proteins-1

Question 1

protein intro

Answer 1

In 1839 Dutch chemist GJ Mulder while investigating
substances such as those found in milk, egg found that

they could be coagulated on heating and were nitroge-
nous compounds. Swedish scientist JJ Berzelius

suggested to Mulder that these substances should be
called proteins. The term is derived from Greek word
Proteios means “primary”, or “holding first place” or
“pre-eminent” because Berzelius thought them to be most
important of biological substances. And now we know
that proteins are fundamental structural components of
the body. They are nitrogenous “macromolecules”
composed of many amino acids.

Question 2

biomedical importance of proteins

Answer 2

• Proteins are the main structural components of the
  cytoskeleton. They are the sole source to replace Nitrogen
  of the body.
• Biochemical catalysts known as enzymes are proteins.
• Proteins known as immunoglobulins serve as the first line
  of defence against bacterial and viral infections.
• Several hormones are protein in nature.
• Structural proteins furnish mechanical support and some
  of them like actin and myosin are contractile proteins and
  help in the movement of muscle fibre, microvilli, etc.
• Some proteins present in cell membrane, cytoplasm and
  nucleus of the cell act as receptors.
• The transport proteins carry out the function of trans-
  porting specific substances either across the membrane

or in the body fluids.
* Storage proteins bind with specific substances and store
them, e.g. iron is stored as ferritin.
* Few proteins are constituents of respiratory pigments and
occur in electron transport chain or respiratory chain, e.g.
cytochromes, hemoglobin, myoglobin.
* Under certain conditions proteins can be catabolised to
supply energy.
* Proteins by means of exerting osmotic pressure help in
maintenance of electrolyte and water balance in body.

Question 3

composition of proteins

Answer 3

In addition to C, H, and O which are present in carbo-
hydrates and lipids, proteins also contain N. The nitrogen

content is around 16 per cent of the molecular weight of
proteins. Small amounts of S and P are also present. Few
proteins contain other elements such as I, Cu, Mn, Zn
and Fe, etc.

Question 4

amino acids

Answer 4

acids: Protein molecules are very large molecules
with a high molecular weight ranging from 5000 to
25,00,000. Protein can be broken down into smaller units
by hydrolysis. These small units the monomers of
proteins are called as amino acids. Proteins are made up
from, 20 such standard amino acids in different sequences
and numbers. So an indefinite number of proteins can be
formed and do occur in nature. Thus proteins are the
unbranched polymers of L- α-amino acids.
R is called a side chain and can be a hydrogen,
aliphatic, aromatic or heterocyclic group. Each amino
acid has an amino group –NH2, a carboxylic acid group
– COOH and a hydrogen atom each attached to carbon
located next to the – COOH group. Thus the side chain
varies from one amino acid to the other.

Question 5

classification of amino acids

Answer 5

Amino acids can be classified into 3 groups depending
on their reaction in solution.
A. Neutral
B. Acidic and
C. Basic.

Question 6

neutral amino acids

Answer 6

A. Neutral amino acids: This is the largest group of
amino acids and can be further subdivided into aliphatic,
aromatic, heterocyclic and S-containing amino acids.

Question 7

aliphatic amino acids (neutral)-simple monoamino monocarboxy-
lic acids.

Answer 7

1. Glycine (Gly) or α-amino acetic acid.
2. Alanine (Ala) or α-amino propionic acid.
3. Valine (Val) or α-amino-isovaleric acid.
4. Leucine (Leu) or α-amino-isocaproic acid.
5. Isoleucine (Ile) or α-amino-β-methyl valeric acid.

Question 8

hydroxy group amino acids

Answer 8

6. Serine (Ser) or α-amino-β-hydroxy propionic acid.
7. Threonine (Thr) or α-amino-β-hydroxybutyric acid.

Question 9

aromatic amino aciids

Answer 9

Second subgroup of neutral amino acids consists of
aromatic amino acids.
8. Phenylalanine (Phe) or α-amino-β-phenyl propionic
acid.

9. Tyrosine (Tyr) or parahydroxy phenylalanine or
   α-amino-β-parahydroxy phenylpropionic acid.

(c) Heterocyclic Amino Acids: Third group belongs to
heterocyclic amino acids.
10. Tryptophan (Trp) or α-amino-β-3-indole propionic
acid. This amino acid is often considered as aromatic
amino acid since it has aromatic ring in its structure.

11. Histidine (His) or α-amino-β-imidazole propionic
    Histidine is basic in solution on account of the
    imidazole ring and often considered as Basic Amino acid.
    (d) Imino Acids
12. Proline (Pro) or Pyrrolidone-2-carboxylic acid.
13. Hydroxyproline (Hyp) or 4 hydroxy pyrrolidone-2
    carboxylic acid.

