a short theory on amino acid basics with past mcq's for competitive exam preparartions

1. Biomolecules
amino acids - 1
ANNUJA ANANDARADJE

2. INTRODUCTION
A biomolecule is an organic molecule that is
produced by a living organism.
Biomolecules act as building blocks of life and
perform important functions in living organisms.
Biomolecules consist primarily of carbon, oxygen,
hydrogen and nitrogen which together makeup
99% of the cell mass.

3. Cont.,,
 Most biomolecules can be considered as derivatives of
hydrocarbon.
 sugars, fatty acids, amino acids &nucleotides constitute
the four major families of biomolecules in cells.
 The biomolecules found within the cells are
macromolecules. These macromolecules are proteins,
carbohydrates, lipids &nucleic acids.

4. Cont.,,
Nucleic acid &proteins are informational
biomolecules.
The polysaccharides have 2 major functions 1)
energy yielding fuels
2) extracellular structural
elements..

5. AMINO ACIDS & PROTEINS
Amino acids are compounds containing carbon,
hydrogen, oxygen and nitrogen.
They serve as monomers of proteins that are
composed of amino group, carboxyl group,
hydrogen atom, side chain and an alpha carbon.
 The various alpha amino acids differ with respect
to the side chain attached to the alpha carbon.

6. GENERAL STRUCTURE OF AMINO
ACID

7. ACID-BASE BEHAVIOUR OF AMINO
ACID
At Low Ph , the positively charged species
predominates. As the ph increases ,the electrically
neutral zwitter ion becomes predominant. At higher
ph, the negatively charged species predominates.

8. Optical properties
All amino acids except glycine are optically active.
Optically active molecules contain chiral carbon. A
tetrahedral carbon atom with 4 different
constituents are said to be chiral.
All amino acids except glycine have a chiral carbon
& hence considered to be optically active.

9. Cont.,,
 Optically active compounds can rotate the plane of
polarized light either in clockwise / anticlockwise
direction.
 Rotation in clockwise direction : dextro- rotatory
 Rotation in anti-clockwise dirction: levo-rotatory
 This is designated by signs: + ( dextro)
-(levo).
Optical activity is measured by polarimeter.
A quantitative measure of optical activity is called optical
rotation.(alpha)α.

10. ABSORBANCE
 (α)T
D = A0/l*c
 A0 = observed rotation in degrees.
 (α)T
D = specific rotation of compound in temperature T &
wavelength λ. If the wavelength of light used is 539 nm the
symbol used is D.
 C = the concentration of the solution in g/ml.
 l= length of light path in decimeter.

11. ABSOLUTE CONFIGURATION
 An amino acid with chiral carbon can exist in two
configurations that are non-superimposable mirror
images of each other.
 These two configurations are called enantiomers.
 All amino acids exist in these two different enantiomeric
forms other than glycine.
 When the hydroxyl group attached to the chiral carbon
is on the left – L configuration.
 When the hydroxyl group is on the right – D
configuration.

12. Enantiomers

13. However the amino acids ribosomically
incorporated into proteins exhibit L-α configuration.
Therefore all are L-α amino acids. The basis for
the preference of such amino acids is not known.
D form amino acids do also exist in nature
e.g, peptide antibiotics, peptidoglycan of bacteria.

14. STANDARD & NON STANDARD
AMINO ACIDS
Only 22 amino acids participate in protein
synthesis ribosomically.
Such amino acids are called proteinogenic amino
acids.
The 4 most abundant amino acids in a protein are
leucine, lycine, serine, glutamic acid.
The two least abundant amino acid in a protein are
tryptophan and methionine.

15. AMINO ACIDS WITH NON
POLAR SIDECHAIN
Glycine
Alanine
Valine
isoleucine
Leucine

16. AMINO ACIDS WITH POLAR
UNCHARGED SIDE CHAIN
Serine
Thronine
Tyrosine
cysteine

17. AMINO ACIDS WITH POLAR
CHARGED SIDE CHAIN
+ charge:
Lysine
Arginine
Histidine
- charge
Aspartate
Glutamate

18. Aliphatic amino acids:
 Glycine ,
 Alanine
 Valine
 Leucine.
Aromatic amino acids:
 Tyrosine
 Phenylalanine
 Tryptophan.
Sulphur containing amino acids:
 Cysteine
 Methionine
Imino:
 Proline
Amino
 Lysine
 Arginine
Amide
Aspartate
 glutamate

19. NON STANDARD AMINO ACIDS
Apart from 22 all the other amino acids are non
standard / non proteinogenic amino acids.
e.g; 4-hydroxyproline
5-hydroxylysine
Desmosine
N –formylmethionine,,,

20. TITRATION OF AMINO ACIDS
An amino acid when becomes electrically neutral is
called isoelectric point.
Because there is no net negative charge at iso
electric point, amino acids become
electrophoretically immobile & least soluble at this
Ph
pI = Pk1 +pK2/2.

