Undenatured proteins

Ideally microbial cells should be consumable directly as food or food ingredients. However, because of their nucleic acid content the presence of undesirable physiologically active components the deleterious effects of cell wall material on protein bioavailability and the lack of requisite and discrete functional properties, rupture of cells and extraction of the protein is a necessary step. Importantly, for many food uses (particularly as a functional protein ingredient) an undenatured protein is required. For these reasons and for many potential applications of yeast protein(s) it is very desirable to separate cell wall material and RNA from the protein(s) for food applications. Much research is needed to develop a practical method for isolation of intact, undenatured yeast proteins from the yeast cell wall material to ensure the requisite nutritional and functional properties. 

In contrast to the caseins, whey proteins retain their solubility in the pH 4.5-5.0 range, provided they have not been denatured. It is therefore relatively difficult to recover and purify undenatured protein concentrates on a commercial scale. Processes that separate the whey proteins from the low molecular weight, nonprotein components of whey have been used with only moderate success to date (18). Such processes utilize ultrafil-tration/reverse osmosis membrane technology, gel filtration by the basket centrifuge technique, polyvalent ion precipitating agents... 

After a 75-hour HC1 hydrolysis of the native, iron-containing, G. gouldii protein only 5% of the threonine was destroyed while the same treatment on the iron-free species resulted in 37% degradation of this residue (169). A related finding is that nitrated, heat-denatured apo-hemerythrin is hydrolyzed by pepsin faster than acid denatured nitro-hemerythrin (201). Furthermore, carboxypeptidase A, which reacted immediately on the iron-free protein, did not release any amino acid from native methemerythrin even after incubation for 8 hours, suggesting the conformation of the undenatured protein prevents access of the peptidase to the carboxyl terminus residue of the polypeptide chain (169). Of similar implications, but related to the amino terminus, is the earlier observation that pork kidney leucine amino peptidase does not release any amino acid from hemerythrin (206). 

The dissociation of a quaternary structure or denaturation of proteins is required prior to emulsification. Therefore, casein micelles are adsorbed at an interface in a semi-intact form (Oortwijn et al., 1977). The thermal denaturation of globular proteins prior to emulsification was reported to improve the emulsifying properties. The high level of the thermally denatured whey protein fraction in mixed proteins (of denatured and undenatured proteins) increased the emulsion viscosity and coalescence stability compared with the low-level denatured fraction (Britten et al., 1994). 

In the raw state, mature soybeans and many other plant foodstuffs contain protease Inhibitors that diminish the proteolytic activities of trypsin and chymotrypsln in the Intestinal tract, cause pancreatic hypertrophy and suppress growth. Trypsin Inhibitors (TI) account for about 40% of the pancreatic hypertrophic effect and growth-inhibitory capacity of raw soy proteins. The resistance of the raw undenatured protein to tryptic digestion accounts for the remaining 60%. The practical significance of residual TI activity In heat-processed soy protein products and the biochemical effects of other protease Inhibitors have been reviewed (40). 

While proteins frequently denature under conditions of reversed-phase chromatography, due to the need for organic solvents for elution and/or the strength of the interaction with the stationary phase, the mild elution conditions and mild hydrophobic interaction of HIC packings allow the elution and recovery of undenatured proteins. 

Chemical Description Undenatured protein of pulverized silk Trade Name Crosilk Powder... 

Wu (218) successfully employed low temperature-ethanol precipitation in the preparation of undenatured proteins and Ferry, Cohn and Newman (71, 72) employed ethanol-water, at —5° C., at low ionic strengths, as a medium for studying the solubility of egg albumin and horse hemoglobin in solvents of low dielectric constant. They showed that the solvent action of neutral salts under these conditions was much greater than in water and that both these proteins could be successfully recrystallized, after exposure to 15-25 per cent ethanol at —5 for long periods, provided the ethanol was removed before the temperature was raised. 

We begin with electrophoretic studies on undenatured proteins. Stastna, et al. report the mobility of bovine serum albumin and glucose-6-phosphate dehydrogenase through three polymers(34), as shown in Figure 3.13. The values of... 

The variation in the performance of different types of egg albumen in different systems is almost certainly caused by variations in the degree of denaturation of the protein. Those products that work best with fresh egg white clearly need an undenatured product. 

Polar groups contribute greatly to adhesion of proteins to carbohydrates and to their cohesion. In denatured or unfolded proteins, such as animal glues, the peptide amide groups play an important role in adhesion but in the undenatured collagen, most peptide groups are associated in helical conformations. 

Casein can be precipitated from solution by any of several salts. Addition of (NH4)2S04 to milk to a concentration of 260 g 1 1 causes complete precipitation of the casein together with some whey proteins (immunoglobulins, Ig). MgS04 may also be used. Saturation of milk with NaCl at 37°C precipitates the casein and Igs while the major whey proteins remain soluble, provided they are undenatured. This characteristic is the basis of a commercial test used for the heat classification of milk powders which contain variable levels of denatured whey proteins. 

Serum Albumin. With a denaturation temperature of 64°C (Table 11.2), bovine serum albumin is denatured almost as easily as a-lactalbumin. Since its denaturation is not as reversible as that of a-lactalbumin, it appears to be the most easily denatured serum protein (de Wit and Klarenbeek 1984). It precipitates between 40° and 50 °C as a result of hydrophobicity-directed unfolding (Lin and Koenig 1976 Macritchie 1973). Some serum albumin remains undenatured even after prolonged heating at 65°C. This may be because already denatured albumin is able to protect native proteins from denaturation (Ter-ada et al. 1980). 

Many variants of the two-hybrid system have been devised.d For example, a green fluorescent protein reporter can be used/ Because significant biological protein-protein interactions often require that three or more proteins interact,d hybrid systems involving more than two proteins have been developed. Two-hybrid systems for bacteria have also been devised.1" A virtue of the two-hybrid methods is that they work with undenatured, if not totally natural, proteins. This is in contrast to widely used methods that involve separation of denatured proteins on gels or columns. 

Tt is well recognized that the nutritional value of dietary proteins de-pends primarily on the content of their constituent amino acids, especially of their essential amino acids. Because of deficiencies of lysine and methionine, and to a lesser extent of a few other amino acids, proteins from plants and other alternative sources have low biological quality. Moreover, incomplete digestion of the protein also may result in a lack of complete availability (I) of the essential amino acids and may further reduce its value. In many raw plant foodstuffs such as soybeans, common beans, or unprocessed protein foods, undenatured... 

Untoasted and flash-desolventized meal in which the protein is undenatured and highly soluble (>70 PDl and preferably >90 PDl) is the preferred starting material in manufacturing soy protein isolates. Under some conditions, extruded-expelled meal can be used, but the yield of soy isolate is reduced. The meal is ground in water adjusted to pH 8.0 with sodium hydroxide and centrifuged to remove insoluble fiber. The soluble fraction is acidified to pH 4.5, and the protein precipitates. The precipitated protein curd is separated from the soluble sugars by centrifuging. The protein curd may be washed, neutralized, and spray-dried. 

The heat classification, based on the amount of undenatured whey protein in the milk powder, is still being used as a general guide to the physical functionality of milk powders and the selection of powders for... 

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