amino acids organic molecules that are the building block of · 2019. 3. 28. · •because amino...
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Amino acids
• Organic molecules that are the building block of
proteins.
• There is 20 α-amino acids commonly found in proteins
• They have a carboxyl group and an amino group bonded
to the same carbon atom [the α-carbon]).
• Amino acids differ from each other in their side chains
(R-group) which vary in structure, size, and electric
charge and which influence the solubility of the amino
acids in water.
• All amino acids have free α-carboxyl group.
• All amino acids except proline have free unsubstituted α-
amino group.
• The common amino acids of proteins have been assigned
three-letter abbreviations and one letter symbols.
Example:
Alanine: Ala
Serine : Ser
Classification on the basis of their R-group: depend on the polarity
of their R-groups [tendency to interact with water at biological PH
(near PH 7.0)]
• The polarity of the R-groups varies widely from non-polar and
hydrophobic (water-insoluble) to highly polar and hydrophilic
(water-soluble).
1) Amino acids with non-polar (hydrophobic) R-groups.
2) Amino acids with aromatic R-groups .
3) Amino acids with polar, uncharged R-group
4) Amino acids with positively charged (basic) R-group.
5) Amino acids with negatively charged (acidic) R-group.
Optical activity of amino acids
• All standard amino acids except glycine have an asymmetric
carbon atom [α-carbon atom bound to four different substituent
groups (i.e., a carboxyl group, amino group, R-group, and a
hydrogen atom).
• The asymmetric α-carbon atom is thus a chiral centre.
• Because amino acids that obtained from the hydrolysis of proteins
have one or more asymmetric carbon atoms
so all amino acids except glycine are optically active.
The α-carbon atom of all amino acids except glycine is
asymmetric, and thus amino acids exist in two stereoisomeric
forms: L-isomer, D-isomer.
• L- and D-isomer of amino acid depend on the configuration
of the four different substituents around the asymmetric
carbon atom.
Example:
Alanine
• Nearly all biologically occurring compounds containing
asymmetric carbon atom are found in nature in only one
stereoisomers form either D or L.
• Except for glycine, the amino acids present in protein
molecules are L-stereoisomers.
• When a compound has two or more asymmetric carbon
atom, it has 2n possible stereoisomers (n = number of
asymmetric carbon atoms).
Peptides
• Two or more amino acids covalently joined by peptide bonds.
• Two amino acid molecules can be covalently joined by
peptide bond to yield dipeptide.
• This linkage is formed by removal of the element of H2O
from the α-carboxyl group of one amino acid and the α-
amino group of the other by the action of strong reducing
agents.
• Three amino acids can be joined by two peptide bonds to form
a tripeptide. Similarly we have tetrapeptides and
pentapeptides. When there are many amino acids joined in the
same way (the structure is called a polypeptide).
• The amino acid residue at the end of peptide having a free α-
amino group is the amino terminal residue (N-terminal
residue).
• The residue at the opposite end, which has a free carboxyl
group, is the carboxyl terminal residue (C-terminal residue).
• Peptides contain only one free α-amino group and one free α-
carboxyl group at their terminal residues.
Proteins
• Proteins are made up of large numbers of amino acids
linked into chains by peptide bonds joining the amino
group of one amino acid to the carboxyl of the next.
• The number of amino acids present varies from about a
hundred to several thousands in different proteins.
• Some proteins are composed of only one polypeptide chain
while others are composed of two or more polypeptide
chains (multi-subunit proteins) held together by non-
covalent bonds.
Classification of proteins:
A) proteins can be classified
on the basis of the chemical
composition.
B) proteins can be classified
on the basis of shape.
C) proteins can be
classified on the basis
of their biological
function.
A) proteins can be classified on the basis of the chemical
composition:
a) Simple proteins b) conjugated proteins
a) Simple proteins:
are those proteins which upon hydrolysis give only amino acids.
Example:
ribonuclease A, chymotrypsin.
b) Conjugated proteins:
are proteins which yield upon hydrolysis organic or
inorganic components in addition to the amino acids.
•The non-protein part is called the prosthetic group.
•Conjugated proteins are classified on the basis of the
chemical nature of their prosthetic groups:
i) nucleoproteins
ii) glycoproteins
iii) lipoproteins
iv) hemoproteins
v) metalloproteins
vi) phosphoproteins
B) proteins can be classified on the basis of shape:
a) Globular proteins b) Fibrous proteins
a) Globular proteins:
• They are generally soluble in water.
• The polypeptide chains are tightly folded into a globular shape.
Example:
enzymes, hemoglobin, myoglobin
b) Fibrous proteins:
• They are insoluble in water
• Their polypeptide chains are arranged in long
strands (elongated in the form of fibers).
Example:
Collagen, keratin
C) proteins can be classified on the basis of their
biological function:
i) enzymes
ii) Transport proteins
[e.g., hemoglobin iii) Nutrient and
lipoprotein] storage proteins
[e.g., casein
ferritin] iv) contractile or
mobile proteins
(e.g., actin, myosin)
v) structural
proteins
vi) defense proteins (e.g., α-keratin,
[e.g., immunoglobulins(antibodies) collagen)
fibrinogen and thrombin]
vii) Regulatory proteins
(protein hormones)