Objectives:
Chapter 7 - Protein
• Review the basic structure of proteins and their component amino acids
• Learn which are the essential amino acids and why
• Understand that there are various amino acid “pools” within the body
including dietary and body tissue pools
• Learn how amino acids can be used for
 The synthesis of new proteins for growth or replacement of cellular
proteins
 Production of important non-protein nitrogen containing molecules
 Oxidation as an energy source
 Synthesis of glucose, ketones or fatty acids
• Understand how the liver and muscle handle amino acid metabolism, and
how brain function is closely related to some amino acid metabolism
• Learn how to determine the “quality” of the protein you eat, what the
recommended intakes are and why
Functional roles of proteins:
Structural organization
Fig. 7-1, p. 175
Table 7-1a1, p. 178
Table 7-1a2, p. 178
Key Ideas: the tertiary structure of a protein is determined by its amino acid
composition and amino acid side chain chemistry determines what our bodies can
do with them metabolically
Table 7-1b, p. 179
Sources of Protein
Exogenous Sources
• Animal products such as meat, poultry, fish, and dairy
products (not including butter, sour cream, or cream
cheese)
• Plant products such as grains, grain products,
legumes, and vegetables
Endogenous Sources
• Desquamated mucosal cells (50g protein/day)
• Digestive enzymes and glycoproteins (17g/day)
(total daily requirement for protein for males is 56g/day, for
females it is 46 g/day)
Digestion of Proteins – pepsin only works on exogenous
proteins, all other enzymes work on both exogenous and
endogenous proteins
Table 7-2, p. 182
Step 2
Amino
acid
Na+
Step 1
Amino
acid
Cell membrane
Na+
Amino acid
transporter
Na+
Cytosol
Amino
acid
Intracellular
amino acid
metabolism
Na+
Step 3
Na+ /K +
ATPase
Na+
Amino
acid
Na+
Amino
acid
Step 4
Step 5
Fig. 7-6, p. 183
Table 7-3, p. 184
Amino Acid Supplements
• Amino acids that use the same
carrier system may create,
depending on the amount ingested,
a competition between the amino
acids for absorption
• Supplements may result in
impaired or imbalanced amino acid
absorption
• Some amino acid supplements can
cause serious illness
•L-tryptophan
•L-phenylalanine
• Some amino acids are useful
•L-lysine
Enterocyte
Basolateral
Membrane
Lumen
(small intestine)
Peptide
Peptide
Peptide
Carrier
H1
H1
H1
Na1
H1
Na1
K1
Na1
K1
Na1
Brush border
Membrane
Fig. 7-7, p. 184
Basolateral
Membrane
Enterocyte
Na1
Na1
Lumen
ATP
Na1
Na1
ADP 1 Pi
K1
K1
Amino
Acid
Amino
acids
Carrier
Na1
Required for
some amino
acid transport
Brush
border
membrane
Peptides
Na1
Amino
Acids
Fig. 7-8, p. 185
Table 7-4, p. 185
Many dietary amino acids are used by enterocytes and never make it into the
circulation. This is an example of the fates of glutamine in the enterocyte.
Fig. 7-9, p. 186
Reduced Glutathione can also be made from glutamine in enterocytes
Fig. 7-10, p. 186
Metabolic Fates of Amino Acids
• Synthesis
– Plasma proteins (albumin, retinol-binding protein, acute phase
proteins, heat shock proteins)
– Nitrogen containing non-protein compounds (glutathione, carnitine,
creatine, carnosine, choline)
– Purines and pyrimidines
• Catabolism
•Transamination and deamination to keto-acids
•Energy
•Glucose
•Ketone bodies
•Cholesterol
•Fatty acids
•Ammonia disposal – urea cycle
Synthesis of plasma proteins
Synthesis of nitrogen containing, nonprotein compounds
• Glutathione
• Carnitine
• Creatine
• Carnosine
• Choline
• Purine and pyrimidine bases
Table 1. Effect of Oral Creatine Supplementation on Specific Activities
Activity Summary of Studies
Comments
Stationary Several studies support an
cycling
ergogenic effect in repeated
sprints
sprints but are not
convincing for single sprints
Field studies with bicycles on a track are
needed to simulate actual competition
Running
sprints
(repeated
or single)
Conflicting results regarding
performance enhancement
