1. Amino Acids: The Building Blocks
of Life

2. Protein are polymers of α-amino acids
•The amino acids used to make proteins are
2-aminocarboxylic acids.
•The α (alpha) carbon is the carbon to which a
functional group is attached.

3. Properties of amino acids:
•structure and chemical functionality
•chirality
•acid-base properties
•capacity to polymerize

4. Proteins in the Diet
9 of the 20 amino acids must be obtained from the
diet. These are referred to as the essential amino
acids.
– Histidine
– Isoleucine
– Leucine
– Lysine
– Methionine
– Phenylalanine
– Threonine
– Tryptophan
– Valine
Proteins are also the major source of nitrogen in the
diet

6. Properties of amino acids:
•Aliphatic chains: Gly, Ala, Val, Leu, and Ile
Hydrophibicity
•Hydroxyl or sulfur side chains: Ser, Thr, Cys, Met
•Aromatic: Phe, Trp, Tyr
•Basic: His, Lys, Arg
•Acidic and their amides: Asp, Asn, Glu, Gln

7. Amino acids
• Classified according to their capacity to interact with
water
• 4 classes: NON POLAR, POLAR, ACIDIC AND BASIC
• Non polar amino acids contain hydrocarbon R groups
• R groups do not have (+) or (-) charges and interact
poorly with water
• 2 types of hydrocarbon chains: aliphatic and aromatic

8. • Non-Polar Side Chains:
• Side chains which have pure hydrocarbon alkyl groups
(alkane branches) or aromatic (benzene rings) are
non-polar. Examples include valine, alanine, leucine,
isoleucine, phenylalanine.
• The number of alkyl groups also influences the
polarity. The more alkyl groups present, the more
non-polar the amino acid will be. This effect makes
valine more non-polar than alanine; leucine is more
non-polar than valine.

9. Polar Side Chains:
Side chains which have various functional groups such as
acids, amides, alcohols, and amines will impart a more polar
character to the amino acid.
The ranking of polarity will depend on the relative ranking of
polarity for various functional groups
In addition, the number of carbon-hydrogens in the alkane or
aromatic portion of the side chain should be considered along
with the functional group.

10. >
Example: Aspartic acid is more polar than serine because an
acid functional group is more polar than an alcohol group.

11. >
Example: Serine is more polar than tyrosine, since tyrosine
has the hydrocarbon benzene ring.

12. Acid - Base Properties of Amino Acids:
• Acidic Side Chains:
• If the side chain contains an acid functional group,
the whole amino acid produces an acidic solution.
Normally, an amino acid produces a nearly neutral
solution since the acid group and the basic amine
group on the root amino acid neutralize each other in
the zwitterion. If the amino acid structure contains
two acid groups and one amine group, there is a net
acid producing effect.
The two acidic amino acids are aspartic and glutamic

13. • Basic Side Chains:
• If the side chain contains an amine functional group,
the amino acid produces a basic solution because the
extra amine group is not neutralized by the acid
group.
Amino acids which have basic side chains include:
lysine, arginine, and histidine.

14. Hydrophobic Amino Acids (aliphatic)

15. Hydrophobic Amino Acids (aromatic)
• all very
hydrophobic
•Some contain
aromatic group
•Absorb UV at
280 nm

16. Sulfur Containing Amino Acids
•Methionine (Met, M) – “start”
amino acid, very hydrophobic
•Cysteine (Cys,C) – sulfur in
form of sulfhydroyl, important
in disulfide linkages, weak acid,
can form hydrogen bonds.

17. Charged Amino Acids
• Asp and Glu are acidic amino
acids
•Contain carboxyl groups
Negatively charged at
physiological pH, present as
conjugatebases
•Hydrophillic nitrogenous bases
• Carboxyl groups function as
•Positively charged at
nucleophiles in some enzymatic
physiological pH
reactions
•Histidine – imidazole ring
protonated/ionized, only amino
acid that functions as buffer in
physiol range.
•Lysine - diamino acid,
protonated at pH 7.0
•Arginine - guianidinium ion
always protonated, most basic
amino acid

18. Polar Amino Acids 1

19. Polar Amino Acids 2

20. Classification of Amino Acids by Polarity
POLAR Acidic Neutral Basic
Asp Asn Ser Arg
Tyr Cys His
Glu Gln Thr Lys
Gly
POLAR
Ala Ile Phe Trp
NON-
Val Leu Met Pro
Polar or non-polar, it is the bases of the amino acid properties.
Juang RH (2003) Biochemistry

