NUTR 250 Human Nutrition & Metabolisms Chapter 7 Proteins PDF

NUTR 250 Human Nutrition & Metabolisms Chapter 7 Proteins Dr. Keting Li, FDST, UNL Chapter Topics Structures of Proteins Synthesis of Proteins Sources of Proteins Nitrogen Balance Protein Digestion and Absorption Functions of Protein Health Concerns Related to Protein Consumption Amino Acids (AAs) –...

NUTR 250 Human Nutrition & Metabolisms Chapter 7 Proteins Dr. Keting Li, FDST, UNL Chapter Topics Structures of Proteins Synthesis of Proteins Sources of Proteins Nitrogen Balance Protein Digestion and Absorption Functions of Protein Health Concerns Related to Protein Consumption Amino Acids (AAs) – The Building Blocks of Proteins Elements: carbon, hydrogen, oxygen + nitrogen (some contain sulfur) Structure Central carbon Amino group These 4 components are the same for all AAs. Carboxyl group Hydrogen R = Side chain Make each AA unique Determines structure and function Carbon skeleton- amino acid without amino group 20 AAs are supplied by Diet (all) Endogenous synthesis (some) Amino Acids - Essential vs Non-Essential Nonessential Amino Acids (11) Endogenous synthesis; not needed from diet “conditionally essential” – essential only under specific circumstances Infancy Disease Trauma Essential Amino Acids (9) Must be obtained from diet Cannot be synthesized endogenously Cannot make the carbon skeleton Cannot attach an amino group to the carbon skeleton Cannot synthesize fast enough to meet the body’s needs Amino Acids - Essential vs Non-Essential At least you need to know which amino acid is essential! Conditionally Essential Amino Acids People with phenylketonuria (PKU), a rare inherited genetic disorder, are unable to metabolize phenylalanine due to lack of the enzyme phenylalanine hydroxylase (PAH) PAH breaks down phenylalanine to tyrosine Tyrosine becomes conditionally essential amino acid for PKU patients. PAH Phenylketonuria and Aspartame The artificial sweetener aspartame (Equal, NutraSweet) is added to many diet foods and diet sodas (Diet Coke and Coca-Cola Zero). Aspartame is converted into phenylalanine in the body. Aspartame is safe. U.S. FDA requires that all food or related products that contain aspartame bear this warning: "Phenylketonurics: Contains phenylalanine" https://www.coca-colacompany.com/faqs/what-is-aspartame Synthesis of Nonessential Amino Acids Transamination (transaminase) Amino group is transferred from one amino acid to a carbon skeleton Result: new amino acid Synthesis of Nonessential Amino Acids Deamination (deaminase) Amino group is lost (not transferred) Incorporated into urea (liver), transported via bloodstream to kidney, and excreted via urine Peptide Bond Formation A peptide bond is formed by a combination of amino acids in which the amino group of one amino acid has undergone a reaction with the carboxyl group of another amino acid The reaction is a dehydrationcondensation reaction, forming an amide group (CO−NH) Peptide or protein A peptide has two ends: the end with a free amino group is called the N-terminal amino acid residue; the end with a free carboxyl group is called the C-terminal amino acid residue. Protein Nomenclature Peptides 2 – 50 amino acids Proteins >50 amino acids Peptides are named from the N-terminal acid residue to the C-terminal amino acid Letter code (three-letter or single-letter) Amino Acid Composition – Complete vs Incomplete Proteins Complete Proteins Contain all 9 essential AA Animal proteins classified as high-quality proteins meat, milk, eggs, fish (except gelatin) Incomplete Proteins Lack 1 or more essential AA Plant proteins classified as lowquality proteins beans, grains (except soy, quinoa) Complete and Incomplete Proteins Cells require essential AA for protein synthesis Limiting AA: Essential AA in smallest supply in relation to body needs Limits the amount of protein a body can make Complementary proteins: Combination of 2 (or more) incomplete proteins to compensate limiting AAs Complementary proteins Protein Synthesis In transcription, a DNA sequence is rewritten, or transcribed, into a similar RNA "alphabet." In eukaryotes, the RNA molecule must undergo processing to become a mature messenger RNA (mRNA). In translation, the sequence of the mRNA is decoded to specify the amino acid sequence of a polypeptide. The name translation reflects that the nucleotide sequence of the mRNA sequence must be translated into the completely different "language" of amino acids. Protein Synthesis Example of the consequences of errors in DNA coding of proteins – Sickle Cell Anemia Abnormal red blood cell (Sickle shape); become rigid and sticky Due to amino acid substitution valine for glutamate; change the structure of hemoglobin (O2 carrying protein) Normal red blood cell Sickle cells blocking blood flow Sources of Protein 1. Diet (65-110 g/d): Replenish and maintain an adequate AA pool for protein synthesis and repair North American diet: 70% of proteins from meat, poultry, fish, milk, cheese, legumes, and nuts Worldwide: About 35% from animal sources Plants are major protein source 2. Recycling of body protein Utilization of amino acids released during the breakdown of body protein Sources of Protein for American vs Chinese Food Sources of Protein Edible Insects – The new protein source Husker Grad Turns Bugs Into Delicious Grub Evaluation of Food Protein Quality Protein quality indicates a dietary protein’s ability to support body growth and maintenance. Protein quality is determined by Digestibility (amount of amino acids absorbed) Amino acid composition compared with a reference protein that provides the essential amino acids in amounts needed to support growth (e.g., egg white protein) The digestibility of animal protein is relatively high (90-100%), in contrast to that of plant protein (70%). Food Protein Quality - Biological Value How efficiently is dietary protein converted into body protein Determined by measuring how much N from food is retained in body If food protein amino acid pattern matches the body protein amino acid patterns, BV is high Source BV = 100 for egg white protein Most animal proteins: high BV Egg Most plant proteins: lower BV Beef Milk 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑏𝑜𝑑𝑦(𝑔) 𝐵𝑉 = × 100 Wheat 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑓𝑟𝑜𝑚 𝑓𝑜𝑜𝑑 (𝑔) Corn BV 100 78 78 60 54 Food Protein Quality - Protein Efficiency Ratio (PER) How efficiently does protein lead to weight gain? Based on the weight gain of a test subject divided by its intake of a particular food protein during the test period. Comparison to reference protein (casein has PER of 2.5) Reflects BV because the weight gain and growth measured in PER depends on the incorporation of dietary protein into body protein FDA uses PER to set labelling standards for infant foods Source 𝑮𝒂𝒊𝒏 𝒊𝒏 𝒃𝒐𝒅𝒚 𝒎𝒂𝒔𝒔 (𝒈) 𝑷𝑬𝑹 = 𝑷𝒓𝒐𝒕𝒆𝒊𝒏 𝒊𝒏𝒕𝒂𝒌𝒆 (𝒈) PER Egg 4.5 Milk 3.0 Meat (average) 2.8 Soybean 2.0 Wheat 1.7 Food Protein Quality - Chemical Score How does the essential AA composition compare to an “ideal” protein? The “ideal” protein is egg protein. 𝒎𝒈 𝒐𝒇 𝒆𝒔𝒔𝒆𝒏𝒕𝒊𝒂𝒍 𝑨𝑨 𝒑𝒆𝒓 𝒈 𝒐𝒇 𝒑𝒓𝒐𝒕𝒆𝒊𝒏 𝑪𝒉𝒆𝒎𝒊𝒄𝒂𝒍 𝒔𝒄𝒐𝒓𝒆 = 𝒎𝒈 𝒐𝒇 𝒆𝒔𝒔𝒆𝒏𝒕𝒊𝒂𝒍 𝑨𝑨 𝒑𝒆𝒓 𝒈 𝒐𝒇 “𝒊𝒅𝒆𝒂𝒍” 𝒑𝒓𝒐𝒕𝒆𝒊𝒏 Chemical score = Lowest (limiting) AA ratios Limiting amino acids---the essential amino acids in smallest supply in food Range: 0-1.0 Essential Amino Acid What are those nine essential amino acid? Histidine (His), isoleucine (ILE), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), threonine (Thr), tryptophan (Trp) and valine (Val) Practice: Chemical Score What is the chemical score of this food protein? Reference Food AA content (mg/g) 100 80 60 40 20 0 His Ile Leu Lys Met Phe +Cys +Tyr Thr Try Val Chemical Score What is the chemical score of this food protein? Reference Food AA content (mg/g) 100 80 60 40 20 0 His Chemical Score = (or Amino Acid Score) Ile Leu Lys Met Phe +Cys +Tyr limiting AA in food limiting AA in reference = Thr 9 mg 11 mg Try = 0.82 Val Food Protein Quality - Corrected Score Protein Digestibility Corrected Amino Acid Score (PDCAAS) Most used method PDCAAS = chemical score × digestibility; e.g., wheat PDCAAS = 0.47 × 0.9 Highest PDCAAS is 1.0, such as soy protein and most animal proteins Discussion What is the PDCAAS for an incomplete protein? PDCAAS = chemical score × digestibility Nitrogen Balance Method to determine protein requirements For healthy individuals (not growing or recovering) Replace the amount of protein lost in urine, feces, sweat, skin cells, hair, and nails Nitrogen Balance 1. Negative nitrogen balance: Inadequate dietary protein Serious illness or injury Diseases that increase protein breakdown (e.g., AIDS) 2. Positive nitrogen balance: Growth Recovery from trauma, injury, illness Recommendations Adult RDA: 0.8 g/kg body weight The recommended percentage of energy that should come from protein is 10-35% Does not address additional requirements: Recovery