Biomolecules 2 Flashcards
what is meant by amino acids
Amino acids contain amino (–NH2
) and carboxyl (–COOH) functional
groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be
classified as a, b, g, d and so on. Only a-amino
acids are obtained on hydrolysis of proteins.
All a-amino acids have trivial names, which
usually reflect the property of that compound or
its source. Glycine is so named since it has sweet taste (in Greek glykos
means sweet) and tyrosine was first obtained from cheese (in Greek, tyros
means cheese.)
Essential amino acids
TV MILL PATH
T- Tryptophan
V- Valine
M-Methionine
I- Isoleucine
L- Lysine
L-Leucine
P-Phenylalanine
A- Arginine
T-Threonine
H- Histidine
Non essential amino acids
Glycine
Serine
Alanine
Tyrosine
Aspartic Acid
Glutamic acid
Asparagine
Glutamine
Cysteine
Proline
types of amino acids: acidic, basic, neutral
Amino acids are classified as acidic, basic or neutral depending upon
the relative number of amino and carboxyl groups in their molecule.
Equal number of amino and carboxyl groups makes it neutral; more
number of amino than carboxyl groups makes it basic and more
carboxyl groups as compared to amino groups makes it acidic.
types of amino acids: essential and non essential
The
amino acids, which can be synthesised in the body, are known as nonessential amino acids. On the other hand, those which cannot be
synthesised in the body and must be obtained through diet, are known
as essential amino acids
physical properties amino acids
Amino acids are usually colourless, crystalline solids. These are
water-soluble, high melting solids and behave like salts rather than
simple amines or carboxylic acids.
why do amino acids act more like salts than carbo acids or simple amines?
or
explain dipolar ion/ zwitter ionic form of amino acid
This behaviour is due to the presence of both acidic (carboxyl group) and basic (amino
group) groups in the same molecule.
In aqueous solution, the carboxyl group can lose a proton and amino group can accept a proton, giving rise to a dipolar ion known as zwitter ion. This is neutral but contains both positive and negative
charges.
In zwitter ionic form, amino acids show amphoteric behaviour as
they react both with acids and bases.
optical activity of amino acids
Except glycine, all other naturally occurring a-amino acids are
optically active, since the a-carbon atom is asymmetric. These exist
both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids have L-configuration. L-Aminoacids are represented by writing the –NH2 group on left hand side.
what is peptide bond
peptide linkage is an amide formed between –COOH group and –NH2
group.
The reaction between two molecules of similar or different amino acids, proceeds through
the combination of the amino group of one molecule with the carboxyl group of the other.
This results in the elimination of a water molecule and formation of
a peptide bond –CO–NH–.
The product of the reaction
is called a dipeptide because it is made up of two
amino acids.
For example, when carboxyl group of glycine combines with the amino group of alanine we get a dipeptide, glycylalanine.
what are proteins
When the number of such amino acids is more than ten, then
the products are called polypeptides.
A polypeptide with more than hundred amino acid residues, having molecular mass higher than 10,000u is called a protein.
However, the distinction between a polypeptide and a protein is
not very sharp.
Polypeptides with fewer amino acids are likely to be called
proteins if they ordinarily have a well defined conformation of a protein such as insulin which contains 51 amino acids.
distinguish types of protein based on molecular shape
(a) Fibrous proteins
- When the polypeptide chains run parallel and are held together by
hydrogen and disulphide bonds, then fibre– like structure is formed.
- Such proteins are generally insoluble in water.
- Some common examples are
keratin (present in hair, wool, silk) and myosin (present in muscles), etc.
(b) Globular proteins
- This structure results when the chains of polypeptides coil around
to give a spherical shape.
- These are usually soluble in water.
-Insulin and albumins are the common examples of globular proteins.
Primary structure of proteins
Proteins may have
one or more polypeptide chains. Each polypeptide in a
protein has amino acids linked with each other in a
specific sequence and it is this sequence of amino acids
that is said to be the primary structure of that protein.
Any change in this primary structure i.e., the sequence
of amino acids creates a different protein.
Secondary structure of proteins:
- The secondary structure of protein refers to the shape in which a long polypeptide chain can exist.
- They are found to exist in
two different types of structures viz. a-helix and b-pleated sheet structure. - These structures arise due
to the regular folding of the backbone of the polypeptide
chain due to hydrogen bonding between -C=O and
–NH– groups of the peptide bond.
-a-Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds
by twisting into a right handed screw (helix) with the –NH group of each amino acid residue hydrogen bonded to the C O of an adjacent turn of the helix.
- In b-pleated sheet structure all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery and therefore is known as
b-pleated sheet.
Tertiary structure of proteins:
The tertiary structure of proteins represents overall folding of the
polypeptide chains i.e., further folding of the secondary
structure.
It gives rise to two major molecular shapes viz. fibrous and globular.
The main forces which stabilise the 2° and 3° structures of proteins are
hydrogen bonds, disulphide linkages, van der Waals
and electrostatic forces of attraction.
Quaternary structure of proteins:
Some of the
proteins are composed of two or more polypeptide
chains referred to as sub-units.
The spatial
arrangement of these subunits with respect to each
other is known as quaternary structure.
