Download 1. Amino Acids (2017)

AMINO ACIDS
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NB: 20 amino acids enter in protein structure. There are other
amino acids in the body, but they do not enter in protein e.g.
ornithine, citrulline are amino acids with other functions in the
body. They enter in formation of urea in the liver.
Free Amino Acid Pool
• Free amino acids can come from plasma or
muscle
• From muscle due to it’s mass that contains
~75% of the total body free AA
• Still, free AA are thought to contribute only
~1% to metabolically active AA pool.
.
• Amino acids are essential to integrity of skeletal
muscle, their use for energy is of concern
• Estimated amino acids contribute 5-15% of
energy during prolonged exercise
• Because energy demands are so high during
exercise, a small percentage is still substantial
General Structure of Amino Acid
• Carboxylic acid
group
• Amino group
• Side group R
gives unique
characteristics
Examples of Amino Acids
COOH
I
H2N—C — H
I
H
glycine
COOH
I
H2N—C — H
I
CH3
alanine
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Don’t Be Confused
COOH
I
H2N—C — H
I
H
glycine
H2N
I
H —CH — COOH
glycine
H
I
H2N—C — COOH
I
H
glycine
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AMPHOTERIC PROPERTIES OF AMINO ACIDS:
that is they have both basic and acidic groups and so can act
as a base or acid. This is called a zwitter ion
Neutral amino acids (monobasic, monocarboxylic) can exist
in aqueous solution as “ Zwitterion” i.e. contain both
positive and negative charge. Zwitterion is electrically
neutral and can’t migrate into electric field.
The presence of both an acid (carboxyl) and a base
(amine) in the same molecule leads to an interaction
between the two. This interaction results in a transfer of a
hydrogen ion from the acid portion to the base portion of
the molecule.
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The net charge of the zwitterion is 0. This leaves the acid end of the
amino acid with a negative charge (–COO-) and a positive charge at
the base end (–NH3 +). The deprotonated portion (portion that has
lost a hydrogen ion) is a carboxylate group, and the protonated group
(group that has gained a hydrogen ion) is an ammonium group. THE
PRESENCE OF A CHARGE ON THE AMINO ACID MAKES THEM WATER-SOLUBLE.
Figure shows zwitterion formation.
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ZWITTERION
Amino acids are polar
• Due to presence of
– polar covalent bonds N, O and H
– Atoms are capable to form hydrogen bonds
with water
– Carry charges COO- and NH3+
The water solubility of amino acids vary to some
extend, depending of side chain
CLASSIFICATION OF AMINO
ACIDS
CLASSIFICATION ACCORDING TO POLARITY OF SIDE CHAIN (R):
A- Polar amino acids: in which R contains polar hydrophilic group
so can forms hydrogen bond with H2O. : Have an UNEVEN
distribution of electrons such as Acids and Bases In those amino
acids, R may contain:
1- OH group : as in serine, threonine and tyrosine
2- SH group : as in cysteine
3- amide group: as in glutamine and aspargine
4- NH2 group or nitrogen act as a base (basic amino acids ): as
lysine, arginine and histidine
5- COOH group (acidic amino acids): as aspartic and glutamic .
CLASSIFICATION ACCORDING TO POLARITY OF SIDE CHAIN (R):
B- Non polar amino acids: :
Have an EVEN distribution of electrons
R is alkyl hydrophobic group which can’t enter in hydrogen bond
formation. 9 amino acids are non polar ( glycine, alanine, valine,
leucine, isoleucine, phenyl alanine, tryptophan, proline and
methionine)
NONPOLAR AMINO ACIDS
cont ….
Each of these amino acids has a nonpolar side chain that does
not gain or lose protons or participate in hydrogen or ionic
bonds . The side chains of these amino acids can be thought of
as “oily” or lipid-like, a property that promotes hydrophobic
inter-actions.
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C. AMINO ACIDS WITH ACIDIC SIDE
CHAINS
The amino acids aspartic and
glutamic acid are proton donors. At
physiologic pH, the side chains of
these amino acids are fully ionized,
containing a negatively charged
carboxylate group (–COO–). They are,
therefore, called aspartate or
glutamate to emphasize that these
amino acids are negatively charged at
physiologic pH
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D. AMINO ACIDS WITH BASIC SIDE CHAINS
The side chains of the basic amino acids accept protons .
At physiologic pH the side chains of lysine and arginine
are fully ionized and positively charged.
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AMINO ACIDS WITH BASIC SIDE CHAINS
….cont
In contrast, histidine is weakly basic, and
the free amino acid is largely uncharged
at physiologic pH. However, when
histidine is incorporated into a protein,
its side chain can be either positively
charged or neutral, depending on the
ionic environment provided by the
polypeptide chains of the protein. This is
an important property of histidine that
contributes to the role it plays in the
functioning of proteins such as
hemoglobin
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NUTRITIONAL CLASSIFICATION:
1- Essential amino acids: These amino acids can’t be
formed in the body and so, it is essential to be taken in
diet. Their deficiency affects growth, health and protein
synthesis.
H LIT TV LAMP
H= histidine* L= lysine I = isoleucine T = tryptophan
T= threonine V= valine L= leucine
A = arginine*
M= methionine P= phenyl alanine
*= arginine and histidine are semiessential
Nutritional classification:
2- Semiessential amino acids: These are formed in the
body but not in sufficient amount for body
requirements especially in children.
*= arginine and histidine are semiessential
3- Non essential amino acids: These are the rest of amino
acids that are formed in the body in amount enough
for adults and children. They are the remaining 10
amino acids.
METABOLIC CLASSIFICATION: according to metabolic or
degradation products of amino acids they may be:
1- Ketogenic amino acids: which give ketone bodies .
Lysine and Leucine are the only pure ketogenic amino
acids.
2- Mixed ketogenic and glucogenic amino acids: which
give both ketonbodies and glucose.These are: isoleucine,
phenyl alanine, tyrosine and tryptophan.
3- Glucogenic amino acids: Which give glucose. They
include the rest of amino acids. These amino acids by
catabolism yields products that enter in glycogen and
glucose formation.
Ketogenic amino acids
Leucine
Lysine