PROTEINS AND AMINO ACIDS

Question 1

In 1838 when Dutch chemist __________first
identified protein as a substance in all living
things, it is unlikely he realized the importance of
his discovery. Proteins shape our lives. Protein
enzymes break down our food into nutrients the
cells can use. As antibodies they shield us from
disease. Peptide hormones carry messages that
coordinate continuous body activity. They guide
our growth in childhood and maintain our bodies
thereafter; they make us each unique.

Answer 1

Johann Mulder

Question 2

The structural units of proteins—the _______—
are the working currency of protein in body cells.
Amino acids contain carbon, hydrogen, and oxygen
the same three elements that make up
carbohydrates and fats; however, amino acids and
their proteins have the additional element nitrogen.
Protein is about 16% nitrogen. Several amino acids
also contain sulfur.

Answer 2

amino acids

Question 3

The term _____ ______ tells us they have a dual
nature. The word amino refers to a base or alkaline
substance, so at once we have a contradiction. How
can a chemical substance be both a base and an
acid, and why is this important? Consider the
significance of this fact as we learn more about
amino acids and their roles in the body.

Answer 3

amino acid

Question 4

is found in all acids, and in solution carries a negative charge.

Answer 4

carbonyl group

Question 5

contains the essential element nitrogen and in solution carries a positive
charge.

Answer 5

amino group

Question 6

An attached radical (R) group: The R stands for radical, a general term referring to a group of elements attached to a
chemical compound. In this case it refers to the attached side chains on amino acids; each one is different.
The distinctive side chain on an amino acid gives it a

Answer 6

unique size, shape, and set of properties.

Question 7

Compare the structure of the two simplest amino acids—glycine and alanine with the larger and more complex amino
acid arginine with its extended carbon chain (R) and three additional amino groups. Twenty different amino acids are
used to build body proteins. Each has the same core pattern but a specific and different side group.
The dual chemical structure of amino acids, including both acid and base groups, gives them a unique amphoteric
nature, meaning that an amino acid can behave as either an acid or a base, depending on the pH of the solution in
which it is found. This makes it possible for amino acids to act as buffers, important in clinical care.

Answer 7

glycine and alanine

Question 8

Of the 20 amino acids used to build body proteins, nine cannot
be synthesized by the body and must be supplied in food. These
nine amino acids are designated as indispensable (essential)
amino acids.
* Another five of the 20 can be synthesized in the amounts
needed and are termed dispensable (nonessential) amino acids.
The remaining six fall in between and are known as conditionally
indispensable.
* Although the body is able to synthesize the conditionally
indispensable amino acids, it cannot meet the demand when
tissue needs are elevated or the supply of necessary precursors
is inadequate. Arginine is such an amino acid. The quantity that
can be produced in the liver is not sufficient to meet the needs
of the newborn, so additional amounts must be supplied in
food. The concept of dietary essentiality for the indispensable
and conditionally indispensable amino acids is important when
assessing protein quality.

Answer 8

Essential Amino Acids

Question 9

The dual chemical nature of amino acids, with a base group on one
end and an acid group on the other, enables them to form the
unique chain structure found in all proteins. The end amino group
of one amino acid joins with the end carboxyl group of the amino
acid next to it. This joining of amino acids is called a peptide bond.
Specific amino acids are joined in a particular sequence to form
long chains called polypeptides, and specific polypeptides come
together to form proteins. Polypeptides vary in length from
relatively short chains of 3 to 15 amino acids called oligopeptides
to medium- sized polypeptides with chains of 21 to 30 amino acids
such as insulin. Larger still are complex proteins made up of several
hundred amino acids.
* To build a compact structure, long polypeptide chains coil or fold
back in a spiral shape called a helix. Other proteins form a pleated
sheet held together by strengthening cross-links of sulfur and
hydrogen bonds. Learning more about the structure of body
proteins helps medical researchers develop effective medications
and understand how genes influence disease risk.

Answer 9

Peptide Bond

Question 10

• This fibrous protein found in muscle is built from chains of 153
  amino acids that coil and unfold as needed. Shaped into long
  rods, these fibers end in two- headed bundles so that they can
  change shape and bend, making it possible to tighten and
  contract muscles and then relax them.

Answer 10

Myosin

Question 11

• This structural protein contains three separate polypeptide
  chains that wind around each other to produce a triple helix.
  Thus reinforced, collagen is shaped into long rods and bundled
  into stiff fibers to do its job of strengthening bone, cartilage,
  and skin to maintain body form.

