Proteins
Iva Klimešová
• Like other biological macromolecules such
as polysaccharides and nucleic acids,
proteins are essential parts of all living
organisms and participate in every
process within cells.
• The protein of the body tissues, particularly the
muscle tissue, is the large reservoir of energy but
is not used under normal circumstances.
• The building blocks of human proteins are 20 amino
acids that may be consumed from both plant and
animal sources.
• Of these 20 amino acids, 8 are considered to be
essential because their carbon skeletons cannot be
synthesized by human enzymes.
• The remaining 12 that can be manufactured by the
body are termed nonesential. This doesn't mean
that they are unimportant, but simply that the body
can synthesize them from ingested protein and
nonprotein nutrients.
Classification of Amino Acids
Essential Amino Acids
Isoleucine
Leucine
Valine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Nonessential Amino Acids
Alanine
Arginine*
Aspartic acid
Cysteine*
Cystine
Glutamic acid
Glutamine*
Glycine
Proline
Serine
Tyrosine
*These amino acids, along with taurine, may be considered conditionally essential
in that their requirements are increased during periods of catabolic stress.
• Amino acids are joined together by peptide
bonds,
– the joining of two amino acids produces a dipeptide
(three a tripeptide)
– configuration of up to as many as 11-100 amino acids
is known as a polypeptide
– more than 100 amino acids produces proteins
The condensation of two amino acids to form a peptide bond:
• Proteins differ chemically from carbohydrates
and lipids in that they contain nitrogen in
addition to other elements such a sulfur,
phosphorus, and iron.
• Protein is the basic structural material of all cells.
• Biologically active proteins include enzymes,
immunoglobulins, hormones, neurotransmitters,
nutrient transport and storage compounds, and cell
membrane receptors.
• Many proteins are enzymes that catalyze biochemical
reactions, and are vital to metabolism.
• Biological value of a dietary protein is determined by
the amount and proportion of essential amino acids
it provides.
• If any one of the essential amino acids is not available in
sufficient amounts or is present in excessive amounts
relative to other essential amino acids, protein synthesis
will not be supported!
• Egg protein has been found to have the highest
known biological value of any protein in humans.
• Protein from animal sources (meat, fish, dairy
products, egg white) is considered high
biological value protein or a "complete"
protein because all essential amino acids are
present in these proteins.
• Plant sources of protein (grains, legumes, nuts,
and seeds) generally do not contain sufficient
amounts of one or more of the essential amino
acids.
• Plants that are entirely lacking in essential amino
acids are considered incomplete proteins or
sources of low biological value protein.
The recommended Dietary Allowance
• The RDA of protein intake for adults is
0.8 g per kilogram of body mass.
• For children, a higher intake of protein is
recommended 2 to 4 g/kg of body mass.
• Also, protein intake should be increased by
10 g a day for pregnant women and
20 g daily for nursing mother.
• The protein requirements tends to
decrease somewhat with age;
• conversely, stress, disease, injury, and
prolonged heat exposure increase the
protein requirement
Vegetarian approach
• Grains and legumes are excellent proteins
sources, but neither provides the full
complement of essential amino acids (AA).
• Grains lack the essential AA d, while legumes
contain lysine but lack of the sulfur-containing
essential AA methionine for which grains are rich
sources.
• However, because large quantities of those
foods must be eaten to obtain the requirement
quantity of essential AA, most people eat some
animal food.
• Evidence from a few large cohort studies suggests
that vegetarians have lower overall mortality
ratios than the general population, but this is not
the case when vegetarians are compared with
similar non-vegetarian groups who follow a healthconscious lifestyle.
• Vegetarianism has been associated with a
reduction in several of the established risk factors
for coronary heart disease (CHD), including more
favourable blood lipid profile, lower body mass
index and lower blood pressure.
There are hundreds if not thousands of ways to be vegetarian.