from illness or injury Trained athletes During these conditions, protein needs can range from 0.8 – 2.0 g/kg body weight. Digestion and Absorption 1. Cooking: Denatures proteins; softens connective tissues in meat Make many protein-rich foods easier to chew and aids in breakdown in GI tract 2. Enzymatic digestion: Gastric Pepsins Pancreatic Digestive Enzymes Intestinal Peptidases Enzymes of protein digestion Gastric Pepsins Pepsinogens (inactive proenzyme or zymogen) are converted by acidic pH 2 of the stomach into active pepsins. Pepsins are endopeptidases (optimum pH 2-5) that split off proteins. Pepsins become inactive in the small intestine at pH 7-8 when acidic chyme mixes with the bicarbonate-rich secretion from the pancreas. Enzymes of protein digestion Pancreatic Digestive Enzymes Pancreatic juice contains inactive zymogens. All pancreatic digestive enzymes are active at neutral pH. pH of pancreatic juice is neutralized by bicarbonates. Inactive zymogens become active enzymes. Trypsin, chymotrypsin and elastase are endopeptidase. Carboxypeptidases are exopeptidases. The action of pancreatic digestive enzymes produces 40% free amino acids and 60% oligopeptides (2-8 residue peptides). Enzymes of protein digestion Intestinal Peptidases Tripeptidases, dipeptidases, and aminopeptidases are located on the luminal surface of the mucosal cell and some in the cell cytoplasm of small intestine. These peptidases are all exopeptidases and hydrolyze oligopeptides into free amino acids. These cleavage products are absorbed in the jejunum. Regulation of Digestive Enzyme Secretion Acidic chyme in duodenum stimulates CCK (cholecystokinin) and secretin secretion. CCK and vagus nerve stimulate pancreatic juice secretion. Role of CCK and Enteropeptidase CCK (cholecystokinin) is produced throughout small intestinal mucosa. Its secretion is stimulated by long-chain fatty acids, amino acids and oligopeptides passing in the intestinal lumen. CCK causes the release of pancreatic inactive zymogens. CCK stimulates to secretion of enteropeptidase from duodenal epithelial cell. Enteropeptidase initiates the zymogen activation by cleaving pancreatic trypsinogen into active trypsin. Digestion of Proteins Describe the digestion of protein in stomach and small intestine. Describe the digestion of protein in stomach and small intestine. Amino Acid Uptake by Enterocytes Conceptually identical to that of monosaccharides. Amino acid concentration in blood is ~10% that found in mucosal cell, therefore active transport systems are predominant. Active transport is associated with ATPrequiring sodium pump. Transport systems also include facilitated diffusion. Many amino acids have a dual system of uptake; a high capacity (low affinity) in the fed state, and a low capacity (high affinity) in the starved state. Fate of Amino Acids after Uptake 1. AAs reach to liver via portal vein 2. Liver distributes AA: Protein synthesis Energy needs Conversion to CHO or fat Release into bloodstream Protein Functions 1. Producing vital body structures Structural proteins (collagen, actin, myosin) make up more than 1/3 of body protein Provide a matrix for muscle, connective tissues, and bone. Protein Functions 2. Maintaining fluid balance: Blood proteins include albumin, globulin, which maintain fluid balance by Attract fluid back into blood Counteract the force of blood pressure Inadequate protein consumption: Concentration of proteins in the blood eventually decreases Inadequate to pull enough of fluid back into the bloodstream; fluid builds up in tissues (Edema) Protein Functions 3. Contributing to acid-base balance Expressed in terms of pH Proteins act as buffers: negatively charged proteins attract positively charged H+ E.g., proteins in cell membrane pump ions into and out of cells; keep the blood slightly alkaline. http://www.austincc.edu/apreview/EmphasisItems/Electrolytefluidbalance.html Protein Functions 4. Forming hormones, enzymes and neurotransmitters a) Hormones Act as messengers in the body and aid in regulatory functions b) Enzymes Facilitate chemical reactions fundamental to metabolism c) Neurotransmitters Derivatives of amino acids; e.g., dopamine form tyrosine Protein Functions 5. Contributing to immune function Antibodies (aka immunoglobulins) are key component of immune system: Bind to foreign proteins that invade the body and prevent their attack on target cells Immune incompetence develops when protein