Answer 11

Collagen

Question 12

This globular protein includes four globin poly- peptide
chains per molecule of hemoglobin. Each chain has
several hundred amino acids conjugated with a
nonprotein, the iron-containing pigment called heme. The
globin wraps around the heme and forms protective
pockets to secure the iron. The iron in heme has a special
ability to bind oxygen, and as part of the red blood cell
delivers oxygen to the tissues and returns carbon dioxide
for excretion via the lungs.

Answer 12

Hemoglobin

Question 13

is the major plasma protein and has a compact
globular shape. It consists of a single polypeptide chain of
584 amino acids, twisted and coiled into helix structures
held together by disulfide bridges. Albumin serves as a
carrier protein for drugs, hormones, enzymes, and trace
elements. It helps maintain fluid balance by exerting
colloidal pressure in the capillaries that forces the flow of
nutrients and fluids into the cells and the return of fluid and
waste products out of the cells. In serious illness, albumin is
broken down to supply amino acids for the synthesis of new
proteins to meet the body emergency.

Answer 13

Albumin

Question 14

• Other proteins with special immune
  system and the blood protein fibrinogen,
  important in blood clotting. Hormones
  such as insulin and thyroxin and the
  enzymes that regulate our day-to-day
  metabolic activities and produce energy
  for work are proteins. structural or
  metabolic roles include the antibodies of
  the

Answer 14

Proteins with Special Roles

Question 15

Dietary protein supplies the amino acid building blocks for the growth and maintenance of body tissues across the life
cycle. It must furnish amino acids in the appropriate patterns and amounts for efficient synthesis of specific structural
molecules. Although we often consider dietary protein to be most important during growth and development when new
tissues are being formed, protein needs are also important for maintenance and well-being after the growth potential has
been realized.

Answer 15

Growth, Tissue Building, and Maintenance

Question 16

Every cell in the human body contains protein so a constant supply is needed to
support the expansion of body tissue as an individual grows from a 7-lb infant to a 160-lb man. Protein is especially
important to support the rapid growth occurring in the first years of life when protein deficits can have lifelong effects.
Protein needs escalate approaching the adolescent growth spurt as bone matrix and muscle tissues expand rapidly.
Good quality protein is needed to support fetal growth and the production of milk.

Answer 16

Infancy, childhood, and adolescence:

Question 17

Following the attainment of full growth and maturity, protein requirements reach a steady state
based on replacing lost cells and worn-out protein molecules. Loss of cells from the skin and gastrointestinal tract; new
cells formed in growth of hair and nails; and formation of enzymes, hormones, and other control proteins constitute
daily protein needs. Individuals who participate in strength training and muscle growth require additional protein.
Protein requirements demand special attention following recovery from serious illness when depleted protein stores
must be replenished.

Answer 17

Adult maintenance:

Question 18

Methionine assists in the formation of choline, a
precursor of acetylcholine, and tyrosine is used to synthesize the neurotransmitters dopamine and epinephrine;
tryptophan is the precursor of the neurotransmitter serotonin. Age-related decreases in the neurotransmitter
dopamine are associated with Parkinson’s disease, with muscle tremors and rigidity.

Answer 18

• Form neurotransmitters for brain and nerve function5:

Question 19

Methionine is the precursor of the conditionally indispensable amino acid cysteine, as
well as carnitine and taurine; carnitine transports long-chain fatty acids into the mitochondria for energy
production, and taurine, found in bile salts, also regulates fluid pressure in the eyes.

Answer 19

*Form other amino acids:

Question 20

Phenylalanine is the precursor of the conditionally indispensable amino acid tyrosine needed to
make thyroxin and epinephrine.

Answer 20

*Form hormones:

Question 21

Protein is needed to make anti- bodies for the immune system. Protein molecules
referred to as cytokines participate in the acute phase response that brings about alterations in body metabolism
needed to handle the demands resulting from critical illness, burns, or trauma.

Answer 21

*Support immune function:

Question 22

Particular amino acids are beneficial in treating catabolic illness when breakdown and loss of skeletal muscle
become critical. Branched chain amino acids—leucine, isoleucine, and valine—have positive effects on protein
metabolism in cancer patients for whom malnutrition and body wasting are life threatening.
* Leucine reverses the muscle wasting associated with bed rest by stimulating protein synthesis while decreasing
protein breakdown. Although increased amounts of certain amino acids can improve patient status, this should not
give license to self-medication with over-the-counter amino acid supplements.
* Before amino acids can be burned for energy the nitrogen-containing amino group must be removed, and then the
remaining carbon skeleton is converted to either glucose or fat. In adults about 17 to 25 g of every 100 g of dietary
protein is not needed for tissue building or repair and is oxidized for energy.3 This breakdown of amino acids to a
form that can be burned for energy in itself requires energy, leading to the popularization of high-protein diets for
weight reduction.
* Adequate carbo- hydrate spares protein for its primary purpose of tissue growth and repair.

