scholarly journals Characterisation and modelling of a polyhydroxyalkanoate synthase fromAquitaleasp. USM4 reveals a mechanism for polymer elongation

2017 ◽  
Author(s):  
Aik-Hong Teh ◽  
Nyet-Cheng Chiam ◽  
Go Furusawa ◽  
Kumar Sudesh
Keyword(s):  
Pseudomonas Aeruginosa ◽  
Substrate Specificity ◽  
Class Ii ◽  
Class I ◽  
Common Mechanism ◽  
The Third ◽  
Dimeric Interface ◽  
Key Enzyme ◽  
Polyhydroxyalkanoate Synthase ◽  
Key Residues

AbstractPolyhydroxyalkanoate synthase, PhaC, is a key enzyme in the biosynthesis of PHA, a type of bioplastics with huge potential to replace conventional petroleum-based plastics. While two PhaC structures have been determined recently, the exact mechanism remains unclear partly due to the absence of a tunnel for product passage. The PhaC fromAquitaleasp. USM4, PhaCAq, was characterised and showed aKmof 394 µM and akcatof 476.4 s−1on the 3HB-CoA substrate. A model based on the structure of the closely related PhaC fromCupriavidus necator, PhaCCnrevealed a three-branched tunnel at the dimeric interface. Two of the branches open to the solvent and serve as the putative routes for substrate entrance and product exit, while the third branch is elongated in a PhaC1 model fromPseudomonas aeruginosa, indicating a function of accommodating the hydroxyalkanoate (HA) moiety of the HA-CoA substrate. Docking of the two tetrahedral intermediates formed during catalysis suggests a PHA elongation mechanism that requires the HA moiety of the ligand to rotate ~180°. Both classes I and II PhaCs share a common mechanism for polymer elongation, and substrate specificity is determined in part by a bulky Phe/Tyr/Trp residue in the third branch in class I, which is conserved as Ala in class II to create room for longer substrates. The PhaCAqmodel provides fresh insights into a general PhaC mechanism, pinpointing key residues for potential engineering of PhaCs with desirable characteristics.

Lexical stresses in Macedonian and Polish

Phonology ◽  
1989 ◽  
Vol 6 (1) ◽  
pp. 19-38 ◽  
Author(s):  
Michael Hammond
Keyword(s):  
Ad Hoc ◽  
Class Ii ◽  
The Other ◽  
Class I ◽  
The Third ◽  
Strong Claim ◽  
Lexical Accent

SummaryWe have presented two different analyses of stress in Macedonian and Polish: one in terms of CGs and the other in terms of QS/ROB footing. Further, we showed how there were a number of problems with the CG account. We now consider those problems again in light of the latter analyses above.The first problem was that the emergence of unmarked stress under suffixation did not find a uniform explanation in Macedonian and Polish. This is not true of the revised analyses. Both are couched in terms of accent and the reemergence of regular stress in both languages follows as a consequence of distribution of lexical accent. In fact, aside from the contribution of secondary stresses in Polish and their absence in Macedonian, the analyses differ minimally. Footing in Macedonian is QS; footing in Polish is ROB.A second problem for the CG was that the reemergence of unmarked stress in Polish did not find a uniform explanation. Some cases were handled with lexical extrametricality, while others were handled with the special extrametricality rule.(44)This is also not a problem for the revised analysis. Class I and class II now differ minimally in the placement of accent.The third problem discussed above was the ad hoc character of the special extrametricality rule (44). This too is no longer a problem since this rule is no longer a part of the analysis. In fact, perhaps we can make the strong claim that rules of this type are to.be excluded in principle. An even stronger claim which might be supportable, though it would require we reconsider a number of analyses, is that lexical accent can never be marked with extrametricality. We leave this latter hypothesis open.Summarising, the best analysis of Macedonian and Polish requires a theory that makes use of two different kinds of footing – quantity-sensitive (QS) and revised obligatory-branching (ROB) – and that marks accent in terms of a diacritic feature rather than in terms of grid marks without constituency. Only by adopting such a theory can we capture in a satisfying fashion the generalisation exhibited in Macedonian and Polish, that regular stress reemerges when a word with exceptional stress undergoes sufficient affixation.

scholarly journals Bragg resonance of waves in a two-layer fluid propagating over bottom ripples. Part I. Perturbation analysis

2009 ◽  
Vol 624 ◽  
pp. 191-224 ◽  
Author(s):  
MOHAMMAD-REZA ALAM ◽  
YUMING LIU ◽  
DICK K. P. YUE
Keyword(s):  
Internal Wave ◽  
Internal Waves ◽  
Incident Wave ◽  
Class Ii ◽  
Class I ◽  
Bragg Resonance ◽  
Class Iii ◽  
Third Order ◽  
The Third ◽  
Bragg Resonances

