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Published on November 1, 2007

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PROTEIN QUALITY Methods for Evaluating Protein Quality of Food Proteins:  PROTEIN QUALITY Methods for Evaluating Protein Quality of Food Proteins An Educational Programme* by Dupont Protein Technologies in cooperation with the American Soybean Association *1999:Reviewed by Vernon R. Young, PhD, DSc, Professor of Nutritional Biochemistry Massachusetts Institute of Technology, Cambridge MA, U.S.A. Update 2002: Prof. Dr. H. Steinhart / Dr. C. Reimers, Institute of Biochemistry and Food Chemistry, University of Hamburg, Germany; Dr. Jean-Michel Lecerf, Institut Pasteur de Lille, France FUNCTION AND COMPONENTS OF PROTEIN:  FUNCTION AND COMPONENTS OF PROTEIN Protein is essential to building and maintaining body tissues Amino acids are building blocks of protein Plant and animal proteins are made of up of 20 common amino acids Mahan, L.K., Escott-Stump, S. Krause’s Food, Nutrition, & Diet Therapy, 9th edition. Philadelphia: W.B. Saunders Company, 1996. Rehner, G., Daniel, H.: Biochemie der Ernährung, Spektrum Akademischer Verlag Heidelberg, 1999 Nelson, D., Cox, M., Lehninger, A.: Biochemie, 3. Auflage, Springer Verlag Berlin, 2001 PROTEIN COMPOSITION:  Proteins consist of 20 -amino acids in the L-configuration Amino acids consist of carbon, hydrogen, oxygen, nitrogen, and partly sulphur Amino acids differ in their side chains PROTEIN COMPOSITION BASIC STRUCTURE OF AN AMINO ACID:  BASIC STRUCTURE OF AN AMINO ACID Mahan, L.K., Escott-Stump, S. Krause’s Food, Nutrition, & Diet Therapy, 9th edition. Philadelphia: W.B. Saunders Company, 1996. Rehner, G., Daniel, H.: Biochemie der Ernährung, Spektrum Akademischer Verlag Heidelberg, 1999 Nelson, D., Cox, M., Lehninger, A.: Biochemie, 3. Auflage, Springer Verlag Berlin, 2001 R = sidechains of different composition PROPERTIES OF AMINO ACIDS:  Characteristic side chain (R) influences physiological and physico-chemical properties of amino acids and also those of proteins Division into five groups in relation to the different side chains: non-polar aliphatic side chains polar, not charged (hydrophilic) side chains positively charged side chains negatively charged side chains aromatic side chains Nelson, D., Cox, M., Lehninger, A.: Biochemie, 3. Auflage, Springer Verlag Berlin, 2001 PROPERTIES OF AMINO ACIDS AMINO ACIDS IN PROTEINS:  1: Non polar aliphatic side chains alanine (Ala) glycine (Gly) isoleucine (Ile) leucine (Leu) methionine (Met) valine (Val) 2: Polar, not charged (hydrophilic) side chains asparagine (Asp-NH2) cysteine (Cys) glutamine (Glu-NH2) proline (Pro) serine (Ser) threonine (Thr) 3: Positively charged side chains arginine (Arg) histidine (His) lysine (Lys) 4: Negatively charged side chains aspartic acid (Asp) glutamic acid (Glu) 5: Aromatic side chains phenylalanine (Phe) tryptophan (Trp) tyrosine (Tyr) AMINO ACIDS IN PROTEINS AMINO ACID PROPERTIES:  Amino acids may form ions in aqueous solution Some amino acid properties follow from the uneven charge distribution In aqueous solution, amino acids appear as amphoteric molecules: AMINO ACID PROPERTIES AMINO ACID STEREOCHEMISTRY:  All amino acids (except glycine) have optical activity due to an asymmetric C-atom in -position Amino acids in animal and vegetable proteins always are L-isomers D-Alanine L-Alanine AMINO ACID STEREOCHEMISTRY LINKING AMINO ACIDS INTO PEPTIDES and PROTEINS:  Amino acids are linked by amide bonds (also peptide bonds) Up to 10 amino acids: peptides More than 10 amino acids: proteins C O H N CR2 COO- H R1 and R2 are side chains of amino acids R1C H +NH3 LINKING AMINO ACIDS INTO PEPTIDES and PROTEINS PROTEIN TYPES:  Proteins can be divided into two groups: Homoproteins, containing amino acids only Heteroproteins, containing an extra non-protein part or prosthetic group nucleo-, lipo-, glyco-, phospho-, hemo-, flavo-, metalo-proteins PROTEIN TYPES PROTEIN STRUCTURE:  Proteins are characterised by their amino acid sequence and their conformation or three-dimensional structure: Primary structure: the amino acid sequence Secondary and tertiary structure: the threedimensional arrangement of the polypeptide chain Quarternary structure: the arrangement of several polypeptide chains together PROTEIN STRUCTURE MAIN PROTEIN CLASSES:  Fibrous proteins Mainly structural tasks Consist of simple and repeating secondary structures (-helix and -sheet structure) For example: keratin, collagen (-helix), silk fibrin (-sheet structure) Globular proteins Biologically active proteins Complex tertiary structure often with several types of