Proteins with the same molecular weight have the same amino acid composition

Calculations and Examples

The EAA composition and protein digestibility of the food or mixed diet being tested are determined. Then the percentage or fractional value of the most limiting EAA (noncorrected amino acid score) is multiplied by the percentage or fractional value of ‘true’ protein digestibility to obtain the corrected score, which is equivalent to protein quality. This value can be used as such or it can be expressed in relation to the corrected amino acid score of a reference protein or food, usually casein or an animal food (milk, egg, or beef).

Proteins that have no limiting amino acids are assigned an amino acid score of 100% (or 1.00) that must be only corrected for digestibility. Similarly, if the clinical or experimental assessment of ‘true’ protein digestibility gives a value greater that 100% (generally due to experimental variability), a digestibility correction factor of 100% (or 1.00) is applied to the amino acid score. Table 6 shows examples of calculations for a single food as protein source. The same procedure can be used for food mixtures using a weighted average procedure based on the protein content, amino acid composition, and digestibility of the individual components. Table 7 shows an example of those calculations. For simplicity, the example uses only the four EAAs that are most often limiting.

Table 6. Calculation of amino acids scores of single protein sources corrected for digestibility and in relation to the protein quality to cow's milk

FoodMost limiting amino acidNoncorrected amino acid scoreTrue protein digestibilityCorrected amino acid scoreProtein quality relative to milkCow's milkNone>100→100%×95%=95%–Polished riceLysine 36 mg per g protein(36/58)×100=62%×88%=55%(55/95)×100=58%Egg whiteNone>100→100%×97%=97%(97/95)×100=102%

Table 7. Calculation of protein quality of a mixed diet based on whole wheat, polished rice, and chicken breast

Raw ingredientsData from analysis of literatureQuantities calculated for the mixed dietWeight (g)Protein (g per 100 g)LysSAAThTrpTrue digestibility (%)Total protein (g) H=A×B/100Lys (mg) I=H×CSAA (mg) J=H×DThr (mg) K=H×ETrp (mg) L=H×F(mg per g protein)ABCDEFGWhole wheat300112837291186339241221957363Polished rice2007363833138814504532462182Chicken breast15019833840129528.5236610831140342Total75.5379428362559887M Weighed mean digestibility of the mixed diet (sum of (G×H) for each food component divided by total protein, H)0.90N mg amino acid per g protein (total for I, J, K, or L divided by total H)50383412P Amino acid scoring pattern, mg amino acid per g protein58253411Q Score for each amino acid in the mixed diet (N/P)0.861.521.001.09R Amino acid score adjusted for digestibility (Q of the limiting amino acid multiplied by M)0.86×0.90=0.77 (or 77%)

Lys, lysine; SAA, sulfur-containing amino acids; Thr, threonine; Trp, tryptophan.

Protein Quality

The amino acid composition of protein is of great importance if only small amounts of protein are consumed (or absorbed). The bottom line is that protein synthesis depends on adequate amounts of each constituent amino acid. If even a single amino acid is acids cannot be produced in the body, by definition, and each has to be supplied in the needed quantity with food. If one is in short supply, malnutrition will result over the long term. The limiting amino acid in protein from a particular food is indicated in the US Department of Agriculture (USDA) nutrient database. Various protein quality measures aim to capture the ability of food protein to supply all necessary amino acids. The classical assessment of the biological value of a protein uses animals to determine net nitrogen utilization.

What counts in the end, however, is the combination of food proteins consumed over the short term, not single food items. This means that foods that are low in one essential amino acid may complement other foods that have enough of this amino acid but lack another.

Protein

Amino acids, peptides, and proteins are important constituents of food. They supply the required building blocks for protein biosynthesis. In addition, they directly contribute to the flavor of food and are precursors for aroma compounds and colors formed during thermal or enzymatic reactions in production, processing, and storage of food. Other food constituents, e.g., carbohydrates, take part in such reactions. Proteins also contribute significantly to the physical properties of food through their ability to stabilize gels, foams, doughs, emulsions, and fibrillar structures.

Table 1 shows the most important protein sources and their contribution to world-wide production of protein. Cereals contribute to protein production by more than half, followed by oil seeds and meat. Besides plants and animals, algae (Chlorella, Scenedesmus, and Spirulina spp.), yeasts and bacteria may be used for protein production (single-cell protein (SCP)). Common carbon sources are glucose, molasses, starch, waste water, higher n-alkanes, and methanol. Yeasts of the genus Candida can be grown, for example, on paraffins, and they deliver about 0.75 t of protein per tonne of alkane. Bacteria of the genus Pseudomonas deliver about 0.3 t of protein per tonne of alcohol in aqueous methanol. Because of the high nucleic acid content of yeasts and bacteria (6–17% of dry mass), isolation of the protein from biomass is necessary. The importance of SCP in the future will depend on the total protein market, especially on prices and functional properties of individual proteins.

