CPG Control for Biped Hopping Robot in Unpredictable Environment

A CPG control mechanism is proposed for hopping motion control of biped robot in unpredictable environment.Based on analysis of robot motion and biological observation of animal's control mechanism,the motion control task is divided into two simple parts:motion sequence control and output force control.Inspired by a two-level CPG model,a two-level CPG control mechanism is constructed to coordinate the drivers of robot joint,while various feedback information are introduced into the control mechanism.Interneurons within the control mechanism are modeled to generate motion rhythm and pattern promptly for motion sequence control; motoneurons are modeled to control output forces of joint drivers in real time according to feedbacks.The control system can perceive changes caused by unknown perturbations and environment changes according to feedback information,and adapt to unpredictable environment by adjusting outputs of neurons.The control mechanism is applied to a biped hopping robot in unpredictable environment on simulation platform,and stable adaptive motions are obtained.     

Mechanisms of domain closure in proteins

Certain enzymes respond to the binding of substrates and coenzymes by the closure of an active site that lies in a cleft between two domains. We have examined the mechanism of the domain closure in citrate synthase, for which atomic co-ordinates are available for "open" and "closed" forms. We show that the mechanism of domain closure involves small shifts and rotations of packed helices within the two domains and at their interface. Large motions of distant segments of the structure are the cumulative effect of the small relative shifts in intervening pairs of packed segments. These shifts are accommodated not by changes in packing but rather by small conformational changes in side-chains. We call this the helix interface shear mechanism of domain closure. The relative movements of packed helices follow the principles suggested by our recent study of insulin. This mechanism of domain closure is quite different from the hinge mechanisms that allow the rigid body movements of domains in immunoglobulins. The large interface between the domains of citrate synthase precludes a simple hinge mechanism for its conformational change. The helix interface shear mechanism of conformational change occurs in other enzymes that contain extensive domain-domain interfaces.     

A basis of echinocytosis and stomatocytosis in the disc-sphere transformations of the erythrocyte

A mechanism of erythrocyte shape control has been previously hypothesized in which Band 3, the anion exchange protein, controls the shape. In essence, the mechanism hypothesizes that the membrane skeleton is used to generate different shapes and the alternate influx and efflux of anions mediated by Band 3, which recruit Band 3 to an inward-facing and an outward-facing conformation, contract and relax the skeleton by folding and unfolding spectrin. Spectrin is bound to Band 3 by the intermediary of ankyrin. The mechanism is shown to be consistent with rapid shape deformations of the erythrocyte in blood circulation. We have examined whether the mechanism could provide a basis of echinocytosis and stomatocytosis in disc-sphere transformations of the erythrocyte induced by a wide variety of agents. These agents were classified into four groups: lipids of the bilayer, Donnan equilibrium modifiers, Band 3 anion transport inhibitors and integral membrane protein modifiers. Evidence is presented that the lipids play a secondary function in the control of the erythrocyte shape, as indicated by the mechanism. Two possible functions of the lipids are suggested with respect to the mechanism. Without exception, echinocytogenic and stomatocytogenic Donnan equilibrium modifiers decrease and increase the equilibrium ratio of chloride (Cl-(i)/Cl-(o)), respectively, as predicted by the mechanism. Echinocytosis produced by competitive anion transport inhibitors slowly transported inward by Band 3 and by affinity labels of Band 3 is compatible with the mechanism. Evidence is presented which indicates that echinocytosis and stomatocytosis induced by amphiphilic drugs and detergents occur by inhibition of the Band 3 anion transport. Finally, echinocytosis and stomatocytosis induced by non-covalent and covalent modifiers of integral membrane proteins such as agglutinins and digestive enzymes are consistent with the mechanism.     

Resistance to Mercuric Chloride in Pseudomonas K-62

Mercuric ion-resistant mechanism in Pseudomonas K-62 was investigated by isolating a mercuric ion-sensitive mutant strain (H-2 strain) from a resistant wild-type strain (P strain) and comparing the physiological properties of these two strains. Growth curves obtained in mercuric ion-containing medium suggested that P strain constitutively has a resistant mechanism. The capacity of P and H-2 strains to decrease the concentration of mercuric ions in the medium was found to be almost equal; thus, the presence of a resistant mechanism was expected other than MMR-enzyme, which is an inducible system. From the characteristic property of the inhibitory effect of mercuric ions on respiration of the two strains, a resistant mechanism, a permeability barrier mechanism, was proposed. The barrier mechanism in P strain traps mercuric ions with high selectivity to prevent the penetration of ions to the intracellular space where the functional units are located. Finally, the resistance mechanism to mercuric ions in the bacterium was discussed including the MMR-enzyme system and the barrier mechanism.     

