amino acids

Question 1

amino acids

Answer 1

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.

Question 2

structure of amino acids

Answer 2

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 3

classification of amino acids

Answer 3

Amino acids can be classified into 3 groups depending on their reaction in solution.

A. Neutral
B. Acidic and
C. Basic.

Question 4

neutral amino acids

Answer 4

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 5

aliphati amino acids

Answer 5

(a) Aliphatic Amino Acids:
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
6. Serine (Ser) or α-amino-β-hydroxy propionic acid.
7. Threonine (Thr) or α-amino-β-hydroxybutyric acid.

Question 6

aromatic amino acids

Answer 6

(b) Aromatic Amino Acids
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.

Question 7

heterocyclic amino acids

Answer 7

(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

Question 8

sulphur containing amino acids

Answer 8

d) ‘S’ containing amino acids: The fourth subgroup of neutral amino acids contains two sulphur containing amino acids.
12. 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.
13. Methionine (Met) or α-amino γ-methylthio-η-butyric acid

Question 9

acidic amino acids

Answer 9

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

Question 10

examples of acidic amino acids

Answer 10

14. Aspartic acid (Asp) or α-amino succinic acid.
    Asparagine (Asn) or γ -amide of α-aminosuccinic acid.
15. 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 monocarboxylic acids. Arginine, lysine and hydroxylysine are included in this group.

Question 12

examples of basic amino acids

Answer 12

16. Arginine (Arg) or α-amino- δ-guanidino-n-valeric acid.
17. Lysine (Lys) or α- ε-diamino caproic acid.
18. Hydroxylysine (Hyl) or α-ε-diamino-δ-hydroxy-nvaleric acid.

Question 13

nutritionally amino acids are off

Answer 13

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

Question 14

essential amino acids

Answer 14

(a) Essential amino acids:
These are the ones which are not synthesised by the body and must be taken in diet. Because they are essential for proper growth.
They also called indispensable amino acids.
They include valine, leucine, isoleucine, phenylalanine,
threonine, tryptophan, methionine, and lysine.

Question 15

non-essential amino acids

Answer 15

b) Non-essential amino acids: They can be synthesised by the body and may not be the requisite components of the diet.
They also called Dispensable amino acids.
These are Glutamic acid, aspartic acid, alanine, serine, proline, hydroxy proline.

Question 16

semi-essential amino acids

Answer 16

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

Question 17

physical properties of amino acids

Answer 17

They are colourless
Crystalline substances
more soluble in water than in polar solvents.

Question 18

isomerism

Answer 18

Isomerism:
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.

Question 19

stereoisomerism

Answer 19

All amino acids except glycine exist in D and L isomers. In D-amino acids – NH2 group is on the right hand while in L-amino acids it is oriented to the left.
Natural proteins of animals and plants generally contain L-amino acids.
D-amino acids occur in bacteria.

Question 20

optical isomerism

Answer 20

(b) Optical Isomerism:
All amino acids except glycine have asymmetric carbon atom.
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.
Few amino acids like isoleucine and threonine have an additional asymmetric carbon in their structures

Question 21

Amphoteric Nature and Isoelectric pH:

Answer 21

Amphoteric Nature and Isoelectric pH:
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 amino 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 isoelectric pH.

Question 22

functions of amino acids

Answer 22

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
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.
Some amino acids such as glycine and cysteine are used as detoxicants of specific substances.
Cystine and methionine are sources of sulphur.