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Chapter 6. Proteins and Amino Acids. Chapter 6. Photo courtesy of the USDA. Amino Acids Are the Building Blocks of Protein. Proteins are sequences of amino acid Compounds composed of carbon, hydrogen, oxygen, and nitrogen arranged as strands of amino acids Types of amino acids

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proteins and amino acids



Proteins and Amino Acids

Chapter 6

Photo courtesy of the USDA

amino acids are the building blocks of protein
Amino Acids Are the Building Blocks of Protein
  • Proteins are sequences of amino acid
    • Compounds composed of carbon, hydrogen, oxygen, and nitrogen arranged as strands of amino acids
  • Types of amino acids
    • Essential: most come from diet
    • Nonessential: can be made in the body
amino acids essential non essential amino acids












Aspartic acid


Glutamic acid






Amino AcidsEssential & Non Essential Amino Acids
proteins in the body
Proteins in the Body
  • Some are working molecules
    • Enzymes
    • Antibodies
    • Transport vehicles
    • Hormones
    • Cellular “pumps”
    • Oxygen carriers
  • Some are structural molecules
    • Tendons
    • Ligaments
    • Scars
    • Fibers of muscles
    • Cores of bone and teeth
    • Filaments of hair
    • Materials of nails
amino acids the building blocks of protein
Amino Acids The building blocks of protein
  • Each amino acid has an amine group at one end
    • The nitrogen-containing part
  • An acid group at the other end
  • A distinctive side chain attached to the carbon at the center
      • Sequence of amino acids determines shape
      • Shape of protein determines function
the variety of proteins
The Variety of Proteins


  • Large, globular protein
  • Packed into red blood cells
  • Carries oxygen
  • Made of 4 protein strands
    • Each strand holding an iron atom
the variety of proteins9
The Variety of Proteins
  • Inherited Amino Acid Sequences
    • For each protein there exists a standard amino acid sequence
    • A sequence specified by heredity (genes)
      • If a wrong amino acid is inserted, health can be affected as in Sickle Cell Anemia
    • Normal hemoglobin contains two kinds of protein strands
      • In sickle-cell disease on one of the two strands, the protein is altered so that it is unable to carry oxygen
the variety of proteins11
The Variety of Proteins
  • Sickle-cell disease
    • Red blood cells collapse from the normal disk shape into crescent shapes
    • If too many of these types of red blood cells appear in the blood the results are
      • Abnormal blood clotting
      • Strokes
      • Severe pain
      • Susceptibility to infection
      • Early death
the variety of proteins12
The Variety of Proteins
  • Inherited Amino Acid Sequences
    • Genes determine the sequence of amino acids for each protein
    • Your unique combination of genes directs the making of all your body’s proteins

The nucleus of body cells contains the DNA for making every human protein

      • Each cell does not make every protein
      • Cells specialize in making certain proteins typical of their cell types
      • Example-- Pancreas makes enzymes and hormones
the variety of proteins digestion and absorption
The Variety of ProteinsDigestion and Absorption
  • Denaturing of Proteins

Irreversible changes in a protein’s shape

      • caused by
        • Heat
        • Acids
        • Bases
        • Alcohols
  • Example: Digestion of food protein

Stomach acids open up a protein’s structure

      • Allows digestive enzymes to make contact with the peptide bonds
the variety of proteins14
The Variety of Proteins
  • Cooking denatures proteins
    • Changes that occur to an egg white when an egg is cooked
    • With regard to diet this is important because
      • A protein that binds both biotin and iron is denatured, liberating them for digestion
protein digestion
Protein Digestion
    • When eating a protein source the body must break it down into amino acids
    • These amino acids can then be arranged into human body proteins
  • Mouth
    • Protein is crushed by chewing and moistened with saliva
  • Stomach
    • Acid (1.5 pH) uncoils protein’s strands so the stomach’s protein-digesting enzyme can attack the peptide bonds
      • These enzyme works best in an acidic environment
    • The stomach lining is protected from the acid and enzymes by a mucus coating secreted by stomach cells
protein digestion16
Protein Digestion
  • Small intestine
    • Receives small denatured pieces of protein
    • Alkaline juices from the pancreas neutralize the acid
      • The pH increases to about 7 (neutral)
    • Protein-digesting enzymes from the pancreas and small intestine continue breaking down protein until all that is left are dipeptides, tripeptides, or single amino acids
    • Cells along the small intestine absorb single amino acids
after protein is digested what happens
After Protein Is Digested, What Happens…?
  • Amino acids are absorbed into the bloodstream
    • Carried to the liver
      • Used by the liver or released into the blood to be taken up by other cells
    • Cells can then use the amino acids to
      • Make their proteins for their own use
      • Make proteins that are released into the lymph or blood for other uses
    • When necessary body cells can use amino acids for energy
the roles of proteins in the body
The Roles of Proteins in the Body