Thermal stability of collagen fibers in ethylene glycol

The mechanism that renders collagen molecules more stable when precipitated as fibers than the same molecules in solution is controversial. According to the polymer-melting mechanism the presence of a solvent depresses the melting point of the polymer due to a thermodynamic mechanism resembling the depression of the freezing point of a solvent due to the presence of a solute. On the other hand, according to the polymer-in-a-box mechanism, the change in configurational entropy of the collagen molecule on denaturation is reduced by its confinement by surrounding molecules in the fiber. Both mechanisms predict an approximately linear increase in the reciprocal of the denaturation temperature with the volume fraction (epsilon) of solvent, but the polymer-melting mechanism predicts that the slope is inversely proportional to the molecular mass of the solvent (M), whereas the polymer-in-a-box mechanism predicts a slope that is independent of M. Differential scanning calorimetry was used to measure the denaturation temperature of collagen in different concentrations of ethylene glycol (M = 62) and the slope found to be (7.29 +/- 0.37) x 10(-4) K(-1), compared with (7.31 +/- 0.42) x 10(-4) K(-1) for water (M = 18). This behavior was consistent with the polymer-in-a-box mechanism but conflicts with the polymer-melting mechanism. Calorimetry showed that the enthalpy of denaturation of collagen fibers in ethylene glycol was high, varied only slowly within the glycol volume fraction range 0.2 to 1, and fell rapidly at low epsilon. That this was caused by the disruption of a network of hydrogen-bonded glycol molecules surrounding the collagen is the most likely explanation.     

Donor-strand exchange in chaperone-assisted pilus assembly revealed in atomic detail by molecular dynamics

Adhesive multi-subunit fibres are assembled on the surface of many pathogenic bacteria via the chaperone-usher pathway. In the periplasm, a chaperone donates a β-strand to a pilus subunit to complement its incomplete immunoglobulin-like fold. At the outer membrane, this is replaced with a β-strand formed from the N-terminal extension (Nte) of an incoming pilus subunit by a donor-strand exchange (DSE) mechanism. This reaction has previously been shown to proceed via a concerted mechanism, in which the Nte interacts with the chaperone:subunit complex before the chaperone has been displaced, forming a ternary intermediate. Thereafter, the pilus and chaperone β-strands have been postulated to undergo a strand swap by a ‘zip-in-zip-out’ mechanism, whereby the chaperone strand zips out, residue by residue, as the Nte simultaneously zips in, although direct experimental evidence for a zippering mechanism is still lacking. Here, molecular dynamics simulations have been used to probe the DSE mechanism during formation of the Saf pilus from Salmonella enterica at the atomic level, allowing the direct investigation of the zip-in-zip-out hypothesis. The simulations provide an explanation of how the incoming Nte is able to dock and initiate DSE due to inherent dynamic fluctuations within the chaperone:subunit complex. In the simulations, the chaperone donor strand was seen to unbind from the pilus subunit, residue by residue, in direct support of the zip-in-zip-out hypothesis. In addition, an interaction of a residue towards the N-terminus of the Nte with a specific binding pocket (P*) on the adjacent pilus subunit was seen to stabilise the DSE product against unbinding, which also proceeded in the simulations by a zippering mechanism. Together, the study provides an in-depth picture of DSE, including the first atomistic insights into the molecular events occurring during the zip-in-zip-out mechanism.     

Recent trends in evolutionary ethics: greenbeards!

In recent years, there has been growing awareness among evolutionary ethicists that systems of cooperation based upon “weak” reciprocity mechanisms (such as tit-for-tat) lack scalability, and are therefore inadequate to explain human ultrasociality. This has produced a shift toward models that strengthen the cooperative mechanism, by adding various forms of commitment or punishment. Unfortunately, the most prominent versions of this hypothesis wind up positing a discredited mechanism as the basis of human ultrasociality, viz. a “greenbeard.” This paper begins by explaining what a greenbeard is, and why evolutionary theorists are doubtful that such a mechanism could play a significant role in explaining human prosociality. It goes on to analyze several recent philosophical works in evolutionary ethics, in order to show how the suggestion that morality acts as a commitment device tacitly relies upon a greenbeard mechanism to explain human cooperation. It concludes by showing how some early scientific models in the “evolution of cooperation” literature, which introduced punishment as a device to enhance cooperation, also tacitly relied upon a greenbeard mechanism.     