Proline and Hydroxyproline do not have a free –NH2
group but only a basic pyrrolidone ring in which the
Nitrogen of the Imino group is in a ring but can still
function in the formation of peptides. These amino acids
are therefore called as imino acids.
(e) ‘S’ containing amino acids: The fourth subgroup of
neutral amino acids contains two sulphur containing
amino acids.
14. Cysteine (Cys) or α-amino-β-mercaptopropionic
acid.
Two molecules of cysteine make cystine (cys-cys) or
dithio β, β-α aminopropionic acid. The S–S linkage is
called as disulphide bridge.

15. Methionine (Met) or α-amino γ-methylthio-η-butyric

Question 10

acidic amino acids

Answer 10

These amino acids have two
–COOH groups and one – NH2 group. They are therefore
monoaminodicarboxylic acids.

16. Aspartic acid (Asp) or α-amino succinic acid.

Asparagine (Asn) or γ -amide of α-aminosuccinic acid.

17. Glutamic Acid (Glu) or α-aminoglutaric acid.

Glutamine (Gln)-Amide of Glutamic Acid or δ-amide of
α-aminoglutaric acid.

Question 11

basic amino acids

Answer 11

C. Basic amino acids: This class of amino acids consists
of those amino acids which have one – COOH group and
two –NH2 groups. Thus they are diamino monocarboxy-
lic acids. Arginine, lysine and hydroxylysine are included
in this group.
18. Arginine (Arg) or α-amino- δ-guanidino-n-valeric acid.

19. Lysine (Lys) or α- ε-diamino caproic acid.
20. Hydroxylysine (Hyl) or α-ε-diamino-δ-hydroxy-n-
    valeric acid.

As already mentioned histidine is also classified as
basic amino acid.

Question 12

non-standard similar to amino acids but not in proteins

Answer 12

A. The compounds similar to basic structure of amino
acids but do not occur in proteins. Examples of some of
those are:
* β-alanine: They found in coenzyme A.
* Taurine: They found in bile acids
* Ornithine and citrulline: They are intermediates in
urea cycle
* Thyroxine (T4) and Tri-iodo Thyronine (T3): Thyroid
hormones synthesised from tyrosine.
* γ-aminobutyric acid (GABA): A neurotransmitter
produced from glutamic acid.
* β-amino isobutyric acid: These are end product of
pyrimidine metabolism.

• δ-aminolaevulinic acid (δ-ALA): These are inter-
  mediate in haem synthesis.
• S-adenosyl methionine (SAM): These are methyl
  donor formed from L-methionine
• 3, 4-dihydroxy phenyl alanine (DOPA): A precursor
  of mela nine pigment.

Question 13

non standard D amino acids

Answer 13

B. D-amino acids: These are non-standard amino acids—
Amino acids normally isolated from animal and plants are
L-amino acids. But certain D-amino acids are found in
bacteria and antibiotics and in brain tissues of animals.
* D-glutamic acid and D-Alanine are constituents of
bacterial cell walls.
* D-amino acids are found in certain antibiotics, e.g.
gramicidin-S, Actinomycin-D.
* Animal tissues contain L-amino acids which are
deaminated by L-amino acid oxidase. But there is also
present D-amino acid oxidase the function of which
was not known. Now D-amino acids like D-aspartate
and D-serine have been found in brain tissue. This
explains the existence of D-amino acid oxidase.

Question 14

functions of amino acids

Answer 14

Apart from being the monomeric constituents of proteins and
peptides, amino acids serve variety of functions.
(a) Some amino acids are converted to carbohydrates and
are called as glucogenic amino acids.
(b) Specific amino acids give rise to specialised products, e.g.
* Tyrosine forms hormones such as thyroid hormones,
(T3, T4), epinephrine and norepinephrine and a
pigment called melanin.
* Tryptophan can synthesise a vitamin called niacin.
* Glycine, arginine and methionine synthesise creatine.
* Glycine and cysteine help in synthesis of Bile salts.
* Glutamate, cysteine and glycine synthesise glutathione.
* Histidine changes to histamine on decarboxylation.
* Serotonin is formed from tryptophan.
* Glycine is used for the synthesis of haem.
* Pyrimidines and purines use several amino acids for
their synthesis such as aspartate and glutamine for
pyrimidines and glycine, aspartic acid, Glutamine and
serine for purine synthesis.
(c) Some amino acids such as glycine and cysteine are used
as detoxicants of specific substances.

(d) Methionine acts as “active” methionine (S-adenosyl-
methionine) and transfers methyl group to various

substances by transmethylation.
(e) Cystine and methionine are sources of sulphur.