Speculation is that weight gain offsets any
potential benefit
Swimmin
g sprints
(repeated
or single)
Conflicting results regarding
performance enhancement
Speculation is that weight gain offsets any
potential benefit
Weight
lifting
Some evidence of an
ergogenic effect
Whether creatine supplementation truly
increases muscle synthesis is being
investigated. Creatine-induced weight gain
may make true double-blindness difficult to
achieve
Pyrimidine synthesis
Fig. 7-16, p. 192
Purine synthesis from ribose phosphate
Fig. 7-17a, p. 193
Fig. 7-17b, p. 193
Gout
Amino Acid
Metabolism
• Deamination or
transamination
• Nitrogen
recycling/disposal via
urea cycle
• Use of remaining
carbon skeletons
Fig. 7-20, p. 195
Urea
Deamination
Fig. 7-21, p. 196
Transamination
Fig. 7-22, p. 196
Fig. 7-23, p. 197
A typical blood chemistry panel might include the following tests:
General
Metabolism
Kidney
Function
Electrolytes
Liver Function
Thyroid
Pancreas
GLU (Glucose)
LDH (Lactate
dehydrogenase)
CPK (Creatine
phosphokinase)
BUN
(Blood
Urea
Nitrogen)
CREAT
Creatinine
Na
(Sodium)
K
(Potassium)
Cl
(Chloride)
CA
(Calcium)
PHOS
Phosphorus
ALP (Alkaline
phosphatase)
ALT (Alanyl
amino
transferase)
ALB (Albumin)
GGT (Gammaglutamyl
transpeptidase)
SGPT (Serum
glutamate
pyruvate
transaminase)
TP (Total Protein)
CHOL (Cholester
ol)
GLOB (Globulin)
TBILI (Total
Bilirubin)
T3
Triiodothyro
nine)
T4
Thyroxine
AMY
(Amylase)
LIP (Lipase)
If acetylcoA
is abundant,
but
oxaloacetate
is not
glucogenic
ketogenic
Fate of carbon skeletons from
amino acids
Fig. 7-24, p. 199
Metabolic fates of selected amino acids
• Phenylalanine – tyrosine, melanin, thyroid hormones,
L-dopa, dopamine, norepinephrine, epinephrine, acetyl
coA
• Tryptophan – serotonin, melatonin, picolinate, NAD+,
nicotinamide, NADP+, acetylcoA, pyruvate,
xanthurenic acid
• Methionine – homocysteine, spermidine,
tetrahydrofolate, taurine, sulfate, succinylcoA
• Lysine – carnitine, acetylcoA
• Threonine – succinyl coA, pyruvate, acetaldehyde
Fates of amino acids not used for the synthesis of body
proteins
Fig. 7-31, p. 207
How do organs other than the liver
get rid of the ammonia biproduct of
amino acid metabolism?
• Ammonia to glutamate to glutamine (glutamine
synthetase; ATP; Mg2+ or Mn2+)
• Alanine to pyruvate to glucose (transamination)
• Branched chain amino acids to glutamate to
glutamine to alanine (transamination)
Alanine-glucose cycle
Fig. 7-34, p. 209
Fig. 7-38b, p. 213
Kidneys prefer:
Glycine, alanine,
glutamine, glutamate,
phe, and aspartate
From these it
produces:
Arginine, histidine,
serine and maybe
tyrosine
It is the only organ
other than the
kidney that has
gluconeogenic
enzymes and is
critically important
during fasting.Fig. 7-38c, p. 213
You can also excrete urea via the skin, and can lose nitrogen
via hair and skin cell loss. These losses are referred to as
insensible nitrogen losses.
Table 7-5, p. 214
Fig. 7-38a, p. 213
How to Determine Protein
Quality?
• Digestibility
• Composition with respect to indispensable
amino acids
– High quality or complete
– Low quality or incomplete
• Quantitative evaluation methods
–
–
–
–
–
–
Amino acid score
Digestiblity corrected amino acid score
Protein efficiency ratio
Nitrogen balance
Biological value
Net protein utilization
Table 7-6, p. 220
Table 7-7, p. 222
Food
Weight in
grams
Serving
Protein grams
% Daily Value
Hamburger,
extra lean
6 ounces
170
48.6
97
Chicken,
roasted
6 ounces
170
42.5
85
Fish
6 ounces
170
41.2
82
Tuna, water
packed
6 ounces
170
40.1
80
Beefsteak,
broiled
6 ounces
170
38.6
77
Cottage
cheese
1 cup
225
28.1
56
Cheese pizza
2 slices
128
15.4
31
Yogurt, low fat
8 ounces
227
11.9
24
Tofu
1/2 cup
126
10.1
20
Lentils, cooked
1/2 cup
99
9
18
Skim milk
1 cup
245
8.4
17
Split peas,
cooked
1/2 cup
98
8.1
16
Whole milk
1 cup
244
8
16
Lentil soup
1 cup
242
7.8
16
Kidney beans,
cooked
1/2 cup
87
7.6
15
Cheddar
cheese
1 ounce
28
7.1
14
Macaroni,
cooked
1 cup
140
6.8
14
p. 227a
p. 227b
p. 228