2. Proteins
• Proteins are the prime molecules of life, occuring in
every cell and in every part of the cell.
• Building material for the body structure.
• Protein is what we made of!
• Accounts for 17% of total body weight
• Proteins are composed of carbon, oxygen, hydrogen,
nitrogen and small amounts of sulphur.
• Proteins are important sources of body nitrogen.
3. Functions of proteins
• Structural proteins- Form the matrix of bone
and connective tissue, provide strength and
elasticity to organs and tissues (collagen,
elastin etc).
• Biocatalyst (enzymes). All enzymes are
proteins.
• Antibodies (defense molecules)
• Transporters (hemoglobin and albumin)
4. Functions of protein (contd)
• Hormones (insulin, growth hormone, etc)
• Contractile element in muscles (actin ,
myosin)
• Storage molecules ( ferritin)
• Control and regulate gene expression
(transcription and translation factors)
• Ion channels and pumps in cell membranes
• Stabilise genome structure (histone protein)
5. Proteins are polymers of Amino acids
• Amino Acids are the building blocks of proteins.
• Naturally, there are more than 300 known
amino acid
• Only 20 amino acids occur in (mammalian)
proteins through protein synthesis. (standard
amino acids)
6. Amino Acids
• Amino acids are the structural units
(monomers) of proteins.
• An aminoacid is made up of two functional
groups – amino (-NH2) and carboxyl (-COOH)
• Also contain a hydrogen atom (H) and side
chain (R) linked to the carbon atom.
8. Amino Acids (contd)
• Amino acids are distinguished from one another by their R groups,
attached to the α- carbon
• The α-carbon is chiral (except gly), ie is asymmetrical due to having 4
different groups attached (-NH2, -COOH, -R and -H)
• The two enantiomers of each amino acid are designated by D,L system
according to the D- and L-glyceraldehyde. D: Dextrorotation; L:
Levorotation
• Only the L-amino acids have been found in proteins.
• (D-isomers have been found only in small peptides of bacteria cell walls or
in some peptide antibiotics).
9. Standard Aminoacids
• Not every protein contains all the 20 aminoacids
• At the time of protein biosynthesis only 20 aminoacids
are incorporated into a protein (Standard aminoacids)
• Standard aminoacids possess the genetic code-
specifies the composition and size of a protein
• Biological system- produce protein- by combining the
same 20 aminoacids in different combinations.
10.
11.
12. Modified Protein Amino Acids
• Aminoacids which are modified after they are
incorporated into proteins
• Examples : Hydroxy proline and hydroxy lysine (proline
and lysine undergo hydroxylation)
• Essential for the formation of a mature collagen
molecule
• Some aminoacids are methylated (methyl lysine) and
acetylated (histone proteins and ribosomal proteins)
13.
14. Non Protein Aminoacids
• The non-protein aminoacids are not use to make
proteins, but possess some biological significance
• Occur as free or in combined states.
• Examples Taurine- component of bile acids,
Homocysteine – methionine metabolism
• Ornithine and citrulline occur in the urea cycle
(part of aa breakdown)
15. Non alpha Aminoacids
• The NH2 group is not attached to the alpha
carbon atom but some other carbon atom.
• Examples Beta alanine and γ-amino butyric
acid (GABA)
• Beta alanine (amino group is attached to
betacarbon)
16. The 21st and 22nd Aminoacid
• Selenocysteine is now recognised as the 21st
aminoacid found in protein
• It is structurally similar to cysteine in which
the Sulphur group is replaced by selenium
(Se)
• Pyrolysine 22nd aminoacid( found in methyl
transferase enzymes of certain bacteria)
17. Classification of Aminoacids
• Amino acids are classified based on solubility
properties, structure, chemical nature,
nitritional requirement and so on.
I. Classification Based on structure and
chemical Nature
II. Classification Based on Nutritional
Requirements
III. Classification Based on Metabolic Fate
IV. Classification Based on solubility
18. I. Classification Based on structure
and chemical Nature
1. Amino acids with aliphatic side chains
2. Aromatic aminoacids
3. Sulphur containing Amino acids
4. Hydroxyl (OH) group containing Amino acids
5. Acidic aminoacids and their amides
6. Basic aminoacids
7. Iminoacids
19. I. Classification Based on structure and
chemical Nature
1.Amino acids with aliphatic side chains
Simple aminoacids
• Glycine
• Alanine
Glycine is the only
aminoacid that is
optically inactive
Branched chain amino acids
• Valine
• Leucine
• Isoleucine
20. I. Classification Based on structure and
chemical Nature
2. Aromatic aminoacids (contains aromatic ring)
• Phenylalanine ( with phenyl ring),Tryptophan (with
indole ring), and Tyrosine (with phenol ring),
• Histidine (with imidazole ring) can also be considered as
aromatic aminoacid.