Question 15

essential amino acids types

Answer 15

Nutritionally, amino acids are of two types: (a) Essential
and (b) Non-essential. (c) There is also a third group of
semi-essential amino acids.

Question 16

essential amino acids

Answer 16

These are the ones which are
not synthesised by the body and must be taken in diet.
They include valine, leucine, isoleucine, phenylalanine,
threonine, tryptophan, methionine and lysine. For
remembering the following formula is used—MATT VIL
PHLY.

Question 17

non-essential amino acids

Answer 17

They can be synthesised
by the body and may not be the requisite components of
the diet.

Question 18

semi-essential amino acids

Answer 18

(c) Semi-essential amino acids: These are growth
promoting factors since they are not synthesised in
sufficient quantity during growth. They include arginine
and histidine. They become essential in growing children,
pregnancy and lactating women.

Question 19

occurance of amino acids

Answer 19

All the standard amino acids
mentioned above occur in almost all proteins. Cereals are
rich in acidic amino acids Asp and Glu while collagen is
rich in basic amino acids and also proline and hydroxy-
proline.

Question 20

new amino acids

Answer 20

In addition to 20 L-amino acids that take part in protein
synthesis, recently two more new amino acids described.
They are:
A. Selenocysteine - 21st amino acids
B. Pyrrolysine - 22nd amino acid

Question 21

selenocysteine

Answer 21

Selenocysteine is recently introduced as 21st amino acid.
Selenocysteine occurs at the “active site” of several
enzymes.
Examples include:
* Thioredoxin reductase
* Glutathione peroxidase which scavenges peroxides,

• De-iodinase that converts thyroxine to tri-
  iodothyronine
• Glycine reductase
• Selenoprotein P, a glycoprotein containing 10
  selenocysteine residues, found in mammalian blood.
  It has an antioxidant function and its concentration
  falls in selenium deficiency.
  Selenocysteine arises co-translationally during its
  incorporation into peptides. The UGA anticodon of the
  unusual tRNA designated tRNAsec, normally signals
  STOP.
  The ability of the protein synthesising apparatus to
  identify a selenocysteine specific UGA codon involves
  the selenocysteine insertion element, a stem-loop
  structure in the untranslated region of the m-RNA.
  Selenocysteine-tRNAsec is first charged with serine by
  the Ligase that charges tRNAsec. Subsequent replacement

of the serine oxygen by selenium involves seleno-
phosphate formed by Selenophosphate synthetase.

Question 22

pyrrolysine

Answer 22

Recently it has been claimed as 22nd amino acid by some
scientists. The STOP codon UAG can code for
pyrrolysine.

Question 23

properties of amino acids

Answer 23

-isomerism
-amphoteric nature and isoelectric pH
-chemical properties

Question 24

isomerism

Answer 24

Two types of isomerism are shown by
amino acids basically due to the presence of asymmetric
carbon atom. Glycine has no asymmetric carbon atom
in its structure hence is optically inactive.
(a) Stereoisomerism: All amino acids except glycine exist

in D and L isomers. As described in the chapter on carbo-
hydrates it is an absolute configuration. In D-amino acids

– NH2 group is on the right hand while in L-amino acids
it is oriented to the left. It is the same orientation of – OH
group of the central carbon of glyceraldehyde
Natural proteins of animals and plants generally
contain L-amino acids. D-amino acids occur in bacteria.

Question 25

optical isomerism

Answer 25

All amino acids except glycine have asymmetric carbon atom. Few amino acids like isoleucine and threonine have an additional asymmetric carbon in their structures. Consequently all but glycine exhibit ‘optical’ activities and rotate the plane of plane polarised light and exist as dextrorotatory (d) or laevorotatory (l) isomers. Optical activity depends on the pH and side chain.

Question 26

amphoteric nature and isoelectric pH

Answer 26

The -NH2 and -COOH groups of amino acids are ionizable groups. Further, charged polar side chains of few amino acids also ionise. Depending on the pH of the solution these groups act as proton donors (acids) or proton acceptors (bases). This property is called as amphoteric and therefore amino acids are called as ampholytes. At a specific pH the ami11no acid carries both the charges in equal number and exists as dipolar ion or “Zwitterion”. At this point the net charge on it is zero, i.e. positive charges and negative charges on the protein/amino acid molecule equalizes. The pH at which it occurs without any charge on it is called pI or isoelectric pH. On the acidic side of its pI amino acids exist as a cation by accepting a proton and on alkaline as anion by donating a proton.

Question 27

physical properties

Answer 27

They are colourless, crystalline substances, more soluble in water than in polar solvents. Tyrosine is soluble in hot water. They have high melting point usually more than 200°C. They have a high dielectric constant. They possess a large dipole moment.

