Download 3. Proteins Classification (2017)

CLASSIFICATION OF
PROTEINS
Functions / importance of Proteins
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Type
Structural
Contractile
Transport
Storage
Hormonal
Enzyme
Protection
Energy
Examples
tendons, cartilage, hair, nails
muscles
hemoglobin, albumin
ferritin
eg. insulin, growth hormone
eg. hydroxylases
immunoglobulins
4.1 k.cal/gm
2
Classification of proteins based on
physical properties
CLASSIFICATION OF PROTEINS BASED ON PHYSICAL PROPERTIES
Class
Properties
Examples
I. FIBROUS PROTEINS Insoluble in aqueous solutions
elongated molecules often consisting
of several coiled polypeptide chains
1. Collagens
Can be converted into soluble gelatins The major proteins of
by boiling contain large amounts of
connective tissues
hydroxy-proline and hydroxylysine
but no cysteine or tryptophan
2. Elastin
Similar to collagens but cannot be
converted to gelatins by boiling
Proteins of tendons and
arteries
3. Keratins
Contain large amounts of cysteine
Hair, wool, nails (Hair is
about 14% cysteine)
CLASSIFICATION OF PROTEINS BASED ON PHYSICAL PROPERTIES
Class
Properties
Examples
II GLOBULAR
PROTEINS
Soluble in aqueous solutions spherical or
ellipsoidal in shape
1. Albumins
Readily soluble in pure water coagulated
by heat function as carriers for
hydrophobic molecules
Serum albumin, egg
albumin
2. Globulins
Insoluble or only slightly soluble in pure
water very soluble in aqueous salt
solutions can be coagulated by heat
Enzymes and antibodies
3. Histones
Basic proteins contain large amounts of
arginine and lysine soluble in pure water
Histones in chromatin
4. Protamines
Very basic proteins contain large amounts
of arginine but no tryptophan or tyrosine
Found in sperm cell
chromosomes
Classification of proteins based on
function
Classification of proteins based on function
Class
Properties
Examples
CATALYTIC PROTEINS
I. Enzymes
Catalyze chemical reactions
Lactate dehydrogenase (LDH)
amylase pyruvate
dehydrogenase
II. NONCATALYTIC
PROTEINS
1. Carrier proteins
Carry molecules or ions
through the bloodstream
Hemoglobin, albumin
2. Receptor proteins
Bind hormones and
neurotrans mitters to cell
membranes
The insulin receptor
3. Membrane transport
proteins
Carry molecules across cell
membranes
Na+ K+ Atpase, which
transports K+ ions into cells
and pumps Na+ ions out of
cells
4. Structural proteins
Form extracellular structures
such as hair and nails
Collagen keratin
5. Contractile proteins
Extend or contract to
produce movement of
muscles cells or subcellular
parts
Myosin, tubulin
Classification of proteins based on function……. cont
Class
Properties
Examples
6. Proteins hormones
Messenger molecules that
direct the activities of
various cells and organs
Insulin, adreno corticotropic
hormone (ACTH), growth
hormones
7. Antibodies
Bind to foreign substances
and activate their
elimination from the body
Anti-Rh, Anti –A (antibodies
in Rh factor and to blood
group A)
Another Classification
Simple
Conjugated
Derived
I SIMPLE PROTEINS
• Water soluble , coagulated by heat eg. albumin,
lactalbumin, ovalbumin
• Insoluble in water, soluble in dilute salt solutions.
Globulin
Heat coagulable to variable extent eg lact globulins,
immune globulins, myosine
Globin
• Rich in histidine. Unite with heme to form
hemoglobin
Prolamines • Soluble in 70-80% ethanol and insoluble in water eg.
gliadin of wheat and zein of maize
• Soluble in water but not in ammonium hydroxide.
Histones
Present in nucleus
Protamines • Like histones but present in sperm cells. Soluble in
ammonium hydroxide.
Albuminoids • Also called scleroproteins eg. collagen , elastin and
keratin
• Albumin
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II CONGUGATED PROTEINS
Types
Prosthetic
Group
Properties
Example
Nucleoproteins
Nucletic acid
(DNA,RNA)
Large, compact complexes
Chromatin, ribosomes
Mucoproteins*
Carbohydrate
More than 4%
carbohydrate by weight
Human chorionic
gonadotrophin, a
hormone used to test
for pregnancy
Glycoproteins *
Lipoproteins
Carbohydrate
Lipid
Less than 4% carbohydrate Antibodies
Water soluble
Serum lipoproteins
Proteolipids
Lipid
Not very water soluble,
soluble in nonpolar
solvents
Cell membranes
Hemoproteins
Heme group
Characteristic color
Hemoglobin,
cytochrome c
Metalloproteins
Metal ion (Fe3+, Require a metal ion to
Zn2+ Mg2+, Mn2+ ) function
Carbonic anhydrase
*The distinction between mucoproteins and glycoproteins is somewhat arbitrary
III Derived Proteins
• Primary Derived
• Eg denatured proteins
• Secondary Derived
• Polypeptides, oligopeptides,
proteosis, peptones
Peptides
• Amino acids can be polymerized to form chains:
– Two amino acids  dipeptide  one peptide bond.
– Three amino acids  tripeptide  two peptide bonds.
– Four amino acids  tetrapeptide  three peptide bonds.
– Few (2-20 amino acids)  oligopeptide.
– More (>20 amino acids)  polypeptide.
Peptide Bond (amide bond)
water is eliminated
O
two amino acids
condense to form...
H2N
CH
O
C
OH
H2N
CH
R1
OH
R2
N or amino
terminus
H2N
...a dipeptide. If
there are more it
becomes a polypeptide.
Short polypeptide chains
are usually called peptides
while longer ones are called
proteins.
C
O
CH
C
R1
O
NH
CH
C
R2
peptide bond is formed
residue 1
residue 2
C or carboxy
terminus
OH
+ HOH
Peptide bond
In proteins, amino acids are
joined covalently by peptide
bonds, which are amide
linkages between the ácarboxyl group of one amino
acid and the á-amino group of
another. For example, valine
and alanine can form the
dipeptide valylalanine through
the formation of a peptide
bond. Peptide bonds are not
broken by conditions that
denature proteins, such as
heating
or
high
concentrations of urea
Characteristics of the peptide
bond: The peptide bond has a
partial double-bond character,
that is, it is shorter than a single
bond, and is rigid and planar .
This prevents free rotation
around the bond between the
carbonyl
carbon
and
the
nitrogen of the peptide bond.
However, the bonds between the
á-carbons and the á-amino or ácarboxyl groups can be freely
rotated (although they are
limited by the size and character
of the R-groups). This allows the
polypeptide chain to assume a
variety
of
possible
configurations..
• Each amino acid in a chain makes two peptide bonds.
• The amino acids at the two ends of a chain make only one peptide
bond.
• The amino acid with a free amino group is called amino terminus
or NH2-terminus.
• The amino acid with a free carboxylic group is called carboxyl
terminus or COOH-terminus.
PEPTIDES AND PROTEINS
Amino terminal-
Carboxyl terminal-
N-terminal-
C-terminal
Oligopeptide :a few amino acids ≤ 100
Polypeptide : many amino acids ≥ 100