Amino acids are needed

To build the proteins of new tissue

  • Embryo
  • Muscles of an athlete in training
  • Growing child
  • New blood to replace losses
  • Scar tissue
  • New hair and nails

To replace

  • Worn-out cells and internal cell structures
  • Red blood cells live only 3-4 months
  • Cells lining the intestinal tract live only 3 days
  • Skin cells are constantly being shed and replaced
role of proteins
Role of Proteins
  • Enzymes
    • Thousands of enzymes live inside a single cell
      • Each is a catalyst that facilitates a specific chemical reaction
  • Hormones
    • Chemical messengers that respond to conditions that require regulation
    • Each hormone affects a specific organ or tissue and elicits a specific response
    • Are made from amino acids and lipids
role of protein
Role of Protein
  • Antibodies
    • Large proteins in the blood, produced by the immune system
    • Distinguish between “foreign” and “self” proteins
    • Inactivate foreign proteins. The next time the body encounters the same foreign protein, it destroys it even more rapidly
role of proteins22
Role of Proteins

Maintaining Fluid and Electrolyte Balance

Failure of any part of the fluid balance system causes edema-- seen in protein deficiency

Maintaining Acid-Base Balance

Blood proteins act as buffers to maintain the blood’s normal pH not too acid and not to basic ( pH 7)

Acidosis and alkalosis can denature proteins causing coma or death by denaturing proteins

role of proteins23
Role of Proteins
  • Energy
  • Proteins can provide energy
  • Many amino acids can be converted to glucose to serve the glucose needs of the brain
role of proteins providing energy and glucose
Role of ProteinsProviding Energy and Glucose
  • Glucose is stored as glycogen
  • Fat is stored as triglycerides
  • There is no specialized protein energy-storage compound
    • When protein-sparing energy from carbohydrate and fat is lacking, the body must dismantle its tissue proteins (blood, muscle) to obtain amino acids for energy
      • Energy deficiency (starvation) can cause wasting of lean body tissue as well as loss of fat
providing energy and glucose
Providing Energy and Glucose
  • If amino acids are over supplied, the body cannot store them
    • The body removes and excretes the amine groups-It is wasted
    • The residue (C, H, O) can be
      • Converted to glucose for energy or
      • stored as glycogen or fat
the fate of an amino acid
The Fate of an Amino Acid
  • Wasting occurs when:
    • The body does not have enough energy from other sources
    • Has more protein than it needs
    • Has too much of any single amino acid
      • Such as from a supplement
    • Or the diet supplies protein of low quality, with too few essential amino acids
the fate of an amino acid28
The Fate of an Amino Acid
  • To prevent the wasting of dietary protein and permit the synthesis of needed body protein
    • Dietary protein must be adequate in quantity
    • The diet must supply all of the essential amino acids in the proper amounts
    • Enough energy-yielding carbohydrate and fat must be present to prevent the dietary protein from being wasted
consumer corner protein and amino acid supplements
Consumer Corner: Protein and Amino Acid Supplements

Critically ill patients

Supplemental amino acids may help reverse malnutrition in some critically ill patients