Table 1. World-wide annual protein production (× 106 tonnes)

Source199319941995199619971998Cereals177183177193196191Oil seeds53.160.760.262.165.668.0Legumes11.812.111.811.611.912.2Root crops8.58.18.48.98.48.4Vegetables5.15.35.65.96.06.1Meat37.939.140.240.642.042.9Milk18.018.218.418.518.718.7Fish and aquatic animals11.312.212.613.013.3Eggs4.85.15.35.65.96.0Total328344340359368353a

Data from FAO (1999) FAO Year book Production, vol. 52, 1998. FAO Statistics Series No. 148. Rome: Food and Agriculture Organization and by courtesy of Dr. Rehbein (Federal Research Centre for Fisheries, Hamburg, Germany, for fish and aquatic animals), protein calculated from production with data from Scherz H and Senser F (eds) (2000) Food Composition and Nutrition Tables, 6th edn. Boca Raton, FL: CRC Press.

Table 2 provides data on the average protein content and the nutrient density – the ratio between the amount of protein (g) and the total energy value (MJ) of the digestible constituents – of selected foodstuffs. The protein content varies as follows: > 20% (cheeses, meat, legumes, oil seeds); 10–20% (fish, eggs); 5–10% (cereals); and < 5% (milk, roots, tubers, vegetables, fruits, mushrooms).

Table 2. Protein content (%) of selected foodstuffs

No.aFoodstuffAverage protein contentNutrient density (g MJ−1)Raw productEdible portionMilk and dairy products 1Human milk1.131.133.92 2Cow's milk3.333.3312.05 3Buffalo milk4.014.018.94 4Ewe's milk5.275.2713.17 5Goat's milk3.693.6913.14 6Condensed milk (7.5% fat)6.496.4911.78 7Dried milk whole25.2025.2012.50 8Dried skimmed milk35.0035.0023.05 9Yoghurt (1.5–1.8% fat)3.553.5516.88 10Blue cheese (50% fat dmb)21.1021.1014.35 11Camembert cheese (40 % fat dm)22.5022.5019.72 12Cheddar cheese (50% fat dm)25.4025.4015.43 13Edam cheese (40% fat dm)24.8026.1019.93 14Emmental cheese (45% fat dm)26.9828.7018.02 15Feta cheese (45% fat dm)17.0017.0017.24Eggs 16Chicken egg, whole11.3512.9019.98 17Chicken egg, yolk16.1016.1011.03 18Chicken egg, white11.1011.1053.37Meat 19Beef (muscles only)22.0022.0048.41 20Veal (muscles only)21.3021.3054.33 21Pork (muscles only)22.0022.0049.68 22Mutton (muscles only)20.4020.4043.13 23Lamb (muscles only)20.8020.8042.41 24Corned beef, American25.3025.3028.94 25Luncheon meat14.7014.7012.36 26Meat extract56.6056.6054.13 27Sausage ‘Cervelat’19.8920.3012.43 28Chicken, for roasting14.7319.9028.70 29Duck14.4818.1019.17 30Goose9.8915.7011.10 31Turkey14.7520.2022.48Fish 32Carp9.3618.0037.22 33Cod13.2817.7054.55 34Flounder7.4216.5053.85 35Halibut16.0820.1049.67 36Herring12.7418.2018.80 37Mackerel12.1618.7024.66 38Mullet10.6120.4040.32 39Redfish8.7418.2041.09 40Salmon12.7419.9023.65 41Sardine11.4519.4038.98 42Trout10.1419.5045.09 43Tuna13.1221.5022.90Cereals 44Barley (whole grain)9.849.847.35 45Maize (whole grain)8.548.546.17 46Corn flakes7.157.154.77 47Oats (whole grain)11.6911.697.91 48Rolled oats12.5312.538.09 49Rice, polished6.836.834.68 50Rye (whole grain)8.828.827.08 51Rye flour, type 9976.866.865.22 52Rye bread6.226.226.76 53Sorghum10.3010.306.97 54Wheat (whole grain)11.7311.738.95 55Wheat flour, type 4059.849.846.98 56Wheat flour, type 170011.2311.238.76 57Wheat bread7.617.617.54Roots and tubers 58Beetroot1.191.538.73 59Cassava0.741.001.75 60Celeriac1.131.5520.18 61Potato1.632.046.84 62Potato flakes8.608.606.26 63Sweet potato1.321.633.55 64Taro1.682.004.40 65Topinambur1.682.4418.72 66Yam1.682.004.77Leaves, stems, and flowers 67Artichoke1.152.4025.68 68Broccoli2.013.3029.88 69Brussel sprouts3.474.4529.41 70Cauliflower1.532.4625.94 71Chinese leaves0.941.1922.85 72Red cabbage1.171.5016.24 73Soya bean sprouts4.595.5326.15Vegetable fruits 74Aubergine1.031.2417.12 75Cucumber0.440.6011.59 76Green peppers0.901.1713.65 77String beans2.252.3917.24 78Tomato0.910.9512.98Legumes and oil seeds 79Bean, white dry21.0921.3019.32 80Chick pea, dry19.8019.8017.02 81Lentil, dry23.5023.5017.61 82Pea, dry22.6722.9019.89 83Peanut25.2525.2510.75 84Soya bean, dry28.0033.7324.98Fruits 85Apple0.310.341.49 86Apricot0.820.904.90 87Apricot, dried5.005.004.91 88Banana0.771.153.07 89Cherry (morello)0.800.904.00 90Date, dried1.611.851.58 91Fig1.301.305.00 92Fig, dried3.503.543.34 93Grape0.650.682.38 94Grape, dried (raisin)2.462.462.09 95Orange0.721.005.58 96Peach0.700.764.33 97Pear0.440.472.02 98Plum0.560.602.92 99Strawberry0.800.826.03Mushrooms 100Champignon2.692.7441.10 101Chanterelle0.961.5732.89 102Edible boletus2.883.6042.32 103Edible boletus, dried19.7019.7037.66 104Morel1.381.6641.44 105Oyster mushroom2.352.3552.07 106Truffles5.535.5348.99

