Chapter 5: Protein

Protein
55
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© Cengage Learning 2017
Introduction
• Proteins are “of prime importance”
• Versatile roles
– Facilitate chemical reactions
– Fight infections
– Provide structure in all living tissue

The Chemist’s View of Proteins
• Comprised of carbon, hydrogen, oxygen,
and nitrogen atoms
– Amino: nitrogen containing
• The structure of proteins
– Amino acids: protein building blocks
– About 20 different amino acids in proteins
– Protein chains: dipeptides, tripeptides,
polypeptides

Amino Acid Structure and Examples of Amino
Acids

The Chemist’s View of Proteins (cont’d.)
• The structure of proteins
– Protein shapes
• Amino acid chains fold into complex shapes
• What determines the shape of polypeptide chains?
– Protein functions
• Different shapes enable proteins to perform
different tasks in the body

The Coiling and Folding of a Protein Molecule

The Structure of Hemoglobin

The Chemist’s View of Proteins (cont’d.)
• Nonessential and essential amino acids
– Nonessential amino acids: those the body can
make for itself
– Essential amino acids: those that must be
obtained in foods
– What is a conditionally essential amino acid?

Protein Digestion and Absorption
• Long peptide chains + enzymes
• Short peptides + enzymes
• Tripeptides and dipeptides + enzymes
• Amino acids

Protein Turnover and Nitrogen Balance
• Protein turnover
– Continual process in which proteins are made
and broken down
– What are some specific examples?
– Amino acids needed each day to support new
growth and maintenance of cells
• Nitrogen balance (N in vs. N out)
– Nitrogen equilibrium: intake equals output

Roles of Body Proteins
• As structural components
– Muscle, bones, teeth, tendons, cartilage,
blood vessels, etc.
• As enzymes
– Catalysts essential to all life processes
– Protein circle
• Some break food proteins into amino acids
• Others build long chains of amino acids (proteins)

Roles of Body Proteins (cont’d.)
• As transporters: move substances around
the body
– Protein hemoglobin: carries oxygen
– Lipoproteins: transport lipids
• As regulators of fluid and electrolyte
balance
– Maintain necessary amounts and types of
fluid and minerals in each compartment

One Cell and Its Associated Fluids

• As regulators of fluid and electrolyte
balance
– Proteins can help control fluid distribution this
because they:
• Can’t move freely across membranes
• Are attracted to water
– Cell membrane transport proteins transfer
electrolytes and other substances into and out
of cells to maintain balance

• As regulators of acid-base balance
– Maintain the balance between acids and
bases within the body’s fluids
– What are acidosis and alkalosis?
• As antibodies
– Giant protein molecules produced when body
detects antigens; specific to those antigens
– What is immunity?

• As hormones
– Chemical messengers (some are proteins,
e.g., insulin, glucagon)
• As a source of energy and glucose
– When glucose or fatty acids are limited
– Starvation results in wasting of lean tissue
• Functions of proteins summarized in Table
5-2

Protein and Health
• Protein deficiency
– Slow growth in children, impaired brain and
kidney function, weakened immune defenses,
and impaired nutrient absorption
– Malnutrition
• Protein-energy malnutrition (PEM)
• Severe acute malnutrition (SAM): kwashiorkor and
marasmus
• Chronic malnutrition

Protein and Health (cont’d.)
• Malnutrition
– Severe acute malnutrition
• Wasting: thinness for height, indicating recent rapid
weight loss
– Chronic malnutrition
• Stunting: low height for age, indicating restriction of
potential growth in children
– Rehabilitation
• What are ways to prevent and treat malnutrition?

• Protein excess
– Heart disease
• Animal-derived food sources high in saturated fat
can contribute to heart disease, obesity, and
diabetes risk
– Kidney disease
• High protein intakes can accelerate loss of kidney
function in chronic kidney disease

• Protein and amino acid supplements
– Protein supplements
• Not needed by well-fed athletes
• Use in weight loss unsupported by scientific
studies
– Amino acid supplements
• The body absorbs amino acids from whole proteins
best

• Protein recommendations and intakes
– RDA (adults): 0.8 gram per kilogram (2.2
pounds) of healthy body weight
– AMDR: 10% to 35% percent of kcalories
– Typical U.S. intakes
• Men: 16% of kcalories
• Women, children, and some elderly: 13% to 15%
of kcalories

Protein in Foods
• Protein quality
– High-quality proteins provide adequate
amounts of all of the essential amino acids
– Digestibility: measure of absorption
– Amino acid composition
• Limiting amino acid: essential amino acid available
in the shortest supply relative to the amount
needed to support protein synthesis

Protein in Foods (cont’d.)
• Protein quality
– High-quality proteins
• Generally derived from animal foods
• Soy protein
– Complementary proteins
• Two protein-rich foods with different amino acids
combined to obtain all the essential amino acids in
amounts sufficient to support health

Complementary Proteins

Protein in Foods (cont’d.)
• Protein sparing
– Carbohydrate and fat allow amino acids to be
used to build body proteins
– If they are insufficient, the body will use
protein to meet energy needs
• Protein on food labels
– Quantity required
– % Daily Value reflects quantity and quality

Nutrition in Practice: Vegetarian Diets
• Protein: varied plant protein foods
• Appropriate energy intake
• Vitamin B12: fortified foods/ supplements
• Vitamin D: adequate sun, fortified foods
• Iron absorption increased by vitamin C
• Zinc: legumes, enriched foods, whole
grains

Nutrition in Practice: Vegetarian Diets
• Calcium: dark green leafy vegetables,
legumes, fortified foods, almonds, sesame
seeds
• Essential fatty acids: flaxseed, walnuts,
their oils; soybeans, canola oil
– EPA and DHA: algal oils
• Health advantages of vegetarian diets

Chapter 5: Protein

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