Gluten amino acid composition

Seven diets were constructed from purified natural ingredients obtained from either C3 (beet sugar, rice starch, cottonseed oil, wood cellulose, Australian Cohuna brand casein, soy protein or wheat gluten for protein) or C4 foodwebs (cane sugar, corn starch, com oil, processed corn bran for fiber, Kenya casein for protein) supplemented with appropriate amounts of vitamins and minerals (Ambrose and Norr 1993 Table 3a). The amino acid compositions of wheat gluten and soy protein differ significantly from that of casein (Ambrose and Norr 1993). 

Friedman (21) studied the effect of pH on the amino acid composition of wheat gluten. At pH 10.6 and above (65 C, 3 hours) no cystine was present. LAL increased with pH above 10.6. Lysine decreased over the same range of pH s, while serine and threonine contents dropped sharply at pH 13.9. Friedman concluded that cystine is most sensitive to alkali and that LAL will form most readily if lysine residues are in proximity to the dehydroalanine formed from cystine. Thus, he explained that different steric considerations may explain the different susceptibilities of wheat gluten, casein, and lactalbumin to LAL formation. 

The amino acid composition of alkali-treated casein, lactal-bumin, and wheat gluten are given in Tables I-III. The results show that the following amino acids are destroyed to various extents under basic conditions threonine, serine, cystine, lysine, and arginine, and possibly also tyrosine and histidine. The losses of these amino acids is accompanied by the appearance of lysinoalanine and other ninhydrin-positive compounds. 

For example, the respective values at pH 10.6 are 0.262, 0.494, and 1.04 mole per cent (ratio of about 1 2 4) at pH 11.2 the values are 0.420, 0.780, and 1.32 mole per cent and at pH 12.5 (pH of 1% protein solution in 0.IN NaOH), the respective values are 0.762, 0.780, and 2.62 mole per cent. (Note that the value of casein approaches that of gluten at this pH). The observed differences in lysinoalanine content of the three proteins at different pH values are not surprising since the amino acid composition, sequence, protein conformation, molecular weights of protein chains, initial formation of intra- versus intermolecular crosslinks may all influence the chemical reactivity of a particular protein with alkali. Therefore, it is not surprising to find differences in lysinoalanine content in different proteins treated under similar conditions. These observations could have practical benefits since, for example, the lower lysinoalanine content of casein compared to lactalbumin treated under the same conditions suggests that casein is preferable to lactalbumin in foods requiring alkali-treatment. 

Effect of pH on amino acid composition of wheat gluten. Conditions 1% wheat gluten 65°C 3 hours. 

Also they differ in amino acid composition from the gluten proteins, possessing lower amounts of glutamic acid and more lysine. Unfortunately, because they are present in the wheat endosperm in minor proportions, their presence it is not enough to overcome the lack of lysine in wheat. Analysis of aneuploid stocks of the wheat variety Chinese Spring has revealed that the HMW albumins of 69,63,60, and 45 kDa are controlled by genes on the chromosome arms 4DL, 4AL, SAL and SDL respectively. 

At the present time there is no apparent explanation for the inhibition of the later stage of protein synthesis by wheat gluten, but this analysis suggests that the differences in response to these three dietary proteins involves more differences in amino acid composition and identifies the portion of the pathway which should be the subject of further research. 

Table 15.15. Amino acid composition of protein groups of wheat gluten (cultivar Rektor) ...

The quantitatively predominant low-molecular protein group in gluten (Table 15.14) has the best balanced amino acid composition. Most of the values lie between those of the high and intermediate molecular groups. Only the content of Cys, Val, Met, and Leu is higher (Table 15.15). 

Alkali-extracted proteins from simflower oil cake (89% proteins, Nx6.25) and wheat gluten (76.5% proteins, Nx5.7) were reacted with n-octanol in the presence of an acid catalyst. Temperature, reaction time and catalyst concentration were varied according to an experimental design to maximize the esterification yield. The latter was determined by alkaline hydrolysis and subsequent analysis by gas chromatography. The hydrolysis of the peptide chain was traced by the determination of the amount of free amino groups in esterified proteins using 2,4,6-trinitrobenzene-sulfonic acid (TNBS) assays. The solubility curves of modified proteins in water as a function of pH were obtained by Kjeldahl analysis to determine the composition of the soluble and insoluble parts. 

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