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
R group Acid group Amine group
NH2 O
R C C OH
H
TyrosineSerineProlineValineGlycineTryptophanGlutamineThreonineGlutamic acidPhenylalanineCysteineMethionineAspartic AcidLysineAsparagineLeucineArginineIsoleucineAlanineHistidineNonessential AAEssential AA
Amino acids
9 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 acids11 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 acidsProvide the nonessential amino acids or nitrogen to make the nonessential amino acidsProtein 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
quantitiesLow 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
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 resultsEssential 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 bondDipeptideTripeptideOligopeptide
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
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 inthe 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 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 acidGastrin (hormone) stimulates the release of pepsinogen and acid from cells in the stomachPepsinogen is converted to pepsin by the acid in the stomachPepsin (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 duodenumThe enzymes will break peptones into smaller peptides and amino acidsPeptides and amino acids are ready for absorption
Protein Absorption (fig 7.7)Active absorptionWhole proteins are broken down at the microvilli surface and within the absorptive cellsWhole 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 AbsorptionMouth – no enzymatic digestion taking place; mechanical
breakdown of proteins taking placeStomach 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 enzymesImmune function: antibodies Lack of protein leads to anergy: reduced
immune functionGluconeogenesis Formation of glucose
Energy yielding
Functions of Proteins in theBody
Growth and Maintenance (collagen, muscle)Formation of enzymesFluid balance – proteins attract waterAcid/base balance – act as buffersAntibody formationHormone synthesisAct as transport proteinBlood clotting – fibrin, collagenVision – opsinStored 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 outPositive N balance : N in> N out Growth, pregnancy, building muscles
Negative N balance Wasting of body tissues Loss of weight
RDA for Protein
Promotes equilibrium0.8 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 pregnancyEndurance athletes may need 1.5 - 2 gm/kg healthy weightAbout 8-10% of total kcalsMost of us eat more than the RDA for proteinExcess protein cannot be stored as proteinThe 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 cancerBurden on the kidney; need additional fluid to secrete N. may result in dehydrationIncrease calcium lossNational 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 proteinsSupplement can overwhelm the absorptive mechanismExcess 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 – 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 + legumesCombine legumes + seeds & nutsRice + BeansPeanut butter + BreadChili + CornbreadSplit 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 fatHigh in (soluble) dietary fiber, phytochemicalsLacking in one or more essential amino acid
Soy Protein
Similar to animal protein (used in school lunches)High in linoleic & a-linolenic acidContains Ca for bone healthLowers blood cholesterolContains isoflavones (genistein and diadzein) plantlike estrogen
Sources: tofu, soy milk, soy flour, tempeh, misoRecommend 2-4 servings a weekNot recommended for women WITH breast cancer (or family history)
Evaluation of Protein Quality
Ability to support body growth and maintenanceMeasured under the condition that the amount of protein consumed is < body’s needsProtein exceeding this amount becomes less efficientEgg 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 qualityMeasure protein (AA) retention
Nitrogen retained
Nitrogen absorbedBV = X 100
Protein Efficiency Ratio
Used by FDA to set standards for baby foodCompares the weight gained in a growing rat after 10 days or more eating a standard amount of proteinMeasures BV (protein retention)
Gram weight gain
Gram protein consumedPER =
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 AAThe lowest AA score is the C.S. for that food
Mg of ess. AAn per gm of protein
Required mg needs of the ess. AAn per gm of proteinChem. Score =
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 MalnutritionMarasmus: disease of starvation Seen in hospitalized patients
Kwashiorkor Protein Malnutrition
KwashiorkorLow protein density dietEnergy needs are marginally metSigns 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 deathInsufficient protein, energy, nutrients“skin and bones” appearanceLittle or no subcutaneous fatReduce brain growth