Proteins
• Protein: chain of amino acids joined by peptide bonds
• Amino Acids: The Building Blocks of protein.
Consist of:
 Central carbon bonded to hydrogen
 Amino group (NH2)
 Carboxylic group (-COOH)
 R-group
 20 common amino acids (AA)
 Classified based on the properties of the R groups
 Body uses over 20 amino acids to make proteins
Amino Acid Structure
R
NH2CH
COOH
Protein Structure
O
O
~NHCHC-NHCHC~
R2
R1
Peptide bond
Peptide Bond: Joins amino acids
Peptide Chain
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 Proteins are large organic compounds made of amino acids arranged in a
linear chain and joined together by peptide bonds between the carboxyl and
amino groups of adjacent amino acid residues.
 They play key roles in constructing and maintaining living cells.
 The word protein comes from the Greek ("prota"); meaning "of primary
importance" and these molecules were first described and named by the
Swedish chemist Jöns Jakob Berzelius in 1838. However, proteins' central
role in living organisms was not fully appreciated until 1926, when James
B. Sumner showed that the enzyme urease was a protein.
Proteins are classified based on number of amino acids (structure):
 Dipeptides
 Tripeptides
 Oligopeptides
 Polypeptides
Proteins in the Diet:
9 of the 20 amino acids must be obtained from the diet. These are referred to as
the essential amino acids.
Protein Structure and Function
Proteins are polymers of Alpha-amino acids.
- The amino acids used to make proteins are 2-aminocarboxylic acids.
- The (alpha) carbon is the carbon to which a functional group is attached.
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Classification of amino acids
1- Essential amino acid
– An amino acid that cannot be synthesized by the organism (usually referring
to humans), or can only make in inadequate quantities and therefore must be
supplied in the diet.
– Need to be consumed from the diet
– 8-10 essential amino acids
– Depends on species and physiological state
2- Nonessential amino acid
– Nonessential amino acids are that which is ``made by the body from the
essential amino acids or normal breakdown of proteins.
– The body can make these Amino Acids in large enough quantities
– Made from essential amino acids
– Not necessary to consume these in the diet
– 10-12 nonessential amino acids
3- Conditionally essential amino acid.
– Meaning they are not normally required in the diet but can become essential
and must be supplied exogenously in certain physiologic conditions in
populations that do not synthesize it in adequate amounts.
– Example: Tyrosine becomes essential in people with “Phenylketonuria
(PKU)”
Classification of amino acids:
Essential (10):
Nonessential (10)
Phenylalanine
Valine
Threonine
Tryptophan
Isoleucine
Methionine
Histidine
Arginine
Leucine
Alanine
Asparagine
Aspartic acid
Cysteine
Glutaminc acid
Glutamine
Glycine
Proline
Serine
3
Conditionally essential
(3)
Cysteine
Glutamine
Tyrosine
Types of Proteins:
– Structural: tendons, cartilage, hair, nails
– Contractile: muscles
– Transport: hemoglobin, myoglobin
– Storage: milk, nuts, seeds
– Hormonal: insulin, growth hormone
– Enzyme: catalyzes reactions in cells
– Protection: immune response
Protein metabolism
1- The liver uses amino acids for its own purposes or sends them out into the
blood for the other cells of the body
2- Cells use free amino acids for the functions of that cell. These include:
a) Synthesize protein
b) Provide energy if not enough carbohydrate and fat for this function
3- Store as fat if too many amino acids present
Biological functions of proteins
Enzyme
Building blocks: Cell membrane structure and function
Muscles
Cytoplasm
Storage
Chromosome
Protective
Circulation and Transportation (Blood, P-proteins)
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Hormones and other chemical messengers
Immunity: Immune factors (antibodies)
Fluid balance
Acid-base balance
Source of energy and glucose.
Plasma Proteins
• More than 200
• Most abundant
– Albumin - 4-5 g/100 mL
– g-glubulins - ~1 g/100 mL
– fibrinogen - 0.2-0.4g/100 mL
• Original classification by zone electrophoresis at pH 8.6
Functions of Plasma Proteins:
1. Maintenance of:
a. Colloid osmotic pressure (p)
b. pH
c. electrolyte balance
2. Transport of ions, fatty acids, steroids, hormones etc.
a. Albumin (fatty acids), ceruloplasmin (Cu2+), transferrin (Fe),
lipoproteins (LDL, HDL)
3. Nutritional source of amino acids for tissues
4. Hemostasis (coagulation proteins)
5. Prevention of thrombosis (anticoagulant proteins)
6. Defense against infection (antibodies, complement proteins)
Proteins and disease
– Increase in serum total protein reflects increases in albumin, globulin, or
both.
– Generally significantly increased total protein is seen in volume contraction,
venous stasis, or in hypergammaglobulinemia.
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– Decrease in serum total protein reflects decreases in albumin, globulin or
both.
1- Albumin:
–
MW 66 000
– Single chain, 580 amino acids, sequence is known
– Dimensions - Heart shaped molecule
– 50% a helix
– Synthesis: Mainly liver cells then exported
• Functions
o “Colloid” osmotic pressure of blood is 80% due to albumin and thus
regulates water distribution
o Transport of fatty acids from the Liver to tissues for binding.
Albumin in disease:
– Increased absolute serum albumin content is not seen as a natural
condition.
– Relative increase may occur in hemoconcentration.
– Absolute increase may occur artificially by infusion of hyperoncotic
albumin suspensions.
– Decreased serum albumin is seen in states of decreased synthesis
(malnutrition, malabsorption, liver disease, and other chronic diseases),
increased loss (nephrotic syndrome, many GI conditions, thermal burns,
etc.), and increased catabolism (thyrotoxicosis, cancer chemotherapy,
Cushing's disease, familial hypoproteinemia).
2- Globulin:
• 20% of plasma proteins
• Represents a group of proteins of variable structure
– immunoglobulins
– 5 classes of immunoglobulins: IgG, IgA, IgM, IgD, IgE
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• Main functional task is immunochemical
– Antibodies - combine with specific antigens
Functions
– Primary function is antigen binding (immune response)
– Secondary function is complement binding (after antigen)
Synthesis
– In lymphocytes (T and B)
– Made in response to presence of antigen (“foreign” macromolecule,
virus particle etc.)
Globulin and disease:
– Globulin is increased disproportionately to albumin (decreasing the
albumin/globulin ratio) in states characterized by chronic inflammation and
in
B-lymphocyte
neoplasms,
like
myeloma
and
Waldenström's
macroglobulinemia. More relevant information concerning increased
globulin may be obtained by serum protein electrophoresis.
– Decreased
globulin
may
be
seen
in
congenital
or
acquired
hypogammaglobulinemic states. Serum and urine protein electrophoresis
may help to better define the clinical problem.
3- Fibrinogen:
• Function
– Blood coagulation (clotting)
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