Answer 22

Role in Critical Care

Question 23

• Interdependent checks and balances keep the body
  in working order. This is true for protein ebb and
  flow, with tissues being built and broken down, and
  body materials being stored and released. These
  coordinated activities enable the body to respond
  to any situation disturbing its normal cycles.

Answer 23

Concept of Balance

Question 24

The average man contains about 11 kg of protein. Forty
percent is in the skeletal muscle with the remainder in
skin, blood, kidney, liver, brain, and other organs. Body
distribution of protein changes with growth and
development. The newborn has relatively little skeletal
muscle, with proportionately more protein in the brain
and visceral organs.

Answer 24

Protein Reserves

Question 25

In contrast to the substantial fat reserves held by most
people, body protein reserves are quite limited. What
we call the l______, meaning they are
easily broken down to meet immediate needs, make
up only about 1% of total body protein; like glycogen,
these reserves are sufficient to maintain body
functions for only a short period of time.

Answer 25

labile protein reserves

Question 26

The concept of balance as applied to ____________
refers to the steady state between protein synthesis
(anabolism) and protein breakdown (catabolism). In
periods of growth, such as infancy or childhood, or the
enlargement of muscle as a result of strength training,
protein synthesis exceeds break- down, with a net gain
of new tissue.
* When food intake is drastically reduced, as in famine or
extreme voluntary food restriction, tissue breakdown
exceeds synthesis as body protein is broken down to
yield energy, with a net loss of body protein. Protein
catabolism is accelerated during periods of critical
illness as amino acids are needed for the synthesis of
new proteins to fight infection, and other body
responses to metabolic stress cause protein breakdown
and loss of nitrogen in the urine.

Answer 26

protein balance

Question 27

is the process by which
body proteins are continuously broken down and the released amino acids made into new proteins.3New proteins
replace worn-out proteins, and different proteins are formed to meet changing needs. When an amino acid labeled
with a radioactive carbon atom is incorporated into a protein food and eaten, it can be traced; that is, we can follow
its journey through the body. By these studies we learned that amino acids are rapidly incorporated into body
proteins. Then, when these proteins are broken down, they are reused to form new proteins. Protein turn- over
varies among tissues. Turnover rates are higher in the intestinal mucosa, liver, pancreas, kidney, and plasma, tissues
that are more metabolically active, and lower in muscle, brain, and skin. Protein turnover is very slow in structural
tissues such as collagen. Total body protein turnover is higher in infants and children who are growing rapidly and
lower in older adults.

Answer 27

• Protein turnover:

Question 28

Body protein is divided between two compartments: (1) tissue protein and (2) plasma
protein. Protein from one compartment may be drawn to supply a need in the other. Muscle tissue becomes an
important source of amino acids in periods of stress or low-protein intake. Plasma proteins such as prealbumin have
rapid turnover and are early indicators of protein catabolism brought on by inflammation, surgery, cancer, or other
serious illness. Levels of plasma prealbumin can identify patients at risk of nutritional deficiency or in need of
nutrition intervention. Low plasma albumin is characteristic of children with kwashiorkor, the protein deficiency
disease seen in countries with food shortages. Although albumin is not a specific marker of protein deficiency, it
alerts the clinician to the need for comprehensive nutrition assessment. Plasma protein levels are restored to
normal when the disease or catabolic stress has abated and protein and kilocalories (kilocalories or kcal) become
available.

Answer 28

Protein compartments:

Question 29

Amino acids derived from tissue breakdown or supplied by dietary protein contribute
to a common metabolic “pool.” Amino acids from this pool are used to synthesize body proteins as needed.

Answer 29

Metabolic amino acid pool:

Question 30

is sometimes described as nitrogen
balance. Nitrogen balance involves all of the nitrogen
in the body— protein nitrogen as well as nonprotein
nitrogen in such com- pounds as urea, uric acid, and
ammonia. Nitrogen balance is the net result of
nitrogen gain and loss across all body tissues.

Answer 30

Protein balance

Question 31

occurs when body
nitrogen loss exceeds the nitrogen taken in from food
as occurs in long- term illness, hypermetabolic
wasting disease, and inadequate protein intake.

Answer 31

Negative nitrogen balance

Question 32

occurs during growth or
pregnancy or strength training as more nitrogen is
retained. Healthy persons with an adequate protein
intake are in nitrogen equilibrium with nitrogen
intake and loss about equal on a daily basis.