A considerable number of proteins have been isolated from various foods and characterized by physical and chemical properties. Cereals and cereal products are amongst the most important staple foods of mankind. Proteins provided by bread consumption in industrial countries meet about one-third of the daily requirement. The major cereals are wheat, maize, rice, barley, sorghum, oats, millet, and rye. Wheat and rye have a special role since only they are suitable for bread-making. With the example of wheat, the cereal proteins have been separated by Osborne, on the basis of their solubility, into four fractions: the water-soluble albumins, the salt-soluble globulins, the 70% aqueous ethanol-soluble prolamins, and the remaining glutelins. In the literature, Osborne fractions derived from different cereals are often designated by special names, e.g., gliadin and glutenin for wheat prolamins and glutelins, respectively. The levels of Osborne fractions differ amongst cereals with albumins amounting to 4–44%, globulins 3–12%, prolamins 2–48%, and glutelins 24–77% of the whole protein fraction. Each of the Osborne fractions consists of a larger number of proteins. Albumins and globulins contain the enzymes, whereas prolamins and glutelins are storage proteins. The glutelins can be separated into two subfractions after reduction of the disulfide bonds: the low-molecular-weight (LMW) subunits and the high-molecular-weight (HMW) subunits. Wheat prolamins and glutelins, both fractions together also designated wheat gluten, are responsible for the characteristic rheological properties of wheat dough. Wheat prolamins consist of ω-, α-, and γ-gliadins, designated according to their electrophoretic mobility. In addition, the LMW subunit fraction of wheat glutelins also contains some gliadins (ω5-, ω1,2-, and γ-gliadins). The amino acid sequences of some gluten proteins from wheat are shown in the article ‘PROTEIN | Chemistry’ (HMW subunit of glutenin, x-type and y-type; LMW subunit of glutenin; α-gliadin; γ-gliadin).

Meat and meat products are other important staple foods, in particular in industrial countries. The main meat-producing warm-blooded animals are pig, cattle, poultry, sheep, goats, and buffalo. Meat proteins, i.e., the proteins of the muscle, are divided into three groups: proteins of the contractile apparatus (myofibrillar proteins), soluble proteins (sarcoplasma proteins), and insoluble proteins (connective tissue and membrane proteins). The myofibrillar proteins of a typical mammalian muscle amount to about 60% of total muscle protein, with myosin (29%) and actin (13%) as their predominating components and about 20 minor components including connectin, tropomyosins, troponins, and actinins. The sarcoplasma proteins form about 30% of total protein. They contain most of the enzymes, in particular those of glycolysis and the pentosephosphate cycle, but also considerable amounts of creatine kinase (2.7% of total protein), myoglobin, and some hemoglobin. The insoluble proteins contain collagen as the main component, besides elastin, insoluble enzymes, and cytochrome c. In connective tissue, collagen forms a triple-stranded helix composed of α-helices. Covalent cross-links are formed between the fibers of collagen during maturation and aging, thus improving its mechanical strength. When heated, collagen fibers shrink or are converted into gelatine, depending on the temperature. The structure of the gelatine obtained after cooling depends on the gelatine concentration and temperature gradient. Collagen contains two unusual amino acids, 4-hydroxyproline and 5-hydroxylysine. Since the occurrence of the former is confined to connective tissue, its determination provides data on the extent of connective tissue content of a meat product. The primary structure of bovine skin collagen is given in the first edition of this encyclopedia (Belitz H-D (1993) Protein-Chemistry. In: Macrae R, Robinson RK and Sadler MJ (eds) Encyclopaedia of Food Science, Food Technology, and Nutrition, pp. 3781–3791. London: Academic Press) showing the characteristic three-amino-acid repeats with glycine in the first position, often followed by proline and hydroxyproline.