21. Functional significance
•Hydrophobic amino acids: encountered in the interior
of proteins shielded from direct contact with water
•Hydrophillic amino acids: generally found on the
exterior of proteins as well as in the active centers of
enzymes
•Imidazole group: act as either proton donor or
acceptor at physiological pH
– Reactive centers of enzymes
•Primary alcohol and thiol groups: act as nucleophiles
during enzymatic catalysis
– Disulfide bonds

22. Stereochemistry
• Note that the R group means that the α-carbon is a
chiral center. All natural amino acids are L-amino
acids.

23. L-Form Amino Acid Structure
Carboxylic group -
COO
Amino group
+
H3 N α H
H = Glycine
R group
CH3 = Alanine
Juang RH (2004) BCbasics

24. Mirror Images of Amino Acid
α Mirror
image
α
Same chemical properties
Stereo isomers
Juang RH (2004) BCbasics

25. THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
• Amino acids are zwitterions:
• An amino acid has both a basic amine group
and an acidic carboxylic acid group.

26. • There is an internal transfer of a hydrogen ion
from the -COOH group to the -NH2 group to
leave an ion with both a negative charge and a
positive charge.
• This is called a zwitterion.

27. Adding an alkali to an amino acid solution
• increase the pH of a solution of an amino acid
by adding hydroxide ions, the hydrogen ion is
removed from the -NH3+ group
• The amino acid would be found to travel towards
the anode (the positive electrode).

28. Adding an acid to an amino acid solution
• decrease the pH by adding an acid to a
solution of an amino acid, the -COO- part of
the zwitterion picks up a hydrogen ion.
• the amino acid would move towards the
cathode (the negative electrode).

29. Shifting the pH from one extreme to the other
• Suppose you start with the ion we've just
produced under acidic conditions and slowly
add alkali to it.
• That ion contains two acidic hydrogens - the
one in the -COOH group and the one in the
-NH3+ group.
• The more acidic of these is the one in the
-COOH group, and so that is removed first -
and you get back to the zwitterion.

30. • So when you have added just the right amount
of alkali, the amino acid no longer has a net
positive or negative charge. That means that it
wouldn't move towards either the cathode or
anode during electrophoresis.
• The pH at which this lack of movement during
electrophoresis happens is known as the
isoelectric point of the amino acid. This pH
varies from amino acid to amino acid.

31. • If you go on adding hydroxide ions, you will
get the reaction we've already seen, in which
a hydrogen ion is removed from the -NH3+
group.

32. • You can, of course, reverse the whole process
by adding an acid to the ion we've just finished
up with.
• That ion contains two basic groups - the -NH 2
group and the -COO- group. The -NH2 group is
the stronger base, and so picks up hydrogen
ions first. That leads you back to the zwitterion
again.

33. • . . . and, of course, you can keep going by then
adding a hydrogen ion to the -COO- group.

34. Proton Is Adsorbed or Desorbed
Proton ： abundant and small, affects the charge of a molecule
lone pair High Low
electrons pKa
H+
Amino N H H+ N H
H H
Low pKa High
O H O
Carboxylic C C H+
O O
Ampholyte contains both positive and negative groups on its molecule
Juang RH (2004) BCbasics

35. Amino acids are zwitterions:

36. Acidic environment Neutral environment Alkaline environment
pK2 ~ 9
NH2 H+ NH2 H+ NH2
R-C-H R-C-H R-C-H
COOH COO- COO-
pK1 ~ 2
5.5
+1 0 -1
Isoelectric point
Juang RH (2004) BCbasics

37. Amino Acids Have Buffering Effect
pH 12
★ pK2
9
NH2 H+
6 H-C-R Isoelectric point = pI
COO-
pK1 + pK2
3 ★ 2
pK1
0
[OH] →
Juang RH (2004) BCbasics

38. Buffer pH
Environment pH vs Protein Charge
10
9
8
7
Isoelectric point,
pI 6
5
4
3
+ 0 - -
Net Charge of a Protein
Juang RH (2004) BCbasics

