Chapter 7: Proteins

Major building blocks Much of the body is made of protein 17% of total body weight Mostly muscle tissue/lean body tissue Contain nitrogen (amino acids) What do amino acids contain?

Amino Acid NH2 O R C C OH H R group Acid group Amine group

Tyrosine Serine Proline Valine Glycine Tryptophan Glutamine Threonine Glutamic acid Phenylalanine Cysteine Methionine Aspartic Acid Lysine Asparagine Leucine Arginine Isoleucine Alanine Histidine Nonessential AA Essential AA

Amino acids 9 essential amino acids 11 non-essential amino acids Cannot be made by the body, must come from diet Found in foods that come from animal sources Plant proteins may be combined to make complete proteins containing these essential amino acids 11 non-essential amino acids Body can make these amino acids from essential amino acids or as long as there is sufficient nitrogen

Making an Essential Amino Acid Phenylalanine Tyrosine (Essential AA) phenylalanine hydroxyase (Nonessential AA)

Phenylketonuria Phenylalanine Tyrosine (Essential AA) phenylalanine hydroxyase (Nonessential AA) “ MISSING; LACKING” Control Intake Becomes essential

Synthesis of Nonessential Amino Acids Transamination Addition of an amino grp to a carbon skeleton to form a new non-essential amino acid. Deamination The removal of an amino group from an amino acid Occurs during protein metabolism

Synthesis of Nonessential Amino Acid Transamination and Deamination

Functions of Proteins in Our Diet Provide the 9 essential amino acids Provide the nonessential amino acids or nitrogen to make the nonessential amino acids Protein have many functions Regulate and maintain body functions Cell repair Supply energy (4 kcal/g)

Quality of Protein High quality protein or complete protein Contains ample amount of all 9 essential AA Animal proteins: contain all EAA in sufficient quantities Low quality protein Deficient or low in one or more essential AA Plant proteins (except Soybean) All-or-none principle Diet must supply all 9 EAA for protein synthesis

Limiting Amino Acid CCCCC AAAAAAAA CAR CAR CAR RRRRRR CAR CAR R A A A The EAA in lowest conc in a diet relative to body needs. C is the limiting amino acid in this example CCCCC AAAAAAAA CAR CAR CAR RRRRRR CAR CAR R A A A

Complementary Proteins Food 1 Food 2 CC CCCC AAAA AA CAR CAR CAR RRR RRR CAR CAR CAR

Complementary Protein: (Table 7.2) Mixed diets provides high quality protein because a complimentary protein pattern results Essential amino acids must be 40% of total protein intake for infants and pres-school children Diets need careful planning For adults; EAA -11% of total pn intake

Complementary Protein: (Table 7.2) Beans (legumes) Grains Nuts/seeds Vegetables

Protein Organization Peptide bond Dipeptide Tripeptide Oligopeptide SH SH CH2 CH2 H O CH H2O H O CH H N C OH H N C OH H N C N C OH CH H O CH H O CH3 H2O CH3 Peptide bond Dipeptide Tripeptide Oligopeptide

Protein Synthesis

Turnover and Metabolism

Protein turnover Protein turnover – the degradation and synthesis of endogenous protein. When proteins break down, they free amino acids to join the general circulation…what happens next? Recycled to make new proteins (or other Ncontaining compounds) OR – Nitrogen is removed and remaining part of amino acid is used for energy (Deamination: NH2 is removed to make urea)

Summary of Protein Synthesis in the Body • DNA in nucleus of cell acts as a template • mRNA from cytoplasm of cell enters nucleus • Portion of DNA unwinds and mRNA matches up with it to transcribe the pattern • mRNA leaves nucleus and goes to cytoplasm of cell • mRNA hooks onto ribosomes • tRNA brings in single amino acids one-at-a-time • Amino acid placed on mRNA where so directed by template (called translation) • Amino acids enzymatically joined into a protein • Protein is released into cell fluid

When an essential amino acid (EAA) is missing, protein synthesis stops, and the remaining amino acids are deaminated and the nitrogen is excreted in the urine. Vegetarians must pay CLOSE attention to protein complementation so as to not be deficient in any EAA!