Question 28

chemical properties

Answer 28

due to carboxylic group 1- formation of esters 2-reduction to amino alcohol 3-formation of amines by decarboxylation 4-formation of amides properties due to amino group -formation of acyl derivatives -salt formation with acids -oxidation -reaction with HNO2 -reaction with CO2 -reaction with formaldehyde -specific color reaction properties due to both

Question 29

formation of esters

Answer 29

1. Formation of esters: They can form esters with alcohols. The COOH group can be esterified with alcohol. Treatment with Na2CO3 solution in cold releases the free ester from ester hydrochloride.

Question 30

reduction to amino alcohol

Answer 30

2. Reduction to amino alcohol: This is achieved in pre- sence of lithium aluminium hydride.

Question 31

formation of amines by decarboxylation

Answer 31

3. Formation of amines by decarboxylation: Action of specific amino acid decarboxylases, dry distillation or heating with Ba(OH)2 or with diphenylamine evolves CO2 from the —COOH group and changes the amino acid into its amine (Fig. 6.2). In vivo, the amino acids can be decarboxylated by the enzyme decarboxylase and forms the corresponding amines.

Question 32

formation of amide

Answer 32

4. Formation of amides: Anhydrous NH3 may replace alcohol from its combination with an amino acid in an amino acid ester so that an amide of amino acid and a molecule of free alcohol is produced

Question 33

salt formation with acids

Answer 33

1. Salt formation with acids: The basic amino group reacts with mineral acids such as HCl to form salts like hydrochlorides

Question 34

formation of acyl derivaties

Answer 34

2. Formation of acyl derivatives: Amino group reacts with acyl anhydride or acyl halides such as benzoyl chloride and give acyl amino acids like benzoyl glycine (hippuric acid). Incidentally, this is one of the mechanisms of detoxication in which glycine is used and this also forms the basis of one of the liver function tests.

Question 35

oxidation

Answer 35

3. Oxidation: Potassium permanganate or H2O2 oxidises the NH2 group and converts the amino acid into imino acid which reacts with water to form NH3 and α-keto- acid.

Question 36

reaction with HNO2

Answer 36

4. Reaction with HNO2: Like other primary amines, the amino acids except proline and hydroxyproline react with HNO2 (nitrous acid) libering N2 from NH2 group. This forms the basis of Van Slyke’s method for determining -NH2 group (Nitrogen)

Question 37

reaction with CO2

Answer 37

5. Reaction with CO2: The amino acid anion present in an alkaline solution may react with CO2 through NH2 group to form a carbaminoacid anion.

Question 38

reaction with formaldehyde

Answer 38

Formaldehyde reacts with the-NH2 group to form a methylene compound. Application: Because of the presence of free basic amino group in the amino acid molecule its amount cannot be estimated directly by titration with a standard alkali. On addition of neutral formaldehyde it combines with the amino group to form either methylene amino acid or dimethylol amino acid. Both these products are strong acids and may be estimated by titration with a standard alkali. This is known as “Sorensen’s” formol titration method

Question 39

specific colour reactions

Answer 39

7. Specific colour reactions: Reactions with Ninhydrin, Millon’s test, Sakaguchi test, Hopkins-Cole test are discussed under properties of proteins.

Question 40

properties due to both groups

Answer 40

In addition to the property of reacting with both cation and anion, the amino acids form chelated, co-ordination complexes with certain heavy metals and other ions. These include Cu++, Co++, Mn++ and Ca++. An example of chelated complex of Ca and glycine is given in

Question 41

protein classification

Answer 41

Proteins are classified: I. On the basis of shape and size II. On the basis of functional properties III. On the basis of solubility and physical properties.

Question 42

on the bais of shape and size

Answer 42

* Fibrous proteins: When the axial ratio of length: width of a protein molecule is more than 10, it is called a fibrous protein. Examples: α-keratin from hair, collagen. * Globular protein:When the axial ratio of length: width of a protein molecule is less than 10, it is called as globular protein. Examples: Myoglobin, haemoglobin, ribonuclease, etc.

Question 43

on the basis of functional properties

Answer 43

II. On the basis of functional properties: The second way of classifying proteins makes use of their functional properties, such as: * Defence proteins: Immunoglobulins involved in defence mechanisms. * Contractile proteins: Proteins of skeletal muscle involved in muscle contraction and relaxation. * Respiratory proteins: Involved in the function of respiration, like haemoglobin, myoglobin, cyto- chromes. * Structural proteins: Proteins of skin, cartilage, nail. * Enzymes: Proteins acting as enzymes. * Hormones: Proteins acting as hormones.