39. H first Aspartic acid
HOOC-CH2-C-COOH +1
NH3+ Isoelectric point is the average
pK1 = 2.1 of the two pKa flanking the
zero net-charged form
second H 2.1 + 3.9
HOOC-CH2-C-COO- 0 2
= 3.0
NH3+
Isoelectric point
pK2 = 3.9
H -2
-
OOC-CH2-C-COO- -1 pK3
NH3+ third -1
pK2
pK3 = 9.8 0
H pK1
+1
-
OOC-CH2-C-COO- -2 [OH]
NH2
Juang RH (2004) BCbasics

40. Peptide bond formation:
• Polypeptides are linear polymers composed of amino
acids linked together by peptide bonds
• Peptide bonds are amide linkages formed when
unshared electron pair of α-carboxyl of another
amino acid
• When 2 amino acids reacted with one another, the
product is called a dipeptide.
• Therefore tripeptide contain 3 amino acid residues,
tetrapeptide 4 and so forth

41. Formation of Peptide Bonds by Dehydration
Amino acids are connected head to tail
NH2 1 COOH NH2 2 COOH
Carbodiimide Dehydration
-H2O
O
NH2 1 C N 2 COOH
H
Juang RH (2004) BCbasics

43. • By convention, amino acid residue with free –NH2 group is
called N –terminal residue and is written to the left
• Free –COOH on C-terminal is written on the right. Peptides
are named by using their amino acid sequences beginning
from N-terminal residues,
• E.g:
H2N----Tyr----Ala----Cys----Gly----COOH
Above is a tetratpeptide named tyrosylalanylcysteinylglycine

44. Polypeptide backbone:
• Polypeptides are polymers composed of amino acids linked
together by peptide/amide bonds
• Order of amino acids in polypeptide is called amino acid
sequence
• Disulphide bridges formed by oxidation of Cys residues are an
important structural element in polypeptides and proteins

45. Peptides:
• Less complex than larger protein molecules have significant
biological activities
• E.g: Glutathione, Oxytocin, Vasopressin, substance P and
bradykinin
• Peptides are found in almost all organisms, involved in many
important biological processes:
-protein DNA synthesis
-Drug and environment toxin metabolism
-amino acid transport
-reducing agent (-SH group of cys) protects cells from
destructuve effects of oxidation by reacting with substances
such as peroxidase

46. Disulphide bond
• 2 cysteine - cystine ; 2 R-SH- R-S-S-R (+2H)
(Oxidation reaction)
- Intracellular conditions are maintained sufficiently reducing to
inhibit formation of most disulfide bonds
- Extracellular conditions (as well as those found in some
organelles) are more oxidizing, favouring disulphide formation
- Thus, extracellular proteins containing cysteines often have
disulfides, while intracellular (cytosolic) proteins rarely have
disulfides.

47. Detection, identificationand quantificaton
of amino acids and proteins
• Reaction between the thiol group of cysteine
and Ellman’s reagent
• Produce nitrothiobenzoate anion and since
this product adsorbs light at 410nm it provides
a route for quantifying protein concentration.
• Other reagents for estimating protein
concentration are: ninhydrin, fluorescamine,
dansyl chloride, nitrophenols and
fluorodinitrobenzene (all react with functional
groups)

48. Protein quantitation:
Quick and simple way of estimating protein cncentration
1. Spectrophotometric method at 280 nm using quartz
cuvettes, absorption mainly due to Trp and Tyr
2. Biuret reaction
3. Bradford method: widely used
4. BCA (Bichinchoninic acid)
5. Modified lowry assay
6. fluorescamine protein assay
Note: to understand the principle behind the reaction
used to determine protein concentration, also
sensitivity of method used (eg: detection limits of
protein assay)
Extracts containing protein should be treated with care

49. 1. Absorbance at 280 nm:
Principle:
• Proteins in solution absorb ultraviolet light with
absorbance maxima at 280 and 200 nm.
• Amino acids with aromatic rings are the primary
reason for the absorbance peak at 280 nm.
• Peptide bonds are primarily responsible for the peak
at 200 nm.
• Secondary, tertiary, and quaternary structure all
affect absorbance, therefore factors such as pH,
ionic strength, etc. can alter the absorbance
spectrum.
• Advantage: Quick estimation, protein not consumed,
no additional reagent,incubation needed, no protein
standard needed

50. • Historically use biuret reaction:
solutionofcopper(II) sulphate in alkaline
tartarate solution reacts with peptide bonds
to form purple complex absorbing light at540
nm

51. • Disadvantage: considerable error due to varying
absoprtion characteristics of protein samples
2. Bradford method:
Principle:
• The assay is based on the observation that the
absorbance maximum for an acidic solution of
Coomassie Brilliant Blue G-250 shifts from 465 nm to
595 nm when binding to protein occurs.
• Both hydrophobic and ionic interactions stabilize the
anionic form of the dye, causing a visible color
change.
• Advantage: relatively fast, fairly accurate

52. • Disadvantage:
-The dye reagent reacts primarily with arginine
residues and less so with histidine, lysine, tyrosine,
tryptophan, and phenylalanine residues. Obviously, the
assay is less accurate for basic or acidic proteins.
-The Bradford assay is rather sensitive to bovine
serum albumin, more so than "average" proteins, by
about a factor of two.