secondary interactions within the same polypeptide chain MAIN PROTEIN CLASSES PROTEIN DENATURATION:  Physical: Heating Cooling Mechanical treatment Hydrostatic pressure Radiation Chemical: Acids Bases Metals Organic solvents PROTEIN DENATURATION PROTEIN DENATURATION EFFECTS:  Change in solubility by exposure of hyrophilic or hydrophobic peptide units Change in water-binding capacity Loss of biological activity Higher risk of chemical attack because of exposure of other peptide bonds Change in the viscosity of solutions Loss of crystallisation properties PROTEIN DENATURATION EFFECTS FUNCTIONAL PROPERTIES OF FOOD PROTEINS:  Hydration Solubility Viscosity Gel formation Texture Dough formation Emulsification Foaming Aroma binding Interaction with other food components FUNCTIONAL PROPERTIES OF FOOD PROTEINS CLASSIFICATION OF AMINO ACIDS IN FOOD PROTEIN:  CLASSIFICATION OF AMINO ACIDS IN FOOD PROTEIN Amino acids in food protein can be classified as Essential (indispensable) - body synthesis inadequate to meet needs Non-essential (dispensable) - can be synthesised by the body Conditionally essential (indispensable) - become essential under certain conditions Rehner, G., Daniel, H.: Biochemie der Ernährung, Spektrum Akademischer Verlag Heidelberg, 1999 ESSENTIAL AMINO ACIDS:  ESSENTIAL AMINO ACIDS Histidine*** Methionine* (and Cysteine) Isoleucine Phenylalanine** (and Tyrosine) Leucine Threonine Lysine Tryptophan Valine * necessary for synthesis of cysteine ** necessary for synthesis of tyrosine *** necessary only for infants Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. Elmadfa, I., Leitzmann, C., Ernährung des Menschen, 3. Auflage, Verlag Eugen Ulmer Stuttgart, 1998 COMPLETE AND INCOMPLETE PROTEINS:  COMPLETE AND INCOMPLETE PROTEINS Complete (balanced) protein – single food protein containing all 9 essential amino acids in concentrations sufficient to meet effectively the requirements of humans Incomplete (unbalanced) protein – single food protein deficient in 1 or more of 9 essential amino acids Williams, S.R. Basic Nutrition and Diet Therapy, 10th edition. St. Louis (MO): Mosby, 1995. COMPLEMENTARY PROTEINS1:  COMPLEMENTARY PROTEINS1 People can meet their minimum daily requirements for protein and essential amino acids by – Consuming sufficient quantities of complete protein(s) Consuming a sufficient amount of a variety of incomplete proteins Combining complete proteins with incomplete proteins2 Complementary proteins can be consumed over the course of a day3 Elmadfa, I., Leitzmann, C., Ernährung des Menschen, 3. Auflage, Verlag Eugen Ulmer Stuttgart, 1998 Williams, S.R. Basic Nutrition and Diet Therapy, 10th edition. St. Louis (MO): Mosby, 1995. Young, V.R., Pellett, P.L. Am J Clin Nutr 1994; 59 (Suppl): 1203S-1212S. EVALUATING PROTEIN QUALITY:  EVALUATING PROTEIN QUALITY Amino acid content of food protein Digestibility Amino acid requirements Based on standard amino acid requirement pattern for appropriate population age group Requirements for 2- to 5-year old child used as standard for all above 1 year Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. FDA. Food labeling; general provisions; nutrition labeling; label format; nutrient content claims; health claims; ingredient labeling; state and local requirements; and exemptions; final rules. Food and Drug Administration, Fed Reg 1993; 58 (3):2101-2106 AMINO ACID AVAILABILITY:  Digestion and absorption rate of native Animal protein: 90 % Vegetable protein: 60 – 70 % Limited protein digestibility due to: Protein conformation effects Interaction with metal ions, lipids, nucleic acids, cellulose Antinutrititional factors Size and surface properties of the protein particle Thermal treatment Biological differences between individuals Cheftel, J.C., Cuq, J.L., Lorient, D.: Lebensmittelproteine: Biochemie, funktionelle Eigenschaften, Ernährungsphysiologie, chemische Modifizierung. 1. Auflage, Behrs Verlag Hamburg, 1992 AMINO ACID AVAILABILITY METHODS FOR EVALUATING PROTEIN QUALITY:  METHODS FOR EVALUATING PROTEIN QUALITY Examples for older biological methodologies include1,2 – Biologic value (BV) Net protein utilisation (NPU) Protein efficiency ratio (PER) New methodology is3 – Protein digestibility-corrected amino acid score (PDCAAS) 1 Elmadfa, I., Leitzmann, C., Ernährung des Menschen, 3. Auflage, Verlag Eugen Ulmer Stuttgart, 1998 2 Biesalski, H.K., Grimm, P., Taschenatlas der Ernährung, Georg Thieme Verlag Stuttgart, 1999 3 Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. PROTEIN EFFICIENCY RATIO – PER:  PROTEIN EFFICIENCY RATIO – PER Traditional method of evaluating protein quality. In United States PER was1 – Standard