BCH 282
Dr.Saba Abdi
Building Blocks of Protein
Amino Acids
• 20 common amino acids in food, each with a
different side chain
• More than half amino acids are non-essential (body
can synthesize them)
• Nine amino acids are essential ( body cannot make
or makes in insufficient quantities). Must be supplied
by diet.
• Conditionally essential amino acids are nonessential
amino acids that some time become essential. Eg.
Tyr (NE) synthesized by Phe (E). If diet low in Phe
then Tyr becomes conditionally essential.
Non-essential amino
acids
Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic acid
Glutamine
Glycine
Histidine (essential in children)
Proline
Serine
Tyrosine
(The 11 amino acids that we can
produce)
Essential amino acids
• Leucine
• Isoleucine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine
Amino Acids: structural &
functional roles
• cell membrane
• connective tissue(collagen & elastin)
• inert protein(hair & nails)
• Enzymes
• Hormones(insulin)
• Fluid balance(aquaporins)
• blood proteins(albumin & haemoglobin)
• skeletal & smooth muscle
• antibodies
• energy source
• Acid- base regulator
• Transporters
How Proteins are used in the body
Protein Metabolism
Protein Turnover and amino acid
pool
• AA’s and proteins are continually synthesised
& catabolised in the body this is called protein
turnover.
• The AA’s released during protein break down
form an AA pool.
• daily turnover @3 – 4 g/kg body weight/day
• dietary intake @ ¼ - ½ this amount
Quality of proteins
• The quality of a dietary protein is a measure of its
ability to provide the essential amino acids required
for tissue maintenance.
• The Protein Digestibility-Corrected Amino Acid
Scoring (PDCAAS) is the standard by which to
evaluate protein quality.
• PDCAAS is based on the profile of essential amino
acids and the digestibility of the protein. The highest
possible score is 1.00.
• This amino acid score provides a method to balance
intakes of poorer-quality proteins by vegetarians and
others who consume limited quantities of highquality dietary proteins.
Sources of protein
Proteins from animal sources
• Proteins from animal sources (meat, poultry,
milk, and fish) have a high quality because
they contain all the essential amino acids in
proportions similar to those required for
synthesis of human tissue proteins
• [Note: Gelatin prepared from animal collagen
is an exception; it has a low biologic value as a
result of deficiencies in several essential
amino acids.]
Proteins from plant sources
• Proteins from wheat, corn, rice, and beans
have a lower quality (except soy protein) than
do animal proteins.
• However, proteins from different plant
sources may be combined in such a way that
the result is equivalent in nutritional value to
animal protein.
• Plant Proteins are less digestible.
• [Note: Animal proteins can also complement
the biologic value of plant proteins.]
Mutual supplementation yeilds
Complementary proteins
Complementary proteins
• Legumes
• Cereals
• Combined
IsoLeu
Lys
x
x
Met
x
Trp
X
Measure of Protein Quality
Biological Value (BV)of Protein
• – based on NITROGEN RETENTION
• – higher the protein quality= more Nitrogen
retained= more protein synthesis than
catabolism
• BV of EGG protein = 100%, all absorbed
protein retained
Nitrogen balance
• Nitrogen balance occurs when the amount of
nitrogen consumed equals that of the
nitrogen excreted in the urine, sweat, and
feces. Most healthy adults are normally in
nitrogen balance.
Nitrogen Balance
• Positive nitrogen balance: This occurs when nitrogen
intake exceeds nitrogen excretion. It is observed
during situations in which tissue growth occurs, for
example, in childhood, pregnancy, or during recovery
from an emaciating illness.
• Negative nitrogen balance: This occurs when
nitrogen loss is greater than nitrogen intake. It is
associated with inadequate dietary protein, lack of
an essential amino acid, or during physiologic
stresses, such as trauma, burns, illness, or surgery.
Requirement for protein in humans
• 1 gram protein supplies 17kJ
• The RDA for protein is computed for proteins of
mixed biologic value at 0.8 g/kg of body weight for
adults, or about 56 g of protein for a 70-kg individual.
• Women who are pregnant or lactating require up to
30 g/day in addition to their basal requirements.
• To support growth, children should consume 2
g/kg/day.
• sedentary individuals: balanced diet: 10-15% of
Etotal from protein
• Athletes: balanced diet: 12-17% of Etotal from
Consumption of excess protein
• Protein consumed in excess of the RDA is
deaminated, and the resulting carbon
skeletons are metabolized to provide energy
or acetyl coenzyme A for fatty acid synthesis.
When excess protein is eliminated from the
body as urinary nitrogen, it is often
accompanied by increased urinary calcium,
increasing the risk of nephrolithiasis and
osteoporosis
The protein-sparing effect of
carbohydrate
• When the intake of carbohydrates is low, amino
acids are deaminated to provide carbon skeletons for
the synthesis of glucose that is needed as a fuel by
the central nervous system.
• If carbohydrate intake is less than 130 g/day,
substantial amounts of protein are metabolized to
provide precursors for gluconeogenesis.
• Therefore, carbohydrate is considered to be “proteinsparing,” because it allows amino acids to be used for
repair and maintenance of tissue protein rather than
for gluconeogenesis.
Protein from food
Food with high Protein
content( g/100g):Beef & lamb 28
Cheese
26
Chicken
25
Fish
18
Eggs
12
Beans
7
Peas
5
Milk
3.3
Food Protein g/100g
MEDIUM CONTENT
•
•
•
•
•
•
•
•
Cornflakes 8.6
Bread (white) 7.8
Spaghetti 4.2
Sweetcorn 4.1
Rice 2.2
Cauliflower 1.6
Potatoes 1.6
Cabbage 1.3
Food Protein g/100g
LOW CONTENT
•
•
•
•
•
Apples 0.3
Honey 0.5
Butter & margarine <0.4
Wine 0.1
Soft drink 0
Protein-energy malnutrition(PEM)
• People are deprived of protein, energy or both
result in PEM. Forms of PEM are kwashiorkor
and marasmus.
• In developed contries PEM is seen most
frequently in hospital patients with chronic
illness, or in individuals who suffer from major
trauma, severe infection, or the effects of
major surgery.
• In developing countries, an inadequate intake
of protein and/or energy may be observed.
Kwashiorkor:
• Kwashiorkor occurs when protein deprivation is
relatively greater than the reduction in total calories.
• Significant protein deprivation is associated with
severe loss of visceral protein.
• Frequently seen in children after weaning at about
one year of age, (diet predominantly of
carbohydrates).
• Symptoms: stunted growth, edema, skin lesions,
depigmented hair, anorexia, enlarged fatty liver, and
decreased plasma albumin concentration. Edema
results from the lack of adequate plasma proteins to
maintain the distribution of water between blood
and tissues
Marasmus
• Marasmus occurs when calorie deprivation is
relatively greater than the reduction in protein.
Marasmus usually occurs in children younger than
one year of age when the mother's breast milk is
supplemented with thin watery gruels of native
cereals, which are usually deficient in protein and
calories.
• Symptoms: arrested growth, extreme muscle wasting
(emaciation), weakness, and anemia. Victims of
marasmus do not show the edema or changes in
plasma proteins observed in kwashiorkor
Health effects of Protein-problem
of excess
• Heart Disease-food rich in animal protein are rich in
saturated fats.The amino acid homocysteine may be
a risk factor for heart disease. But arginine may be
protective.
• Cancer-colon, breast , kidney , pancreas & prostrate.
• Adult bone loss (osteoporosis)- Calcium excreation
rises as protein intake increases.
• Weight control-protein rich food are often fat rich
foods.
• Kidney diseases-protein rich diet increases the work
of kidney