53. 3. BCA
Principle:
• BCA serves the purpose of the Folin reagent in
the Lowry assay, namely to react with complexes
between copper ions and peptide bonds to produce a
purple end product.
• The advantage of BCA is that the reagent is fairly
stable under alkaline conditions, and can be included
in the copper solution to allow a one step procedure.
A molybdenum/tungsten blue product is produced as
with the Lowry
• Disadvantage: greater variability among proteins and
the assay is less sensitive

54. 4. Modified lowry assay
Principle:
• Under alkaline conditions the divalent copper ion forms a
complex with peptide bonds in which it is reduced to a
monovalent ion.
• Monovalent copper ion and the radical groups of
tyrosine, tryptophan, and cysteine react with Folin
reagent to produce an unstable product that becomes
reduced to molybdenum/tungsten blue
• Advantage: fairly accurate
• Disadvantage: proteins are consumed and proteins with
an abnormally high or low percentage of tyrosine,
tryptophan, or cysteine residues will give high or low
errors, respectively.

56. 5. Fluorescamine protein assay:
Principle:
• Fluorescamine react with amino acids containing primary
amines such as lysine and n-terminal amino acid to yield a
highly fluorescent product. Fluoresence measure using a
standard fluorometer with the excitation wavelenght at 390
nm and emission at 475nm
• Advantage: sensitive (nano gram range), fast, reaction is
instantaneous
• Disadvantage: reagents hydrolyzed very rapidly therefore
rapid mixing is required to produce reproducible results as
fluorescamine react with primary amine, primary amine buffer
eg: tris and glycine cant be used

57. Secondary stucture:
• Secondary structure of polypeptides consists of several
repeating structures most common types: α-helix and β-
pleated sheet
• α-helix and β-pleated sheet stabilize by H bonds between
carbonyl and NH groups (interactions with other amino acids
in close proximity) in polypeptide backbone
• α-helix : rigid, rodlike structure that forms when a
polypeptide chain twists into right-handed or left-handed
helical conformation.

58. Nonstandard amino acids
chemically modified after they have been incorporated into a
protein (termed a “posttranslational modification”)
- γ-carboxyglutamic acid, a calcium-binding amino acid residue
found in the blood-clotting protein prothrombin (as well as in
other proteins that bind calcium as part of their biological
function).
- collagen: Significant proportions of the amino acids in
collagen are modified forms of proline and lysine: 4-
hydroxyproline and 5-hydroxylysine.
- Phosphate molecule to the hydroxyl portion of the R groups of
serine, threonine, and tyrosine. This event is known as
phosphorylation and is used to regulate the activity of proteins
in the cell. Serine is the most common in proteins, threonine is
second, and tyrosine is third.

59. - Glycoproteins are widely distributed in nature and provide
the spectrum of functions already discussed for unmodified
proteins. The sugar groups in glycoproteins are attached to
amino acids through either oxygen (O-linked sugars) or
nitrogen atoms (N-linked sugars) in the amino acid residues.
- Selenocysteine: Although it is part of only a few known
proteins, there is a sound scientific reason to consider this the
21st amino acid because it is in fact introduced during protein
biosynthesis rather than created by a posttranslational
modification. Selenocysteine is actually derived from the
amino acid serine (in a very complicated fashion), and it
contains selenium instead of the sulfur of cysteine.

61. A helix has the following features:
•every 3.6 residues make one turn,
•the distance between two turns is 0.54 nm,
•the C=O (or N-H) of one turn is hydrogen bonded to N-H (or
C=O) of the neighboring turn.
•Hydrogen bonds play a role in stabilizing the a helix
conformation. However, the size and charges of sidechains
are also important factors. Alanine has a greater propensity
to form a helices than proline.

