Proteins: Building Blocks of Life

CHAPTER 5
Proteins

Eleanor D. Schlenker

Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

Physical and Chemical Nature
of Protein
 In 1838 Dutch chemist Johann Mulder first
identified protein as a prime substance in all
life forms

Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 2

Physical and Chemical Nature
of Protein – Cont’d
 Protein enzymes break down our food
 Antibodies shield us from disease
 Peptide hormones carry messages
 Structural proteins in our bodies


The Nature of Amino Acids
 Amino acids are composed of the elements
carbon, hydrogen, oxygen, and nitrogen
 Several contain sulfur
 Amino acids are the building blocks for body
tissues
 Word amino refers to a base or alkaline
substance


General Pattern and Structure
 Common structural pattern holds for all amino
acids
 This pattern is built around a central alpha-
carbon with several attached chemical
groups:
 Amino (base, NH2) group
 Carboxyl (acid, COOH) group
 Attached radical (R) group


Essential Amino Acids
 20 amino acids are used to build body
proteins
 Indispensable (essential) amino acids: 9 that
cannot be synthesized by the body and must be
supplied in food
 Dispensable (nonessential) amino acids: 5 that
can be synthesized by the body in adequate
amounts
 Conditionally indispensable amino acids: 6 that fall
in between; the body is unable to synthesize the
amount needed under certain conditions


The Building of Protein
 Ability of amino acids to form peptide linkages
and arrange themselves into peptide chains
is basic to all proteins and body tissues
 Peptide bonds: the joining of the end amino
group of one amino acid with the end
carboxyl group of the amino acid beside it


The Building of Proteins – Cont’d
 Polypeptides: specific amino acids joined in a
particular sequence to form long chains of
amino acids
 Oligopeptides: short chains of 3 to 15 amino
acids


Types of Protein
 Myosin: fibrous protein found in muscle fiber,
built from chains of 153 amino acids that coil
and unfold as the muscle contracts and
relaxes
 Collagen: contains three separate
polypeptide chains that wind around each
other to produce a triple helix


Types of Protein – Cont’d
 Hemoglobin: globular type of protein that
includes four globin polypeptide chains per
molecule of hemoglobin
 Albumin: major plasma protein that has a
compact globular shape, consists of a single
polypeptide chain of 584 amino acids


Functions of Protein
 Dietary protein supplies building material for
the growth and maintenance of body tissues
 Dietary protein furnishes amino acids in the
appropriate patterns and amounts for efficient
synthesis of specific structural molecules


Physiologic Roles
 All amino acids participate in tissue growth
and maintenance


Physiologic Roles – Cont’d
 Some have important metabolic roles of their
own:
 Form neurotransmitters for brain and nerve
function
 Form other amino acids
 Form hormones
 Support immune function
 Maintain fluid balance


Role of Protein in Critical Care
 Certain amino acids appear to have a
particular role in recovery from severe
infection, trauma, or life-threatening
malnutrition
 Recovery in these situations may be
improved with specific amino acid
supplements


Role of Protein in Critical Care –
Cont’d
 Leucine: following surgery and trauma
 Branched chain amino acids—leucine,
isoleucine, and valine: cancer with
malnutrition, liver disease
 Arginine and methionine: burn patients
 Cysteine: recovery from protein-energy
malnutrition
 Dangers of self-medication

Energy Source
 Seldom used for energy in the fed state
 May be used for energy in the fasting state
 Nitrogen-containing amino group must be
removed
 Remaining keto acid is converted to glucose
or fat


Protein and Nitrogen Balance
Concept of Balance
 Interdependent checks and balances

 Constant ebb and flow of tissue building and
breakdown
 Coordinated activity to maintain normal

function


Body Protein Reserves
 The average man contains approximately 11
kg of protein
 Nearly half this protein (43%) is found in skeletal
muscle
 The remainder is in skin, blood, kidney, liver,
brain, and other organs


Body Protein Reserves – Cont’d
 Body distribution of protein changes with
growth and development


Body Protein Reserves – Cont’d
 Labile protein reserves
 Are easily broken down to meet immediate needs
 Make up only about 1% of total body protein
 Are intended to provide amino acids for an
emergency


