Protein
Chapter 17
What is Protein?
Protein molecules are very large and
complex. Made up of:
– Hydrogen, carbon, oxygen ,nitrogen and
sometimes other elements
– Because of protein large size, protein molecules
are often called macromolecule
Macromolecule = large molecule containing
many atoms.
– Nitrogen is a crucial part of protein
Nitrogen is the site of the chemical bond that
gives proteins their variety and versatility
Structure of Protein
Protein is made of chains of amino
acids.
– Amino Acids = type of organic acid.
– Organic acids are molecules that contain a
carboxyl group, plus the amino acids also contain
an amine group.
– Amine group = composed of two atoms of
hydrogen and one atom of nitrogen.
– (Look on page 258, figure 17-1, both carboxyl
group and the amine group are attached to a
central carbon.)
– With four bonds needed and one is left open, this
is what makes one amino acid different from
another.
Peptide Bonds
Peptide bonds = bonds between
the nitrogen of one amino acid and
the carbon of a second amino acid.
(example is on page 258, figure 17-3)
Through peptide bonds, amino acids chain
together, creating a polypeptide.
Polypeptide = single protein molecule
containing then or more amino acids linked
in peptide chains
– Peptide bonds hold chains of amino acids
together, that are called helixes (ex. page 259,
figure 17-4)
Protein Structure and Function
Protein’s shape determine its
function
Protein molecules that form rope-like fibers
are called fibrous protein ex is collagen and
elastin
Protein molecules that can be compared to a
ball of steel wool are called globular proteins
– The shape makes them convenient carriers ex
hemoglobin (transports oxygen in the blood)
Denaturation of Protein
Denaturation = this process changes the
shape of a protein molecule without
breaking its peptide bonds
As the molecule unfolds, some of the protein’s
original properties are diminished or lost.
This change is unique to protein and because each
protein is unique the process varies from one
molecule to the next.
Denaturation in the first step in the process of
coagulation
Coagulation = changes a liquid into a soft, semisolid
clot or solid mass. (look pg. 260, figure 17-6)
– Ex is scrambled eggs. Beating the egg denatures its
protein. The protein coagulates as the egg cooks.
Other Means of Denaturation
Heat is the most common agent in
denaturing protein.
The temperature is significant, the degree of
denaturation increase 600 times for every 10°C.
Protein molecules may unfold in reaction
to the following:
Freezing, pressure, and sound waves
Mechanical treatment, such as beating eggs and
kneading bread
Very high or very low pH. (adding lemon juice for
example can sour milk, causing the milk to separate
from the liquid – forming curds and whey)
Certain metal ions
Protein in Eggs
Eggs
Complex biological system, containing almost every
vitamin and mineral you need. All that lacks is vit. C
and calcium which is found in the egg shell
Look on pg. 262, figure 17-7 for a diagram of an egg
– Albumen = egg white – substance makes up 54% of
the inner contents of an egg
– Chalaza = a twisted, ropelike structure that keeps the
egg yolk centered
– Egg Yolk – main component of the yolk include the
globular protein livetin and both high-and-low-density
lipoproteins
Storage
– Do not exposed eggs to light and temperature changes
Eggs lose quality as compounds in the egg white
break down, forming water
Protein in Meat
Muscle tissue, the lean part of meat,
is about 15-20% protein
– Other components include: water, fat
and minerals
– Muscle is composed of fibrous proteins
called actin and myosin. These proteins
form bundles of fiber, which are held
together by connective tissue made of
collagen and elastin
Protein in Fish, Nuts and Legumes
Fish
– Shorter segmented muscle fibers are layered
between thin sheets of connective tissue. This
is why some books say fish is cooked when
you can “flake with a fork”
– Far less connective tissue than meat, it’s also a
type that liquefies easily
Nuts and Legumes
– Cholesterol free, but fat content ranges
– Soybeans are excellent source of protein,
they’re composed of 40% protein, high in fiber
and polyunsaturated fatty acids. Soybeans
have also been associated with reducing the
risk of coronary heart disease
Protein in Cooking
Emulsifiers
Certain proteins contain amino acids that our polar.
One end is attracted to water (forming a hydrogen
bond), the other end avoids water and bonds with oil.
This is why an egg can emulsify oil and lemon juice
Foams
Foam = air bubbles incorporated and trapped in a
protein film by whipping
– Foam begins when a protein-containing liquid (egg
whites or cream) are whipped, introducing air and
denatures the protein molecules
– Fats can be trouble for foams, they keep protein
molecules from bonding with each other, making the
end product watery
Protein in Cooking Continued
Gelatin
Used to set desserts and thicken meat sauces in
animal protein
Made by using heat and water to hydrolyze the
collagen on the inner layer of hides and bones
Gelatin can bind 100 times its weigh in water
Gluten
Gluten = an elastic substance formed by mixing
water with the proteins found in wheat
Gluten includes both fibrous and globular proteins,
making the reactions between the water and proteins
both stretch and springy
Gluten is developed as dough is kneaded, denaturing
the protein molecules
Protein in the Body
During digestion, proteins are denatured by
hydrochloric acid in the stomach, making the
peptide bonds easier for enzymes to break.
Most proteins enter the bloodstream as amino
acids (the body creates ALL of its proteins from
20 different amino acids)
Functions of Body Proteins
– Structural protein is needed by every cell in the body
– New growth requires a continuous supply of protein to
replace and repair cells
– Some proteins pick up, deliver and stores nutrients in
cells
– Proteins called antibodies help you ward off disease
Antibodies = very large proteins that weaken or destroy
foreign substances in the body.
Essential Amino Acids
Of the 22 amino acids the body
needs to build protein, it
manufactures 18 the other 9 needs
to be digested. Look on pg 269, figure 17-8
– Complete Protein = a protein that contains
all the essential amino acids ex. ground beef
– Incomplete protein = lacking one or more
essential amino acids, among plants only
soybeans provide complete protein
High-quality Protein = contains all the essential
amino acids in proportion to the body’s need for them
Protein Allowances
The RDA for a healthy adult is 0.8g of high-
quality protein per kilogram of ideal body
mass (take your weight in kg and multiply
by 0.8 gives you the amount of protein
needed in a day (young males under 18
multiply by 0.9)
RDA for children ages 11-14 is higher 1.0g
of protein per kg of body mass
Most people in the US get enough protein
Look on page 270 figure 17-9 for a list of
protein contents in certain food choices.
Questions
1. Why are amine groups important to
proteins?
2. Compare peptide bonds and hydrogen
bonds in protein formation.
3. Describe the two basic protein shapes.
What is the main function of each?
4. What occurs during Denaturation?
5. Can proteins coagulate without
denaturation? Why or why not?
6. How does the egg yolk compare in
composition to the egg white?
Questions Continued
7. A friend refrigerates eggs still in their carton
immediately after returning form the
supermarket. Explain whether this is a good
idea.
8. Compare the protein structure of meat to that
in fish.
9. Suppose your egg whites aren’t making a very
impressive foam. What might be some
reasons?
10. Why is kneading the dough an important step
in making bread?
11. Why might injury and illness be more serious
for someone with a long-term protein
deficiency?
12. Should you be concerned if your diet lacks just
one essential amino acid? Explain.
13. Can a diet of fruits, vegetables, and grains
supply all of the essential amino acids?
Explain.