Protein Organization Primary structure Secondary structure Sequence of AA; affects protein’s shape Related to sickle cell anemia (fig 7.3) Secondary structure Bends and folds held together by bonds Formed by interactions of AA at various positions Tertiary structure 3-D shape: affects function of protein Quaternary structure 2 or more proteins interacting together

Denaturation of Proteins Heat/acid/alkaline/enzymes Result in alteration of the protein’s three dimensional structure

Digestion of Protein in the Stomach Proteins are denatured by the cooking and the stomach acid Gastrin (hormone) stimulates the release of pepsinogen and acid from cells in the stomach Pepsinogen is converted to pepsin by the acid in the stomach Pepsin (an enzyme produced in the stomach) breaks down proteins into peptones

Digestion of Protein in the Small Intestine Partially digested pns from stomach stimulate the release of CCK (cholesystokinin) Causes pancreas to release the protein splitting enzymes: trypsin, chymotrypsin, and carboxypeptidase into the duodenum The enzymes will break peptones into smaller peptides and amino acids Peptides and amino acids are ready for absorption

Protein Absorption (fig 7.7) Active absorption Whole proteins are broken down at the microvilli surface and within the absorptive cells Whole proteins are eventually broken down to amino acids Many different amino acid transport mechanisms Amino acids are sent to the liver via portal circulation

Protein Digestion and Absorption Mouth – no enzymatic digestion taking place; mechanical breakdown of proteins taking place Stomach HCl acid uncoils the large protein molecule so that Pepsin can begin to break the protein chain apart into smaller polypeptides, tripeptides, and dipeptides Gastrin is released in the stomach to stimulate HCl acid production

Protein Digestion & Absorption Cont. • Small Intestine In the lumen, pancreatic and intestinal proteases break polypeptides into smaller tripeptides, dipeptides, and amino acids At the brush border, intestinal tripeptidases and dipeptidases break down the tripeptides and dipeptides, respectively into amino acids for absorption Note: these enzymes are “amino-acid” specific Amino acids are absorbed into blood stream most of the absorption takes place in the lower part of the small intestine

Functions of Proteins Building blocks of body components Muscle, connective tissue, mucus, blood clotting factors, bone, Collagen, actin, myosin, hemoglobin, keratin Maintain fluid balance Albumins and globulins – blood proteins Prevent excessive build up of fluid in the extracellular spaces Reduces risks of edema Contribute to acid/base balance Maintenance of normal pH in blood Act as buffers: regulate ion concentration blood and cells

Functions of Proteins Building blocks for hormones and enzymes Immune function: antibodies Lack of protein leads to anergy: reduced immune function Gluconeogenesis Formation of glucose Energy yielding

Functions of Proteins in the Body Growth and Maintenance (collagen, muscle) Formation of enzymes Fluid balance – proteins attract water Acid/base balance – act as buffers Antibody formation Hormone synthesis Act as transport protein Blood clotting – fibrin, collagen Vision – opsin Stored as fat if consumed in excess!

Measuring protein utilization and nitrogen balance (fig 7.9) Protein not used is deaminated Nitrogen is excreted in urine 16% of protein molecule is N Normally N intake = N out Positive N balance : N in> N out Growth, pregnancy, building muscles Negative N balance Wasting of body tissues Loss of weight

RDA for Protein Promotes equilibrium 0.8 gm of protein / kg of healthy body weight 154 lb. = 70 kg 2.2 kg/lb. 70 kg x 0.8 g protein = 56 g protein kg healthy body wt

RDA for Protein Increased by ~10-15 gm /day for pregnancy Endurance athletes may need 1.5 - 2 gm/kg healthy weight About 8-10% of total kcals Most of us eat more than the RDA for protein Excess protein cannot be stored as protein The Food and Nutrition Board does not support any higher needs

