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TOPIC 6:
PROTEINS: CRUCIAL
COMPONENTS OF
ALL BODY TISSUES

What Are Proteins?
■
–
–
–

Proteins: large, complex molecules found in tissues of all living things
Dictated by genetic material (DNA)
Contain carbon, hydrogen, oxygen, nitrogen
Made from 20 different amino acids

How proteins differ from starch

Amino Acids
■ Nine essential amino acids
– Cannot be produced by the body in
sufficient quantities to meet
physiological needs
– Must be obtained from food
■ Nonessential amino acids
– Can be synthesized by the body in
sufficient quantities
– do not need to be consumed regularly
in our diet

The Body Can Make Nonessential AA
■ Transamination
– Our bodies can make nonessential amino acids by transferring the amine group
from an essential amino acid to a different acid group and side chain (R group)
...


The Body Can Make Nonessential
Amino Acids
■ Conditionally essential amino acid
– Nonessential amino acid becomes essential under certain circumstances
...

-

Someone with PKU cannot metabolize phenylalanine (an essential amino acid)
...


-

If PKU is not diagnosed immediately after birth, it results in irreversible brain
damage
...


How Are Proteins Made?
■ Proteins are long chains of amino acids
■ Peptide bonds join amino acids together forming dipeptides, tripeptides,
oligopeptides, and polypeptides
■ As a polypeptide chain grows longer, it begins to fold into any of a variety of complex
shapes that give proteins their sophisticated structure
...


-

DNA: is a complex compound made up of molecules called nucleotides,
each of which consists of a deoxyribose sugar and phosphate backbone
and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C),
or thymine (T)
...


How Are Proteins Made? (cont
...


■ Translation: The process that occurs when the genetic information carried by
messenger RNA is translated into a chain of amino acids at the ribosome

Protein turnover
■ Protein turnover : involves the synthesis of new proteins and the breakdown of
existing proteins to provide building blocks for new proteins
...

– Urea is produced as a waste product from any excess nitrogen, which is then
excreted by the kidneys
...
In contrast, the proteins that form red blood cells are globular in shape
...

■
–
–
–

Protein function is lost when a protein is denatured:
High fever
Blood pH out of normal range
During digestion

Protein in the Diet
■ For protein synthesis, all essential amino acids must be available to the cell
■ Limiting amino acid
– Essential amino acid that is missing or in the smallest supply
– Slows down or halts protein synthesis
■ Inadequate energy consumption also limits protein synthesis

-

If there is not enough energy available from our diets, our bodies will use any
accessible amino acids for energy, thus preventing them from being used to build
new proteins
...
)
■ Incomplete protein (low quality): insufficient essential amino acids
– Does not support growth and health

■ Complete protein (high quality): sufficient amounts of all nine essential amino acids
– Derived from animal and soy protein

Protein in the Diet (cont
...


■ The two foods involved in this process are called complementary proteins
...
)
■ Digestion continues in the small intestine
■ Proteases: enzymes that continue the breakdown of polypeptides in the small
intestine included:
-

Trypsin

-

Chymotrypsin

-

Carboxypeptidase

■ The cells in the wall of the small intestine then absorb the single amino acids, dipeptides, and tripeptides
...


Protein Digestion (cont
...


Protein Quality
Methods for estimating protein quality

■ Chemical score: A method used to estimate a food’s protein quality; it is a
comparison of the amount of the limiting amino acid in a food to the amount of the
same amino acid in a reference food
...
Proteins with higher digestibility are more complete
...

- Legumes are also highly digestible (about 70% to 80%)
...


Functions of Proteins
■

Proteins serve many important functions:

-

Enabling growth, repair, and maintenance of body tissues

-

Acting as enzymes and hormones

-

Maintaining fluid and electrolyte balance

-

Maintaining acid–base balance

-

Making antibodies, which strengthen the immune system

-

Providing energy when carbohydrate and fat intake are inadequate

-

Transporting and storing nutrients

-

Producing compounds such as neurotransmitters, fibrin, and collagen
...


Protein Adequacy
■ Nitrogen balance determines protein needs
– Positive nitrogen balance: consuming more nitrogen than is excreted
Growth
Pregnancy
Recovery from illness or protein deficiency
– Negative nitrogen balance: excreting more nitrogen than is consumed
Starvation or low-energy diets
– In nitrogen balance: intake equals excretion

Protein Adequacy

RDA for Protein
■ RDA for sedentary people is 0
...


■ Most Americans meet or exceed the RDA for protein

Too Much Protein Can Be Harmful
■ Heart disease
– High protein diets of mostly animal sources are associated with higher blood
cholesterol levels
– Consuming proteins low in saturated fat may help

Too Much Protein Can Be Harmful
■ Kidney disease
– High protein diets can increase risk of kidney diseases in susceptible people
– Maximum of 2 g of protein per kilogram body weight each day is safe for healthy
people
– Important to drink more water with higher protein intake

...
Fluid needed to flush excess urea from kidneys

Protein Sources
■ Meats
■ Dairy products
■ Eggs
■ Legumes (including soy)
■ Whole grains
■ Nuts
■ "New foods" (quinoa which provides 8 g of protein in a 1-cup serving
...
As a result, phenylalanine and its
metabolic by- products build up in tissues and can cause brain damage
...

■ Sickle cell anemia: A genetic disorder that causes red blood cells to be shaped like a
sickle or crescent
...

■ Cystic fibrosis: A genetic disorder that causes an alteration in chloride transport,
leading to the production of thick, sticky mucus that causes life- threatening
respiratory and digestive problems

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