Question 44

on the basis of solubility and physical properties

Answer 44

However, both the above classification schemes have many overlapping features. Therefore a third most acceptable scheme of classification of proteins is adopted. According to this scheme proteins are classified on the basis of their solubility and physical properties and are divided in three different classes. A. Simple proteins: These are proteins which on complete hydrolysis yield only amino acids. B. Conjugated proteins: These are proteins which in addition to amino acids contain a non-protein group called prosthetic group in their structure. C. Derived proteins: These are the proteins formed from native protein by the action of heat, physical forces or chemical factors.

Question 45

simple protein

Answer 45

These are further subclassified based on their solubilities and heat coagulabilities. These properties depend on the size and shape of the protein molecule. Major subclasses of simple proteins are as follows: 1-protamines 2-histones 3-albumin 4-globulin 5-gliadins(prolamines) 6-glutelins 7-scleroproteins on albuminoids

Question 46

protamines

Answer 46

These are small molecules and are soluble in water, dilute acids and alkalies and dilute ammonia and non- coagulable by heat. They do not contain cysteine, tryptophan and tyrosine but are rich in arginine. Their isoelectric pH is around 7.4 and they exist as basic proteins in the body. They combine with nucleic acids to form nucleoproteins. Examples: Salmine, sardinine and cyprinine of fish (sperms) and testes.

Question 47

histones

Answer 47

These are basic proteins, rich in arginine and histidine, with alkaline isoelectric pH. They are soluble in water, dilute acids and salt solutions but insoluble in ammonia. They do not readily coagulate on heating. They form conjugated proteins with nucleic acids (DNA) and porphyrins. They act as repressors of template activity of DNA in the synthesis of RNA. The protein part of hemoglobin, globin is an atypical histone having a predominance of histidine and lysine instead of arginine. Examples: Nucleohistones, chromosomal nucleoproteins and globin of haemoglobin. For details of Histone case refer to chaper on chemistry of Nucleic acids.

Question 48

albumins

Answer 48

These are proteins which are soluble in water and in dilute salt solutions. They are coagulable by heat and are changed to products that are insoluble in water and solutions of salt. The albumins may be precipitated (salted out) of solution by saturating the solution with ammonium sulphate. Albumins have low isoelectric pH of pI 4.7 and therefore they are acidic proteins at the pH 7.4. They are generally deficient in glycine. Examples: Plant albumins: Legumelin in legumes, leuco- sin in cereals. Animal source: Ovalbumin in egg, lactalbumin in milk.

Question 49

globulins

Answer 49

Globulins are insoluble in water but soluble in dilute neutral salt solutions. They are also heat coagulable. Vegetable globulins coagulate rather completely. They are precipitated (salted out) by half saturation with ammonium sulphate or by full saturation with sodium chloride. Globulins bind with heme, e.g. hemopexin, with metals, e.g. transferrin, ceruloplasmin and with car- bohydrates, e.g. immunoglobulins. Examples: In addition to above, ovoglobulin in eggs, lactoglobulin in milk, legumin from legumes.

Question 50

gliadins (prolamines)

Answer 50

Alcohol soluble plant proteins, insoluble in water or salt solutions and absolute alcohol, but they dissolve in 50 to 80 per cent ethanol. They are very rich in proline, but poor in lysine. Examples: Gliadin of wheat and hordein of barley.

Question 51

glutelins

Answer 51

Alcohol soluble plant proteins, insoluble in water or salt solutions and absolute alcohol, but they dissolve in 50 to 80 per cent ethanol. They are very rich in proline, but poor in lysine. Examples: Gliadin of wheat and hordein of barley.

Question 52

secleroprotein

Answer 52

These are fibrous proteins with great stability and very low solubility and form supporting structures of animals. In this group are found keratins, collagens and elastins. (a) Keratins: These are characteristic constituents of chidermal tissue such as horn, hair, nails, wool, hoofs and feathers. All hard keratins on hydrolysis yield as part of their amino acids, histidine, lysine and arginine in the ratio of 1:4:12. The soft or pseudokeratins such as those occurring in the outermost layers of the skin do not have these amino acids in the same ratio. In neurokeratin the ratio is 1:2:2. Human hair has a higher content of cysteine than that of other species it is called α-keratin. β-keratins are deficient in cysteine and, rich in glycine and alanine. They are present in spider’s web, silk and reptilian scales. (b) Collagen: A protein found in connective tissue and bone as long, thin, partially crystalline substance. Insoluble in all neutral (salt) solvents. Is converted into a tough, hard substance on treatment with tannic acid. This is the basis of tanning process. Collagen can be easily converted to gelatin by boiling by splitting off some amino acids. Gelatin is highly soluble and easily digestible. It forms a gel on cooling and is provided as diet for invalids and convalescents. It is not a complete protein as it lacks an amino acid tryptophan which is an essential amino acid. (c) Elastins: These are the proteins present in yellow elastic fibre of the connective tissue, ligaments and tendons. They are rich in non-polar amino acids such as alanine, leucine, valine and proline. They do not contain cysteine, methionine, 5-hydroxylysine and histidine. They are formed in large amount in the uterus during pregnancy. Elastins are hydrolysed by pancreatic elastase enzyme.