The mirror system and its role in social cognition

Experiments in monkeys have shown that coding the goal of the motor acts is a fundamental property of the cortical motor system. In area F5, goal-coding motor neurons are also activated by observing motor acts done by others (the 'classical' mirror mechanism); in area F2 and area F1, some motor neurons are activated by the mere observation of goal-directed movements of a cursor displayed on a computer screen (a 'mirror-like' mechanism). Experiments in humans and monkeys have shown that the mirror mechanism enables the observer to understand the intention behind an observed motor act, in addition to the goal of it. Growing evidence shows that a deficit in the mirror mechanism underlies some aspects of autism.     

Preprotein-controlled catalysis in the helicase motor of SecA

The cornerstone of the functionality of almost all motor proteins is the regulation of their activity by binding interactions with their respective substrates. In most cases, the underlying mechanism of this regulation remains unknown. Here, we reveal a novel mechanism used by secretory preproteins to control the catalytic cycle of the helicase 'DEAD' motor of SecA, the preprotein translocase ATPase. The central feature of this mechanism is a highly conserved salt-bridge, Gate1, that controls the opening/closure of the nucleotide cleft. Gate1 regulates the propagation of binding signal generated at the Preprotein Binding Domain to the nucleotide cleft, thus allowing the physical coupling of preprotein binding and release to the ATPase cycle. This relay mechanism is at play only after SecA has been previously 'primed' by binding to SecYEG, the transmembrane protein-conducting channel. The Gate1-controlled relay mechanism is essential for protein translocase catalysis and may be common in helicase motors.     

F plasmid ccd mechanism in Escherichia coli.

The ccd mechanism specified by the ccdA and ccdB genes of the mini-F plasmid determines fate of plasmid-free segregants in Escherichia coli (Jaffé et al., J. Bacteriol. 163:841-849, 1985). The killing function in plasmid-free segregants by the ccd mechanism did not affect cell growth of coexisting cells in the same culture. Elongated cells and anucleate cells caused by the ccd mechanism were clearly detected by flow cytometry in cultures of bacterial strains harboring Ccd+ Sop- mini-F plasmids defective in partitioning. This indicates that the defect in correct partitioning of plasmid DNA molecules into daughter cells also induces the ccd mechanism to operate.     

Energy dissipation in the brown fatty tissue of warm-blooded animals

The uncoupling mechanism of respiration and phosphorylation in brown adipose tissue as well as factors affecting this process are discussed. An assumption has been suggested and substantiated experimentally that the uncoupling of respiration and phosphorylation in liver mitochondria caused by vitamin K alimentary deficit may have mechanism similar to the unique uncoupling mechanism of brown fat mitochondria.     

微生物产氢机理的研究进展

微生物可以利用工业废弃物产生氢气,其产氢机理可以分成两种:光合产氢和发酵产氢。前者利用光能,后者利用代谢过程中产生的电子,分解有机物产氢。氢酶是产氢过程中的关键酶,催化氢的氧化或质子的还原。氢酶主要有[NiFe]氢酶和[Fe]氢酶两种,具有不同的结构,但催化机理是相似的。本文主要综述产氢微生物的种类、微生物产氢代谢途径和关键酶催化机理,并展望微生物产氢研究的发展方向。     

Role of ATP-Hydrolysis in the Dynamics of a Single Actin Filament

We study the stochastic dynamics of growth and shrinkage of single actin filaments taking into account insertion, removal, and ATP hydrolysis of subunits either according to the vectorial mechanism or to the random mechanism. In a previous work, we developed a model for a single actin or microtubule filament where hydrolysis occurred according to the vectorial mechanism: the filament could grow only from one end, and was in contact with a reservoir of monomers. Here we extend this approach in two ways—by including the dynamics of both ends and by comparing two possible mechanisms of ATP hydrolysis. Our emphasis is mainly on two possible limiting models for the mechanism of hydrolysis within a single filament, namely the vectorial or the random model. We propose a set of experiments to test the nature of the precise mechanism of hydrolysis within actin filaments.     

