Chapter Five
The Proteins
& Amino
Acids
I. What Proteins Are Made of
 Amino
Acids
Amine Group
– Acid Group
– Side Group
–
 All
protein molecules contain
nitrogen atoms.
A. Essential & Nonessential
Amino Acids



Nine amino acids
Cannot be made
(synthesized) by
the body from other
amino acids
Protein foods must
be eaten daily that
contain these
amino acids


11 amino acids
Can be made from
other parts
– Nitrogen (other
proteins)
– Backbone
(carbohydrates
and fats)
Cont’d


Some amino acids
are considered
essential at
different stages of
life or in states of
illness.
“Conditionally
essential”
B. Synthesis of Proteins
 Peptide
bond
 Between amino end &
acid end
B. Synthesis of Proteins
 Different
places along the
chain of amino acids & side
chains may be attracted to
others
B. Synthesis of Proteins
 Strand
coils & folds back on
itself, giving the protein a
distinct shape.
 Shapes of proteins affect how
they are used in the body
C. Denaturation of Proteins
First step in its destruction
 Excess heat, alcohol, acids or salts of
heavy metals can damage the body’s
proteins
 Stomach acid opens up the protein’s
structure and permits digestive
enzymes to act upon the protein.

II. The Functions of Proteins
A.
Growth & Maintenance
 Dietary protein ensures that amino
acids are available to build the
proteins needed for new tissue.
 Nearly all cells are constantly
replaced, requiring protein.
B. Enzymes
 Proteins
such as enzymes are
catalysts that help chemical
reactions take place.
 Each enzyme is specific for a
particular reaction.
C. Hormones
 Some
hormones, but not all, are
proteins
 Hormones signal the appropriate
enzymes to act.
D. Antibodies
Formed in response to foreign
substances in the body.
 Each antibody is specific to one
foreign substance.
 Once an antibody is made for the
substance, the body develops
immunity to that substance.

E. Fluid Balance
Fluid Balance:
 Proteins help regulate the quantity of
fluids to help maintain fluid balance.
 Cells and the spaces between cells
must contain a constant flux of and
amount of fluid.
 Water can diffuse freely in and out of a
cell; proteins can not
F. Acid-Base Balance
Normal body processes produce acids
& bases that must be excreted
 pH: concentration of hydrogen ions
 Acids: release hydrogen ions
 Bases: accept hydrogen ions
 Acid-Base Balance: equilibrium
between acids and bases in body

Cont’d

Blood proteins act as buffers for acidbase balance.
– Buffers: can give up or add extra
hydrogen to maintain balance

Acidosis or alkalosis may occur if
acid-base balance is upset
G. Transport Proteins

Move nutrients and other molecules in and
out of cells
–
–

Turn on and off
Hormones do the switching
Move substances from one organ to
another
–
–
Lipoproteins
Vitamins and minerals
G. Energy



Protein may be sacrificed to provide energy
if insufficient carbohydrate and fat are
eaten.
Amino acids are degraded for energy.
Amino acids will only make proteins if
carbs and fat are providing protein-sparing
energy.
– Protein-sparing: Leave amino acids alone
to make proteins
III. Digestion & Absorption of
Protein
Begins in the stomach with
denaturation
 Breaks into dipeptides, tripeptides
and free amino acids into small
intestine

HOW BODY HANDLES PROTEIN




Amino acids taken up by cells
Protein synthesis takes place
If “non-essential” amino acid is missing, the
body can make it
If “essential” amino acid is missing, the protein
synthesis stops and all of the amino acids in
place are dismantled and the protein is not
made
IV. Protein Quality



A measure of the essential amino
acid content of a protein relative to
the essential amino acid needs of the
body
Biological value (BV) – how well
supports nitrogen balance
Reference protein – egg white
protein, the standard to which other
proteins are compared = 100 score
Complete and Incomplete
Proteins



Complete protein:
contains all the
essential amino acids
in the right proportion
relative to need
Meat, dairy products,
and many soybean
products
High Biological Value



Incomplete protein:
lacking or low in one or
more of the essential
amino acids
Most plant based
proteins
Low Biological Value
Complimentary
Proteins

Two or more food proteins whose
amino acid assortments
complement each other in such a
way that the essential amino
acids limited in or missing from
each are supplied by the others.
V. Recommended Protein
Intakes

Covers the need to
replace proteincontaining tissue
that people lose
and wear out daily
TWO WAYS TO ESTIMATE




The RDA for the
healthy adult is 0.8
grams of protein per
kilogram of body
weight
Use desirable weight
220 pounds = 100 kg
100 kg x 0.8 = 80 grams





Percentage of total
calories
Dietary Goals
12-15 % of total
calories
2500 calories x .12 to
.15 = 300 – 375 calories
75 – 94 grams
VI. Protein & Health
A. Too Little Protein
Protein-Energy Malnutrition (PEM)
 The world’s main form of malnutrition
1. Kwashiorkor: a deficiency disease
caused by inadequate protein in the
presence of adequate food energy
2. Marasmus: an energy-deficiency
disease; starvation
B. Too Much Protein
No benefits to eating too much
protein
 Risks associated with
overconsumption
 Many protein-rich animal sources
are high in fat
 Zinc loss is observed in animal
studies

Cont’d
If amino acids are in excess of needs,
the body will remove the amine group
and convert the residues to glucose
and glycogen or to fat.
 High protein intake also increased
excretion of calcium
 Extra stress on kidneys

C. Choosing Protein Wisely



More is not necessarily better
Select 1/3 or less of protein from animal
sources
Include legumes:
–
–

Garbanzo beans, great northern beans, kidney
beans
Lentils, lima beans, pinto beans, split peas, white
navy beans, soybeans
High quality protein, fat free, high in fiber