F. MacRitchie

Proteins at interfaces.

Publisher Summary The chapter discusses the proteins at interfaces and aims to present protein adsorption and its ramifications, such as the effects of adsorption on conformation and reactivity from a fundamental physicochemical approach. Although many proteins are very soluble in water, adsorption leads to stable monolayers that are extremely difficult to desorb. Adsorption may be followed at fluid/fluid interfaces by measuring changes in interfacial pressure, potential, or viscosity, using spread monolayers for calibration purposes. Before a protein molecule can adsorb and exert its influence at a phase boundary or take part in an interfacial reaction, it must arrive at the interface by a diffusion process. At fluid/fluid interfaces, it is established that proteins lose their tertiary structure. One of the advantages of the fluid/fluid type interface is that the distribution between adsorbed (trains) and displaced segments (loops) may be quantitatively studied by carrying out measurements with a film balance. This approach has been used for spread monolayers of protein. The high free energy of adsorption of proteins results in all segments being at the interface at low interfacial pressures. The conformational changes that accompany desorption from an interface have direct bearing on the important subject of protein biosynthesis. The chapter also discusses the equilibrium aspects of adsorption, reactions at interfaces, and proteins at interfaces in biological systems.

Biochemical, Genetic, and Molecular Characterization of Wheat Glutenin and Its Component Subunits

Of all the cereal grains, wheat is unique because wheat flour alone has the ability to form a dough that exhibits the rheological properties required for the production of leavened bread and for the wider diversity of foods that have been developed to take advantage of these attributes. The unique properties of the wheat grain reside primarily in the gluten-forming storage proteins of its endosperm. It is these dough-forming properties that are responsible for wheat being the most important source of protein in the human diet. The bread and durum wheats are polyploid species containing three (AABBDD) and two (AABB) related genomes, respectively. The genetic constitution of wheat is important because all quality traits result from the expression of genes and their interaction with the environment. The full spectrum of wheat-endosperm proteins has been exhibited in proteome studies involving the two-dimensional fractionation of the polypeptides (after disulfide-bond rupture), followed by dissection of the individual components for identification. This display (Fig. 1) shows that there are at least 1,300 polypeptides, over 300 of which have been identified by N-terminal amino

Protein adsorption at solid-liquid interfaces: reversibility and conformation aspects.

Abstract The patterns for the adsorption of human plasma albumin are similar at sorbent surfaces that differ with respect to electrical charge and hydrophobicity. It seem, therefore, that the adsorption behavior is determined by properties of the albumin molecule, rather than by the sorbent surface. With all systems studied, dilution did not lead to significant desorption. However, the protein could be (partly) removed from most surfaces by adjusting the pH or the ionic strength or by adding a displacer. In the desorbed samples the α-helix content, as calculated from circular dichroism spectra, is somewhat lower than in the native albumin molecule. It is reasoned that the structural rearrangements involve an entropy gain related to an increased rotational mobility along the polypeptide chain. This entropy increase may be sufficiently large to compensate for the positive adsorption enthalpy.

Physicochemical properties of wheat proteins in relation to functionality

Publisher Summary This chapter discusses the physicochemical properties of wheat proteins in relation to functionality. One of the major obstacles to the characterization of wheat proteins has been the lack of methods to solubilize the total protein. On the basis of chemical composition, there are two broad classes of proteins in wheat—the albumidglobulins and the gliadidglutenin, or gluten, proteins. Whereas albumin or globulin proteins are readily soluble in aqueous solution, the gluten proteins present problems for their solubilization. Several methods have been applied for studying the surface hydrophobicities of wheat proteins. These include hydrophobic interaction chromatography (HIC), reversed-phase high-performance liquid chromatography (RP-HPLC), and methods involving interactions with apolar ligands in solution. The solubility of proteins is hindered by hydrophobic effects resulting from the high content of nonpolar side chains. This effect is usually minimized by folding of molecules so as to bury a large proportion of the nonpolar side chains in the interior of the molecule, where they are shielded from interaction with water molecules.

The adsorption of proteins at the solid/liquid interface

Abstract The adsorption isotherms of Bovine Serum Albumin at some solid/water interfaces have been measured. A comparison between adsorption at hydrophobic and hydrophilic silica surfaces showed that a large adsorption energy is associated with adsorption at the hydrophobic surface. At a hydrophilic surface, adsorption is weaker and more sensitive to electrical factors. A comparison with the adsorption isotherm at the air/water interface suggested that the nature of the process is similar at the two types of interface. The role of interfacial coagulation and slow desorption is discussed in relation to the reversibility of adsorption.