Question 53

conjugated proteins

Answer 53

B. Conjugated Proteins Conjugated proteins are simple proteins combined with a non-protein group called prosthetic group. Protein part is called apoprotein, and entire molecule is called holoprotein. -nucleoproteins -muco proteins or mucoids -glycoproteins -chromoproteins -phosphoproteins -lipoproteins -metalloproteins

Question 54

nucleoproteins

Answer 54

The nucleoproteins are compounds made up of simple basic proteins such as protamine or histone with Nucleic Acids as the prosthetic group. They are proteins of cell nuclei and apparently are the chief constituents of chromatin. They are most abundant in tissues having large proportion of nuclear material such as yeast, asparagus tips in plants, thymus, other glandular organs and sperm. Deoxyribonucleoproteins: It contain DNA as prosthetic group, are found in nuclei, mitochondria and chloro- plasts. Ribonucleoproteins: It occur in nucleoli and ribosome granules. They have RNA as prosthetic group. Examples: Nucleohistone and nucleoprotamine.

Question 55

mucoproteins

Answer 55

Mucoproteins are the simple proteins combined with mucopolysaccharides (MPS) such as hyaluronic acid and the chondroitin sulphate. They contain large quantities of N-acetylated hexosamine (>4%) and in addition substances such as uronic acid, sialic acid and mucopolysaccharides are also present. Water soluble mucoproteins have been obtained from serum, egg white (α-Ovomucoid) and human urine. These water soluble mucoproteins are not easily denatured by heat or readily precipitated by picric acid or trichloroacetic acid. They have hexosamine and hexose sugars as the prosthetic groups. Mucoproteins are present in large amounts in umbilical cord. They are also present in all kinds of mucins hormones such as FSH, LH and HCG are mucoproteins. Insoluble mucoproteins are found in egg white (β-ovomucoid), vitreous humour and submaxillary glands.

Question 56

glycoproteins

Answer 56

Glycoproteins are the proteins with carbohydrate moiety as the prosthetic group. Karl Meyer suggested that these proteins carry a small amount of carbohydrates <4 per cent such as serum albumin and globulin. Carbohydrate is bound much more firmly in the glycoproteins than the mucoprotein. Glycoproteins include mucins, immuno- globulins, complements and many enzymes. They carry mannose, galactose, fucose, xylose, arabinose in their oligosaccharide chains.

Question 57

chromoproteins

Answer 57

These are proteins that contain coloured substance as the prosthetic group. (a) Haemoproteins: All hemoproteins are chromoproteins which carry haem as the prosthetic group which is a red coloured pigment found in these proteins. * Haemoglobin: Respiratory protein found in RB Cells (See chapter on haemoglobin for details). * Cytochromes: These are the mitochondrial enzymes of the respiratory chain. * Catalase: This is the enzyme that decomposes H2O2 to water and O2. * Peroxidase: It is an oxidative enzyme. (b) Others * Flavoprotein: It is a cellular oxidation-reduction protein which has riboflavin a constituent of B- complex vitamin as its prosthetic group. This is yellow in colour. * Visual purple: It is a protein of the retina in which the prosthetic group is a carotenoid pigment which is purple in colour.

Question 58

phosphoproteins

Answer 58

These are the proteins with phosphoric acid as organic phosphate but not the phosphate containing substances such as nucleic acids and phospholipids. (a) Casein and (b) Ovovitellin are the two important groups of phosphoproteins found in milk, egg-yolk respectively. They contain about 1 per cent of phosphorus. Similar proteins are stated to be present in fish eggs. They are sparingly soluble in water, and very dilute acid in cold but readily soluble in very dilute alkali. The phosphoric acid which is esterified through the –OH groups of serine and threonine is liberated from organic combination by warming with NaOH and can only be detected by Ammonium Molybdate.

Question 59

lipopreotein

Answer 59

The lipoproteins are formed in combination with lipids as their prosthetic group (Refer chapter on Metabolism of Lipids).

Question 60

metalloproteins

Answer 60

As the name indicates, they contain a metal ion as their prosthetic group. Several enzymes contain metallic elements such as Fe, Co, Mn, Zn, Cu, Mg, etc. Examples: Ferritin: Contains Fe, Carbonic Anhydrase: Contains Zn, Ceruloplasmin: Contains Cu.

