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Contents

C H A P T E R

CONTENTS

10

• Classification based on the
source of protein moleucle

• Classification based on th
shape of protein molecule

• Classification based on
composition and solubility

• Classification based on
biological function

Proteins
Classification

T

he proteins are all remarkably similar in structure
insofar as they contain amino acids
...

However, a few systems based on one or the other cirterion
are given below
...
This molecular
movement is translated into movement of
the entire animal, excitingly depicted in da
Vinci’s rearing horse
...
1504) from the Royal
Collection ® Her Royal Majesty Queen
Elizabeth II
...
Animal proteins are the proteins derived from
animal sources such as eggs, milk, meat and fish
...
On the other hand, plant proteins are called
lower-quality proteins since they have a low content
(limiting amount) of one or more of the essential amino
acids
...
Although plant proteins have limiting amounts
of some (but not all) amino acids, it should not be construed
that they are poor protein sources
...
Limiting amino acids in some plant proteins
Food
Cereal grains and millets
Rice and soybeans
Legumes (peas and beans)
Groundnuts
Sunflower seeds
Green leafy vegetables

Amino acid(s)
Lysine, Threonine
Methionine
Methionine, Tryptophan
Methionine, Lysine, Threonine
Lysine
Methionine

CLASSIFICATION BASED ON THE SHAPE OF PROTEIN MOLECULE
On the basis of the shape of protein molecule, the proteins have been grouped under two categories :
globular and fibrous
...
Globular or Corpuscular Proteins
...
These are usually soluble in water or in aqueous media
containing acids, bases, salts or alcohol, and diffuse readily
...
Tertiary and quaternary structures are usually
associated with this class of proteins
...

A simple functional classification of globular proteins is not possible because of 2 reasons :
(a) Firstly, these proteins perform a variety of different functions
...

However, Conn and Stumpf (1976) have classified globular proteins as follows:
Cytochrome C
Blood proteins
Serum albumin
Glycoproteins
Globular
Antibodies (= Immunoglobulins)
Proteins
Hemoglobin
Hormones
Enzymes
Nutrient proteins
2
...

These have axial ratios greater than 10 and, henceforth, resemble long ribbons or fibres in shape
...
Most fibrous proteins serve in a structural or
protective role
...
These are:
(a) They can stretch and later recoil to their original length
...
e
...
A large scar, for example, creeps to a smaller size if there is no

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206

FUNDAMENTALS OF BIOCHEMISTRY

tension on the scar
...

It is a heterogeneous group and includes the proteins of connective tissues, bones, blood vessels,
skin, hair, nails, horns, hoofs, wool and silk
...
Collagens
...
More than half the total protein in
another
...

in sulfur since cysteine and cystine are lacking
...
Elastins
...

III
...
These are of ectodermal origin; form the major constituents of epithelial tissues
(skin, hair, feathers, horns, hoofs, nails); usually contain large amounts of sulfur in the form of cystine–
human hair has about 14% cystine
...
Fibroin
...


CLASSIFICATION BASED ON COMPOSITION AND SOLUBILITY
This is nowadays the most accepted system of classification and is based on the proposals made
by the committees of British Physiological Society (1907) and the American Physiological Society
(1908)
...
, simple,
conjugated and derived
...
Simple Proteins or Holoproteins
...
This group includes
proteins containing only amino acids, as structural components
...

These are further classified mainly on their solubility basis as follows:
1
...
These are basic proteins and occur almost entirely in animals,
mainly in sperm cells; possess simplest structure and lowest molecular weight (approximately 5,000);
soluble in water; unlike most other proteins, not coagulated by heat; strongly basic in character owing
to high content of basic amino acids (lysine, arginine); form salts with mineral acids and nucleic
proteins
...
Histones are somewhat
weaker bases and are, therefore, insoluble in NH4OH solution, whereas the protamines are soluble
...
g
...

2
...
These are widely distributed in nature but more abundant in seeds; soluble in
water and dilute solutions of acids, bases and salts;
precipitated with a saturated solution of an acid salt like Pseudoglobulins and euglobulins dif(NH4)2
...
SO4; coagulated by heat
...
g
...
A pseudoglobulin is
ovalbumin from white of egg, serum albumin from soluble at very low ionic strength,
blood plasma, myosin of muscles and lactalbumin whereas euglobulin remains sparingly
soluble until the ionic strength is
of milk whey
...
Globulins
...
Euglobulins remain soluble at
the isoelectric point (pl), whereas
and euglobulins*
...

in nature than the pseudoglobulins; either soluble

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CLASSIFICATION OF PROTEINS

207

(pseudoglobulins) or insoluble (euglobulins) in water; precipitated with half saturated solution of
(NH4)2
...

e
...
, pseudoglobulins— pseudoglobulin of milk whey
...

