1. Name :- Pruthwiraj Satish Wankhede
Roll No :- 213704
Class TYBSC ( Botany )
Paper :- Plant Physiology and Biochemistry
(BoT 3601 )
Mo no :- 8080690495
Email :- wankhedepruthwiraj@gmail.com
Topic :- Protein
2. Protein
Protein is by far the most important organic substance known so far in a living
cell.
The name protein was suggested by Berzelius (1938).
It is derived from proteios, a Greek word, meaning ‘first rank’.
Proteins are complex substances of high molecular weight ranging up to several
millions and contain nitrogen in addition to carbon, hydrogen, and oxygen.
Sometimes elements like phosphorus, sulphur, iron, zinc, and iodine may also be
present.
However, elementary composition of most proteins is very similar (approximate
percentage is C = 50 - 55, H = 6- 8, O = 20 - 23, N = 15 - 18 and S = 0 - 4).
Proteins are made up of several simple nitrogen containing organic molecules
called amino acids, i.e., proteins are polymerised forms or polymers of amino
acids. Thus, amino acid is the basic unit of protein.
3. Amino acid :-
These are organic compounds with at least one amino group and one carboxyl group.
Other functional groups may also be present. The chemical properties of amino acids
reflect the chemical properties of amino, carboxyl or other functional groups.
Theoretically limitless number of amino acids are possible.
In nature large number of them are found in free and combined forms. A partial list of
amino acids identified at least from one biological source includes 28 amino acids.
However, the names and structures of 20 amino acids which combine to form one or
other proteins are listed in the Table 3.1. Of these, all other but cysteine and
hydroxyproline are commonly found in proteins. These two are rarely known.
The general formula for majority of amino acids can be
4. A. Monoaminomonocarboxylic
1. Glycine (Neutral)
2. Alanine
3. Valine
4. Leucine
5. Isoleucine
D. Hydroxyl-containing
11. Threonine (Neutral)
12. Serine
E. Sulphur-Containing
13. Cysteine (Neutral)
14. Methionine
B. Monoaminodicarboxylic
1. Glutamic acid (Acidic)
2. Aspartic acid
F. Aromatic
15. Phenylalanine
16. Tyrosine
C. Diaminomonocarboxylic
1. Arginine (Basic)
2. Lysine
3. Hydroxylysine
G. Heterocyclic
17. Tryptophan
18.Proloine
19.Hydroxyproline (Basic)
20. Histidine
Table 3.1 Types of natural amino acids arranged groupwise.
5. H
ǀ
H2N - C- COOH
ǀ
R
Where R represents any one of the great variety of structures, eg. In glycine R=H,
In serine R = OH and in alanine R=CH3
Amino acids are amphoteric compounds, or they possess both acidic and basic groups.
The electrical charges on the amino acids influence their properties markedly. After losing
water molecules different kinds of amino acids link up with peptide bonds to form long
chains i.e different kinds of proteins.
Amino acids are the principal building blocks for proteins. Some amino acids like
tyrosine is converted into hormones thyroxin and adrenaline, glycine is involved in
formation of heme and tryptophan in the formation of vitamin nicotinamide and hormone
IAA.
6. Peptides : Peptides are primarily made up of amino acids. As
intermediates in the hydrolysis of proteins, peptides are formed.
Theoretically it is possible to combine quite many amino acids into
enormous number of peptides. These compounds are found in many
biological materials but even a rough estimate of naturally occurring
peptides is not yet available due to shortcomings in isolation and
separation techniques. Some investigators believe that the proteins are
formed directly from amino acids without intervening peptides, however,
others do not agree with this concept.
Proteins: as already stated these are chains of amino acids still larger
than peptides. These compound always per a giant macromolecule with
molecular weights in the range of 104 to 107. Although the list of 20 amino
acids in protein molecule is limited, the possible variety of protein is not.
The number of each kind of amino acids per protein molecule is widely
variable. The sequence of amino acids in the peptide chain is specific for
each protein and potentially capable of enormous diversity. Proteins may
differ in number of peptide chains per protein molecule. Amino acids form
peptides and peptides form proteins. Two peptides may be held together
with S bonds.
7. Classification of Proteins
Because of structural complexities, the classification of proteins cannot be a perfect one.
However, based on their solubility and chemical properties, these have been classified into
three major groups:
I. Simple proteins, II. Conjugated proteins, and III. Derived proteins,
(I) Simple Proteins:
Simple proteins contain only ordinary α-amino acids. These have been further divided into
seven major groups based on their solubility.
1. Albumins: Albumins are water soluble proteins but are also soluble in dilute salts, acids,
and alkalis. These coagulate by heat. Common albumins are legumelin from legume seeds,
β-amylose from barley, leucosin from cereals and albumins from soybeans.
2. Globulins. These are sparingly soluble in water but dissolve in salt solution of moderate
concentration. Globulins are less soluble in concentrated salt solutions than albumins. On
heating, these coagulate. Among examples legumin of peas and tuberin of potato are
common.
3. Glutelins: Glutelins are insoluble in water and salt solutions but are soluble in weak
acids or alkalies. These are not heat sensitive and are confined to plants. Common
examples are seed proteins particularly of cereals such as glutenin in wheat and oryzenin in
rice.
8. 4. Prolamines: These are insoluble in water but frequently dissolve in
70-80 percent ethanol (ethyl alcohol). These have relatively few polar
groups and are usually high in proline and amide groups. Examples are
gliadin (C635 H1068 N196 O211 S5 from wheat, hordein from barley and zein
(C736 H1161 N174 O208 S3) from maize.
