67 research outputs found
Friends in need: how chaperonins recognize and remodel proteins that require folding assistance
Chaperonins are biological nanomachines that help newly translated proteins
to fold by rescuing them from kinetically trapped misfolded states. Protein
folding assistance by the chaperonin machinery is obligatory in vivo for a
subset of proteins in the bacterial proteome. Chaperonins are large oligomeric
complexes, with unusual seven fold symmetry (group I) or eight/nine fold
symmetry (group II), that form double-ring constructs, enclosing a central
folding chamber. Dramatic large-scale conformational changes, that take place
during ATP-driven cycles, allow chaperonins to bind misfolded proteins,
encapsulate them into the expanded cavity and release them back into the
cellular environment, regardless of whether they are folded or not. The theory
associated with the iterative annealing mechanism, which incorporated the
conformational free energy landscape description of protein folding,
\textit{quantitatively} explains most, if not all, the available data.
Misfolded conformations are associated with low energy minima in a rugged
energy landscape. Random disruptions of these low energy conformations result
in higher free energy, less folded, conformations that can stochastically
partition into the native state. Group I chaperonins (GroEL homologues in
eubacteria and endosymbiotic organelles), recognize a large number of misfolded
proteins non-specifically and operate through highly coordinated cooperative
motions. By contrast, the less well understood group II chaperonins (CCT in
Eukarya and thermosome/TF55 in Archaea), assist a selected set of substrate
proteins. Chaperonins are implicated in bacterial infection, autoimmune
disease, as well as protein aggregation and degradation diseases. Understanding
the chaperonin mechanism and their substrates is important not only for the
fundamental aspect of cellular protein folding, but also for effective
therapeutic strategies.Comment: 26 pages, 4 figures, to be published in Frontiers in Molecular
Bioscience
Rubisco is not really so bad
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the most widespread carboxylating enzyme in autotrophic organisms. Its kinetic and structural properties have been intensively studied for more than half a century. Yet important aspects of the catalytic mechanism remain poorly understood, especially the oxygenase reaction. Because of its relatively modest turnover rate (a few catalytic events per second) and the competitive inhibition by oxygen, Rubisco is often viewed as an inefficient catalyst for CO2 fixation. Considerable efforts have been devoted to improving its catalytic efficiency, so far without success. In this review, we re-examine Rubisco's catalytic performance by comparison with other chemically related enzymes. We find that Rubisco is not especially slow. Furthermore, considering both the nature and the complexity of the chemical reaction, its kinetic properties are unremarkable. Although not unique to Rubisco, oxygenation is not systematically observed in enolate and enamine forming enzymes and cannot be considered as an inevitable consequence of the mechanism. It is more likely the result of a compromise between chemical and metabolic imperatives. We argue that a better description of Rubisco mechanism is still required to better understand the link between CO2 and O2 reactivity and the rationale of Rubisco diversification and evolution.C. B. and G. D. F. acknowledge funding by the Australian Government through the Australian Research Council Centre of Excellence for Translational Photosynthesis (Project CE140100015), and G. T. thanks the Australian Research Council for its support via a Fellowship under contract FT140100645
Commentary: Directions for Optimization of Photosynthetic Carbon Fixation: RuBisCO's Efficiency May Not Be So Constrained After All
The authors thank the Australian Research Council for its support through a Future Fellowship grant, under contract FT140100645
The Very Young Radio Pulsar J1357-6429
We report the discovery of a radio pulsar with a characteristic age of 7300 years, making it one of the 10 apparently youngest Galactic pulsars known. PSR J1357-6429, with a spin period of P = 166 ms and spin-down luminosity of 3.1e36 ergs/s, was detected during the Parkes multibeam survey of the Galactic plane. We have measured a large rotational glitch in this pulsar, with Delta P/P = -2.4e-6, similar in magnitude to those experienced occasionally by the Vela pulsar. At a nominal distance of only ~ 2.5 kpc, based on the measured free electron column density of 127 pc/cc and the electron distribution model of Cordes & Lazio, this may be, after the Crab, the nearest very young pulsar known. The pulsar is located near the radio supernova remnant candidate G309.8-2.6
Spectroscopy of Ultra Steep Spectrum Radio Sources
We present optical spectroscopy of 62 objects selected from several samples
of ultra steep spectrum (USS) radio sources. 46 of these are from our primary
catalog, consisting of 669 sources with radio spectral indices alpha < -1.30
(S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on the
basis of their faint optical and near-IR identifications. Most are identified
as narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19.
Ten objects are at z>3, nearly doubling the number of such sources known to
date. Four of the USS radio sources are identified with quasars, of which at
least three have very red spectral energy distributions. The source TN
J0936-2242 is identified with an extremely red object (ERO, R-K>5); both it and
a close companion are at z=1.479. The spectrum of the ERO closely resembles
that of previously discovered radio galaxies at z~1.5. Five sources show
continuum emission, but fail to show any clear emission or absorption features,
despite integrations of ~1 h with the Keck telescope. We suggest that these
objects could be (i) radio galaxies with faint emission lines in the ``redshift
desert'' at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, which
are dominated by a stellar continuum observed with insufficient S/N, or (iii)
pulsars. Three radio sources identified with faint objects in the K-band images
remain undetected in 50-90 min spectroscopic integrations with the Keck
telescope, and are possible z>7 candidates.Comment: 59 Pages, including 59 PostScript figures. Accepted for publication
in the Astronomical Journal (March 2001 issue). Tables 5 and 6 will be
available in electronic format on the AJ website or upon reques
Erratum: 'A precise mass measurement of the intermediate-mass binary pulsar PSR J1802-2124' (2010, ApJ, 711, 764)
Figure 1 as originally published did not match its corresponding caption; although the image was meant to be updated, it was unintentionally left unchanged. The correct Figure 1 with its corresponding caption is shown below
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