Healthy people

Protein supplements used as a replacement for food can be dangerous

Especially when used in weight loss diets

“Liquid protein” diets have caused deaths and heart problems in the past

consumer corner protein and amino acid supplements30
Consumer Corner: Protein and Amino Acid Supplements
  • Lysine supplements
    • Touted to prevent or relieve the infections that cause herpes sores
      • Does not cure herpes infections
      • Unknown if it reduces outbreaks
      • Unknown if it is safe
  • Blends of amino acids leucine, isoleucine and valine
    • Popular with athletes hoping for a competitive edge
      • Value of these supplements is unclear
consumer corner protein and amino acid supplements31
Consumer Corner: Protein and Amino Acid Supplements
  • Tryptophan supplements
    • Tryptophan is a precursor of the neurotransmitter serotonin
    • Advertised to relieve
      • Pain
      • Depression
      • Insomnia
    • DRI committee concludes that high doses of tryptophan may cause
      • Sleepiness
      • Severe nausea
      • Skin disorders
protein and amino acid supplements
Protein and Amino Acid Supplements
  • People vulnerable to harm from protein supplements
    • Women of child-bearing age
    • Pregnant or lactating women
    • Infants, children, adolescents, and the elderly
    • People with inborn errors of metabolism that affect their bodies’ handling of amino acids
protein and amino acid supplements33
Protein and Amino Acid Supplements
  • DRI committee reviewed the available research on amino acid supplements
    • Not enough research exists to support long-term use of supplements by healthy people
    • More research into adverse effects is needed
  • Canada
    • Bans the sale of single amino acid supplements to consumers
food protein
Food Protein
  • Protein quality

Determines how well a diet supports the growth of children and the health of adults

Influenced by

      • A protein’s digestibility
      • A protein’s amino acid composition
which kinds of protein rich foods are easiest to digest
Which Kinds of Protein-Rich Foods Are Easiest to Digest?
  • Digestibility of a protein varies from food to food
    • The amino acids from animal sources are more easily digested
    • Animal sources: 90+% digested and absorbed
    • Legumes: ≈80%-90% digested and absorbed
    • Grains and other plant foods:≈70%-90% digested and absorbed
amino acid composition
Amino Acid Composition
  • High-quality proteins

Dietary proteins containing all of the essential amino acids in relatively the same amounts that human beings require

May also contain nonessential amino acids

amino acid composition37
Amino Acid Composition
  • Limiting Amino Acids
    • Is an essential amino acid present in dietary protein in a small amount
      • Thereby limiting the body’s ability to build protein
    • Lack of availability will slow protein synthesis
      • When the limiting amino acids are available again cells resume their normal protein synthesis
amino acid composition38
Amino Acid Composition
  • Complementary proteins
    • Mutual supplementation
      • The strategy of combining two incomplete proteins sources so that the amino acids in one food make up for those lacking in the other food
amino acid composition41
Amino Acid Composition
  • For people in areas where food sources are less reliable, protein quality can make the difference between health and disease
    • When food energy intake is limited or when the variety of foods is severely limited the primary food source of protein is important
measuring protein quality
Measuring Protein Quality
  • Protein digestibility-corrected amino acid score (PDCAAS)
    • A measuring tool used to determine protein quality
    • Reflects a protein’s digestibility as well as the proportions of amino acids that it provides
measuring protein quality43
Measuring Protein Quality
  • 100 - 0 scale
    • 100: egg white, ground beef, tuna fish, etc.
      • Protein sources that are most readily digested and most perfectly balanced for meeting human needs.
    • 94: soybean
    • 60s-50s: most legumes
    • 25: wheat protein; gluten
how much protein do people need
How Much Protein Do People Need
  • DRI
    • Designed to cover the need to replace protein-containing tissue that healthy adults lose and wear out every day
      • Depends on body size: larger people have a higher protein need
    • For adults: 0.8g/kg body weight/day
    • Minimum: 10% of total calories
    • Athletes may need slightly more
      • Increased need is well covered by a regular diet
how much protein do people need45
How Much Protein Do People Need
  • Nitrogen balance
    • The amount of nitrogen consumed is compared with the amount excreted in a given time period
    • Studies of nitrogen balance underlie the DRI Committee’s recommendations
how much protein do people need47
How Much Protein Do People Need

Positive nitrogen balance

  • Growing children add new blood, bone, and muscle cells to their bodies every day

They have more protein, and more nitrogen, at the end of the day than the beginning

