Protein
•After water, the most prevalent substance in our bodies
Protein…
•Is as much as 50% of a cell’s dry weight
•Is made up of smaller building blocks called amino acids
•Contains carbon, hydrogen, oxygen, and nitrogen
Generic Amino Acid Structure
Side chain
H2N
(Amino group)
C
COOH
(Carboxyl end)
H
•Animals can only utilize L-amino acids
Peptide bonds join two amino acids together
O
O
•AA1-C-N-AA2-C-N-AA3….
Essential Amino Acids
•Body cannot make their side chains
•Therefore, must be in diet daily
•Phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine,
lysine (table 6.1, p. 182)
Non-essential amino acids
•Body can swap amine groups with other compounds to form these amino acids
•Some of non-essential amino acid carbon skeletons come from carbohydrate metabolism
intermediates
Conditionally essential amino acids
•Essential under certain conditions
•Persons with PKU lack the enzyme to convert phenylalanine (essential) to tyrosine
(non-essential); therefore, tyrosine is an essential amino acid to people with PKU
•Glutamine may be essential to patients under physiologic stress (e.g., surgery)
Amino acid side chains have different characteristics
•Some contain sulfur
•Some have an additional carboxyl or amino group
•Some side chains are hydrophobic
•The characteristics of the amino acids can dictate the final 3-D structure and function of
a protein
A change in a single amino acid can change the function of a protein
•Sickle cell anemia patients have abnormal hemoglobins (see fig. 6-8)
•May render an enzyme inactive, which could result in a birth defect
Protein structures
•Primary--the amino acid sequence
•Secondary--bonds between side chains
•Tertiary--folding
•Quaternary--final 3-D structure with different protein subunits
Protein Synthesis
•You have all of the genes you need in each nucleated cell, but each cell only makes a
fraction of the proteins
•For example, only your beta cells produce insulin
•You also have lots of “junk” DNA that needs to be edited out before protein synthesis
can take place
Nutrients can influence gene expression
•High concentrations of iron stimulate production of the iron storage protein ferritin
•The active form of vitamin D stimulates production of a protein in the small intestine to
increase calcium absorption from the diet
Protein and Amino Acid Functions in the Body
•Structural--from cells to muscle to bone
•Blood proteins (albumin) keep the plasma water from leaking into the tissues
•Maintenance of acid-base balance
•Enzymes
•Some hormones (insulin), antibodies, and neurotransmitters
A Review
•How are dietary proteins digested and absorbed?
Once inside the intestinal mucosa cell
•Di- and tripeptides are hydrolyzed to single amino acids
•Amino acids enter bloodstream
•First stop…
Protein Metabolism
•Carbon skeletons can be used for energy when carb is limited and in situations of stress
(injury, illness)
•Amino acids from diet and from broken-down proteins used for protein synthesis in all
tissues
Final disposition
•Transamination--donating amino group to a carbon skeleton to make a new amino acid
•Deamination--removing an amino group without giving it to another compound
–Leftover amino groups are converted to urea, then excreted in urine
–Other nitrogen compounds are also excreted in urine--uric acid, creatinine
Nitrogen balance
•Protein in - protein out
•Measurement is cumbersome
More nitrogen balance
•If 0, you are excreting as much as you’re taking in
•If positive, you may be in a period of growth, recovery from illness, or pregnancy
•If negative, you may be ill or malnourished
2002 DRI’s for Protein
•Adult RDA 0.8 g/kg body wt./day
•Acceptable Macronutrient Distribution Range 10-35% of Calories
•No UL
•Average adult intake in US 75 g/day
•Men may consume 100 g+/day
“How can I be eating all that protein?” Easy!