in food industry for evaluating protein quality of food proteins 1 Young, V.R., Pellett, P.L. J Nutr 1991; 121: 145-150. PROTEIN EFFICIENCY RATIO – PER:  PROTEIN EFFICIENCY RATIO – PER Measures the ability of a protein to support growth in young, growing rats, not humans Growing rats need more sulphur- containing amino acid, methionine Sulphur-containing amino acids considered “limiting” amino acid in soy protein Using amino acid needs of rats led to – Overestimation of protein quality in animal protein Underestimation of protein quality in plant protein Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE – PDCAAS:  PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE – PDCAAS Factors used in calculating PDCAAS include – Essential amino acid content of food protein Digestibility Ability to supply indispensable amino acids in amounts adequate to meet human needs Uses amino acid requirements of 2- to 5-year old child – most demanding requirements in any group except infants Highest possible score is 1.0 CALCULATING PDCAAS:  1. Analyse for nitrogen content 2. Calculate protein (N x 6.25) 3. Analyse for essential amino acid (IAA) content 4. Calculate amino acid score (uncorrected) mg of IAA in 1 g test protein mg of IAA in 1 g amino acid requirement pattern* 5. Determine digestibility 6. Calculate PDCAAS: PDCAAS = lowest uncorrected IAA Ratio x true protein digestibility CALCULATING PDCAAS * 1985 FAO/WHO/UNU 2- to 5-year old requirement pattern. Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. CALCULATION OF PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE OF ISOLATED SOY PROTEIN:  CALCULATION OF PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE OF ISOLATED SOY PROTEIN INDISPENSABLE AMINO ACID REQUIREMENT PATTERNS (FAO/WHO/UNU):  INDISPENSABLE AMINO ACID REQUIREMENT PATTERNS (FAO/WHO/UNU) HIS = Histidine; ILE = Isoleucine; LEU = Leucine; LYS = Lysine; MET + CYS = Methionine + Cysteine; PHE + TYR = Phenylalanine + Tyrosine; THR = Threonine; TRY = Tryptophan; VAL = Valine * Amino acid values for isolated soy protein based on analysis of SUPRO® Brand Isolated Soy Protein provided by Protein Technologies International. † Suggested pattern of requirements from Energy and Protein Requirements, Report of the Joint FAO/WHO/UNU Expert Consultation. Technical Report Series No. 724, 1985. PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE – PDCAAS:  PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORE – PDCAAS In 1993 U.S. Food and Drug Administration (FDA) adopted PDCAAS as standard for calculating percent Daily Value (%DV) of protein of food labels because – PDCAAS is based on human amino acid requirements, making it more appropriate for evaluating protein quality of foods consumed by humans PDCAAS is recommended by Food and Agricultural Organisation/World Health Organisation (FAO/WHO), international organisation experienced in establishing such standards FDA. Food Labeling. Fed Reg 1993; 58(3):2101-2106. PLANT AND ANIMAL PROTEIN SOURCES:  PLANT AND ANIMAL PROTEIN SOURCES Traditionally – Animal proteins considered complete according to the composition of the amino acids All plant proteins considered incomplete according to the composition of the amino acids Actually – Some soy protein products and some animal proteins have a complete composition of amino acids PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORES OF SELECTED FOOD PROTEINS:  PROTEIN DIGESTIBILITY-CORRECTED AMINO ACID SCORES OF SELECTED FOOD PROTEINS * Values for SUPRO® Brand Isolated Soy Protein provided by Protein Technologies International as determined through actual analysis. † Protein Quality Evaluation, Report of the Joint FAO/WHO Expert Consultation. Rome: FAO Food and Nutrition Paper No. 51, 1991. PROTEIN QUALITY - A SUMMARY:  PROTEIN QUALITY - A SUMMARY Amino acids are the building blocks of protein, which is essential to building and maintaining body tissues The body cannot synthesise essential (indispensable) amino acids in amounts adequate to meet its needs; therefore, they must be obtained from the diet The protein quality of food protein depends on its digestibility and its ability to provide all essential (indispensable) amino acids to meet human requirements SUMMARY:  SUMMARY PDCAAS is superior to other methods of evaluating the protein quality of food proteins for humans Isolated soy protein, with a PDCAAS of 1.0, is a complete protein and has the same score as milk protein and egg white Teaching Objectives:  Teaching Objectives Key Learnings:  Key Learnings Key Learnings:  Key Learnings References:  References

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