      • Same for a pregnant woman
how much protein do people need48
How Much Protein Do People Need
  • Negative nitrogen balance
    • Occurs when muscle or other protein tissue breaks down and is lost
      • Illness or injury trigger the body to break down some of the less vital proteins
      • They provide amino acids and energy to help the body fight the illness
what happens when people consume too little protein
What Happens When People Consume Too Little Protein?
  • Protein-energy malnutrition (PEM)

protein-calories malnutrition (PCM)

    • World’s most widespread malnutrition problem
    • Includes both marasmus and kwashiorkor and states in which they overlap
what happens when people consume too little protein50
What Happens When People Consume Too Little Protein?
  • Marasmus
    • Chronic inadequate food intake
      • Inadequate energy, vitamins, minerals, and protein intake
    • Person is shriveled and lean all over
  • Kwashiorkor
    • Swollen belly and skin rash are present
    • Severe acute malnutrition
      • Too little protein to support body functions
  • Marasmus and kwashiorkor can occur in combination
too little protein
Too Little Protein
  • Marasmus
    • children from 6 - 18 months of age in overpopulated city slums
    • subsist on a weak cereal drink with little energy and protein of low quality
    • Without adequate nutrition
      • Muscles, including the heart muscles, waste and weaken
      • Brain development is stunted
        • learning is impaired
      • Metabolism is so slow that body temperature is subnormal
      • Little or no fat under the skin to insulate against the cold
too little protein53
Too Little Protein
  • Body cuts down on any expenditure of protein not needed for the heart, lungs, and brain to function
  • Growth ceases
  • Skin loses its elasticity and moisture
    • Tends to crack, sores develop and fail to heal
  • Digestive enzymes are in short supply
    • Digestive tract lining deteriorates and absorption fails
  • Blood proteins, including hemoglobin, are no longer produced
    • Child becomes anemic and weak
  • Lack the protein to heal broken bones
too little protein54
Too Little Protein

Antibodies are degraded to provide amino acids for other uses

  • Leaving the child an easy target for infection
    • Dysentery
      • An infection of the digestive tract causes diarrhea
      • Depletes the body of nutrients, especially minerals
    • Measles
      • Might make a healthy child sick for 1-2 weeks
      • Kills a child with PEM within 2-3 days
too little protein55
Too Little Protein
  • Once an infection sets in kwashiorkor often follows and the immune system weakens further
  • Infections that occur with malnutrition are responsible for two-thirds of the deaths of young children in developing nations
too little protein56
Too Little Protein
  • Marasmus progresses to the point of no return
    • The body’s machinery for protein synthesis, itself made of protein, has been degraded
    • At this point, attempts to prevent death by giving food or protein fail
too little protein57
Too Little Protein
  • If caught in time, the starvation of a child can be reversed by careful nutrition therapy
  • Years after PEM is corrected a child may experience deficits in learning compared with well-nourished peers
too little protein58
Too Little Protein
  • Kwashiorkor
    • In countries where kwashiorkor is seen, each baby is weaned from breast milk as soon as the next one comes along
      • The older baby no longer receives breast milk, which contains high-quality protein
      • Instead, they get a watery cereal with low quality protein
    • Rare in the United States, but not totally unknown
too little protein59
Too Little Protein
  • PEM in the USA and Canada
      • The poor living
        • On U.S. Indian reservations
        • In inner cities
        • In rural areas
      • Many elderly people
      • Homeless children
      • Those suffering from anorexia nervosa
  • Hungry children:
    • Do not learn as well as fed children
    • Are not as competitive
    • Are ill more often
      • Have higher absentee rates from school
      • When they attend, they cannot concentrate for long
too much protein
Too Much Protein
  • Often associated with obesity
  • Animal protein sources in particular can be high in saturated fat
    • A known contributor to atherosclerosis and heart disease
      • Substituting plant protein for animal protein improves indicators of heart disease risk
too much protein61
Too Much Protein
  • Protein intake and calcium--Research
    • Evidence is mixed whether high protein intake from animal sources, accelerates adult bone loss
    • Feeding purified protein to human subjects causes calcium to be spilled in the urine
    • Diets high in animal protein, correlate with a higher incidence of hip fractures in some populations
    • The effect of excess protein on bones was considered but there was insufficient evidence to set a Tolerable Upper Intake Level