We investigate, via perturbation analyses, the mechanisms of nonlinear resonant interaction of surface-interfacial waves with a rippled bottom in a two-layer density-stratified fluid. As in a one-layer fluid, three classes of Bragg resonances are found to exist if nonlinear interactions up to the third order in the wave/ripple steepness are considered. As expected, the wave system associated with the resonances is more complicated than that in a one-layer fluid. Depending on the specifics of the resonance condition, the resonance-generated wave may be a surface or internal mode and may be transmitted or reflected. At the second order, class I Bragg resonance occurs involving two surface and/or internal waves and one bottom-ripple component. The interaction of an incident surface/internal wave with the bottom ripple generates a new surface or internal wave that may propagate in the same or the opposite direction as the incident wave. At the third order, class II and III Bragg resonances occur involving resonant interactions of four wave/ripple components: two surface and/or internal waves and two bottom-ripple components for class II resonance; three surface and/or internal waves and one bottom-ripple components for class III resonance. As in class I resonance, the resonance-generated wave in class II resonance has the same frequency as that of the incident wave. For class III resonance, the frequency of the resonant wave is equal to the sum or difference of the two incident wave frequencies. We enumerate and represent, using Feynman-like diagrams, the possible cases and combinations for Bragg resonance up to the third order (in two dimensions). Analytical regular perturbation results are obtained and discussed for all three classes of Bragg resonances. These are valid for limited bottom patch lengths and initial/finite growth of the resonant waves. For long bottom patches, a uniformly valid solution using multiple scales is derived for class I resonance. A number of applications underscoring the importance and implication of these nonlinear resonances on the evolution of ocean waves are presented and discussed. For example, it is shown that three internal/surface waves co-propagating over bottom topography are resonant under a broad range of Bragg conditions. The present study provides the theoretical basis and understanding for the companion paper (Alam, Liu & Yue 2008), where a direct numerical solution for the general nonlinear problem is pursued.

The classification and representation of main group element compounds that feature three-center four-electron interactions

2016 ◽  
Vol 45 (47) ◽  
pp. 18784-18795 ◽  
Author(s):  
Malcolm L. H. Green ◽  
Gerard Parkin
Keyword(s):  
Class Ii ◽  
Main Group ◽  
Group Element ◽  
Class I ◽  
The Third ◽  
Main Group Element ◽  
Electron Interactions

Compounds that feature 3-center 4-electron interactions may be classified in terms of the number of electrons that each atom contributes to the interaction: Class I are those in which two atoms provide one electron each and the third atom provides a pair of electrons, while Class II are those in which two atoms each provide a pair of electrons.

Electron microscopy analysis of degenerating pancreatic ß cells in transgenic mice expressing the MHC Class I antigen Dd

1990 ◽  
Vol 48 (3) ◽  
pp. 548-549
Author(s):  
T. A. Stewart ◽  
D. Liggitt ◽  
S. Pitts ◽  
L. Martin ◽  
M. Siegel ◽  
...  
Keyword(s):  
Type I Diabetes ◽  
Disease Process ◽  
Class Ii ◽  
Class I ◽  
Type I ◽  
Aberrant Expression ◽  
Mhc Molecules ◽  
Endogenous Insulin ◽  
Class Ii Mhc ◽  
Ifn Γ

Insulin-dependant (Type I) diabetes mellitus (IDDM) is a metabolic disorder resulting from the lack of endogenous insulin secretion. The disease is thought to result from the autoimmune mediated destruction of the insulin producing ß cells within the islets of Langerhans. The disease process is probably triggered by environmental agents, e.g. virus or chemical toxins on a background of genetic susceptibility associated with particular alleles within the major histocompatiblity complex (MHC). The relation between IDDM and the MHC locus has been reinforced by the demonstration of both class I and class II MHC proteins on the surface of ß cells from newly diagnosed patients as well as mounting evidence that IDDM has an autoimmune pathogenesis. In 1984, a series of observations were used to advance a hypothesis, in which it was suggested that aberrant expression of class II MHC molecules, perhaps induced by gamma-interferon (IFN γ) could present self antigens and initiate an autoimmune disease. We have tested some aspects of this model and demonstrated that expression of IFN γ by pancreatic ß cells can initiate an inflammatory destruction of both the islets and pancreas and does lead to IDDM.

Pictish Horse Carvings

1991 ◽  
Vol 17 (1) ◽  
pp. 53-62
Author(s):  
Irene Hughson
Keyword(s):  
Class Ii ◽  
Class I ◽  
Early Historic Period ◽  
High Quality ◽  
Historic Period ◽  
Agricultural Economy ◽  
Different Types ◽  
Early Historic

Summary This paper examines the horse carvings to be found on Class I and Class II Pictish sculptured stones and considers their reliability as evidence of the sort of horses and ponies that would have existed in the Early Historic Period. An attempt is made to show that the availability in Britain of good sized, high quality riding horses during that period is not inconsistent with what is known of the development and distribution of different types of horses in pre-hislory. The importance of horses and ponies in Early Historic societies is stressed and inferences drawn about the agricultural economy that could support horses and the skilled specialists required to look after them.

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