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Oxidation of methionine residues in calmodulin (CaM) lowers the affinity for calcium and results in an inability to activate target proteins fully. To evaluate the structural consequences of CaM oxidation, we used infrared difference spectroscopy to identify oxidation-dependent effects on protein conformation and calcium liganding. Oxidation-induced changes include an increase in hydration of α-helices, as indicated in the downshift of the amide I′ band of both apo-CaM and Ca2+-CaM, and a modification of calcium liganding by carboxylate side chains, reflected in antisymmetric carboxylate band shifts. Changes in carboxylate ligands are consistent with the model we propose: an Asp at position 1 of the EF-loop experiences diminished hydrogen bonding with the polypeptide backbone, an Asp at position 3 forms a bidentate coordination of calcium, and an Asp at position 5 forms a pseudobridging coordination with a calcium-bound water molecule. The bidentate coordination of calcium by conserved glutamates is unaffected by oxidation. The observed changes in calcium ligation are discussed in terms of the placement of methionine side chains relative to the calcium-binding sites, suggesting that varying sensitivities of binding sites to oxidation may underlie the loss of CaM function upon oxidation. 相似文献
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Kinetically-resolved absorbance measurements during extended, or steady-state illumination are typically hindered by large,
light-induced changes in the light-scattering properties of the material. In this work, a new type of portable spectrophotometer,
the Non-Focusing Optical Spectrophotometer (NoFOSpec), is introduced, which reduces interference from light-scattering changes
and is in a form suitable for fieldwork. The instrument employs a non-focusing optical component, called a compound parabolic
concentrator (CPC), to simultaneously concentrate and homogeneously diffuse measuring and actinic light (from light-emitting
diode sources) onto the leaf sample. Light passing through the sample is then collected and processed using a subsequent series
of CPCs leading to a photodiode detector. The instrument is designed to be compact, lightweight and rugged for field work.
The pulsed measuring beam allows for high sensitivity (typically < 100 ppm noise) and time resolution (∼ 10 μs) measurements
in the visible and near infrared spectral regions. These attributes allow high-resolution measurements of signals associated
with energization of the thylakoid membrane (the electrochromic shifting of carotenoid pigments), as well as electron transfer,
e.g., the 820-nm changes associated with electron transfer through Photosystem I (PS I). In addition, the instrument can be
used as a kinetic fluorimeter, e.g., to measure saturation-pulse fluorescence changes indicative of Photosystem II (PS II)
quantum efficiency. The instrument is demonstrated by estimating electron and proton fluxes through the photosynthetic apparatus
in an intact tobacco leaf, using respectively the saturation-pulse fluorescence changes and dark-interval relaxation kinetics
(DIRK) of the electrochromic shift. A linear relationship was found, confirming our earlier results with the laboratory-based
diffused-optics flash spectrophotometer, indicating a constant H+/e− stoichiometry for linear electron transfer, and suggesting that cyclic electron flow around PS I is either negligible or
proportional to linear electron flow. This type of measurement should be useful under field conditions for estimating the
extent of PS I cyclic electron transfer, which is proposed to operate under stressed conditions.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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Abstract The palatability to common carp, Cyprinus carpio L. of three newly developed differently flavoured floating pellets made from a high proportion (40%) of brewer's spent grain (BSG) was tested using a multiple-offer feeding experiment. The addition of ‘bold’ flavours, such as vanilla or strawberry essence, may help mask the unpleasant taste of some piscicides; however, their inclusion must not compromise uptake by carp. There were no significant differences between the consumption rates of the three varieties, and all flavours were readily consumed. Therefore, it is suggested that highly flavoured pellets made with BSG have a strong potential to mask the flavour of an unpalatable toxin, and further research is now needed to test this hypothesis. 相似文献
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Kavish J Bhansing Martin Lammens Hanneke KA Knaapen Piet LCM van Riel Baziel GM van Engelen Madelon C Vonk 《Arthritis research & therapy》2014,16(3):R111