DRI committee recommendation

    • Diet should contain no more than 35% of calories from protein
      • To decrease risk of chronic diseases
getting enough protein
Getting Enough Protein

Protein in the Food Groups

High-quality protein in abundance

  • Meat, poultry, fish, legumes, eggs, and nuts group
  • Milk, yogurt, and cheese group

Smaller amounts of protein that can add up to significant quantities

  • Vegetables group
  • Grains group
getting enough protein65
Getting Enough Protein
  • Protein-rich foods
      • Are high in many vit and minerals including vitamin B12 and iron
      • Tend to lack vitamin C and folate
getting enough protein66
Getting Enough Protein
  • The Advantages of Legumes
    • The protein quality of some legumes is almost comparable to that of meat
      • The quality of soy protein can be considered the equivalent to that of meat
      • Excellent sources of B vitamins, Iron, Calcium and other minerals
    • Lack
      • Vitamin A, Vitamin C, Vitamin B12
Planned vegetarian diets
  • Obesity rates 
  • Heart disease rates 
  • High blood pressure rates 
  • Cancer rates 
  • Life span 
controversy vegetarian and meat containing diets
Controversy: Vegetarian And Meat-Containing Diets

Why eat less meat?

  • For health reasons
  • For environmental reasons
    • Production of meat protein requires a much greater input of resources than does an equal amount of vegetable protein
  • Fear of contracting diseases-- food poisoning, mad cow
  • There is strong evidence linking vegetarian diets with reduced incidences of chronic diseases
controversy vegetarian and meat containing diet vegetarian diets and cancer
Controversy: Vegetarian And Meat-Containing Diet Vegetarian Diets and Cancer
  • Vegetarians have significantly lower rates of certain cancers than the general population
      • May be associated more with the abundance of fruits and vegetables in the diet than with the exclusion of all animal products
  • Colon cancer appears to correlate with moderate-to-high intakes of
    • Fatty red meats and processed meats
    • Alcohol
    • Total food energy
controversy vegetarian and meat containing diets72
Controversy: Vegetarian And Meat-Containing Diets
  • What Diet Offers the Best Support During Pregnancy and Infancy?
    • Women who eat meat, eggs, and milk products get enough vitamin B12, vitamin D, calcium, iron, zinc and protein to support pregnancy and breastfeeding
    • Lacto-ovo vegetarian diet also supports preg and lactation
    • Vegans, do not supply enough B12, Vit D, Calcium, iron, zinc and protein
controversy vegetarian and meat containing diets73
Controversy: Vegetarian And Meat-Containing Diets
  • What Diet Offers the Best Support During Childhood?
    • Children who eat small servings of meat, poultry, and fish receive abundant protein, iron, vitamin B12 and food energy
    • Children eating well-planned vegetarian diets that include eggs, milk, and milk products also do well
    • Vegan children tend to be slightly shorter and lighter in weight and may need fortified animal free foods and supplements of Vitamin D, Calcium, Vitamin B12, Iron and Zinc
controversy vegetarian and meat containing diets74
Controversy: Vegetarian And Meat-Containing Diets

Poorly planned vegetarian diets typically lack:

Zinc, Calcium, Vitamin B12, Vitamin D and Iron

Poorly planned meat eater’s diet may lack:

Vitamin A, Vitamin C, Folate and Fiber

Careful planners can get needed nutrients

  • Soy milk and tofu are fortified with Calcium, Vitamin D, and Vitamin B12
  • Fortified juices, soy milk, breakfast cereals, tofu, leafy greens provide calcium
  • Dark green vegetables and legumes help meet Iron and Zinc needs
controversy vegetarian and meat containing diets75
Controversy: Vegetarian And Meat-Containing Diets
  • Planning a Vegetarian Diet
    • Vegetarians are often advised to follow the USDA Food Guide with a few modifications
      • Using legumes, nuts and seeds in place of meats
    • Those who include milk products and eggs can follow the regular guide plan