62. The hydrogen bonds that stabilize the helix are parallel to the long axis of the helix.

63. Beta strand
In a beta strand, the torsion angle of N-Ca-C-N in the
backbone is about 120 degrees. The following figure shows
the conformation of an ideal b strand. Note that the
sidechains of two neighboring residues project in the
opposite direction from the backbone

64. Beta sheet
A beta sheet consists of two or more hydrogen bonded b
strands. The two neighboring b strands may be parallel if
they are aligned in the same direction from one terminus (N
or C) to the other, or anti-parallel if they are aligned in the
opposite direction.

67. Structural motif (supersecondary structure):
a structural motif is a three-dimensional structural
element or fold within the chain, which appears also
in a variety of other molecules. In the context of
proteins, the term is sometimes used
interchangeably with "structural domain," although a
domain need not be a motif nor, if it contains a
motif, need not be made up of only one.

69. Rossman fold: Decarboxylase enzyme

70. What are domains of proteins?
• A domain is a basic structural unit of a protein structure-
distinct from those that make up the conformation
• Part of protein that can fold into a stable structure
independently
• different domains can impart different functions to proteins
•Proteins can have one too many domains depending on
protein size
•In an unbranched chain-like biological molecule, such as a
protein or RNA, a structural motif is the three dimensional
structural element within the chain, which appears also ina
variety of other molecules.

71. Pyruvate kinase

72. Tertiary structure:
• 3D conformation as a result of interactions betweeen side
chains in their primary structure
• Hydrophobic intercations: as polypeptide folds, R groups are
brought into close proximity
• Electrostatic interactions: strongest electrostatic interaction
between ionic groups of opposite charge
• H bonds: significant number of H bonds forms within interior
of protein, polar amino acids interact with water or with
polypeptide backbone
• Covalent bonds: most important, covalent bonds in tertiary
structure are disulfide bridges found in many extracellular
proteins

73. Quaternary structure:
• Proteins esp high M.W composed of several polypeptide
chains
• Each polypeptide is called a subunit
• Subunits in a protein complex may be identical or quite
different
• Multisubunit proteins in which some or all subunits are
identical are called oligomers
• Polypeptide units assemble and held together by noncovalent
interactions such as
-hydrophobic interactions
-electrostatic interactions
-H bonds
-covalent cross links

74. Hydrophobic interactions play an important role in protein
folding as well as covalent crosslinks help stabilize
multisubunit proteins
Eg: disulfide bridges in immunoglobulins, the desmosine and
lysinonorleucine linkages in certain connective tissues
Eg: desmosine cross links connects 4 polypepide chains in the
rubberlike connective tissue called elastin
Lysinonorleucine: crosslink structure found in elastin and
collagen
Interactions between subunit are also affected by binding of
ligands

75. In allostery, control of protei fundtion through ligand binding to
specific site in protein triggers conformational change that
alters its affinity for other ligands
Ligand induced corformational changes in such proteins are
called allosteric transitions, ligands which trigger them are
called effectors or modulators
Loss of protein structure:
• Protein sensitive to environmental factors
• Disruption of native conformation is called denaturation
• Factors: physical and chemical
Denaturing agents:
1. Strong acids or base
2. Organic solvents

76. Hydrogen bonding in a protein

77. 3. Detergents
4. Reducing agents
5. Salt concentrations
6. Heavy metal ions
7. Temperature changes
8. Mechanical stress

78. Antibody family:
• A family of proteins that can be created to bind almost any
molecule
• Ntibodies (imminoglobulin) are made in response to a foreign
molecule i.e: bacteria, virus, pollen..callled and antigen
• Bind together tightly and therefore inactivates the antigen or
marks it for destruction

79. Protein folding:
• The peptide bond allows for rotation around it and therefore
the protein can fold and orient the R groups in favourable
positions
• Weak non covalent interactions will hold the protein in its
functional shape-these are weak and will take many to hold the
shape.
• H bonds form between 1) atoms involved in the peptide bonds
2) peptide bond atoms and R groups, 3) R groups
Protein folding:
• Protein shape is determined by the sequence of the amino
acids
• The final shape is called the conformation and has the lowest
free energy possible

80. 3 main classes of protein folding accessory proteins:
Allow protein to fold within few minutes in cell (in vivo)
a) Protein disulfide isomerases
b) Peptidyl prolyl ci-trans isomerases
c) Molecular chaperones
• Denatured proteins may renature or refold if chemical
compound that causes denaturation can be removed
• Molecular chaperons are small proteins that help guide the
folding and help keep the new protein from associating with
the wrong partner

