Introduction to
Proteins and Amino acids
Contents
– Introduction to proteins
– Classification of aminoacids
Objective
At the end of this lecture, student will be
able to
– Explain
general concept of proteins
– Classify amino acid
– Explain properties of amino
acid
– Explain
the significance of amino acid
• Proteios:
holding the first place
• 10
– 12kg in adult body proteins
• Proteins
are the most abundant organic molecules of living system and forms the
fundamental basis of structure and function of life
• Large
molecules
• Made
up of chains of amino acids
• Are
found in every cell in the body
• Functions
are broadly grouped as structural and dynamic function
• Proteins
are polymers of amino acid
Elements of proteins
• Carbon
– 50%
• Hydrogen
– 6%
• Oxygen
– 19%
• Nitrogen
– 13%
• Sulfur
– 5%
• Other
elements are P, Fe, Cu, I, Mg, Mn, Zn etc
Classification of proteins
Structure of Proteins
• Proteins
on complete hydrolysis with conc. HCl yield L-α-amino acid and is the common properties of all proteins
• As
many as 300 amino acid occur in nature, out of these only 20 are known as
standard amino acid and are repeatedly found in structure of proteins
• Made
up of chains of amino acids; classified by number of amino acids in a chain
– Peptides: fewer than 50 amino acids
• Dipeptides:
2 amino acids
• Tripeptides:
3 amino acids
• Polypeptides:
more than 10 amino acids
– Proteins: more than 50 amino acids
• Typically
100 to 10,000 amino acids linked together
• Chains
are synthesizes based on specific bodily DNA
• An
adult has about 100g of free amino acid, which represents amino acid pool of
the body
• Four
levels of structure
– Primary
structure
– Secondary
structure
– Tertiary
structure
– Quaternary
structure
Any
alteration in the structure or sequencing changes the shape and function of the
protein
Denaturing
• Alteration
of the protein’s shape and thus functions through the use of
– Heat
– Acids
– Bases
– Salts
– Mechanical
agitation
• Primary
structure is unchanged by denaturing
Functions of protein
– Provide
structural and mechanical support
– Maintain
body tissues
– Functions
as enzymes and hormones
– Help
maintain acid base balance
– Transport
nutrients
– Assist
the immune system
– Serve
as a source of energy when necessary
Best
Sources of Protein
• Proteins
are abundant in
– Dairy
foods
– Meats
– Poultry
– Meat
alternatives such as dried beans, peanut butter, nuts, and soy
• Cooked
meat, poultry, or fish
– Provides
21–25 grams of protein
– About
7 g
– About
the size of a deck of cards
– Adequate
amount for one meal
Adults should consume 0.8 g/kg/d of protein
Amino acid
• Amino
acid are the group of organic compounds containing two functional groups ie.
Amino and carboxyl groups
• Amino
group is basic and carboxyl group is acidic in nature
• Amino
acid mostly exist in ionized form in biological system
• α– amino acid: If both –COOH and –
NH2 is attached to same carbon atom
Amino acid structure
Peptide Bonds Link Amino Acids
• Form
when the acid group (COOH) of one amino acid joins with the amine group (NH2)
of a second amino acid
• Formed
through condensation
• Broken
through hydrolysis
Condensation and
Hydrolytic Reactions
Classification of Amino acid
• Amino
acids are classified into different ways based on structure, Polarity,
nutritional requirement etc..
1. Classification based on structure:
• Each
amino acid is assigned with 3 letter or 1 letter symbol and are commonly used
to represents the amino acid in proten structure
• 20
amino acid found I proteins are divided into seven groups
I. Amino acid with aliphatic side chain
a.
Glycine – Gly or G
b.
Alanine – Ala – A
c.
Valine – Val – V
d.
Leucine – Leu – L
e.
Isoleucine – Ile – I
II. Amino acid containing hydroxyl group
f.
Serine – Ser – S
g.
Threonine – Thr – T
III. Sulfur containing amino acid
h.
Cysteine – Cys- C Cystine
– Cys- C
i.
Methonine – Met- M
IV. Acidic amino acid and their amides
j.
Asparitc acid – Asp – D
k.
Asparagine – Asn – N
l.
Glutamic acid – Glu – E
m.
Glutamine – Gln – Q
V. Basic amino acid
n.
Lysine – Lys – k
o.
Arginine – Arg – R
p.
Histidine – His – H
IV. Aromatic amino acid
q.
Phenylanaline – Phe – F
r.
Tyrosine – Tyr – Y
s.
Tryptophane – Trp – W
VII. Imino acid
t.