Is a High-Protein Diet Harmful? Likely to limit fruits and veg. and decrease fiber, vitamins, phytochemicals Intake of animal protein increases risk for heart disease (high in saturated fat) Excessive intake of red meat is linked with colon cancer Burden on the kidney; need additional fluid to secrete N. may result in dehydration Increase calcium loss National Academy of Sciences recommends no more than 2 x RDA for protein

Individual Amino Acid (AA) Supplement Supplement may cause imbalances and toxicity (especially with methionine and tyrosine) Body is designed to handle whole proteins Supplement can overwhelm the absorptive mechanism Excess of one AA can hamper absorption of other AAs

Vegetarian Without animal proteins, a diet can be deficient in essential amino acids – Complete protein (animal) – contains all of the amino acids essential in human nutrition in amounts adequate for human use Incomplete protein (plant) – limiting amino acid -EAA in the shortest supply, relative to the amounts needed for protein synthesis Complementary proteins – proteins that have different amino acid profiles, but when put together, resemble that of a complete protein source

Types of Vegetarians Semi-vegetarian – Lactovegetarian – some animal products, included in diet such as poultry and fish Lactovegetarian – will consume milk products in the diet Ovo-vegetarian – will consume eggs in the diet Lacto-Ovo vegetarian – will consume milk and eggs in the diet Strict Vegetarian (vegan) – no animal sources consumed, only foods of plant origin

Protein Complementation Combine cereal grains + legumes Combine legumes + seeds & nuts Rice + Beans Peanut butter + Bread Chili + Cornbread Split pea soup + Sesame crackers

Protein Sources Supplies protein in abundance: Meats Milk and dairy products Supplies a moderate amount of protein: Vegetables Breads & Cereals Supplies NO protein: Fruits Fats

Vegetarianism: Plant Protein Somewhat less efficient (than animal protein) No cholesterol and low in saturated fat High in (soluble) dietary fiber, phytochemicals Lacking in one or more essential amino acid

Soy Protein Similar to animal protein (used in school lunches) High in linoleic & a-linolenic acid Contains Ca for bone health Lowers blood cholesterol Contains isoflavones (genistein and diadzein) plantlike estrogen Sources: tofu, soy milk, soy flour, tempeh, miso Recommend 2-4 servings a week Not recommended for women WITH breast cancer (or family history)

Evaluation of Protein Quality Ability to support body growth and maintenance Measured under the condition that the amount of protein consumed is < body’s needs Protein exceeding this amount becomes less efficient Egg protein – has the highest quality protein; used as the “standard” from which all proteins in food are measured.

Biological Value Biological Value – the amount of protein nitrogen that is retained for growth and maintenance, expressed as a percentage of the protein nitrogen that has been digested and absorbed; a measure of protein quality Measure protein (AA) retention Nitrogen retained Nitrogen absorbed BV = X 100

Protein Efficiency Ratio Used by FDA to set standards for baby food Compares the weight gained in a growing rat after 10 days or more eating a standard amount of protein Measures BV (protein retention) PER = Gram weight gain Gram protein consumed

Chemical Score of Protein Amount of each essential AA in a gram of protein in the food divided by an “ideal” amount for that essential AA The lowest AA score is the C.S. for that food Chem. Score Mg of ess. AAn per gm of protein Required mg needs of the ess. AAn per gm of protein =

Protein Digestibility Corrected AA Score (PDCAAS) Most widely used (on food labels) Maximum value is 1.0 (= milk, eggs, soy protein) PDCAAS = Chem. Score x (~0.9-1.0) Range of digestibility of that protein

Malnutrition Protein-Energy Malnutrition Marasmus: disease of starvation Seen in hospitalized patients Kwashiorkor Protein Malnutrition

Kwashiorkor Low protein density diet Energy needs are marginally met Signs and symptoms: Apathy, listlessness, failure to grow, poor weight gain, change in hair color, nutrient deficiency, flaky skin, fatty infiltration in the liver, massive edema in the abdomen and legs

Marasmus Starving to death Insufficient protein, energy, nutrients “skin and bones” appearance Little or no subcutaneous fat Reduce brain growth