Milk and dairy products form a further important group of staple foods. Milk generally means cow's milk, but milk from buffalo, goats, and sheep is of importance in some regions. Milk proteins, in particular the caseins, play an important role in processing to yield dairy products such as cheese and sour milk products. The caseins, first isolated by Hammarsten in 1877, make up about 80% of total milk proteins. They have been separated later into different fractions: αs1-, αs2-, κ-, β-, and γ-caseins, constituting 34, 8, 9, 25, and 4% of total protein, respectively. Each of these fractions occurs in the form of different genetic variants, designated A, B, C, etc., depending on the breed from which they have been isolated. In cheese-making, the specific cleavage of κ-casein by chymosin (EC 3.4.23.4) into para-κ-casein and a glycopeptide (so-called, though not always containing a sugar moiety) reduces the solubility of the casein complexes and casein micelles, thus leading to their aggregation followed by gel formation (curd formation). The whey proteins (about 20% of total protein) consist of β-lactoglobulins, α-lactalbumins (both in different genetic variants), serum albumin, immunoglobulins, and proteose-peptone. Also, more than 40 enzymes occur in the whey protein fraction, but in much lower quantities than the other components. Whey proteins can be incorporated into the curd using several new processing methods of cheese-making in order to increase the yield and also to reduce waste water or whey treatment costs. The primary structures of some proteins from bovine milk are shown in the first edition of this encyclopedia (Belitz H-D (1993) see above) (αs1-, αs2-, β-, and κ-casein; α-lactalbumin; β-lactoglobulin).

Legumes (pulses) are very important staple foods in some parts of the world, e.g., soya beans in South-east Asia and common beans in Latin America. Other legumes, some of greater regional importance, include peas, peanuts, chick peas, broad beans, and lentils. Legume proteins, when fractionated, according to Osborne, in a similar way to cereal proteins, yield three fractions: albumins, globulins, and glutelins. The portion of the fractions varies, depending on the legume species, but globulins always predominate. The globulins are subdivided, initially according to sedimentation during ultracentrifugation, into 11S and 7S globulins (legumins and vicilins, respectively). Again, the subfractions derived from different legumes are sometimes designated by special names, e.g., glycinin and arachin for soya bean and peanut legumin, as well as β-conglycinin and phaseolin for soya bean and common bean vicilin, respectively. Soya protein isolates, produced by diluted alkali extraction of defatted soya bean flakes followed by acid precipitation, are texturized and flavored for use as meat substitutes or are added to foods to raise their protein level and improve their processing qualities such as the water-binding capacity or emulsion stability. The isolates contain about 95% protein and consist of 11S and 7S globulins. The similarity between the caseins from bovine milk and the globulins from soya beans is reflected by the production of some typical Asian foods such as soy milk, soy curd (tofu), and soy cheese (sufu). The primary structures of some legume proteins are shown in the article ‘PROTEIN | Chemistry’ (glycinin, pea legumin; β-conglycinin, pea vicilin, phaseolin.)

Eggs are used as a food not only because of their excellent nutritional quality but also because of their functional properties. Eggs generally means chicken eggs; those of other birds (geese, ducks, plovers, seagulls, quail) are less important. Egg proteins are divided into those of egg white and those of egg yolk. Egg white proteins (about 10% of total egg white) are ovalbumin, conalbumin (ovotransferrin), ovomucoid, ovomucin, lysozyme, ovoglobulin G2, ovoglobulin G3, and some minor components (54, 12, 11, 3.5, 3.4, 4, 4, and 2.5% of total egg white protein, respectively). Ovalbumin, conalbumin, ovomucin, and the ovoglobulins G contribute to foam formation and foam stability. Yolk proteins (about 17% of total yolk) are phosvitin, the livetins, and the protein moieties of high-density lipoproteins (HDL) and low-density (LDL) lipoproteins (13, 31, 36, and 20% of total yolk protein, respectively). Apart from phospholipids, LDL and proteins are responsible for the emulsifying effect of whole eggs or egg yolk alone. Owing to the ability of all egg proteins, except ovomucoid and phosvitin, to coagulate when heated, egg products are important food-binding agents.