There are four major types of lifestyles and eating habits that vegetarians
may include in a diet:
Type
Prohibited Foods
Typical Health Practices
Lacto vegetarian
Meat, fish, poultry,
eggs
These will include dairy products as part of their
meals. Some use vitamin and mineral
products; others do not.
Lacto-ovovegetarian
Meat, fish, poultry
This is the major form of vegetarian lifestyle that
many take part in. These include eggs as well
as dairy products. More likely than
omnivores to be physically active and not
smoke or use alcohol.
Semivegetarian
Links to conventional health care system tend to
Meat, but may
be stronger than those of vegans.
include small
amounts of fish
and poultry in the
diet
Vegan
Meat, fish, poultry,
eggs, dairy,
honey
Vegans tend to be physically active, avoid drugs
and tobacco products, and rely on
unconventional rather than conventional
health care.
Exceptions include macrobiotics who often smoke
cigarettes and Rastafarians who smoke
marijuana.
Possible benefits of vegetarian eating:
Characteristic
Comments/Possible Mechanisms
Leanness
Vegetarians tend to be more physically active than nonvegetarians.
Higher intakes of dietary fiber may decrease absorption of food
by 2-3 % and contribute to a feeling of fullness.
Lower blood
pressure
Vegans, who consume a diet very low in fat, tend to have blood
pressures 10 to15 mm Hg lower than nonvegetarians of similar
age and gender. Much of this effect appears to be related to body
weight rather than other dietary variables.
Lower serum
cholesterol
Total blood cholesterol levels are lower in vegans than in
lactovegetarians or nonvegetarians. Whole-fat milk products and
eggs tend to raise serum blood lipids due to their saturated fat and
cholesterol content. Vegetarians often use non- or lowfat milk, and
vegans use no milk or eggs at all
Less colon
cancer
Diets high in meat may increase the incidence of colon cancer by
increasing the fecal concentration of various carcinogens. A high
intake of animal fat also may increase the risk of colon cancer. It is
also possible that carcinogens are produced by cooking meat at
very high temperatures.
Possible risks of vegetarian eating:
The major source that may be lacking in this type of lifestyle is protein. Because of
this, it is important to make sure that protein is eaten through grains, beans, tofu,
nuts, eggs or peas.
The second nutrient that is needed in a vegetarian diet is calcium. If you eat dairy,
this will be easier to consume. However, if not, you can find calcium in darker
greens. Vitamin D, Iron and Vitamin B 12 are the other three nutrients that may be
lacking in this diet.
Risk
Comments
Osteoporosis
There is little evidence that a vegetarian diet causes
osteoporosis.
Iron-deficiency
anemia
Low serum ferritin levels (a sensitive measure of iron storage
status) were found in 5% of male and 27% of female lactoovo-vegetarians
Anemia
Signs of vitamin B12 deficiency have been observed in some
breast-fed infants of women who are strict vegetarians.
Slowed growth
Excessive leanness and/or slow growth are have been noted
among vegan and vegetarian infants.
Meat in the Diet
• Meat makes an important contribution to
intake of a number of nutrients, in particular
micronutrients such as iron and zinc.
• Iron deficiency anaemia is one of the most common
nutritional deficiencies in the westernized society,
particularly in young children and women of childbearing
age.
• When red meat is included in the diet, it should be lean,
and poultry should be consumed without skin. Lower fat
versions of traditional meat products such are better
choices than the standard versions, which may be higher
in fat.
• Meat products that include high fat ingredients, such as
pastry, should not be consumed in excess.
• If meat is eliminated from the diet, a small
increase in oil-rich fish such as mackerel,
salmon, sardines or trout, would compensate for
any deficit in intake of long chain fatty acids and
vitamins D and B12.
• Care should be taken, however, to ensure
adequate intakes of iron and zinc. Inclusion of
legumes, nuts and fortified cereals will help
contribute to intake of these
minerals for those who choose
to avoid animal products.