21. PROB.
 All 20 standard amino acids found in proteins have atleast
one asymmetric carbon atom.
False : glycine has no asymmetric carbon atom.
 An equimolar mixture of D- &L- alanine does not rotate the
plane of polarized light.
True.

22. ABSORPTION OF AMINO ACIDS
Tyrosine & tryptophan : 280nm
Phenylalanine : 257.4 nm
Aromatic amino acids absorb uv light.

23. MCQ
 Which of the following statements is not true about amino acids?
a) Only 22 amino acids are used in protein synthesis
b) All amino acids exist in two sterioisomeric forms
c) Amino acids have N terminus, C terminus and R groups
d) There are more than 300 amino acids in a cell.
 Which of the following is an imino acid?
a) Histidine
b) Glycine
c) Cycteine
d) proline

24. Cont.,,,
 Which pair of amino acids will have highest absorbance?
a) Thr and his
b) Phe and pro
c) Trp and tyr
d) Phe and his.
 Which of the following amino acid has a non polar and aliphatic R group?
a) Tyrosine
b) Histidine
c) Glutamate
d) leucine

25. Cont.,,
 Which amino acid is most likely to be found in water soluble globular
proteins?
a) Ser
b) Arg
c) Asp.
d) Val
 Non polar amino acids are mostly found in ?
a) Core of proteins
b) Surface of proteins
c) On alpha helices
d) Non specific region

26.  The mean molecular weight of an amino acid in typical globular protein is?
a) 70
b) 110
c) 150
d) 90
 If the molecular mass of an amino acid is 150 daltons, the molecular mass of
a tripeptide will be ?
a) 450
b) 486
c) 504
d) 414.
 The closest estimate for the number of amino acid residues in a protein with
the molecular mass of 85kda ?
a) 710
b) 7100
c) 8500
d) 85

27.  A protein x was fused with GFP in a vector and expressed in E. COLI . The
length of the protein x is 1000 amino acid residues and the molecular weight
of GFP is 27 kda. What is the total approximate molecular mass of the fusion
protein in daltons?
a) 137000
b) 138000
c) 83000
d) 270000.
 Imagine you are generating a site-directed mutant of a protein in which a
given residue is replaced by another residue. Which one of the following
substitutions will result in highest isoelectric point of the mutant compared to
the wild type ?
a) Asp to arg
b) Arg to asp
c) Tyr to lys
d) Gly to arg.

28.  Which of the following statement is false ?
a) Amino acids tend to be least soluble in water at their isoelectric point
b) Sickle cell Hb and normal Hb have the same value of negative charge.
c) Amino acids are made visible on chromatogram by treatment with
ninhydrin.
d) The net charge on the amino acid is a function of the ph of the solution.
 From the following sets, pick up the one that contains exclusively
hydrophobic amino acids:
a) Asp, glu , lys, arg, ser.
b) Arg, phe, tyr, trp, asn.
c) Ala, ile, leu, phe, val.
d) Ala, arg, phe, leu, tyr.
 Which of the following peptides can be easily detected by absorbance at
280nm?
a) Leu-tyr-met-ala-glu
b) Ser-met-arg
c) Tyr-trp-phe

29.  Choose the mismatch:
a) Tyrosine- phenolic group
b) Arginine-guanidino group
c) Histidine-imodazole group
d) Cysteine-imino group.
 The high solubility of amino acids In water is due to :
a) Presence of side chain
b) Dipolar ion structure
c) Unipolarity
d) The hydrophilic nature of amino group.
 Amino acids found in proteins that are formed by post translational
modification of one of the common amino acids( derived amino acids) include
which of the following?
a) Isoleucine
b) Threonine
c) 4-hydroxyproline

30.  Choose the incorrect statement:
a) At isoelectric point, protein has minimum solubility
b) Among standard amino acids, tryptophan has highest molecular weight.
c) Selenocysteine is derived from cisteine
d) Histidine contains 3 ionisable groups.
 Which of the following substitutions introduced by site directed mutagenesis,
in a protein is expected to bring about the max change in the isoelectric point
in the mutant protein compared to the wild type protein?
a) Asp to ser
b) Lys to arg
c) Arg to glu
d) Glu to asp.
 The peptide bond has a backbone of atoms in which of the following
sequence?
a) C-N-N-C
b) C-C-C-N
c) C-C-N-C

31.  ASPARTAME, THE SYNTHETIC SWEETNER IS:
a) A dipeptide
b) Also secreted by the posterior pitutary gland in response to thirst
c) A glucose derivative
d) Not digested by humans.
 which of the following has partial double bond character?
a) C-C
b) Cα-N
c) C-O
d) C-N
 The peptide bond in proteins is :
a) Planar, but rotates to three preferred dihedral angles
b) Nonpolar, but rotates to preferred dihedral angles.
c) Nonpolar, and fixed in a trans conformation
d) Planar, and usually found in a trans configuration

32.  The property of resonance as applied to protein structure is
responsible for:
a) The prevention of rotation about the alpha carbon
b) The partial double bond character
c) The planar nature of the peptide bond
d) The ability of cochlear cells to detect sound waves by
mechanotransduction