The amino acid compositions of selected foodstuffs are shown in Table 3. The nutritional quality of a food protein depends on the absolute content of essential amino acids, the relative proportions of essential amino acids, and their ratios to nonessential amino acids. In addition, the digestibility of the food protein, the influence by other food components such as dietary fibers, polyphenols, or proteinase inhibitors, and also the total food energy intake are of importance. (See ‘PROTEIN | Requirements’; PROTEIN | Quality.) The daily requirements of humans at different ages for essential amino acids are compiled in Table 4. During pregnancy and lactation, the first 6 months, and after 6 months, the daily requirement increases by 13, 24, and 18%, respectively. The biological value of a protein is generally limited by the following amino acids:

Table 3. Amino acid compositions of selected foodstuffs

Amino acidsFoodstuffsa12345678910111213Alanine0.0560.1300.1300.2200.1400.2700.9101.250.1600.7300.8900.7600.890Arginine0.0510.1300.1100.1800.1300.2300.9201.280.1301.650.8200.9001.02Aspartic acid0.1200.2900.3100.4600.3000.5501.992.740.2800.8301.671.812.00Cystine0.0240.0280.0480.0600.0830.0660.2300.3100.0270.1200.1300.2100.250Glutamic acid0.2200.7900.4801.070.7801.505.517.570.7004.974.956.626.15Glycine0.0360.0760.1100.0740.1800.5600.7700.0860.5100.4600.4700.550Histidine0.0310.0950.0780.1300.0790.1700.6600.9200.0950.9900.7300.8000.930Isoleucine0.0770.2200.2000.2800.2300.4501.612.240.2201.191.521.811.48Leucine0.1300.3600.3700.5300.3900.7202.473.430.3802.142.192.522.65Lysine0.0860.2800.4600.3400.5201.962.720.2802.381.652.072.37Methionine0.0240.0900.0970.1400.0940.1700.6200.8600.0920.5200.5900.7700.780Phenylalanine0.0540.1800.1620.2600.1800.3401.221.700.1901.221.201.451.44Proline0.1200.3400.5500.4700.7202.553.500.4202.352.383.052.91Serine0.0590.2100.2300.3200.2100.4101.431.970.2201.331.291.581.57Threonine0.0630.1600.1800.2400.2300.3301.161.610.1600.9200.8400.9801.13Tryptophan0.0220.0490.0530.0700.0500.0880.3500.4900.0420.2100.3100.2900.400Tyrosine0.0560.1800.1830.2600.2400.3201.281.780.1801.021.081.301.33Valine0.0810.2400.2200.3200.2800.4801.732.400.2701.461.611.811.88Amino acidsFoodstuffsa14161718192021222324252829Alanine0.9200.8901.030.8301.691.641.531.441.291.44Arginine1.000.8901.280.6801.541.541.531.441.391.630.7901.391.10Aspartic acid1.581.461.761.232.342.402.432.151.892.27Cystine0.1700.3100.3100.2900.2800.2800.3100.2900.1700.3200.1200.300Glutamic acid5.761.812.201.644.133.973.914.303.053.69Glycine0.5100.5300.6200.5001.561.341.421.431.091.40Histidine1.020.3300.4400.2800.8500.8000.9900.6300.6000.8900.3600.6100.410Isoleucine1.730.9301.090.7401.251.291.271.211.021.240.5601.290.940Leucine2.991.261.631.081.951.891.921.801.691.921.031.781.40Lysine2.390.8901.300.7402.312.052.202.001.892.131.012.041.56Methionine0.7900.4500.4700.4700.6500.6000.7200.5600.5300.6900.3300.6400.450Phenylalanine1.610.8000.7900.7601.061.020.9800.9200.8200.9100.5400.9100.710Proline3.730.5900.7800.5001.281.151.211.020.9701.05Serine1.661.151.620.9201.141.151.121.040.8200.920Threonine1.140.7101.010.5801.151.131.251.090.8901.110.5501.010.790Tryptophan0.4300.2300.2900.2000.2900.3000.3100.2900.2000.2300.1200.280Tyrosine1.610.5900.7800.4600.8900.8800.9100.7700.8200.8500.4600.760Valine2.121.121.240.9801.321.311.421.181.121.270.7101.180.870Amino