Download Lecture 9

What Are Proteins?
Proteins: large complex molecules
composed of amino acids.
Lecture 9:
Proteins
– Contain carbon, hydrogen, oxygen,
nitrogen
– Primary source of nitrogen in our diets
– 20 different amino acids are used to
make proteins
Nutrition 150
Shallin Busch, Ph.D.
– The shape of a protein molecule is
important for its function
Amino Acid Structure
Examples of Amino Acids
Side group
varies
Amino
group
Acid
group
Glycine
Alanine
Aspartic acid
Phenylalanine
Two Amino Acids can form a Dipeptide
Amino acid
+
Amino acid
Dipeptide
Proteins fold to form complex structures
Amino acid
sequence
determines the
shape of the
polypeptide chain
Polypeptides are chains of 3+ amino acids.
Digestion of Proteins
Protein Digestion in Stomach
Hydrochloric acid breaks down protein
structure
Hydrochloric acid activates pepsin
Pepsin: an enzyme that breaks down proteins
into short polypeptides and amino acids.
Pancreas
Pancreatic enzymes called proteases
complete the digestion of proteins into
single amino acids
Small intestine
• Specific carriers on intestinal cells
transport amino acids, dipeptides, and
tripeptides
• Enzymes in small intestine cells
convert dipeptides and tripeptides to
single amino acids
• If not used in intestinal cell, amino
acids absorbed into the blood stream
where they travel to liver
Functions of Proteins
in the Body
Swelling due to edema caused by protein deficiency
and resultant fluid imbalance.
– Cell growth, repair, and maintenance
– Enzymes
– Hormones
– Fluid and electrolyte balance
– pH balance
– Antibodies to protect against disease
– Energy source
Adequate supplies of protein in the bloodstream keep
fluids out of the tissues.
Enzymes:
Facilitate both digestion (breakdown) and the formation of new complexes.
B
A
A
A
Enzyme
B
The enzyme forms a
complex with A and B.
Recommended Dietary Allowance (RDA)
– 0.8 grams protein per kg body weight
Enzyme
The separate compounds,
A and B, are attracted to
the enzyme’s active site,
making a reaction likely.
New
compound
B
How Much Protein
Should We Eat?
Enzyme
The enzyme is unchanged,
but A and B have formed
a new compound, AB.
How Much Protein
Should We Eat?
Proper protein intake depends on
– 12-20% of total energy intake should be
from protein
How Much Protein
Should We Eat?
People who require more protein include
– Age
– Children
– Health status
– Adolescents
– Activity level
– Pregnant or lactating women
– Vegetarians
Body’s Amino Acids Needs
• To build the body’s many protein, all amino
acids are needed. If the body lacks an
amino acid, the protein will not be made.
• Essential amino acids
– Cannot be produced by our bodies
– Must be obtained from food
• Nonessential amino acids
– Can be made by our bodies
Proteins in the Diet
• Incomplete protein: does not contain all
essential amino acids.
– Not sufficient for growth and health
– Considered a “low quality” protein
– Typically from vegetables (soy is exception)
• Complete protein: contains sufficient
amounts of all 9 essential amino acids.
– Considered a “high quality” protein
– Typically from animal products
Essential Amino Acids
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Non-essential Amino Acids
Alanine
Arginine
Asparagine
Aspartic Acid
Cysteine
Glutamic Acids
Glutamine
Glycine
Proline
Serine
Tyrosine
Proteins in the Diet
• Mutual supplementation: using two
incomplete proteins together to make a
complete protein.
• Complementary proteins: two protein
sources that together supply all 9 essential
amino acids.
– Examples: beans and rice, hummus (garbanzo
beans and sesame seeds), peanut butter and
bread, tofu and broccoli with almonds, spinach
salad with pine nuts and kidney beans
Complementary Proteins
Isoleucine
Lysine
Protein Quality (PDCAAS)
Methionine Tryptophan
• Amino acid composition (Does it have
amino acids in ratio that humans
need?)
Legumes
Grains
– Score dictated by limiting amino acid
Together
• Digestibility
– Determined by studies in rats
Table 6-3
Page 196
Risks of Too Much Protein?
– High cholesterol and heart disease
• Diets high in protein from animal sources are
associated with high cholesterol
– Possible bone loss
• High protein diets MAY cause excess calcium
excretion leading to bone loss
– Kidney disease
• High protein diets are associated with an
increased risk of kidney disease
• Especially for people who may be susceptible
to kidney disease
Protein Supplements?
• Inappropriate for MAJORITY of people
• Single and unusual combinations of amino
acids can be harmful
– Toxicity
– Deficiency in other amino acids due to
competition for absorption
– Can lead to diseases
• Ingestion of amino acids in foods is optimal
for healthy individuals
Health Benefits of
Vegetarianism
– Lower intake of fat and total energy
– Lower blood pressure
– Reduce the risk of heart disease
– Reduce the risk of some types of cancer
– Fewer digestive problems
Vegetarian Diets
• Vegetarianism: restricting the diet to
foods of plant origin and, in some
cases, consuming some animal
products
• There are many reasons to adopt a
vegetarian diet both for health and
ethics
Challenges of Vegetarianism
•Vegetarian diets can be low in some vitamins
(B12, Vitamin D) and minerals (iron, calcium,
zinc).
•Vegetarians must plan a balanced and
adequate diet (complementary proteins)
•Vegetarians can use soy products as a
protein source (soy is a complete protein)
Protein–Energy Malnutrition
Disorder caused by inadequate intake
of protein and energy.
Marasmus
• Disease resulting from severely inadequate
intakes of protein, energy, and other
nutrients.
• Marasmus symptoms include
• There are two common forms:
– Marasmus
– Kwashiorkor
Kwashiorkor
• Disease resulting from extremely low
protein intake due to diet or infection
• Kwashiorkor symptoms include
– Some weight loss and muscle wasting
– Edema resulting in distention of the belly
– Retarded growth and development
– Severe wasting of muscle tissue
– Stunted physical growth
– Stunted brain development
– Anemia
Child with kwashiorkor
Belly is swollen due to:
•Edema
•Enlarged liver
•Potentially parasites