Molecular Weight Distribution of Wheat Proteins

ABSTRACT The molecular weight distribution (MWD) of wheat proteins is becoming recognized as the main determinant of physical dough properties. Studies of high polymers have shown that properties such as tensile strength are related to a fraction of polymer with molecular weight above a critical value and the MWD of this fraction. Elongation to break is treated as a kinetic process with energies of activation for breaking noncovalent bonds and for chain slippage through entanglements. These considerations are related to tensile properties of wheat flour doughs such as those measured by the extensigraph. The MWD of wheat proteins is determined by the relative amounts of monomeric and polymeric proteins and the MWD of the polymeric proteins. The latter, in turn, depends on the ratio of high molecular weight glutenin subunits (HMW-GS) to low molecular weight glutenin subunits (LMW-GS), the specific HMW-GS that result from allelic variation, and the presence of modified gliadins that act as chain terminators....

Evaluation of contributions from wheat protein fractions to dough mixing and breadmaking

The proteins from glutens of six wheat varieties, three of high and three of low baking performance, were fractionated into either nine or ten fractions by successive extraction with dilute HCl. Fractions were added to base flours so as to increase protein level by one percentage point; the fortified flours were assessed in terms of their Mixograph peak development times and loaf volumes in an optimised breadbaking test. Early-extracted fractions (gliadins) decreased mixing requirements and slightly depressed loaf volumes. Intermediate fractions (glutenins) induced large increases in both dough development times and loaf volumes, but this trend was reversed by the latest-extracted fractions, including the final residue. Amino acid analyses of fractions reflected the changing pattern in functional properties. Most notably, glutamic acid and proline contents were substantially lower in the latest fractions. Using 0·75 M NaCl, larger amounts of protein (believed to be the main protein present in the latest fractions) were extractable from glutens of the poorer flours than those of the better-performing flours. The relative proportion of these globulin-type proteins to the glutenins appears to be important in determining baking quality.

Studies of the methodology for fractionation and reconstitution of wheat flours

A systematic study of the different steps in the separation, fractionation and reconstitution of wheat flours was undertaken to determine where detrimental changes to functional properties might arise. The effects of different variables are summarised. Some of the more important precautions that need to be observed are the extraction of lipid with an appropriate solvent as a first step, the washing out of gluten at an optimum temperature (15°C), minimisation of contact time for gluten protein fractions with acid solution, neutralisation of extracting solutions to the correct pH and grinding of freeze-dried fractions to a suitable particle size. A scheme is outlined for separating, fractionating and reconstituting flour components with complete recovery of original flour functional properties as determined by dough mixing and baked loaf parameters. Numerical results are given for two flours as a guide for workers wishing to adopt the procedure. Separation of the gluten protein of each flour into two approximately equal fractions, followed by interchange of fractions between the reconstituted flours, confirmed that baking quality differences are located mainly in the more insoluble (sediment) fraction.

Kinetics of adsorption of proteins at interfaces. Part I. The role of bulk diffusion in adsorption

Abstract Owing to their low diffusion constants and the (in general) irreversibility of their adsorption, proteins can be used with advantage in elucidating the general mechanism of adsorption at interfaces. In the present study of adsorption at the air/water interface this has been followed firstly, in the absence of an appreciable film pressure, that is in the very initial stages, and secondly, in the later stages when an appreciable film pressure exists, due either to the protein itself or to a spread monolayer. In the former case bulk diffusion accounts very reasonably for the observed rates; in the latter the observed rates are much slower than calculated, suggesting the building up of a barrier associated with the film pressure.

Biochemical basis of flour properties in bread wheats. II. Changes in polymeric protein formation and dough/gluten properties associated with the loss of low Mr or high Mr glutenin subunits

Abstract Polymeric protein plays a critical role in governing the functional properties of wheat flour. Wheat genetic lines lacking high M r and, similarly, low M r glutenin subunits from one, two or all three Glu-1 or Glu-3 loci, respectively, were thus used to investigate the effects of these polypeptides on glutenin polymer formation and dough/gluten properties. Polymer formation (quantity, size distribution) was studied by size-exclusion high-performance liquid chromatography (SE-HPLC) using extractable, unextractable and total protein from flour, as well as by diagonal electrophoresis using total protein extracts. The loss of Glu-1 or Glu-3 subunits had significant effects on the quantity of total, extractable and unextractable polymeric protein and on the dough and gluten properties of these lines. Dough and gluten properties were significantly correlated with the proportions of both total and unextractable polymers (a measure of the relative molecular size distribution of polymeric protein), although more strongly with the proportions of unextractable polymers in the case of Glu-1 null lines. The proportion of total polymeric protein decreased more markedly when all the Glu-3 subunits were deleted than when all the Glu-1 subunits were absent, which was in accordance with the relative quantities of these two types of the subunits in the grains. In contrast, loss of all the Glu-1 subunits, on an equal weight basis, reduced the amounts of the larger polymers to a much greater extent than the loss of all the Glu-3 subunits, reflecting more than the molecular size differences in these subunits. SE-HPLC and diagonal electrophoresis of total protein extracts from the triple Glu-1 and Glu-3 null lines also revealed that Glu-1 or Glu-3 subunits form large polymers on their own. When both high and low M r glutenin subunits were present together, however, the amount of large polymer was much greater than the sum of the amounts when only one group was present, suggesting a positive interaction between these two groups of subunits with respect to polymer formation.