acidsFoodstuffsa30313233343536373839404142Alanine0.9701.451.421.241.771.521.881.551.681.671.601.55Arginine0.9801.211.271.211.161.371.181.161.361.191.331.311.40Aspartic acid1.412.312.012.072.152.312.122.902.282.222.322.36Cystine0.2700.1800.2500.1500.2400.2400.2300.3100.2200.2900.2200.170Glutamic acid2.343.193.133.183.013.233.173.473.303.233.043.33Glycine0.9901.000.9400.9301.151.131.411.381.101.631.241.47Histidine0.4400.5200.4200.5200.4500.4800.5200.8400.6100.4200.6600.4600.570Isoleucine0.7401.011.000.9900.9201.271.041.091.091.141.161.191.07Leucine1.321.471.681.691.601.941.751.801.951.781.771.871.78Lysine1.241.742.112.051.821.561.751.732.041.902.022.282.02Methionine0.3800.5300.5900.6000.5800.8000.6600.6400.6600.6400.7000.6400.660Phenylalanine0.6600.7700.8900.8400.7000.6800.7500.8400.8600.8400.9100.9100.920Proline0.7600.6400.8200.6000.8100.8400.7900.7900.7501.000.8500.850Serine0.6200.9900.9900.8601.271.050.9000.9501.041.011.100.970Threonine0.7000.8101.040.9700.9200.9901.040.9701.021.011.111.121.08Tryptophan0.2000.1600.2100.2400.2100.2600.2100.2700.3200.2000.2600.2400.240Tyrosine0.5100.2800.7400.7100.6400.6800.6700.6400.7400.5600.7200.8100.680Valine0.9700.9501.051.091.061.301.211.211.241.041.391.451.25Amino acidsFoodstuffsa43444546474849505152535455Alanine1.610.5600.7900.8000.7200.7900.5000.5200.4100.3000.8800.5100.370Arginine1.250.5600.4200.2400.8500.8700.5700.4900.3300.4200.3800.6200.430Aspartic acid2.880.6800.6200.5401.111.290.7800.6800.6100.4800.7100.7000.480Cystine0.2900.2200.1400.1600.3200.3900.1100.1900.1400.1300.1000.2900.240Glutamic acid3.522.811.781.862.903.081.582.572.051.922.294.083.66Glycine1.170.5400.4300.3400.7800.8500.4100.5000.4300.3200.4300.7200.420Histidine1.090.2100.2600.2400.2700.3000.1700.1900.1800.2400.2200.2800.220Isoleucine1.210.4600.4300.3300.5600.6100.3400.3900.3200.2600.5800.5400.460Leucine2.170.8001.221.241.021.130.6600.6700.5400.4701.360.9200.820Lysine2.210.3800.2900.1800.5500.5000.2900.4000.2800.3000.2600.3800.240Methionine0.6100.1800.1900.1700.2300.2400.1700.1400.1200.0600.2000.2200.170Phenylalanine1.050.5900.4600.4300.7000.7800.3900.4700.3600.3500.4400.6400.550Proline0.8801.261.020.9700.8700.8400.4201.250.8400.7201.551.561.45Serine1.050.5400.5200.4700.7400.7100.4100.4500.3400.3500.4200.7100.660Threonine1.180.4300.3900.3200.4900.5300.2800.3600.3100.2500.4400.4300.320Tryptophan0.3000.1500.0700.0500.1900.1900.0900.1100.0700.0500.1100.1500.120Tyrosine0.9700.3900.3800.2700.4500.5700.2600.2300.2200.1700.2500.4100.320Valine1.420.5800.5100.4400.7900.8100.4900.5300.4100.3300.5800.6200.490Amino acidsFoodstuffsa56575860616263686970717274Alanine0.4900.2400.0860.1100.350Arginine0.6000.3100.0270.0440.1200.6100.0650.1900.2800.1100.0800.1100.042Aspartic