Proline – Pro –P
2. Classification based on polarity: 4 types
a. Non Polar amino Acid:
Amino acid are also referred as hydrophobic (water heating). They have no
charge in Rl group. e.g.: Alanine,
Leucine, Isoleucine, Valine etc
b. Polar amino acid
with no charge on Rl group: amino acid with no charge on Rl
group. However they possess other groups such as hydroxyl, sulfhydryl &
amide and precipitate in hydrogen bonding of protein synthesis. e.g.: glycine, serine, threonine,
cystine, tyrosine
c. Polar amino acid
with + ve charge on Rl group: e.g. Leucine, Argenine, Histidine
d. Polar amino acid
with – ve charge on Rl group: e.g. Glutamic acid, Aspartic acid
3. Nutritional classification of amino acid
20 amino acid are required for the synthesis of various
proteins
Based on nutritional requirement. They are 2 types
a. Essential amino
acid: Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methonine,
Phenylalanine, Threonine and Tryptophan
b. Non-essential
amino acid: Remaining all
4. Amino acid
classification based on their metabolic fate
Carbon skeleton of amino acid can serve as a precursor for
the synthesis of glucose, fats or both
a. Glycogenic Amino
acid: amino acid are precursor for the formation of glucose or glycogen
e.g.: Alanine, Aspartate, Glycine
b. Ketogenic amino
acid: amino acid are precursor for the formation of fats. e.g.: Leucine and
Lysine
c. Glycogenic &
ketogenic amino acid: amino acid are precursor for the formation of glucose
and fats e.g.: Isoleucine, Phenylalanine, Tryptophan, Tyrosine etc.
Properties of Amino acid
Amino acid differ in their physiochemical properties which
determine the characteristics of proteins
I.
Physical properties
- Solubility: Most are soluble in water and
insoluble in organic solvent - Melting
point: melts at high temperature (above 200oc) - Taste:
may be sweet, tasteless, or bitter - Optical
properties: All amino acid except glycine possess optical isomers due
to presence of asymmetric carbon atom - Amino acid as ampholytes: it
contains both functional group i.e. acidic & basic group. They can
donate a proton or accept a proton. Hence regarded as ampholytes - Zwitterion or Dipolar ion: Zwitter
means hybrid. - zwitterion
is a hybrid molecule contain both +ve and –ve ionic group. - Isoelectric PH (PI):
is defined as the PH at which molecules exists as Zwitterion,
thus molecules are electrically neutral
II. Chemical properties
Based on two functional group
- Reaction
due to –COOH group
1.
Amino acid forms salt (-COONa) with base
& esters with alcohol (-COORI)
2.
Decarboxylation: Amino acid undergoes
decarboxylation to form amines
3. Reaction
with ammonia: the carboxylic group of dicarboxylic amino acid react with NH3
to form amide
Aspartic
acid + NH3 Asparagine
Glutamic
acid + NH3 Glutamine
b. Reaction due to – NH2 group:
- Amino
acid group behaves as bas and combined with acid to form salts - Reaction with Ninhydrin: Amino
acid react with ninhydrin to form a purple, blue or pink colour complex - Transamination: transfer of amino
group from one amino acid to a ketoacid to form a new amino acid. Imp in
amino acid metabolism - Oxidative deamination: Amino acid
undergoes oxidative deamination to liberate free ammonia
• Amino
acid are chiral molecules. Only L-amino acid are found in proteins, D form
occurs in bacterial peptides
Peptides
• Peptides (from
Greek word meaning “digested”) are biologically occurring short
chains of amino acid monomers linked by peptide (amide)
bonds
Functions
• Act
as biological catalyst called enzymes
• Provide
structural frame work of cells and tissues
• Act
as transport media in blood stream
• Act
as hormones or regulatory proteins for controlling biological process
• Perform
mechanical work (skeletal muscle contraction, pumping of heart)
• Serve
as essential nutrient
• Act
as antibodies in the blood stream
• Act
in clotting mechanism
Some important
biologically active peptides in human body
Protein Turnover
• Protein
turnover is the balance between protein synthesis and protein degradation.
• More
synthesis than breakdown indicates an anabolic state that builds lean tissues,
more breakdown than synthesis indicates a catabolic state that burns lean
tissues.
Nitrogen Balance
• Nitrogen
balance is the measure of nitrogen input with the nitrogen output subtracted
from it
• Nitrogen
Balance = Nitrogen intake – Nitrogen loss
• Blood
urea nitrogen can be used in estimating nitrogen balance, as can the urea
concentration in urine
• A
positive value is often found during periods of growth, tissue repair or
pregnancy.
• A
negative value can be associated with burns, fevers, wasting diseases and other
serious injuries and during periods of fasting.
• This
means that the amount of nitrogen excreted from the body is greater than the
amount of nitrogen ingested
• A
negative Nitrogen balance can be used as part of a clinical evaluation of
malnutrition
Summary
• Protein
are polymers of amino acid
• Amino
acid are the group of organic compounds containing two functional group i.e.
amino and carboxyl group
• Amino
acid exhibit zwitterion
• Nitrogen
balance is the measure of nitrogen input with the nitrogen output subtracted
from it