82. Useful protein:
• There are many diferent combinations of amino acids that
can make up proteins and that would increase if each one had
multiple shape
• Proteins usually have one useful conformation because
otherwise it would not be efficient use of energy available to
the system
• Natural selection has elimited proteins that donot perform a
specific function in the cell
• Have similarities in amino acid sequence and 3d structure
• Have similar functions such as breakdown proteins but do it
differently

83. Proteins –multiple peptides
• non covalent bonds can form interactions between
individual polypeptide chains
• binding site-where proteins interact with one another
•Subunit-each polypeptide chain of large protein
• dimer –protein made of 2 subunits

84. Oxygen binding protein:
• Hemoglobin:
Carry O2 in blood from lungs to other tisues in body; function
is to supply O2 to cells for oxidative phosphorylation
• Myoglobin
stores O2 in tissues of body, available when cells reuire it;
highest concentration of myoglobin in skeletal and cardiac
muscle which require large amounts of energy
Myoglobin: small protein, 17.8 Kda, made up of 153 amino acids
in a single polypeptide

85. • Globular protein have a highly folded compact structure with
most of the hydrophobic residues found in the interior while
polar residues on surfaces
• Structure of hemoglobin determined by Max Perutz was the
first protein structure determined via x-ray crystallography
• Secondary structure: α-helix, 8 α-helices, heme prosthetic
group is found in hydrophobic crevice formed by folding of
polypeptide chains
• Hemoglobin made up of 4 polypeptide chains
• Each have similar 3D of single polypeptide chain in myoglobin
even though aino acid sequences differ at 83 % of residues
• This highlight relatively common theme in protein structure:
different primary sequence can specify very similar 3D
structures

86. • Major tyoe of hemoglobin found in adults (HbA):
• Made of 2 diferent polypeptide chains:
- α-chain: 141 amino acid
-β-chain: 146 amino acid
• Each chain has 8 α-helices, each containing heme prosthetic
group; therfore hemoglobin can bind 4 molecules of O2
• 4 polypeptide chains are α2β2, consists of 2α and 2β packed
tightly together ina tetrahedral array to form spherical
shaped molecule held together by multiple noncovalent
interactions

89. Important fibrous proteins:
• Intermediate filaments of the cytoskeleton
-structural scaffold inside the cells
-keratin in hair, horns and nails
• Extracellular matrix
-binds cells together to make tissues
-secreted from cells and assemble in long fibers
-collagen: fiber with a glycine every third amino acid in the
protein
-Elastin: unstructured fibers that give tissues an elastic
characteristic

90. Fibrous proteins:
• Typically contain high proportion of regular secondary
structures such as α-helices and β-pleated sheets
• E.g: alpha-keratin, collagen, silk fibroin
alpha-keratin:bundles of helical polypeptides twisted
together into large bundles
• Alpha-keratin found in hair wool, skins, horns, fingernails are
alpha-helical polypeptides.

92. Globular protein:
Stabilization of cross linkages
• Cross linkages can be between 2 parts of a protein or
between 2 subunits
• Disulphide bonds (-S-S-) form between adjacent –SH groups
on the amino acid cystein

93. Proteins at work:
• Conformation of a protein gives it a unigque function
• To work proteins must interact with other molecules, usually 1
or a few molecules from the thousands .
• Ligand: the molecule that a protein can bind
• Binding site
-part of a protein that interacts with the ligand
-consists of a cavity formed by a specific arrangment of amino
acids
• The binding site forms when amino acids from within the
protein come together
• The remaining sequence may play a role in regulating the
protein’s activity

95. Chemical characteristic of proteins:
• Proteins have ionic and hydrophobic sites both internally
(within folds of tertiary structure) and on surfaces where
primary structures come in contact with the environment
• Ionic sites are provided by charged amino acids at physiological
pH and by covalentl attached modifying group (eg:
carbohydrates and phosphate)
• Net charge on protein contributed by free alpha-amino of N-
terminal residue, free alpha-carbonyl group of c terminal
residue, ionizable R groups and unique array of modifications
attached to proteins
• At isoelectric point (pI): no of (+) and (-) charges on protein are
equal. Protein is electrically neutral
• Protein has net (+) at pH values below its pI and (-) charge
above is pI

96. Blue: positive charge, red: negative charge