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Proteins
Proteins and Amino Acids
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Constitutes 17% of body weight
Protein functions
 Supplies nitrogen in a form we can use
 Aids in regulation and maintenance
 Blood clotting
 Fluid balance
 Hormone and enzyme production
 Visual processes
 Cell repair
Protein can be made from the 20 different amino acids
Amino acids are the building block of proteins
Amino acids
 contain carbon, hydrogen, and oxygen
 have a central carbon bonded to four groups – three (nitrogen or amine,
hydrogen, and an carboxylic acid) always found in each amino acid and a
fourth group unique group
 This fourth group determines the type of amino acid
Essential and nonessential amino acids
 Nonessential amino acids can be synthesized by a healthy body in sufficient
amounts
 Essential amino acids cannot be synthesized by humans in sufficient amounts and
must be included in the diet
 Histidine, isoleucine, leucine, lysine, methionine, phenylalanine,
threonine, tryptophan, valine
 Amino acid metabolism
 Transamination - process of producing nonessential amino acids
 Deamination - removal of amino group (may lead to the formation of
urea)
Dietary aspects
 PKU individuals - cannot metabolize the essential amino acid phenylalanine
 High-quality (complete) proteins – contain all the nine essential amino acids –
example animal protein
 Low-quality (incomplete) proteins – low or lack one or more essential amino
acids – plant protein
 Limiting amino acid – essential amino acid in lowest concentration in a food or
diet relative to body needs
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Complementary proteins – two food protein sources that make up for each
other’s inadequate supply of specific essential amino acids
Protein Synthesis, Turnover, Organization, and Denaturation
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Peptides: dipeptides, oligopeptides, and polypeptides
Synthesis
 DNA transcribe into RNA in nucleus
 RNA leaves the nucleus (now called mRNA)
 mRNA codons read by ribosomes attached to endoplasmic reticulum
 tRNA delivers amino acids to ribosome
 Amino acids are bound together
 mRNA is continually read until chain of amino acids is complete
 polypeptide will undergo further cell metabolism in order to become functional
Turnover
 Body cells are mostly composed of protein
 Cells die and are replaced – therefore protein is recycled through transamination
and deamination reactions
 300 grams of protein degrade in body daily but only 65-95 grams are consumed
daily
Organization
 Primary, secondary, tertiary, quaternary structures
 Organization is based on types of bonding that occurs
Denaturation
 Alteration of a protein’s three dimensional structure, usually because of treatment
by heat, enzymes, acid or alkaline solutions, or agitation
 Denaturation causes proteins to lose function
 Examples
 Precipitation of milk proteins in yogurt
 Acids in stomach denatures bacteria, plant hormones, some active
enzymes, and other forms of protein in food
 Heat during cooking denatures proteins and make food safer to eat
Protein Digestion
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Protein digestion begins in the stomach by enzymes
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An inactive enzyme pepsinogen in the presence of increased acidity will be activated
into an active form of the enzyme called pepsin
Pepsin breaks only a few peptide bonds, therefore does not break proteins into individual
amino acids
Pepsin is regulated by gastrin which is a hormone that stimulate stomach cells to produce
acids
Partially digested proteins enter the small intestine (duodenum) from the stomach and
stimulates the release of the hormone cholecytokinin (CCK)
CCK enters the bloodstream and causes pancreas to release protein digesting enzymes
(trypsin, chymotrypsin, and carboxypeptidase)
Digestive enzymes from pancreas break proteins down into short peptides and individual
amino acids – both of which are actively absorbed in the sm. Intestine
Functions of Proteins
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Every cell contains proteins
Half of body protein is made up of the structural proteins collagen, actin, myosin, and
hemoglobin
Measurement of the amounts of body proteins are used as indicators of health and disease
Fluid balance - blood proteins – albumins and globulins – maintain body fluid balance
Acid-Base balance – maintained by both protein pumps on cell membranes and protein
buffers in blood
Hormone and enzyme formation
 Thyroid hormone and insulin are synthesized using amino acids; one amino
acid, tyrosine for TH and 48 amino acids for insulin
 Enzymes are mostly composed of amino acids and are used in catalyzing
reactions
Immune function – antibodies produced by B-lymphocytes are proteins
Glucose production – glucose can be made from amino acids
Provide energy – 4 kcal/g – provide energy during prolonged exercise
Protein Needs
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Recommended protein intake = 10-15% of daily calories
0.8 grams of protein per kilogram of healthy body weight
example: 220lbs man/2.2 lbs per kg = 100 kg X 0.8 grams/ kg = 80 grams
Are High Protein Diets Harmful?
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May be low in fiber, some vitamins, and some minerals
Can increase calcium loss in urine
Excessive red meat consumption linked to colon cancer
May burden kidney by forcing them to excrete excess nitrogen as urea
Some amino acids may be toxic at high levels
Protein in Foods
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Plant proteins versus animal proteins
Protein Quality
 Biologic Value
 Measure of how efficiently food protein can be turned into body tissues
 Amino acids needed versus amino acids available in food consumed
 Protein Efficiency Ratio – (intended for infants) the amount of weight (in grams)
gained divided by the amount of protein (in grams) consumed
 Chemical Score – amount of each essential amino acid provided by a gram of a
foods protein is divided by an “ideal” amount for that essential amino acid per
gram of the food protein
 Protein Digestibility Corrected Amino Acid Score – chemical score of a
protein multiplied by the digestibility (factor) of the protein
RDA versus protein quality
 RDA = 0.8 g / kg of body weight
 Doesn’t take into account protein quality
Protein-Energy Malnutrition
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Protein-energy malnutrition (PEM) – condition resulting from regularly consuming
insufficient amounts of energy and protein
 Marasmus – PEM
 Kwashiorkor – PEM in conjunction with another disease
 Both common in infants
Vegetarian Diets
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Vegan - primarily eats only plant foods
Fruitarian - primarily eats fruits, honey, and vegetable oils
Lactovegetarian - consumes plant products and dairy products
Lactoovovegetarian - consumes plant products, dairy products, and eggs
Chapter Objectives
After reading chapter seven - A student should be able to
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Discuss the structure of amino acids
Identify the number of (how many) amino acids that exist
Identify the nine essential amino acids and describe why they are considered essential
Define: high quality proteins, low quality proteins, complementary proteins, and limiting
amino acid
5. Describe in detail the process of protein synthesis
6. Discuss the process of protein turnover in the human body
7. List and discuss the four type of protein structure
8. Define: denaturation, transamination, deamination
9. Discuss in detail the process of protein digestion
10. List and discuss various function of protein in the human body
11. Identify the recommended intake of proteins?
12. List and discuss the four types of methods to determine protein quality
13. Discuss the various types of vegetarian diets