4
...
These have been isolated only from plant seeds; insoluble in water, dilute salt
solutions and alcohol solutions but soluble in dilute acids and alkalies; coagulated by heat
...
g
...

5
...
These have also been isolated only from plant seeds; insoluble in water and
dilute salt solutions but soluble in dilute acids and alkalies and also in 60 – 80% alcohol solutions; not
coagulated by heat
e
...
, gliadin from wheat, zein from corn, hordein from oat, etc
...

6
...
These occur almost entirely in animals and are, therefore,
commonly known as the ‘animal skeleton proteins’; insoluble in water, dilute solution of acids, bases
and salts and also in 60–80% alcohol solutions; not attacked by enzymes
...
g
...

B
...

These are also of globular type except for the pigment in chicken feathers which is probably of
fibrous nature
...
The prosthetic group may be either a metal or a compound
...

Their further classification is based on the nature of the prosthetic group present
...
(Instead of metalloproteins, chromoproteins etc
...
, are sometimes used
...
Metalloproteins
...
These may be of stable
nature or may be more or less labile
...
Metals strongly bound by
metalloprotein and constitutes about 30% of the total plasma
proteins
...
It has a molecular weight of about 90,000 and is cametals (Hg, Ag, Cu, Zn)
pable of binding 2 atoms of iron per mole
...

with proteins like
Ceruloplasmin is an important blue copper-binding protein in
collagen, albumin, casein
the blood of humans and other vertebrates
...
, through the —SH
tains about 90% of copper in serum
...

of about 150,000 and contains 8 atoms of copper per mole
...
It probably functions by reversibly releasing and
for Fe (siderophilin*) and
binding copper at various sites in the body, whereby regulatCu (ceruloplasmin**)
...
In its deficiency, the Wilson's disease
these cases, the following
develops in man which is characterized by hepatolenticular
pattern of binding may be
degeneration
...
Metals bound weakly by proteins
...
Here the binding takes place
with the help of radicals possessing the electron charge
...
Metals which do not couple with proteins
...
These form
compounds with nucleic acids where apparently electrostatic bonds are present
...
Chromoproteins
...
Such pigments have
also been found among the enzymes like catalase, peroxidase and flavoenzymes
...
The chloroplastin dissolves in water
as a colloid and is readily denatured
...
g
...

3
...
These
are the proteins containing carbohydrate as prosthetic
group
...

e
...
, glycoproteins— egg albumin, elastase
certain serum globulins
and also certain serum
albumins
...
Elastase is a protease found in serum
...

Note that the oligosaccharide chains have
4
...
These are proteins linked
substantial size even for this protein, which has
a relatively low level of glycosylation
...

e
...
, casein from milk and ovovitellin from egg
yolk
...
Lipoproteins
...

e
...
, lipovitellin and lipovitellenin from egg yolk; lipoproteins of blood
...
The classification of lipoproteins is frequently
based on an operational definition, i
...
, the migration of the fraction in a density gradient separation
...
These have densities greater than 1
...

(b) High density lipoproteins (HDLs)
...
063 to 1
...

(c) Low density lipoproteins (LDLs)
...
05 and 1
...

(d) Very low density lipoproteins (VLDLs)
...
93 to 1
...

Table 10–2 also lists the compositions of the fractions floated at the respective densities, along
with proposed functions
...
Classification of lipoproteins based on density
Name

Density range

Composition by weight
(in blood plasma)

Half-life

Function

Very high density
lipoprotein

> 1
...
063 – 1
...
05 – 1
...
93 – 1
...
Nucleoproteins
...
, protamines
and histones
...
They are present in nuclear
substances as well as in the cytoplasm
...

e
...
, nucleoproteins from yeast and thymus and also viruses which may be regarded as large
molecules of nucleoproteins; nucleohistones from nuclei-rich material like glandular tissues;
nuclein
...
Derived Proteins
...

This group also includes the artificially-produced polypeptides
...
Primary derived proteins
...

1
...
Insoluble in water; appear as first product produced by the action of acids, enzymes
or water on proteins
...
g
...

2
...
Insoluble in water but soluble in dilute acids or alkalies;
produced by further action of acid or alkali on proteins at about 30–60°C
...
g
...

3
...
Insoluble in water; produced by the action of heat or alcohol on proteins
...
g
...

II
...
These are derivatives of proteins in which the hydrolysis has
certainly occurred
...