Haemostatic Mechanism in the Uterine Circulation during Placental Separation

The haemostatic mechanism in the uterus during parturition was investigated in 12 patients being delivered by caesarean section. Detailed sequential study of the blood coagulation and fibrinolytic systems in the uterine circulation showed that placental separation is accompanied by a striking local activation of the clotting mechanism. Uterine vein blood draining the placental site while the placenta was separating showed a pronounced shortening of the whole-blood clotting-time, a significant shortening of other clotting-tests, and a sharp increase in factor VIII activity, though these changes were transitory. After delivery the level of fibrinogen and circulating platelets steadily increased and factor VIII activity remained high.Activation of the clotting mechanism during placental separation appears to play an essential part in controlling uterine haemorrhage. The subsequent changes in the haemostatic mechanism in the puerperium are likely to predispose to thromboembolic complications.     

A Model for the Censoring Process in Incomplete Repeated Measurement Experiments

Premature terminations or dropouts occur often in repeated measurement experiments. A number of methods have been proposed to analyze such data but most of them assume that the censoring mechanism is, within each group, unaffected by the mechanism generating the response variables. In this paper, we propose a model for the censoring mechanism that generates dropouts. We then show how this model can be used to check whether the censoring mechanism is affected by the response variables and other covariates. Finally, the methods of the paper are applied to the “Halothane” data set.     

Two mechanisms of synaptic vesicle recycling in rat brain nerve terminals

KCl and 4-aminopyridine (4-AP) evoke glutamate release from rat brain cortical nerve terminals by voltage clamping or by Na(+) channel-generated repetitive action potentials, respectively. Stimulation by 4-AP but not KCl is largely mediated by protein kinase C (PKC). To determine whether KCl and 4-AP utilise the same mechanism to release glutamate, we correlated glutamate release with release of the hydrophobic synaptic vesicle (SV) marker FM2-10. A strong correlation was observed for increasing concentrations of KCl and after application of phorbol 12-myristate 13-acetate (PMA) or staurosporine. The parallel increase in exocytosis measured by two approaches suggested it occurred by a PKC-independent mechanism involving complete fusion of SVs with the plasma membrane. At low concentrations of 4-AP, alone or with staurosporine, glutamate and FM2-10 release also correlated. However, higher concentrations of 4-AP or of 4-AP plus PMA greatly increased glutamate release but did not further increase FM2-10 release. This divergence suggests that 4-AP recruits an additional mechanism of release during strong stimulation that is PKC dependent and is superimposed upon the first mechanism. This second mechanism is characteristic of kiss-and-run, which is not detectable by styryl dyes. Our data suggest that glutamate release in nerve terminals occurs via two mechanisms: (1) complete SV fusion, which is PKC independent; and (2) a kiss-and-run-like mechanism, which is PKC dependent. Recruitment of a second release mechanism may be a widespread means to facilitate neurotransmitter release in central neurons.     

Feeding mechanisms in anuran larvae

Larvae of the neotropical frogs Phyllomedusa are distinctive in that they feed normally in mid-water on phytoplankton, maintaining neutral buoyancy by means of an independently beating tail filament. The feeding mechanism of larval Phyllomedusa trinitatis was studied morphologically and experimentally. The primary feeding mechanism involves a buccal rasp which may in some circumstances render food into small particles, a pumping mechanism which forces water through the buccal cavity and the gill filters, an entanglement system which traps particles in mucous strings produced in special organs, and the formation of mucous cords which transport particles to the oesophagus. In mid-water feeding and surface feeding the buccal rasp serves only its other function in preventing backflow of the respiratory stream. The primary feeding mechanism is discussed and compared with schemes proposed for Rana temporaria and R. agilis. Little agreement exists between these schemes and that which is here proposed. It is concluded that the primary feeding mechanism is the same in the three forms but that there are behavioural differences in feeding generally. Some comment is made on the primary feeding mechanism in the Microhylidae.     

植物胞质雄性不育及育性恢复的分子机制研究进展(综述)

本文从与雄性不育有关的线粒体基因引起雄性不育的机理、雄性不育育性恢复机制以及育性恢复基因的克隆等方面,介绍国内外对植物细胞质雄性不育分子机理的研究进展,并对今后的研究进行讨论。