The oxidation of Cys51 appears to have trapped the structure into a stable decamer, as confirmed by sedimentation analysis. A comparison with two previously reported dimeric Prx structures reveals that the catalytic cycle of 2-Cys Prx requires significant conformational changes that include the unwinding of the active-site helix and the movement of four loops. It is proposed that the stable decamer forms in vivo under conditions of oxidative stress. Similar decameric structures of TPx-B have been observed by electron microscopy, which show the protein associated with the erythrocyte membrane.

Calcium phosphate nanoclusters were prepared under standardised conditions using 10 mg ml Ϫ1 of the 25-amino-acid N-terminal tryptic phosphopeptide of bovine β-casein as a stabilising agent. The M r determined by sedimentation equilibrium was 197 600Ϯ 13700 and the apparent radius of gyration determined by X-ray scattering was 2.80Ϯ0.05 nm. A small-angle neutron scattering contrast variation study in 1 H 2 O/ 2 H 2 O mixtures was performed and gave radii of gyration at the calculated match points for the calcium phosphate (88.2 % 2 H 2 O) and phosphopeptide (41.3 % 2 H 2 O) of 3.39Ϯ 0.08 nm and 1.85Ϯ 0.05 nm, respectively. Measurements at larger scattering wave vector showed a subsidiary maximum at about Q ϭ 1.6 nm Ϫ1 .The results are consistent with a model of the nanoclusters comprising a spherical core of 355Ϯ 20 CaHPO 4 · 2 H 2 O units, density 2.31 g ml Ϫ1 and radius 2.30Ϯ 0.05 nm surrounded by 49Ϯ 4 peptide chains with a partial specific volume of 0.7 cm 3 g Ϫ1 , forming a tightly packed shell with an outer radius of 4.04Ϯ 0.15 nm.This model suggests that the phosphopeptide is able to arrest the process of growth of the precipitating phase of calcium phosphate at its earliest stages. A similar role for whole casein could be vital to the normal functioning of the mammary gland during milk secretion.Keywords : casein ; calcium phosphate ; neutron scattering; X-ray scattering; phosphopeptide.In cow's milk, about two thirds of the calcium and half the of calcium phosphate, without causing uncontrolled precipitation P i is found in the casein micelles, largely as calcium phosphate of calcium phosphate in the mammary gland [10, 11]. clusters bound to the phosphate centres of the A s1 -, A s2 -and β-The highly phosphorylated caseins, along with many other caseins [2Ϫ5]. The radius of the micellar calcium phosphate phosphoproteins found in calcifying tissues, have been shown to particles isolated from casein micelles by exhaustive proteolytic be effective in inhibiting the precipitation of calcium phosphate digestion has been shown by electron microscopy to be about from solutions at low to moderate supersaturation [10, 12Ϫ14]. 2.5 nm [6, 7], the exact value depending on the thickness as-Indeed, it has been suggested that the open and flexible (rheosumed for the carbon coating; other electron microscopical morphic [11]) conformation in solution of many of these highly studies have revealed electron-dense nanometer-sized bodies phosphorylated phosphoproteins is an adaptive feature of pepdistributed fairly uniformly throughout the protein matrix of the tides required to interact speedily with calcium phosphate nuclei casein micelle [8, 9]. The biological significance of the structure in order to control ectopic calcification in biological tissues. β-of casein micelles is not easily established but it has been sug-Casein shows a number of similarities of primary structure to the gested that casein micelles allow the total calcium and phosphate class of proline-rich phosphoproteins of saliva. In this context, concentr...

Phosphorylation is ubiquitous in control of protein activity, yet its effects on protein structure are poorly understood. Here we investigate the effect of serine phosphorylation in the interior of an alpha-helix when a salt bridge is present between the phosphate group and a positively charged side chain (in this case lysine) at i,i + 4 spacing. The stabilization of the helix is considerable and can overcome the intrinsically low preference of phosphoserine for the interior of the helix. The effect is pH dependent, as both the lysine and phosphate groups are titratable, and so calculations are given for several charge combinations. These results, with our previous work, highlight the different, context-dependent effects of phosphorylation in the alpha-helix. The interaction between the phosphate(2)(-) group and the lysine side chain is the strongest yet recorded in helix-coil studies. The results are of interest both in de novo design of peptides and in understanding the structural modes of control by phosphorylation.

To reduce time-dependent aggregation phenomena and achieve true "molecular" solution, the "pressure cell" solubilization method of Vorwerg and co-workers was applied to solutions of guar galactomannans (three samples of different molecular weights), using various heating, time, and pressure profiles. Physicochemical characterization of the guar samples before and after pressure cell treatment included measurements of intrinsic viscosity [eta] by capillary viscometry and M(w) and radius of gyration from size exclusion chromatography coupled to multiangle laser light scattering (SEC/MALLS). Heating the guar solutions (100-160 degrees C) without pressurization produced chain degradation with [eta] and M(w) values being reduced significantly, whereas this effect was reduced substantially for samples subject to initial pressurization ( approximately 5-10 bar). The constants in the Mark-Houwink-Sakurada equation, relating [eta] and M(w) were established and the characteristic ratio C(infinity) and chain persistence length L(p) were calculated using both the Burchard-Stockmayer-Fixman (BSF) method for flexible and semiflexible chains and the Hearst method more appropriate for stiffened chains. Definitive conclusions can now be drawn on the flexibility of the guar chain backbone, with L(p) approximately 4 nm from the BSF plot, in good agreement with previously published work using such geometric methods. This contrasts with the higher values obtained from extrapolation of data for polyelectrolytes with a similar backbone geometry, such as sodium carboxymethyl cellulose, to "infinite" ionic strength.