acid0.6600.3900.1640.4301.83Cystine0.2500.1800.0040.0200.1400.0250.0100.030Glutamic acid3.753.150.2830.4601.89Glycine0.6300.2900.0470.1200.340Histidine0.2500.1800.0210.0240.0400.2900.0290.0630.1100.0490.0260.0270.021Isoleucine0.5200.3800.0490.0480.1000.3300.0680.1300.2100.1100.0430.063Leucine0.8600.5900.0530.0750.1400.5900.0840.1600.2300.1700.0610.077Lysine0.3500.2000.0820.0740.1300.6900.0660.1500.2500.1400.0580.0710.034Methionine0.2100.1300.0050.0180.0300.0800.0280.0500.0400.0480.0320.0140.007Phenylalanine0.5900.4200.0260.0470.1000.4200.0790.1200.1500.0770.0470.0320.054Proline1.570.9600.0400.1100.470Serine0.7400.3900.0490.1000.460Threonine0.3900.2500.0330.0440.0900.3600.0680.1200.1600.1100.0520.0420.043Tryptophan0.1500.0800.0130.0120.0300.0700.0280.0370.0500.0340.0200.0120.011Tyrosine0.3700.2100.0250.0800.5000.0710.0350.039Valine0.6000.3900.0470.0730.1300.4900.1100.1700.2400.1500.0700.0460.073Amino acidsFoodstuffsa75767778798081828384858890Alanine0.0260.7401.290.4800.8101.530.0150.046Arginine0.0450.0230.1000.0181.491.482.243.713.462.360.0080.0540.040Aspartic acid0.1212.453.161.923.313.990.1010.115Cystine0.0240.0010.2300.2800.2500.4500.4300.5900.0010.002Glutamic acid0.3374.334.493.465.636.490.0250.105Glycine0.0180.9501.300.5901.641.420.0090.042Histidine0.0080.0140.0490.0130.7000.5300.7100.7700.7100.8300.0060.0770.040Isoleucine0.0190.0450.1100.0231.491.141.191.881.231.780.0100.0380.060Leucine0.0250.0450.1400.0302.261.462.112.342.032.840.0160.0850.062Lysine0.0260.0500.1400.0291.871.371.892.131.101.900.0150.0570.044Methionine0.0070.2600.2600.2200.3500.3100.5800.0030.0090.022Phenylalanine0.0140.0540.0730.0241.400.9601.401.391.541.970.0090.0340.051Proline0.0160.9801.220.4901.431.820.0100.040Serine0.0281.381.510.9801.831.690.0120.049Threonine0.0160.0490.0930.0231.150.7001.121.570.8501.490.0080.0380.049Tryptophan0.0040.0090.0270.0060.2300.1600.2500.3500.3200.4500.0020.0180.049Tyrosine0.0500.0120.9700.6600.8401.221.191.250.0050.021Valine0.0210.0320.1300.0231.630.9801.391.821.451.760.0120.0570.076Amino acidsFoodstuffsa9294959699100101102103105106Alanine0.1600.0910.0290.0390.0440.044Arginine0.0900.3050.0730.0170.0370.2000.0760.2600.650Aspartic acid1.140.0870.1220.0900.191Cystine0.1400.0060.0030.0090.0070.0140.1200.2902.080.150Glutamic acid0.3800.1180.0660.1390.126Glycine0.2600.0630.0230.0150.0340.035Histidine0.0900.0510.0120.0170.0160.0570.0280.2201.590.0500.100Isoleucine0.1400.0470.0200.0130.0190.1100.0400.0300.2100.1100.160Leucine0.1400.0750.0320.0280.0440.1200.0860.1200.8400.1700.400Lysine0.1400.0710.0390.0290.0340.1700.0390.1901.350.1500.490Methionine0.0400.0130.0080.0300.0010.0230.0090.0580.4200.050Phenylalanine0.1200.0470.0200.0180.0250.0740.0650.1000.7300.1000.190Proline0.1300.1570.1890.0270.0270.040Serine0.2600.0510.0430.0330.0330.049Threonine0.1200.0550.0200.0270.0260.0870.0880.1100.7500.1200.380Tryptophan0.0400.0050.0070.0050.0150.0240.0480.2101.460.0300.020Tyrosine0.2200.0100.0130.0200.0290.0660.0580.1200.8600.180Valine0.2900.0710.0330.0390.0250.0900.0520.0780.5600.1400.250

