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