNA decreases upon such bending. A new model for DNA folding in the nucleosome is proposed on the basis of these findings according to which the DNA molecule is kinked each fifth base pair to the side of the minor and major grooves alternatively. The model explains the known contradiction between a C-like circular dichroism for the nucleosomal DNA and the nuclease digestion data, which testify to the B-form of DNA.     

Overseas doctors and the staffing structure of hospitals.

The introduction of the new immigration rules will considerably reduce the number of overseas doctors available for junior hospital appointments over the next five years. To offset this sponsorship schemes should be offered by universities and colleges comprising training courses, entry to examinations, and hospital appointments, and training programmes should be adapted to make objectives attainable in the four years available to overseas doctors. Employment of overseas doctors can ease the transition within the staffing structure to fewer juniors and more consultants, but planning is urgently required, and in particular the service and manpower needs of the National Health Service must be considered.