Meat, poultry and fish are rich sources of high biological value protein.
Plant sources of protein (legumes, nuts, and seeds) contribute additional
amounts of protein.
Food
Protein (grams)
DAIRY
Skim milk, 1 cup
8.3
Whole milk, 1 cup
8.0
Ice cream, 1 cup
5.0
Yogurt,low-fat,1cup
10.7
Cottage cheese,1cup
28.0
American cheese,1oz
7.0
Egg, 1 large
6.3
FISH, MEAT & POULTRY
Tuna, 3 oz drained
21.7
Salmon, 3 oz ckd
16.8
Ground beef, 3 oz
25.7
Beef, 3 oz ckd
27.0
Pork chop, 3 oz ckd
24.5
Ham, 1 oz
5.9
Chicken breast, 3 oz
18.9
Chicken,dark meat,3oz
23.6
Turkey breast, 3 oz
25.7
Turkey,dark meat,3oz
24.3
Food
Protein(grams)
MEAT SUBSTITUES
Tofu, 3 oz
Peanut butter, 2Tbl
Almonds, 1 oz
Sesame seeds, 1 oz
Black beans, 1/2 cup
Pinto beans, 1/2 cup
Garbanzo beans, 1/2 cup
FRUITS
Banana, 1 medium
Apple, large
Orange, large
VEGETABLES
Corn, ckd, 1/2 cup
Carrots, ckd, 1/2 cup
Green beans, ckd, 1/2 cup
Green peas, ckd, 1/2 cup
Potatoes, white, 1/2 cup
6.9
8.1
5.4
7.5
7.5
7.0
7.3
1.2
0
1.7
2.2
0.8
1.0
4.1
1.2
• Vegetarianism based on sound nutrition
principles can be a healthful choice, but neither
vegetarians nor omnivores have a monopoly on
healthful eating.
• Vegetarians are just as diverse in their health
status as are nonvegetarians. Similar health
benefits can be gained from both well-selected
omnivorous and vegetarian diets.
Digestion and absorption
• Through the process of digestion, animals break down ingested protein
into free amino acids that can be used for protein synthesis.
• The digestion of protein begins in the stomach by the action of the
enzyme pepsin.
• Pepsin is particularly effective in the highly acid stomach medium
(hydrochloric acid) in breaking down the collagenous connective
tissue fibers in meat.
• The hydrochloric acid:
–
–
–
–
activates pepsin,
helps to keep stomach free from bacteria,
improves absorption of the mineral iron and calcium,
inactivates plant and animal hormones.
• Stomach enzymes and acids attack the long, complex protein. They
are degraded to smaler units called polypeptides and peptides.
• The peptide fragments are dismantled in the small intestine by
alkaline enzymes released from the pancreas.
• The smaller protein fragments are then absorbed and transported to
the liver for further assimilation.
•
When amino acids reach the liver, one of
three things happens. They are:
1. converted to glucose,
2. converted to fats, or
3. released directly into bloodstream as plasma
protein such as albumin, or as free amino
acids to serve the anabolic requirements of
various tissues.
• Dietary protein consumed in excess of
requirements is not stored, but is deaminated, or
its storage as glycogen or fat, depending upon
the specific amino acid and the energy balance
at the time. The nitrogen waste generated is
excreted in the urine as either urea or
ammonia.
• High protein intakes can increase urinary
calcium excretion.
• Other health effects of high protein intakes are
less clear including the relationship of long-term
high protein intakes to risk of renal disease or of
diabetic nephropathy.
Summary
• Experts recommend that protein intake make up only
10-20 % of our daily calorie intake.
• Proteins are made of smaller units called amino acids.
• Animal proteins, found in such food as eggs, milk, meat,
fish, and poultry, are considered complete proteins
because they contain all of the essential amino acids
our bodies need.
• Plant proteins, found in vegetables, grains, and beans,
lack one or more of the essential amino acids.