Question 61

derived proteins

Answer 61

This class of proteins includes those protein products formed from the simple and conjugated proteins. It is not a well defined class of proteins. These are produced by various physical and chemical factors and are divided in two major groups. 1-primary derived proteins -proteans -metaproteins -coagulated proteins 2-secondary derived -proteoses or albumoses -peptone peptides

Question 62

coagulated proteins

Answer 62

3. Coagulated proteins: The coagulated proteins are insoluble products formed by the action of heat or alcohol on native proteins. Examples: include cooked meat, cooked egg albumin and alcohol precipitated proteins.

Question 63

proteases

Answer 63

1. Proteans: These are insoluble products formed by the action of water, very dilute acids and enzymes. They are predominantly formed from certain globulins. Example: Myosan: From myosin, Edestan: From elastin and Fibrin: From fibrinogen.

Question 64

metaproteins

Answer 64

2. Metaproteins: They are formed from further action of acids and alkalies on proteins. They are generally soluble in dilute acids and alkalies but insoluble in neutral sol- vents, e.g. acid and alkali metaproteins.

Question 65

primary derived protein

Answer 65

Denatured or coagulated proteins are placed in this group. Their molecular weight is the same as native protein, but they differ in solubility, precipitation and crystallisation. Heat, X-ray, UV rays, vigorous shaking, acid, alkali cause denaturation and give rise to primary derived proteins. There is an intramole- cular rearrangement leading to changes in their properties such as solubility. Primary derived proteins are synony- mous with denatured proteins in which peptide bonds remain intact.

Question 66

secondary derived proteins

Answer 66

(b) Secondary derived proteins: These are the proteins formed by the progressive hydrolysis of proteins at their peptide linkages. They represent a great complexity with respect to their size and amino acid composition. They are roughly called as proteoses, peptones and peptides according to relative average molecular size.

Question 67

proteoses or abumoses

Answer 67

1. Proteoses or albumoses: These are the hydrolytic products of proteins which are soluble in water and are coagulated by heat and are precipitated from their solution by saturation with Ammonium Sulphate.

Question 68

peptones

Answer 68

2. Peptones: These are the hydrolytic products of proteo- ses. They are soluble in water, not coagulated by heat and not precipitated by saturation with Ammonium sulphate. They can be precipitated by phosphotungstic acid. Examples: Protein products obtained by the enzymatic digestion of proteins.

Question 69

peptides

Answer 69

3. Peptides: Peptides are composed of only a small number of amino acids joined as peptide bonds. They are named according to the number of amino acids present in them. * Dipeptides—made up of two amino acids, * Tripeptides—made of three amino acid, etc. Peptides are water soluble and are not coagulated by heat, are not salted out of solution and can be precipitated by phosphotungstic acid. Hydrolysis: The complete hydrolytic decomposition of a protein generally follows the stages given below: Protein→ Protean→ Metaprotein→ Proteose Aminoacids← Peptides← Peptone Note: The products from protein to peptone give a positive Biuret reaction and are relatively large molecules. The dipeptide and amino acids do not give Biuret positive reaction and therefore are called as Abiuret products.

Question 70

taste

Answer 70

Taste: They are tasteless. However, the hydrolytic pro- ducts (derived proteins) are bitter in taste.

Question 71

odour

Answer 71

Odour: They are odourless. When heated to dryness they turn brown and give off the odour of burning feather.

Question 72

molecular weight

Answer 72

Molecular weight: The proteins in general have a large molecular weight. Proteins therefore are macromolecules. Molecular weight is determined by physical methods such as osmotic pressure measure- ment, depression in freezing point, light scattering effect, X-ray diffraction, turbidity measurement and now by methods such as analytical ultracentri- fugation, molecular sieving by gel filtration and SDS- polyacrylamide gel electrophoresis. Mol. Wt. of some common proteins is shown in Table 6.1.

Question 73

viscocity of protein solutoons

Answer 73

Viscosity of protein solutions: The viscosity of protein varies widely with the kind of protein and its concentration in solution. The viscosity is closely related to molecular shape, long molecules (fibrous proteins) being more viscous than globular proteins. Thus fibrinogen can form a more viscous solution than albumin.

Question 74

hydration proteins

Answer 74

* Hydration of proteins: Polar groups of proteins such as -NH2 and -COOH become hydrated in presence of water and swell up when electrolytes, alcohol or sugars that form complexes with water are added to protein solutions. There is competition for water and the degree of hydration of protein is decreased. They dehydrate protein and precipitate it from solution.

Question 75

heat coagulation poteins

Answer 75

Heat coagulation of proteins: Several proteins coagulate forming an insoluble coagulum. Coagu- lation is maximum at the isoelectric pH of the protein. During coagulation, protein undergoes a change called as denaturation. Denatured proteins are soluble in extremes of pH and maximum precipitation occurs at isoelectric pH (pI) of the protein.