3. Sulphur containing Amino acids (contains sulphur
atom)
• Cysteine, methionine,
• cystine- condensation of 2 molecules of cysteine linked
through disulphide bridge
• cystine- contributes to higher levels of structural
organisation of proteins
21. I. Classification Based on structure and
chemical Nature
4. Hydroxyl (OH) group containing Amino acids
• Threonine and Serine,
• Tyrosine also contain OH group.
5. Acidic aminoacids and their amides
• Contains more than one carboxylic acid group
• Examples – Aspartate and glutamate are dicarboxylic
monoaminoacids
• Asparagine and Glutamine are their respective amides
6. Basic aminoacids
• Contains more than one amino group
• Lysine, Arginine and Histidine
• Dibasic monocarboxylic acids
7. Iminoacids
• Proline- contains an imino group (= N+H2)
22. II. Classification Based on Nutritional
Requirements
• Based on Nutritional requirements aminoacids
are classified into 3 groups
1. Essential amino acids (EAA)- These amino
acids cannot be synthesized in the body and
hence need to be supplied through diet
Examples- Arginie, Histidine, Valine, Isoleucine,
Leucine, Lysine, Methionine, Threonine,
Tryptophan, and Phenylalanine.
23. Essential amino acids (EAA) contd
MATT VILL PHLY (Mat will fly)
M- Methionine
A- Arginine
T- Threonine
T- Tryptophan
V-Valine
IL-Iso Leucine
L- Leucine
P- Phenylalanine
Ly- Lysine
H-Histidine
24. II. Classification Based on Nutritional
Requirements
2.Non-essential amino acids
• These amino acids are synthesized in the body, thus their absence in the
diet does not adversely affect the growth.
• Some are conditionally essential, which means still need to be
supplemented by diet on some occasions (growing age, pregnancy,
lactation and during recovery period of prolonged illness)
3. Semi- essential aminoacids
These amino acids can be synthesized in the body (not during infancy but
can be partly made by the adult) but the rate of synthesis is lesser than
the requirement (e.g. during growth, repair or pregnancy)
Examples-Arginine and Histidine
25. III. Classification based on their
metabolic fate
• The carbon skeleton of aminoacids is used to
synthesise lipids, glucose or both. Based on
this amino acids are classified into 3 groups
1. Ketogenic amino acids
• Fats can be synthezied from these amino
acids. Two amino acids Leucine and Lysine
are exclusively ketogenic.
26. III. Classification based on their
metabolic fate
2. Glycogenic and Ketogenic amino acid
• The four amino acids Isoleucine, Phenylalanine,
Tryptophan, Tyrosine are precursor for synthesis
of glucose as well as fat.
3. Glucogenic amino acids
• These amino acids serve as precursors for the
formation of glucose and glycogen.
• The remaining 14 amino acids will come under
this group
27. IV. Classification Based on solubility
• Amino acids are classified into 2 groups based on
solubility in water
1. Polar (hydrophilic- water loving)
Aminoacids with ionisable group (charged groups)
Acidic aminoacids: aspartate and glutamate (with
negatively charged groups)
Basic aminoacids: lysine, arginine and histidine
(with positively charged groups)
28. IV. Classification Based on solubility
• Certain aminoacids containing groups like
hydroxyl, sulphhydryl and amide are essentially
polar (glycine, serine, threonine, cysteine,
asparagine, glutamine and tyrosine)
2. Nonpolar (hydrophobic- water fearing)
• Contain uncharged groups
• Alanine,valine,
leucine,isoleucine,methionine,phenylalanine,tryp
tophan and proline are hydrophobic aminoacids.
29. Properties of Aminoacids
1. Isomerism- Exist in L-Form, Bcateria contains D-
form
2. Optical isomers- All Aa exihibits optical activity
excepy glcine
3. Solubility- Aa in general soluble in water
4. Melting point- High melting point (> 200 degree
celsius)
5. Taste- sweet, bitter, tasteless (Monosodium
glutamate- derivative of aminoacids, is widely
used in food preparations to increase taste and
flavour of foods)
30. Zwitter ion: at a specific pH, each aminoacid
carries both positive and negative charges in
equal numbers and exists as a Zwitter ion
Isoelectric point: Each aminoacid has a
characteristic pH at which it exists as a zwitter
ion, and carries no net, making the molecule
electrically neutral, known as isoelectric pH
(pI)