•1 ounce meat or cheese has 7 g protein
–A “Whopper” without cheese has 27 g protein
•1 cup milk has 8 g protein
•1 cup cooked dried beans has 15 g protein
•1 slice bread or 1/2 cup cooked grain or pasta has 3 g protein
Protein Quality
•Measured in different ways
Protein Digestibility
•Animal protein highest (~90%)
•Legumes 80%
•Grains 60-80%
•All proteins more digestible after cooking
–Cooking denatures proteins
•Moist heat is best
Cooking also denatures bad proteins in food
•Trypsin inhibitor in raw legumes
•Avidin in raw egg white
–Inhibits the vitamin biotin
Protein quality also depends on the amino acid content
•Humans need all of the essential amino acids in the right amounts every day
–If one is lacking in the diet, cells adjust their protein synthesis accordingly
•Non-essential amino acids can be made by the body
Complete Proteins
•Contain all of the essential amino acids in the right amounts
–In general, animal proteins are complete
–Exception: gelatin
Incomplete Proteins
•Lack at least one essential amino acid, or have them in low amounts
–Plant proteins are generally incomplete
–Soy protein, however, is considered complete
Limiting Amino Acid
•The essential amino acid that’s lacking in a particular food
–Corn and grains lack lysine
–Legumes lack methionine
How to overcome limiting amino acids? Consume complementary protein sources
•Combine two proteins with different limiting amino acids
–Red beans and rice
–Tofu stir-fry over noodles
–Refried black beans with corn tortillas
–Peanut butter sandwich
Measurements of protein quality
•Chemical Score-Compares the mg of essential amino acids in food with a standard
protein (e.g., egg white)
•Protein efficiency ratio (PER)-examines wt gain in animals with protein intakes relative
to a reference (milk casein)
Protein quality measurements, 2
•Net Protein Utilization (NPU) = nitrogen retained divided by nitrogen intake x 100
•Biological Value (BV) = nitrogen retained divided by nitrogen absorbed x 100
•Protein digestibility-corrected amino acid score (PDCAAS) = chem score x %
digestibility of protein
Protein-Energy Malnutrition (PEM)
•See pages 198 and 199
•Kwashiorkor
•Marasmus
Kwashiorkor
•First described in African children
•Children getting adequate Calories, inadequate protein
•Swelling (edema) in hands, feet, legs, and belly
•Hair may lighten in color
•Rapid onset
Marasmus
•Protein and Calorie deficit
•Develops more slowly than kwashiorkor
•Wasting of muscle
•In young children, marasmus can permanently stunt brain development
•Both kwashiorkor and marasmus can be fatal
Does PEM occur in developed countries? Yes
•Low-income persons, especially children
•Elderly
•Persons with certain types of cancer or AIDS
•Anorexia nervosa
•David Blaine
Protein Excess
•There is no evidence that athletes need more than 1.5 g/kg body weight of protein
•There is even less evidence that protein supplements are needed in athletes
–In 1990, people taking tryptophan supplements that were contaminated developed
eosinophilia-myalgia syndrome, a potentially fatal complication
Protein Excess, continued
•Excess protein may increase urinary calcium excretion
•Excess protein may stress kidneys of people whose kidney function is iffy to begin with
Vegetarianism
•Can be healthy IF the right dietary choices are made
–Complementary proteins
–Not going overboard on fat
–Not eliminating fat from diet
Why do people become vegetarians?
Vegetarian Varieties
•Vegans--Do not eat any animal-derived products
•Fruitarians--Consume only raw or dried fruits, raw vegetables, nuts, and seeds
•Macrobiotic--Progressively restrictive; people in advanced phases consume only brown
rice & herb tea
More Veggie Varieties
•Ovo-lacto-vegetarians--Will eat eggs and dairy products
•Semivegetarians--may exclude red meat and/or poultry, but eat fish, eggs, and dairy
•Occasional vegetarians
Nutrients of Concern for Vegetarians and Vegans
•Vitamin B12--found only in animal products
•Calcium--Vegans need to seek out calcium-fortified soy milk or orange juice
•Vitamin D--Vegans may need supplements
•Iron--Vegetarians should consume iron sources with a source of vitamin C