Amino acids protein quality improvement

Limiting essential amino acids covalently attached to proteins by using activated amino acid derivatives can improve the nutritional quality and change the functional properties of proteins. The best chemical methods for incorporating amino acids into water-soluble proteins involve using car-bodiimides, N-hydroxysuccinimide esters of acylated amino acids, or N-carboxy-a-amino acid anhydrides. The last two methods can give up to 75% incorporation of the amount of amino acid derivative used. With the anhydride method, as many as 50 residues of methionine have been linked to the 12 lysine residues of casein. The newly formed peptide and isopeptide bonds are hydrolyzed readily by intestinal aminopeptidase, making the added amino acids and the lysine from the protein available nutritionally. 

On a qualitative and quantitative basis, baobab leaf appears to be a good source of protein for those populations for whom this plant material is a staple (2). The amino acid profile has been previously reviewed (2, 6, 7, 58). Because Nordeide et al. (6) showed results in units (mg of amino acid/g N) that differ from those found by other researchers (2, 7) (mg of amino acid/g dry weight), these data are not compared to each other in this review. Nordeide et al. (6) concluded that baobab leaves are potentially protein sources to be used to complement the amino acid profile to improve the overall protein quality of the local diet. Baobab leaf contains 10.6% protein on a dry weight basis and significant amounts of all the common amino acids (Table Xll) (2). 

B. Protein Quality Improvement by Addition of the Umiting Amino Acid... 

B. PROTEIN QUALITY IMPROVEMENT BY ADDITION OF THE LIMITING AMINO ACID... 

Once an electron density map has become available, atoms may be fitted into the map by means of computer graphics to give an initial structural model of the protein. The quality of the electron density map and structural model may be improved through iterative structural refinement but will ultimately be limited by the resolution of the diffraction data. At low resolution, electron density maps have very few detailed features (Fig. 6), and tracing the protein chain can be rather difficult without some knowledge of the protein structure. At better than 3.0 A resolution, amino acid side chains can be recognized with the help of protein sequence information, while at better than 2.5 A resolution solvent molecules can be observed and added to the structural model with some confidence. As the resolution improves to better than 2.0 A resolution, fitting of individual atoms may be possible, and most of the... 

Cottonseed, peanut, sesame seed, and soybean flours, when used as supplements, add to the quality of the protein. The improvement of protein quality was due to the flours compensating for limiting amino acids. Supplementation using various flours improved total protein, amino acid content and some physical characteristics in akamu, yeast bread and sugar cookies. 

The literature on concentrated sweet potato protein is sparse. Amino acid patterns for sweet potato protein isolates have been reported by three groups (16, 45, 46). One report showed that when compared to the FAO standard (47), no amino acids were limiting. The other reports showed total sulfur amino acids and lysine to be limiting (Table III). The patterns indicate a nutritionally well balanced protein. The improvement in nutritional quality, when compared to amino acid patterns from whole sweet potato, is due to the fact that whole sweet potatoes contain substantial amounts of NPN, which consists mainly of nonessential amino acids. This effectively dilutes the EAA and lowers the amino acid score. 

The plastein reaction usually involves two steps hydrolysis of protein and resynthesis of peptide links. Yamashita et al. (20) found similar BV, digestibility, and weight gain for denatured soy meal and soy plastein. This same group (49,50) had described a one-step process by which amino acids may 5 enzymatically incorporated in intact protein to improve protein quality. With soy protein they applied a racemic mixture of D,L-methionine ethyl ester and were able to enzymatically incorporate L-methionine. As Schwimmer (51) has pointed out, one expects the methionine so incorporated to be highly available due to its location at the end of the polypeptide chains. 

All of the above-mentioned factors affecting the biological quality of proteins should be taken into consideration when dealing with the nutritional improvement of food and feed proteins. Furthermore, the covalent attachment of essential amino acids to proteins by chemical methods must avoid damage to the biological quality of proteins if it is to have potential applications. 

Amino acids are monomeric units of polypeptides and proteins. They are widely used in the food and chemical industries as flavor enhancers, seasonings and sweeteners e.g. for the improvement of bread quality, also in the production of drugs, cosmetics, synthetic leather and surfactants, in medicine for infusions and as therapeutic agents. Amino acids are produced by chemical synthesis or extraction from protein hydrolyzate. They may be also produced by microbiological methods. 

Such chemical changes may lead to compounds that are not hydrolyzable by intestinal enzymes or to modifications of the peptide side chains that render certain amino acids unavailable. Mild heat treatments in the presence of water can significantly improve the protein s nutritional value in some cases. Sulfur-containing amino acids may become more available and certain antinutritional factors such as the trypsin inhibitors of soybeans may be deactivated. Excessive heat in the absence of water can be detrimental to protein quality for example, in fish proteins, tryptophan, arginine, methionine, and lysine may be damaged. A number of chemical reactions may take place during heat treatment including decomposition, dehydration of serine and threonine, loss of sulfur from cysteine, oxidation of cysteine and methio-... 

The seed contains a relatively high amonnt of essential amino acids (55) (Table VII). In contrast to other plant seed protein profiles, baobab seed protein contains a high amonnt of lysine. Becanse lysine is limited in most cereal plants, it may be possible to nse baobab seed protein to improve cereal protein quality, especially in weaning food mixtmes (55). The high protein solubility at acidic and alkaline pH snggests that the baobab seed protein could be an adequate food ingredient (55). 

The amino acid profile of tamarind leaves showed that the leaves of T. indica were potentially acceptable protein sources that would complement the amino acid profile and thus improve the protein quality of local diets (5). These same authors (5) also report that tamarind leaves only contained traces of a and B-carotene. 

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