intake is insufficient Protein Functions 6. Transporting nutrients ▪ Hemoglobin – carries oxygen from lungs to cells ▪ Lipoproteins – transport large lipid molecules from small intestine to body cells ▪ Retinal-binding protein – transport Vitamin A from the liver to other tissue ▪ Transferrin and ferritin – carrier and storage proteins for iron ▪ Ceruloplasmin – carrier protein for copper Protein Functions 7. Provides Glucose/Energy Health Concerns Developing Countries Developed Countries Protein deficiency Overconsumption Protein-energy malnutrition May increase health and disease risks Food allergies Protein-Energy Malnutrition (PEM) Protein deficiency often in combination with energy deficiency PEM is a very serious public health concern in many lowincome countries. PEM affects people of all ages, but has devastating effects on children: Fail to grow normally Diarrhea, infections, diseases Child mortality increased PEM usually occurs as either marasmus or kwashiorkor Protein-Energy Malnutrition Marasmus The result of chronic PEM Caused by diet containing minimal amounts of energy, protein and other nutrients Skin and bone appearance, little/no subcutaneous fat Cognitive and intellectual capacity impaired Kwashiorkor A word from Ghana - “the disease that the first child gets when the new children comes” Diet abruptly change from nutritious human milk to starchy roots Severe protein deficiency Accompanied by infection or disease Edema Moderate weight loss Growth impairment Fatty liver High Protein Diets In those with impaired kidney function, high protein diets may overburden kidney’s capacity to excess nitrogen as urea. Usually in diet high in animal proteins and low plant-based foods: Lower fiber, some vitamins, some minerals, phytochemicals Higher saturated fat and cholesterol Vegetarian Diets Eating patterns of vegetarians vary considerably, depending on the extent to which animal products are excluded. 1. Vegans: Follow the most restrictive diet, only plant foods, low in: High biological value protein; certain vitamins (B-12, D) and minerals (Ca, Fe, Zn) 2. Lacto-vegetarians Similar to vegans but include dairy products 3. Lacto-ovo-vegetarians Exclude meat, poultry, and fish but include eggs and dairy -> Complementary protein sources (beans and rice) Food Allergies Food proteins (allergens) Mistakenly react to food and trigger immune response Production of antibodies (mostly IgE) Symptoms: Runny nose Sneezing Itchy skin Hives Digestive upset Anaphylaxis (life threatening) Big 9 Food Allergens Big 8 Food Allergen Labeling and consumer Protection Act of 2004 Eight foods as major food allergens: milk, eggs, fish, shellfish, tree nuts, peanuts, what and soybean Now Big 9 Sesame was declared as the 9th major food allergen in 2021 The change was effective on Jan 1, 2023 Food Allergies 1. Prevalence Increased in last 30 years; about 5-8% of children and 2-4% of adults have food allergies Some children may outgrow 2. Treatment: avoid trigger foods Read labels Ask questions when eating out Prevent cross-contamination Celiac Disease A serious autoimmune disorder where the ingestion of gluten leads to damage in the small intestine. Not a food allergy Precise cause isn’t known, genes and diet contribute to celiac disease Affects ~0.7% of the U.S. population Healthy small intestinal lining; note the villi that protrude into the intestinal lumen and absorb nutrients. The small intestinal lining from a patient with untreated celiac disease; note the absence of villi - the so-called 'flat lesion‘. Celiac Disease – Gluten free Causes: Gluten from wheat, rye, barley, and related grains Classic symptoms: intestinal gas, bloating, diarrhea, constipation, abdominal pain, weight loss or gain. Long-term: severe malnutrition and increased risk of GI tract cancer A gluten-free diet is essential for managing signs and symptoms of celiac disease. Protein Powders & Muscle Muscle size and strength is built from diet and resistance training Consume 20 g of high-quality protein shortly after resistance exercise workouts Purity, dose, and safety of supplements are not guaranteed High-quality protein from food is effective Excess protein (over 2 g/kg/day) not incorporated into muscle http://www.ift.org/food-technology/past-issues/2019/march/columns/consumer-trends-foods-beverages-withprotein.aspx?token=12be231d-6ee1-43fe-9643-361de96beb66 High-Protein Trends Are Impacting the Food & Beverage Industry!