Answer 32

Positive nitrogen balance

Question 33

To be of use to the body, amino acids must be released from other food components and made
available for absorption. If nondigestible components in the food prevent this breakdown, then the amino acids are lost in
the feces.

Answer 33

Protein digestibility:

Question 34

All 20 amino acids needed to make body proteins must be available at the same time for new
tissues to be formed; therefore food proteins should supply each of the indispensable amino acids in the amount required.
Other sources of amino (NH2) groups in a food protein can be used

Answer 34

Amino acid composition:

Question 35

The nutritive value of a food protein is often expressed as its amino acid score, a value based on both its digestibility
and amino acid composition. The amino acid reference pattern used to evaluate proteins is quite similar to the
pattern of egg white, the reference protein often used in nutrition studies. When evaluating an amino acid score, it is
important to identify the l_________ (or acids).

Answer 35

limiting amino acid

Question 36

is any indispensable amino acid falling below the amount recommended in the amino acid
reference pattern. For protein synthesis to take place, all indispensable amino acids must be available in the required
amount. Thus the limiting amino acid “limits” or hinders the body from effectively using the other amino acids in that
protein regardless of their adequacy. When one or two limiting amino acids are identified in a food protein having
optimal levels of the other indispensable amino acids, that protein can be combined with another food protein that
will supply the amino acid or acids needed.

Answer 36

A limiting amino acid

Question 37

Plant and animal foods differ in their content of indispensable amino acids. Animal foods contain all of the
indispensable amino acids in the amounts and ratio needed to support protein synthesis and are referred to as

Answer 37

complete proteins.

Question 38

These foods include eggs, milk, cheese, meat, poultry, and fish Plant proteins vary in quality but are ___________ ______; either they supply less than the required amount of one or more indispensable amino acids, or they are
missing an indispensable amino acid However, several plant proteins eaten together or combined with a small
amount of animal protein can supply the amino acids required to support tissue growth and repair.

Answer 38

incomplete
proteins

Question 39

This concept of ______________, by which one protein helps meet the amino acid shortage in another, has
been at work in the traditional food patterns of families worldwide.
* For examples of complementary vegetable proteins. Soy protein, although lower in methionine than animal protein,
meets the amino acid needs of adults.

Answer 39

complementary proteins

Question 40

• The quality of dietary protein and its amino acid composition influence the type and amount needed. Individuals
  who consume some animal protein along with plant protein will have an adequate supply of all amino acids. Those
  who eat no animal protein should emphasize a wide variety of complementary plant proteins to ensure appropriate
  amounts of the indispensable amino acids.

Answer 40

Protein Quality

Question 41

• Food processing and food preparation affect amino acid availability. Heat applied in drying or baking bonding
  between some sugars and amino acids, forming compounds that cannot be digested. Lysine, methionine, and
  cysteine can be lost in this way. Protein digestion and amino acid absorption are influenced by the time interval
  between meals, with longer intervals lowering the competition for available enzymes and absorption sites.

Answer 41

Protein Digestibility

Question 42

Protein requirements reflect the growth patterns of infancy, childhood, and adolescence. Pregnancy and lactation
increase protein needs.

Answer 42

Tissue Growth

Question 43

When carbohydrate intake is sufficient to meet energy needs, dietary protein can be used exclusively for tissue
building. This is often referred to as the protein-sparing action of carbohydrate. Dietary carbohydrates also support
protein syn- thesis by stimulating the release of insulin, which promotes the use of amino acids for protein
synthesis.

Answer 43

Energy Content of the Diet

Question 44

Conditions and diseases that increase the rate of protein turn- over and tissue breakdown raise the protein
requirement. After trauma or surgery, amino acids are needed for wound healing and formation of new tissue, as
well as the production of infection-fighting immune factors. Serious burns with extensive tissue destruction
substantially increase protein needs. In critically ill patients protein requirements may reach 1.5 to 2.0 g/kg of body
weight or 2 to 212 times the usual recommendation. Amino acid supplementation helps limit the loss of muscle
protein that occurs with immobilization. Muscle protein synthesis is influenced not only by the amount of protein
available but also by the time frame in which the protein is consumed. Individuals undergoing resistance training
can benefit from eating some high-quality protein during or immediately following the activity.

Answer 44

5. Health Status

Question 45

Protein intake differs widely across population groups, with some getting far more and others far less than they
need. Protein-energy malnutrition (PEM) is a major health problem in many developing countries where protein
intakes are low in both quantity and quality. More than 6 million children die each year from protein-related
deficiencies.