Protein Balance
 Finely tuned mechanisms control protein
balance and regulate protein synthesis and
breakdown across all body tissues


Protein Balance – Cont’d
 Sometimes described as nitrogen balance
 Net result of nitrogen gain and loss across all body
tissues


Protein and Nitrogen Balance

 Protein balance
 The steady state that exists between protein
synthesis (anabolism) and protein breakdown
(catabolism)
 Protein turnover
 Higher rate: intestinal mucosa, liver, pancreas,
kidney, plasma
 Lower rate: muscle, brain, skin
 Very slow: structural tissues such as collage
 Higher in infants and children


Protein and Nitrogen Balance –
Cont’d

 Protein compartments
 Tissue protein
 Plasma protein

 Metabolic amino acid pool


Protein Quality
Evaluating Food Proteins
 Protein quality depends on the following two

characteristics:
 Protein digestibility
 Amino acid composition


Protein Quality – Cont’d
Comparing Food Proteins
 Nutritive value of a food protein = its amino

acid score
 A value based on both its digestibility and amino
acid composition


Comparing Food Proteins – cont’d
 When evaluating an amino acid score, it is

important to identify the limiting amino acid(s)
 The limiting amino acid(s) is any indispensable
amino acid present in a lower amount than
recommended in the Amino Acid Reference
Pattern


 The Amino Acid Reference Pattern indicates

the appropriate proportion of each
indispensable amino acid needed to produce
a body protein


 Other methods used in animal studies to

evaluate protein digestibility and composition:
 Biologic value (BV)
 Net protein use (NPU)
 Protein efficiency ratio (PER)


Amino Acid Content of Plant and
Animal Foods

 Complete proteins: animal foods, which contain
all of the indispensable amino acids in the
amounts and ratio needed to support protein
synthesis
 Include eggs, milk, cheese, meat, poultry, and fish


Amino Acid Content of Plant and
Animal Foods – Cont’d

 Incomplete proteins: plant proteins
 Supply less than the required amount of one or more
indispensable amino acids or are missing an
indispensable amino acid


Factors Influencing Protein
Requirements
Protein Quality
 Protein requirements are influenced by the

quality of dietary protein and its amino acid
pattern
Protein Digestibility
 Effects of food preparation

 Digestibility and absorption is influenced by
the time interval between meals, with longer
intervals lowering the competition for
available enzymes and absorption sites

Requirements – Cont’d
Tissue Growth
 Periods of rapid growth—as for fetal and

maternal tissues in pregnancy—require
added protein
Energy Content of the Diet
 Sufficient amounts of carbohydrates must be
available to meet energy needs so dietary
protein can be used exclusively for tissue
building


Requirements – Cont’d
Health Status
 Critical illness and diseases that increase the

rate of protein turnover and tissue breakdown
(catabolism) raise the protein requirement


Protein Requirements
 Recommended Dietary Allowance (RDA) for
adults
 Furnishes the protein and amino acids needed to
maintain or replace body tissues and replace
nitrogen lost via the urine, feces, and sweat, and
lost cells such as those lost from the inner surface
of the digestive tract


Protein Requirements – Cont’d
 Recommended Dietary Allowance (RDA) for
adults – cont’d
 0.8 g/kg body weight or 56 g/day for men
 0.8 g/kg body weight or 46 g/day for women
 An additional 25 g/day is needed for pregnancy
and lactation


 Protein requirements of infants and children
vary according to age and growth patterns


 Acceptable Macronutrient Distribution Range
 10% to 35% of total kcalories


Protein Energy Malnutrition
 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
 Available protein sources in many developing
countries tend to be plant sources with poor amino
acid patterns


Protein Energy Malnutrition –
Cont’d
 Marasmus
 Chronic extreme starvation
 Deficit of energy, protein, and micronutrients
 Little or no body fat and extreme wasting

 Kwashiorkor
 Deficient in protein; low in energy and
micronutrients
 Edema, hypoalbuminemia, skin lesions, fatty liver


Amounts and Types of Protein
 Extremes
 Research to increase indispensable amino acid
content of grains and vegetables
 Excessive protein intakes in the United States


