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HSD 240 – ONLINE
CHAPTER 6: THE PROTEINS AND AMINO ACIDS
Protein
 Of prime importance
 Working proteins -- enzymes, antibodies, transport vehicles, hormones
 Structural proteins -- muscle tendons (muscle to bone), ligaments (bone to bone), hair, material of nails
Structure of Proteins
 Contain carbon, hydrogen, oxygen AND nitrogen
 The nitrogen atoms give the name amino (nitrogen containing) to amino acids
 Amino acids are the building blocks of protein
 The amino acids in a strand of protein are different from on another – can contain 20 different kinds of
amino acids
Amino Acids
 All amino acids have a chemical backbone consisting of single carbon atoms with both an amine group (the
amine group) and an acid group (carboxyl group)
 The differences in amino acids depends on the chemical side chain that is attached to the center carbon of
the backbone
 It is the side chain that gives identity and chemical nature to each amino acid
 The side chains makes the amino acids differ in size, shape, and electrical charge
 There are 20 amino acids; the body can make most of them from fragments of carbohydrate or fat to make
the backbone, and nitrogen from other sources to make the amine group
Essential Amino Acids
 The body cannot make these amino acids
 Without them, the body cannot make the proteins it needs to do its work
 Therefore, we must eat the foods that provide these essential amino acids
 Phenylalanine, an essential amino acids
o Phenylalanine  tyrosine (a non-essential amino acid)
o If the enzyme needed to convert phenylalanine to tyrosine is absent – condition called PKU
o Phenylalanine builds up in the blood and this results in a decrease in IQ in infants
Building Protein
 Amino acids are connected by a peptide bond – formed between the amine group and the acid group of the
next amino acid
 Proteins take on different shapes depending on their charges
o If side chain electrically charged – proteins are attracted to water
o If side chain are neutral – repelled by water; tuck themselves away from body fluids
 Several strands of protein may cluster together in a functioning unit
 Some proteins need a vitamin or mineral to activate it
 The different shapes of protein allow them to perform different tasks in the body
 Enzymes – act on other substances to change them chemically; they are proteins
 Hemoglobin – a globular protein that carries oxygen in the red blood cells
 No restriction on length of amino aids – they can make up thousands of different proteins
 The amino acids that make up a protein molecule are specified by hereditary; for each protein there is a
proper amino acid sequence; if a wrong amino acid is inherited, the result could be disastrous to health e.g.
sickle cell disease
Denaturation of Protein
 Proteins shape can be changed by heat, acids, bases, and alcohol
 Denaturation = change in shape
 Denaturation of protein is the first step in its destruction
 During digestion of protein, the stomach acid opens up the proteins structure
 Once the structure is open, digestion can break the peptide bonds
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The Digestion of Protein
Stomach
 Where protein digestion begins
 Acids (HCl) in the stomach uncoils (denatures) protein into strands
 Stomach enzymes (pepsin) then begin to break the chemical (peptide) bonds into amino acids
 Protein ----------------polypeptides
--------------tripeptides
--------------dipeptides
--------------single amino acids
--------------few whole proteins
Small Intestine
 Acid from the stomach is neutralized by alkaline juice (bicarbonate) secreted by the pancreas
 Protein digesting enzymes from the pancreas (trypsin, chymotrypsin, and carboxypeptidase) and intestines
(peptidase) continue to break peptide bonds between tripeptides and dipeptides
 Intestinal cells absorb amino acids and transfer amino acids into the bloodstream
 Bloodstream transports amino acids to all the body’s cells
 Protein synthesis occurs in the cells – DNA, mRNA, tRNA – gene expression
The Roles of Protein in the Body
1. Regulation of gene expression
2. Providing structure and movement
3. Building enzymes, hormones and other compounds
4. Building antibodies
5. Transporting substances
6. Maintaining fluid and electrolyte balance
7. Maintaining acid-base balance
8. Blood clotting
9. Provide energy and glucose
Protein energy
 Will be used for energy if it has to
 1 gram of protein yields 4 kcal
 When used for energy it loses its amine
 Amine group either used elsewhere or turned into urea by the liver (deamination)
 Urea sent to the kidney for excretion in the form of urine
 Fragments that remain – C, H, O2 – can be used to build CHO and fats
 Amino acids can be converted to glucose
 Amino acids can be converted to fat, and fats will be stored
 So, if need be, protein can help maintain a steady blood glucose and also serve the glucose needs for the
brain
For Energy
In Storage
Carbohydrates offer energy
Glucose is stored as glycogen
Fats offer concentrated energy
Fat is stored as triglycerides
Proteins offer energy plus nitrogen
Protein is not stored – available ONLY as active working
molecular and structural components of tissue
Summary
Amino acids in a cell can be
 Used to build proteins