Question 76

amphoteric nature of proteins

Answer 76

Amphoteric nature of proteins: In any protein molecule there are amino acids which carry -COOH or -NH2 groups in their side chains. These groups can undergo ionisation in solution producing both anions and cations. In addition to the side chains of polar amino acids, N-terminal -NH2 group and C-terminal -COOH group may also ionise. Depending on the pH few groups act as proton donors while few as proton acceptors. Therefore proteins are ampholytes and act both as acids and bases. At a specific pH called an isoelectric pH (pI) a protein exists as a dipolar ion or “Zwitterion” or “Hybrid” ion, carrying equal number of positive and negative charges on its ionizable groups. So the net charge on protein molecule at its isoelectric pH is zero. On the acidic side of its isoelectric pH, a protein exists as a cation by accepting a proton and migrates towards anode in an electrical field; while on the alka- line side of its pI a protein exists as anion by donating a proton and migrates towards cathode. This property is made use of in electrophoresis to separate different proteins depending on the charge present in them at a particular pH.

Question 77

percipitation proteins

Answer 77

Precipitation of proteins: Proteins can be precipitated from solutions by a variety of +ve and –ve ions. Such precipitation is of importance in the isolation of protein, in the deproteinisation of blood and other biological fluids and extracts for analysis and in the preparation of useful protein derivatives. 1. Precipitation By +ve Ions: The +ve ions most commonly used are those of heavy metals such as Zn+2, Ca+2, Hg+2, Fe+3, Cu+2, and Pb+2. These metals precipitate protein at the pH alkaline to its isoelectric pH. At this pH, protein is dissociated as an anion-proteinate. The metal ions combine with the –COO– group to give insoluble precipi- tate of metal proteinate. On acidification the metal ions can be removed from the protein or by preci- pitation of metals by sulphuric acid. Metal protein precipitates are dissolved by addition of strong alkali. 2. Negative ion precipitation: Negative ions combine with proteins when the pH of the medium is on the acidic side of its isoelectric pH. Acidic pH makes the protein to exist as Protein+ and forms precipitate with –ve ions. NH2-group is the reacting group in this case. Among the more common precipitants involving –ve ion precipi- tation are Tungstic acid, phosphotungstic acid, trichloroacetic acid, picric acid, tannic acid, ferrocyanic acid and sulphosalicylic acid. When these agents are added to protein solution at proper pH, the protein precipitates as its salt. The precipitate is found to be soluble in alkali.

Question 78

primary structure

Answer 78

A. Primary Structure: Primary structure is the linear sequence of amino acids held together by peptide bonds in its peptide chain. The peptide bonds form the backbone and side chains of amino acid residues project outside the peptide backbone. The free -NH2 group of the terminal amino acid is called as N-terminal end and the free -COOH end is called as C-terminal end. It is a tradition to number the amino acids from N-terminal end as No. 1 towards the C-terminal end. Presence of specific amino acids at a specific number is very significant for a particular function of a protein. Any change in the sequence is abnormal and may affect the function and properties of protein.

Question 79

seconday structure

Answer 79

B. Secondary Structure: The peptide chain thus formed assumes a three dimensional secondary structure by way of folding or coiling consisting of a helically-coiled, zig- zag, linear or mixed form. It results from the steric relationship between amino acids located relatively near each other in the peptide chain. The linkages or bonds involved in the secondary structure formation are hydro- gen bonds and disulfide bonds.

Question 80

tertiary structure

Answer 80

C. Tertiary structure: The polypeptide chain with secondary structure mentioned above may be further folded, superfolded twisted about itself forming many sizes. Such a structural conformation is called tertiary structure. It is only one such conformation which is biologically active and protein in this conformation is called as native protein. Thus the tertiary structure is constituted by steric relationship between the amino acids located far apart but brought closer by folding. The bonds responsible for interaction between groups of amino acids are as follows:

Question 81

quaternary structure

Answer 81

D. Quaternary Structure: Many proteins are made up of only one peptide chain. However, when a protein consists of two or more peptide chains held together by non-covalent interactions or by covalent cross-links, it is referred to as the quaternary structure. The assembly is often called as oligomer and each constituent peptide chain is called as monomer or subunit. The monomers of oligomeric protein can be identical or quite different in primary, secondary or tertiary structure. Examples: Protein with two monomers (dimer) is an enzyme called creatine phosphokinase (CPK). Haemoglobin and lactate dehydrogenase (LDH) are tertramers consisting of four monomers. Apoferritin, an apoprotein of ferritin, an iron binding and storage protein contains 24 identical subunits. An enzyme aspartate transcarbamoylase has 72 subunits in its structure.