Biochemistry for medics
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Provide the monomer units from which the long
polypeptide chains of proteins are synthesized
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L-amino acids and their derivatives participate in cellular
functions as diverse as:
Nerve transmission and the
Biosynthesis of porphyrins,
Purines, pyrimidines, and urea.
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Short polymers of amino acids called peptides perform
prominent roles in the neuroendocrine system as :
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Each amino acid (except proline) has a carboxyl group, an
amino group and a distinctive side chain bonded to the
alpha carbon atom.
At physiological pH the carboxyl group is dissociated
forming the negatively charged carboxylate ion(-COO-
and the amino group is protonated(-NH3
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Amino acids can be classified in 4 ways:
1.Based on structure
2.Based on the side chain characters
3.Based on nutritional requirements
4.Based on metabolic fate
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1) Aliphatic amino acids
They are classified in three broad categories:
Mono amino mono carboxylic acids They are further
subdivided in 5 groups:
a. Simple amino acids-example: glycine, alanine
b. Branched chain amino acids-e.g.: valine,leucine,isoleucine
c. Hydroxyl group containing amino acids-e.g.: serine, threonine
d. Sulphur containing amino acids-e.g.: cysteine, cystine,
e. Amide group containing amino acids-e.g.-
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e.g.: β-alanine, γ-amino butyric acid (GABA), δ-amino Levulinic
Derived and Incorporated in tissue proteins:
e.g.: Hydroxy-proline, hydroxy-lysine
Derived but not incorporated in tissue proteins:
e.g.: Ornithine, Citrulline, Homocysteine, Arginosuccinic acid
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A. Amino acids with a non-polar side-chain:
e.g.: Alanine, Valine, Leucine, Isoleucine, Phenylalanine,
Each of these amino acids has a side chain that does
not bind or give off protons or participates in
hydrogen or ionic bonds.
Side chains of these amino acids can be thought of as
“Oily” or lipid like, a property that promotes
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e.g. Serine, Threonine, Tyrosine, Cysteine, Asparagine and Glutamine.
These amino acids are uncharged at neutral pH, although the side chains
of cysteine and Tyrosine can lose a proton at an alkaline pH.
Serine , Threonine and Tyrosine each contains a polar hydroxyl group
that can participate in hydrogen bond formation.
Side chains of Asparagine and Glutamine contain a carbonyl group and
amide group, they can also participate in hydrogen bond formation.
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a) Amino acids with a positively charged side-chain:
The basic amino acids- Lysine, Arginine and Histidine
b) Amino acids with a negatively charged side-chain:
The acidic amino acids- Glutamic acid and Aspartic acid
They are hydrophilic in nature.
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I. Essential amino acids:
These amino acids cannot be synthesized in the body and have to be present
essentially in the diet. Examples-
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II. Semi-essential amino acids:
These amino acids can be synthesized in the body but the
rate of synthesis is lesser than the requirement(e.g.
during growth, repair or pregnancy)
Examples-Arginine and Histidine.
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III. Non-essential amino acids:
These amino acids are synthesized in the body, thus their
absence in the diet does not adversely affect the growth.
Alanine, and the other remaining amino acids.
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The carbon skeleton of amino acids can be used either for glucose
production or for the production of ketone bodies, Based on that
I.Both glucogenic and ketogenic amino acids:
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II. Purely Ketogenic amino acids:
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II. Purely Glucogenic amino acids:
The remaining 14 amino acids are glucogenic-
Glycine, Methionine, Cysteine, Cystine,
Aspartic acid, Glutamic acid,
Histidine and Arginine.
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Of the over 300 naturally occurring amino acids, 20 constitute
the monomer units of proteins.
These 20 amino acids are called the Primary or Standard amino
Selenocysteine is the 21st Amino Acid
The other are Pyroglutamate and Pyrolysine.
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