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210

FUNDAMENTALS OF BIOCHEMISTRY

1
...
Soluble in water; coagulable by heat; produced when hydrolysis proceeds beyond
the level of metaproteins; primary proteoses are salted out by half saturation with (NH4)2
...
SO4 but are not precipitated by HNO3 or picric acid
...
g
...

2
...
Soluble in water; noncoagulable by heat; produced by the action of dilute acids or
enzymes when hydrolysis proceeds beyond proteoses; neither salted out by (NH4)2SO4 nor precipitated
by HNO3 or picric acid
...
Polypeptides
...
In fact, the proteins
are essentially long chain polypeptides
...
Although widely accepted, the system outlined above has certain discrepancies:
1
...

2
...

3
...

4
...

The two (second and third) systems of classification, described above, may be merged into one,
as shown on page 197
...
As such diverse proteins may be grouped under following categories,
based on the metabolic functions they perform (Table 10–3):
Table 10–3
...

Collagen, Elastin, Keratin, Fibroin
Myoglobin, Hemoglobin,
Ceruloplasmin, Lipoproteins
Ovalbumin, Casein, Ferritin
Actin, Myosin, Tubulin
Antibodies, Fibrinogen, Thrombin
Insulin, G proteins, Growth hormone
Snake venom, Ricin
...
Enzymic proteins
...
Virtually, all the chemical reactions of organic biomolecules are catalyzed by
the enzymes
...
Chemically, some enzymes are simple proteins,

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CLASSIFICATION OF PROTEINS

211

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212

FUNDAMENTALS OF BIOCHEMISTRY

containing only amino acid residues; others are complex proteins, containing a major protein part
(apoenzyme) and a small nonprotein part (prosthetic group) associated with the protein unit
...
Urease, amylase, catalase, cytochrome C, alcohol dehydrogenase are
some of the examples of enzymic proteins
...
Structural proteins
...
They
maintain the native form and position of the organs
...
Collagen, which has very high tensile strength, is the most abundant
protein of animals
...
Leather is almost pure collagen
...
α keratin constitutes almost the entire dry weight of hair,
wool, feathers, nails, claws, quills, scales, horns, hooves, tortoise shell and much of the outer layer of
skin
...
The wing hinges of some insects
are made of resilin, which has nearly perfect elastic properties
...
Transport or carrier proteins
...
Hemoglobin of
erythrocytes binds O2 as the blood passes through the lungs, carries it to the peripheral tissues, and
there releases it to participate in the oxidation of nutrients
...
Other kinds of transport proteins are present in the
plasma membranes and intracellular membranes of all organisms
...

4
...
Ovalbumin is the major protein of eggwhite
...
The seeds of many plants store nutrient proteins, required for the growth of
the germinating seedlings
...

5
...
Some proteins endow cells and organisms with the ability to
contract, to change shape, or to move about
...
Tubulin is the protein from which microtubules are
built
...
Defense proteins
...
The antibodies (or immunoglobulins), the specialized proteins made by the
lymphocytes of vertebrates, can precipitate or neutralize invading bacteria, viruses or foreign proteins
from another species
...

7
...
Some proteins help regulate cellular or physiological activity
...
The cellular response to many hormonal signal is often mediated
by a class of GTP-binding proteins called G proteins
...

8
...
Some proteins act as toxic substances, such as snake venom, bacterial toxins
and toxic plant proteins like ricin
...


EGG PROTEINS
The eggs and milk are consumed as food by an appreciable portion of the global population
...
Eggs contain 2 types of proteins : the egg
white protein and egg yolk protein
...


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CLASSIFICATION OF PROTEINS

213

Table 10–4
...
8% (on dry basis)
70% of total proteins
9%
13%

Total protein
Ovalbumin
Conalbumin
Ovomucoid
Globulins
Lysozyme (G1)
(G2)
(G3)
Mucin

2%

Avidin

0
...
6%
7%

(Adapted from Fevold HL, 1951 )
When egg yolk is diluted with water, proteins precipitate
...
Egg yolk contains 2 phosphoproteins, lipovitellin and
lipovitellenin, which differ from each other in the following characteristics :
Lipovitellin
Lipovitellenin
Lipid concentration : 17-18%
36-41% (mainly lecithins)
Phosphorus concentration : 1%
0
...

This fraction is called livetin
...

Whole egg is an excellent food because it is a very rich source not only of protein and lipid but
also of most of the vitamins (except vitamin C) and most of the required minerals (except calcium)
...


Milk (Fig
...
6–0
...
7
...
These whey proteins are separated into 2 fractions :
lactalbumin and lactoglobulin
...
7
...
5%) and human milk
(0
...
6%)
...
It is of 3 types : α, β and γ
...


REFERENCES
See list following Chapter 11

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