 Converted to other small nitrogen-containing compounds such as vitamins or niacin
 Converted to other amino acids
Stripped of their nitrogen, amino acids can be
 Converted to glucose
 Burned as fuel
 Stored as fat
CONSUMER CORNER: Evaluating Protein and Amino Acid Supplements
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How much protein do we need? Table 6-3
DRI
Adults – 0.8 grams per kilogram of body weight
The more athletic adult – 1.0-1.2 grams/kg of body weight
Women – 46 grams per day
Men – 56 grams per day
AMDR 10-35% of kcal from protein
USDA Food Guide Pyramid (MyPyramid)
Other factors to consider:
 The Body’s Health
o Malnutrition and infection increase the body’s need for protein
 Other Nutrients and Energy
o Need a full array of macronutrients and micronutrients
 Protein Quality
o Part of a normal fixed diet
Nitrogen balance
 We need enough protein to maintain nitrogen balance – the amount of nitrogen consumed being equal to
the amount of nitrogen excreted
 Adults need to maintain equilibrium
o Infants, children, pregnant and lactating women, body builders, are in positive nitrogen balance –
retaining more nitrogen than they are excreting
o Surgery, space travel, starvation, results in negative nitrogen balance – muscle tissue being
broken down and lost
What Foods Provide High Quality Protein?
 Depends on its amino acid content
 High –quality protein (formerly “complete protein”) – proteins that provide all of the essential amino
acids
 Animal proteins, eggs
 Milk , yogurt, cheese
 Most soybean products
 If the diet fails to provide enough of the essential amino acids
 Internal proteins are broken down to liberate the needed essential amino acid
 Conserve the essential amino acid by limiting the synthesis of protein
 Limiting Amino Acid
 An essential amino acid that is present in dietary protein in an insufficient amount, thereby limiting
the body’s ability to build protein
 Complementary Protein: eating two different foods where essential amino acids are lacking (or low) in
one are provided in the other and vice versa
 Two proteins do not need to be eaten together as long as the diet provides enough energy and total
protein from a variety of sources
 Mutual supplementation
 Legumes – foods of the pea, bean and lentil family that are nitrogen-fixing; their seeds are
rich in protein
 Protein Digestibility
 Some proteins are more digestible than others
 Oats vs. eggs
 Perspective on protein quality
 Meeting protein needs not a problem for most people in the US even if they were to eat no animal
products at all
 Need to pay attention to mutual supplementation
 Varied, adequate in calories, nutrient dense
 What about where food energy is limited (where malnutrition exists) or protein limited
 Fufu – low-protein staple food of the cassava root
Protein Deficiency and Excess
What happens when people consume too little protein
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
Deficiency in protein and energy can result in protein-energy malnutrition (PEM), the most widespread
form of malnutrition in the world
 Marasmus
o 6-18 months of age
o Children fed weak cereal drink, low calorie, low protein quality
o Child looks like a wizened old person
o Muscle wasting
o Little body fat for insulation
o Insufficient protein for hemoglobin, antibodies
o Dysentery – infection of the digestive tract – diarrhea
o Might progress to a point of no return
 Kwashiorkor – Ghanian name for “the evil spirit that infects the first child when the second child is born”
o Breastmilk – high quality protein
o When baby is weaned, low quality protein follows
o Some symptoms resemble marasmus minus the severe wasting
o Edema in belly and legs
o Fatty liver – lack of transport proteins
o Liver needs to filter toxins
 PEM at home
Is it possible to consume too much protein?
 Overconsumption of protein offers no benefits and may pose some risks
 US protein intakes – most consume sufficient amounts of protein; high-protein diets suggesting 65% of
calories for weight loss not warranted
 Heart Disease
o Saturated fat from animal protein and milk, yogurt , cheese (full fat varieties)
o Homocysteine and heart disease; high protein might raise the level of this amino acid and increase
the risk of heart disease
 Kidney disease- protein diets
 Adult bone loss - osteoporosis
 Cancer
o Colon and rectum from well-cooked red meats and highly processed meats
 Is a Low-Gluten Diet Best for Health?
o Gluten and celiac disease
Food Feature: Getting Enough but Not Too Much Protein

Legumes

Textured vegetable protein

Tofu – curd made from soybeans
Controversy 6: Vegetarian and Meat Consuming Diets
Vegetarians -- Table C6-1
 People who consciously choose not to eat animal some or all products
 Fruitarian
 Lacto-ovo vegetarians
 Lacto-vegetarians
 Ovo-vegetarian
 Vegan
 Partial vegetarians
 Pesco-vegetarian
Positive health effects – reduced risk of
o Obesity, heart disease, blood pressure, cancer, diabetes, osteoporosis, diverticular disease
Positive aspects of a meat eater’s diet
o Energy, B12, vitamin D, calcium, iron, zinc, , omega 3, and protein for all stages of the
lifecycle
Planning a Vegetarian Diet
Vegetarian Food Guide Pyramid - In what way do the food groups differ? See page 226
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