5. Scleroprotein. Scleroproteins are insoluble in most of the common
solvents. Examples are structural and fibrous proteins. (Absent in plants)
6. Histones: These are soluble in water but insoluble in dilute ammonia.
Being high in lysine and arginine these are quite basic and are found
biologically associated with certain acidic structures. Globin,
haemoglobin, and protein components of nucleoproteins are common
examples of histones.
7. Protamines. These are small basic proteins soluble in water, dilute
acids and ammonia and are not coagulated by heat. Like histones,
protamines are rich in basic amino acids while tryptophan and tyrosine
are not found. This group is known only in combination with
nucleoprotein of sperms of fishes.
9. (II) Conjugated Proteins:
Proteins which contain non-amino acid components in addition to the amino acids are termed as conjugated
proteins. These additional non-amino acid components are referred to as prosthetic groups. The conjugated
proteins are classified into seven major types based on the nature of prosthetic groups.
1. Nucleoproteins: These are found in the nucleus conjugated with nucleic acid. On hydrolysis nucleoproteins
give rise to simple proteins and nucleic acid. These are weakly acidic and soluble in water.
2. Glycoproteins or mucoids: These are conjugated with carbohydrates usually macromolecular polysaccharides
containing acetylglucosamine, sugars, sugar acids and sulphate or phosphate esters. They tend to have unusual
properties based on the presence of many polar groups Gonadotropic hormone is an example, though not much is
known about it. The cell membranes are also thought to possess some amount of glycoproteins
3. Lipoproteins: They are conjugates of lipids and proteins. The prosthetic groups of them are lipids such as
lecithin and cephalin and are commonly found in cell membranes forming lipo-protein complex Nuclear,
mitochondrial and chloroplast membranes also possess them.
4. Chromoproteins: Chromoproteins possess pigment groups as their prosthetic groups and are coloured. The
colour may be due to metals such as Cu and Va or due to metals with organic groups such as in Fe and Mg-
porphyrins. The chromoproteins include diverse group of compounds such as flavoproteins, carotenoid proteins,
chlorophyll proteins and haemoglobins.
5. Metalloproteins: These proteins represent the group of enzymes which require metals as activators. Many
enzymes involved in respiration represent this group of protein.
6. Lecithoproteins: Lecithoproteins contain fats such as lecithin as prosthetic group, eg, white of an egg.
7. Phosphoproteins. These are soluble in alkalis and insoluble in water and contain phosphoric acid as prosthetic
group in them. The common examples are casein in milk and vitelline in egg.
10. (III) Derived Proteins:
Derived proteins are the degradation products obtained by hydrolysis of natural proteins
with acids, alkalis, and enzymes. Two categories of such proteins are recognised.
A Primary derived protein
B, Secondary derived proteins
Primary derived proteins: These have been further grouped into two:
i) Metaproteins: Meta Proteins are insoluble in water and dilute salt solution but soluble in
acids and alkalis. These are produced by hydrolysis of natural proteins, by alkalis or
prolonged treatment with acids
ii) Coagulated protein: These are insoluble in water and are produced out of action of heat
or alcohol. These are coagulated with heat, eg coagulated egg white
B) Secondary derived proteins: Three categories of these proteins are recognised
(i) Proteoses: Proteoses are soluble in water but are not coagulated by heat. Commonly
these are produced by prolonged hydrolysis of metaproteins e.g. albuminose from albumin.
(ii) Peptones: Like proteoses, these are soluble in water and are not coagulated by heat.
Further hydrolysis of proteoses by action of HCL, H2SO4, or certain enzymes yields
peptones. These give biuret test.
(iii) Peptides: These are soluble in water and are not coagulated by heat. Peptides are
produced by extensive hydrolysis with HCl or H2SO4 of natural proteins. They do not give
biuret test.
11. Structure of Proteins
Four basic structural levels of proteins are recognised by modern
biochemists. They are called the primary, secondary, tertiary, and
quaternary structures.
Primary structure:
The primary structure of protein refers to the linear sequence of amino
acid residues making up its polypeptide chain. Disulphide (s-s-) bond is
the other important characteristic of the primary structure. It may be
found among one or between two polypeptide chains. It does not make
protein functional.
Secondary structure:
It refers to the spirally coiled structure of the polypeptide chain called α-
helical form. It is most common structure. It is stabilised by extensive
hydrogen bonding. Hydrogen bonds between the carboxyl and the amino
acid groups of the peptide bonds. (It is formed between H atom attached
to peptide N, and O atom attached to peptide C). salt links and van der
Waals forces help in maintaining the helical structure. The second type is
β pleated sheet composed of two or more segments of fully extended
peptide chains. In β sheets, the hydrogen bonds are formed between the
neighbouring segments of polypeptide chains.
12. Tertiary structure: The term tertiary structure refers to the
arrangement of secondary structures into a three-dimensional (folded
and super folded) structure. Folding normally occurs from interactions
between amino acid residues relatively far apart in the sequence. It
makes the protein functional.
Quaternary structure: It refers to the association of more than one
polypeptide chain to form a stable unit. For example, enzyme
phosphorylase contains two identical subunits. Each subunit when
alone is catalytically inactive but when joined as a dimer forms the
active enzyme. If in the quaternary structure participating units are
similar, it is called homogeneous quaternary structure and if
dissimilar, it is called heterogeneous quaternary structure. A subunit
may also be called protomer, and a protein made up of more than one
protomer would be an oligomeric protein.