• PEM also contributes to the morbidity and mortality associated with infectious diseases such as malaria, respiratory
  tract infections, and gastroenteritis. Based on more than 13,000 hospital admissions over a period of 5 years in rural
  Africa, malnutrition was estimated to be the underlying cause of half of the inpatient illness and deaths of young
  children.

Answer 45

Protein-Energy Malnutrition

Question 46

• We often associate low-protein diets with the severe PEM evident in famine-ravaged countries; however,
  degrees of malnutrition exist in all areas of the world, including the United States. Children with chronic
  PEM have poor growth (low weight for height) and stunting (low height for age).
• When protein status is marginal immune function is impaired, contributing to the chronic respiratory
  infections and diarrhea that trouble many poorly nourished children and adults. Low protein intake during
  pregnancy increases the risk of a low-birth-weight infant. Inappropriately low protein intake as part of a
  weight reduction regimen, promotes loss of lean body mass.

Answer 46

Low-Protein Diets

Question 47

Protein intake is controlled to prevent the buildup of excessive amino acids that
compete with the treatment drug levodopa for passage across the blood- brain barrier.

Answer 47

Parkinson’s disease:

Question 48

Individuals with this inborn error of metabolism lack the enzyme required to
metabolize the indispensable amino acid phenylalanine, and accumulation of inappropriately high blood
levels of this amino acid results in mental retardation. All infants born in the United States are tested for PKU
at birth, and immediate intervention with a protein-restricted diet limiting the intake of phenylalanine
prevents harmful changes.

Answer 48

Phenylketonuria (PKU):

Question 49

When kidney function declines, protein intake is reduced to minimize the
burden of excreting urea and other nitrogen-containing waste.

Answer 49

*Chronic kidney disease (CKD):

Question 50

Individuals who consume no animal foods and must obtain all of their protein from plant
sources may have marginal protein intakes based on the added bulk of plant protein foods. Protein may
provide only 10% of total kilocalories, the minimum AMDR.

Answer 50

*Vegan diets:

Question 51

1. Soy foods and other legumes supply good-quality protein along with an array of important nutrients and
   phytochemicals with roles in preventing chronic disease.
2. Soy foods contain phytochemicals thought to have estrogen-like activity that may help prevent bone loss in older
   women.
3. Higher intakes of flavonoids, a class of phytochemicals found in soy, peanuts, and many fruits and vegetables, were
   associated with lower risk of stroke and mortality in older adults followed for 7 years.
4. Persons eating legumes at least four times a week had a lower risk of cardiovascular disease than those eating
   these foods less than once a week, and regular use of legumes improved glucose control among patients with type 2
   diabetes.
5. Vegetable protein foods also contain healthy fats.

Answer 51

Health Benefits of Plant Proteins

Question 52

1. Proteins from animal sources—meat, fish, poultry, eggs, and milk—provide all of the indispensable
   amino acids in the appropriate amounts.
2. Used alone or in combination with vegetable proteins, animal proteins meet the amino acid needs of
   children and adults.
3. Animal foods supply trace minerals such as iron and zinc that are difficult to obtain in sufficient
   amounts from plant foods based on their lower content and interfering substances.
4. Dairy foods are rich in calcium and riboflavin and provide preformed vitamin A. Vitamin B12 and
   vitamin D occur naturally in animal foods only.

Answer 52

Nutritional Contributions of Animal Proteins

Question 53

• Vegetarian diets existed in ancient times as described in early Greek culture.
• People choose to follow a plant-based diet and limit or exclude animal foods for many reasons.
• Several major world religions including Hinduism and Buddhism advocate avoidance of meat for the preservation of
  animal life.
• Ethical considerations were the overwhelming motive for selecting a vegetarian or vegan food pattern, followed by
  concerns for health, among an urban and university group

Although diet patterns that emphasize plant foods are generally referred to as vegetarian diets, they are exceedingly
difficult to quantify. Vegetarian diets differ greatly according to the types of animal foods they include or exclude, as follows:

1. Ovo-lacto-vegetarian (includes all plant foods, dairy, and eggs)
2. Lactovegetarian (includes all plant foods and dairy)
3. Pescovegetarians (includes all plant foods, dairy, eggs, and fish)
4. Vegan (includes plant foods only)
5. Semi-vegetarian (includes predominantly plant foods with eggs, dairy, and fish or chicken on occasion)

Answer 53

VEGETARIAN DIETS

Question 54

will support growth and well-being across the life cycle if they
are well planned. Conversely, vegetarian and vegan diets, as well as diets that include all types of animal
foods, will be inadequate if items high in fat and sugar and low in important nutrients are emphasized.
In general, the more restrictive the diet, the more planning is required to avoid nutrient deficiencies.

Answer 54

• Vegetarian diets, including vegan diets,