Data from Scherz H and Senser F (eds) 2000 Food Composition and Nutrition Tables, 6th edn. Boca Raton, FL: CRC Press.

Table 4. Daily requirement of essential amino acids (milligrams per kilogram of bodyweight)

Amino acidInfants (3–4 months)Children (2 years)Schoolboys (10–12 years)AdultsHistidinea28??[8–12]Isoleucine703128–3010Leucine1617344–4514Lysine1036444–6012Threonine873728–357Total SAA582722–2713Total AAA1256922–2714Tryptophan1712.53.3–43.5Valine933825–3310Total714352216–26184

SAA, sulfur amino acids (methionine+cystine).

AAA, aromatic amino acids (phenylalanine+tyrosine).

Data from WHO (1985) Energy and Protein Requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. WHO Technical Report Series 724. Geneva: World Health Organization.

lysine: deficient in proteins of cereals and other plants;

methionine: deficient in proteins of bovine milk and meat;

threonine: deficient in wheat and rye;

tryptophan: deficient in casein, corn and rice.

The biological values of some important food proteins are given in the article ‘PROTEIN | Quality’. The highest value observed so far is for a blend of 35% egg and 65% potato proteins (one chicken egg and 500 g of potatoes).

11.4.8 Amino acid composition

The amino acid composition varies in different varieties of silk. Three major amino acids such as serine, glycine, and alanine may be found in mulberry and non-mulberry varieties. Among the other major amino acids present are tyrosine and valine. In general in mulberry silks glycine, alanine, and serine together constitute about 82%, of which about 10% is serine. Tyrosine and valine may be considered next to these at about 5.5% and 2.5%, respectively. The overall composition of acidic amino groups (i.e. aspartic and glutamic acids) in the mulberry variety is greater than that of the basic amino acids. The other important aspect is the composition of amino acids with bulkier side groups. The presence of bulky side groups can hamper close packing of molecules and hinder crystallization process. In general, a large portion of the mulberry fibroin is made up of simple amino acids such as glycine and alanine, suggesting a favourable condition for crystallization (Sen and Murugesh Babu, 2004)

Compared to the mulberry silks, the total amount of glycine, alanine, and serine constitute about 73% in the non-mulberry variety, less by about 10%. All the non-mulberry silks exhibit a high proportion of alanine compared to that in the mulberry variety. The proportion of alanine is about 34% in tasar, 36% in eri, and 35% in muga. This value is consistent but is lower than that particularly for muga (∼44%). On the other hand, the glycine content in these varieties is about 27%–29%, which is lower than that found in the mulberry varieties (∼43%).

In addition, the non-mulberry varieties have a substantial proportion of amino acids with bulky side groups, especially aspartic acid (4%–6%) and arginine (4%–5%), which means that not only the acidic but also basic amino acid levels are greater. It is interesting to note the presence of sulphur-containing amino acids (i.e. cystine and methionine) in all the varieties of silk. Methionine content in non-mulberry silks is slightly higher (0.28%–0.34%) compared to that found in mulberry varieties (0.11%–0.19%), whereas the cystine content is comparable (Sen and Murugesh Babu, 2004). Amino acid composition of different varieties of silk is presented in Table 11.3.

Table 11.3. Amino acid Composition of Silk Fibres.

Amino acid composition (mol %)Amino acidBombyx mori (mulberry)Antheraea mylitta (Tasar)Antheraea assama (Muga)Phylisomia ricini (Eri)Aspartic acid1.646.124.973.89Glutamic acid1.771.271.361.31Serine10.389.876.118.89Glycine43.4527.6528.4129.35Hystidine0.130.780.720.75Arginine1.134.994.724.12Threonine0.920.260.210.18Alanine27.5634.1234.7236.33Proline0.792.212.182.07Tyrosine5.586.825.125.84Valine2.371.721.51.32Metheonine0.190.280.320.34Cystine0.130.150.120.11Isoleusine0.750.610.510.45Leusine0.730.780.710.69Phenylalanine0.140.340.280.23Tryptophan0.731.262.181.68Lysine0.230.170.240.23

γ-Crystallins.

These have the smallest molecular weight and the lowest electrophoretic mobility at pH 8·3 (Fig. 7a).

The amino-acid compositions of the three crystallins are different, whereas within each category the sub-groups have similar (though not identical) compositions. α-Crystallins are distinctive for their low cysteine and high proline and phenylalanine content, whereas γ-crystallins have a high arginine and tyrosine content and low lysine and alanine contents. The amino-acid sequence of two of the four major chains of bovine α-crystallin and of one of the γ-crystallins has been determined (van der Ouderaa, de Jong, Hilderink and Bloemendal, 1974; Croft, 1972).

Figure 7 shows the separation of the crystallins of bovine and human lens by three methods: (a) migration in an electric field using gel electrophoresis; this method takes advantage of the differences in charge of the protein molecules and also, to some extent, the different size and shape of the molecules; (b) gel chromatography, which depends on the size and shape of the molecules; and (c) ion-exchange chromatography; this method, like electrophoresis, depends upon the charge on the protein molecules, which are passed in solution through a column of charged material to which they may or may not bind.

A Purification of Tryptic Peptides Containing Cysteine Residues

The amino acid composition of the P–450cam as reported by Dus et al (1970), indicates that P–450cam contains 6 cysteine residues. We find, however, on isolation of the peptides that there appears to be 7 cysteine residues per mole of cytochrome P–450.

The methods for purifying the cysteine peptides are summarized in Fig. 1. Not all of the peptides were purified, only the cysteine containing peptides. The initial fractionation of the peptides occurred on a column of Sephadex G–50. Four main fractions were separated, i.e. T–1 to T–4. The cysteine containing peptides were present in T–1 to T–3 pooled fractions. Pooled fraction T–1 contained one of the cysteine peptides, T1B. Pooled fraction T–2 contained the cysteine containing peptides, T2C and T2F. Pooled fraction T–3 contained T3B and T3Dc and T3De, all cysteine containing peptides. Fig. 1 also shows the methods used to further purify each of the cysteine containing peptides.

A second method for demonstrating the cysteine residues is being developed. Clostripain, a sulfhydryl protease which is specific for cleaving on the COOH–side of arginine but not lysine residues, was used. The isolation procedure will not be described here but some of the peptides isolated and their sequences will be described in the next section.

Are all proteins are composed of the same amino acids?

Does an amino acid or a protein have a higher molecular weight?

Does every amino acid have the same molecular structure?

Can proteins with different amino acid sequences have similar structures?