Low-Protein Diets
 Children with protein-energy malnutrition
(PEM) have the following:
 Poor growth with wasting (low weight for height)
and stunting (low height for age)
 Impaired immune function
 Chronic respiratory infections
 Diarrhea


Low-Protein Diets – Cont’d
 Low-protein intake during pregnancy
increases risk of a low-birth-weight infant
 Clinical situations
 Parkinson’s disease
 Chronic kidney disease
 Vegan diets


High-Protein Diets
 Diets with 25% or more of total kcalories
(kcal) from protein appear to promote weight
loss, provide satiety, and can restore effective
energy regulation
 Protein suppresses hunger to a greater
extent than carbohydrates
 High protein intakes appear to protect muscle
mass


High-Protein Diets – Cont’d
 Long-term effects on renal function:
 Animal proteins increase blood flow to the kidney
 Animal and vegetable proteins accelerate renal
impairment
 Attend to fluid needs


Health Benefits of Plant Foods
 Soy foods and legumes supply good-quality
protein and may help to prevent certain
chronic diseases
 Soy foods are sources of phytochemicals that
may help prevent bone loss in older women


Health Benefits of Plant Foods –
Cont’d
 Legumes lower risk of cardiovascular disease
 Vegetable protein foods are lower in
saturated fat
 Soybeans and chickpeas are good sources of
linoleic acid
 Tree nuts and peanuts are rich in
antioxidants, trace minerals, essential fatty
acids, and phytochemicals


Nutritional Contributions of
Animal and Plant Proteins
 Animal sources
 Provide indispensable amino acids
 Meats provide iron and zinc
 Dairy foods provide calcium, riboflavin, preformed
vitamin A
 Vitamin B12 and vitamin D
 Plant sources
 Iron and zinc are less well absorbed
 Add fiber and phytochemicals
 Essential fatty acids
 Fortified soy milk provides calcium and vitamin D

Vegetarian Diets

 Reasons for following vegetarian diets
 Religion
 Sustainability of the food supply
 Protection of the environment


Vegetarian Diets – Cont’d

 Vegetarian diets are generally categorized as
follows:
 Ovolactovegetarian: includes all plant foods, dairy,
and eggs
 Lactovegetarian: includes all plant foods and dairy
 Pescovegetarian: includes plant foods and fish
 Vegan: includes plant foods only
 Flexitarian: includes predominantly plant foods
with animal foods eaten occasionally



 Nutritional Implications of Vegetarian Diets
 Require careful planning, depending on type of
vegetarian diet
 Phytates interfere with absorption of iron and zinc
 Include vitamin C source to increase iron
absorption
 Use of fortified foods
 Effects of oxalate on calcium absorption



 Planning vegetarian diets
 Vegetarian Food Guide
 Vegetarian diets and chronic disease
 Lower coronary heart disease, obesity,
hypertension, and diabetes
 Generally lower risk of cancer, uncertain effects
on specific cancers


Digestion-Absorption-Metabolism
 The food proteins taken into the body must
be broken down into ready-to-use building
units—the amino acids
 Mouth
 The only digestive action on protein taking place in
the mouth is the mechanical effect of chewing


– Cont’d
Stomach
 Three chemical agents in the gastric

secretions:
1. Pepsin
2. Hydrochloric acid
3. Rennin


– Cont’d
Small Intestine
 Enzymes secreted by the pancreas and
glands in the wall of the intestine help protein
digestion:
 Pancreatic secretions: (1) trypsin, (2)
chymotrypsin, (3) carboxypeptidase
 Intestinal secretions: (1) aminopeptidase, (2)
dipeptidase


Absorption of Amino Acids
 Characteristics of amino acid absorption
 Requires active transport system
 Enter portal blood


Absorption of Amino Acids –
Cont’d
 Absorption of peptides and whole proteins
 A few short-chain peptides and smaller intact
proteins escape digestion and are absorbed
 Most undergo hydrolysis within the cells of the
intestinal mucosa and yield their amino acids
 Protein molecules that pass into the blood intact
may play a part in the development of immunity,
protein sensitivity, and food allergies


Proteins: Building Blocks of Life

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