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
NH2
O
C
C
OH
H
R group
Acid group
Amine group
Essential AA
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Nonessential AA
Alanine
Arginine
Asparagine
Aspartic Acid
Cysteine
Glutamic acid
Glutamine
Glycine
Proline
Serine
Tyrosine
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 acids

11 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
(Essential AA)
Tyrosine
phenylalanine hydroxyase
(Nonessential AA)
Phenylketonuria
Phenylalanine
(Essential AA)
Tyrosine
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 acids
Provide the nonessential amino acids
or nitrogen to make the nonessential
amino acids
Protein 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
quantities
Low 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
The EAA in lowest conc in a diet relative to body
needs. C is the limiting amino acid in this example
CCCCC
AAAAAAAA
RRRRRR
CAR CAR CAR
CAR CAR
R
A A A
Complementary Proteins
Food 1
Food 2
CC
AAAA
RRR
CCCC
AA
RRR
CAR CAR CAR
CAR CAR CAR
Complementary Protein: (Table 7.2)
Mixed diets provides high quality protein
because a complimentary protein pattern
results
Essential 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
H
H
O
N
C OH
CH
SH
SH
CH2
CH2
CH
H N C OH
H
CH3
H2O
H
H N
O
H2O
CH
C N C
CH
Peptide bond
Dipeptide
Tripeptide
Oligopeptide
O
CH3
H O
OH
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 in
the 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 acid
Gastrin (hormone) stimulates the release of
pepsinogen and acid from cells in the stomach
Pepsinogen is converted to pepsin by the acid in
the stomach
Pepsin (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 duodenum
The enzymes will break peptones into smaller
peptides and amino acids
Peptides and amino acids are ready for absorption
Protein Absorption (fig 7.7)
Active absorption
Whole proteins are broken down at the
microvilli surface and within the absorptive
cells
Whole 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 Absorption
Mouth

– no enzymatic digestion taking place; mechanical
breakdown of proteins taking place
Stomach
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 enzymes
Immune function: antibodies

Lack of protein leads to anergy: reduced
immune function
Gluconeogenesis

Formation of glucose
Energy yielding
Functions of Proteins in the
Body
Growth and Maintenance (collagen, muscle)
Formation of enzymes
Fluid balance – proteins attract water
Acid/base balance – act as buffers
Antibody formation
Hormone synthesis
Act as transport protein
Blood clotting – fibrin, collagen
Vision – opsin
Stored 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 out
Positive N balance : N in> N out

Growth, pregnancy, building muscles
Negative N balance


Wasting of body tissues
Loss of weight
RDA for Protein
Promotes equilibrium
0.8 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 pregnancy
Endurance athletes may need 1.5 - 2 gm/kg
healthy weight
About 8-10% of total kcals
Most of us eat more than the RDA for
protein
Excess protein cannot be stored as protein
The 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 cancer
Burden on the kidney; need additional fluid to secrete N.
may result in dehydration
Increase calcium loss
National 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 proteins
Supplement can overwhelm the absorptive
mechanism
Excess 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 + legumes
Combine legumes + seeds & nuts
Rice + Beans
Peanut butter + Bread
Chili + Cornbread
Split 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 fat
High in (soluble) dietary fiber,
phytochemicals
Lacking in one or more essential amino acid
Soy Protein
Similar to animal protein (used in school lunches)
High in linoleic & a-linolenic acid
Contains Ca for bone health
Lowers blood cholesterol
Contains isoflavones (genistein and diadzein)

plantlike estrogen
Sources: tofu, soy milk, soy flour, tempeh, miso
Recommend 2-4 servings a week
Not recommended for women WITH breast cancer
(or family history)
Evaluation of Protein Quality
Ability to support body growth and maintenance
Measured under the condition that the amount of
protein consumed is < body’s needs
Protein exceeding this amount becomes less
efficient
Egg 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 quality
Measure protein (AA) retention
Nitrogen retained
BV =
Nitrogen absorbed
X 100
Protein Efficiency Ratio
Used by FDA to set standards for baby food
Compares the weight gained in a growing
rat after 10 days or more eating a standard
amount of protein
Measures BV (protein retention)
Gram weight gain
PER =
Gram protein consumed
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 AA
The lowest AA score is the C.S. for that food
Chem.
Score
Mg of ess. AAn per gm of protein
=
Required mg needs of the ess. AAn per gm of protein
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 Malnutrition
Marasmus: disease of starvation

Seen in hospitalized patients
Kwashiorkor

Protein Malnutrition
Kwashiorkor
Low protein density
diet
Energy needs are
marginally met
Signs 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 death
Insufficient protein,
energy, nutrients
“skin and bones”
appearance
Little or no
subcutaneous fat
Reduce brain growth