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soluble in water, not organic solvents,
amphoteric,
Zwitterions- electrically neutral, dipolar ion
(i) a basic amino group (–NH2)
(ii) an acidic carboxyl group (–COOH)
(iii) a hydrogen atom and
(iv) a variable group known as the R
gp which gives 'uniqueness' to the Aa
Except for glycine, all Aa have at least one
asymmetric C- atom and thus exhibit optical
isomerism
NON-POLAR (Hydrophobic)
- hydrocarbon R groups
- hydrophobic and unreactive
- Localized inside molecule
POLAR uncharged (Hydrophillic)
- polar R groups (–OH & –NH) lots of O
- no net charge.
- hydrophilic in nature.
POLAR, charged (Hydrophilic) :
- -vely / +vely R group, hydrophilic.
- Acidic Aa = net -, (COO- in R)
- Basic Aa = net +, (NH2+ in R)
R groups have important physical (size & shape) and chemical properties that determine the specific properties of a given amino acid.
Glycine (R= H)
Proline (Hydrophobic Aa)
Cysteine
- Hydropohilic / phobic
- does not fit into 2o structure
- Reactive SH gp
- produces kinks or hinges eg:
collagen
- forms S-S group
- Smallest Aa, thus allow
close contact ./. or within pp
> flexible, allow pp to form
hinges / more
PP formation (conden rxn)
- N & C Terminus
- R gp project fr backbone
- Lesser buffering capacity
- Biuret Test for peptide bonds
-Purple = Pos, Blue = Neg
-Intra / inter pp
- stabilizes 3D protein
These levels of structure are dependent on each other. If one level changes, the other levels may change as a
consequence.
Peptide Bonds
Primary
- unique # and linear sequence of Aa
- determined by genetic DNA sequences (KIV: Gene
Expression).
- contains info to specify how pp chains will coil, fold
and interact
- Size, charge, polarity or hydrophobicity of R
groups will affect the bonds present at higher levels
and the ultimate 3-D conformation n function of
protein
α-keratin
- fibrous protein with an α-helical shape.
-3 helices - protofibril (S-S) cross-linkages.
- Tough – d/p on o of S-S bonds
- Stretchable – H-bonds break & reform easily
- Insoluble in water –hydrophobic R groups
on outside of the fibre.
Silk fibroin
 sheet
-entirely
adj // chains, in ops directions
(i) Inelastic – covalently bonded chains
(peptide backbone) is nearly fully extended.
(ii) Very strong – all C=O and –NH groups
are involved in hydrogen bonding.
H-Bonds along pp backbone
Secondary
- regular coiling and folding of pp regions
- resultant repeated patterns is 2o structure
- by regularly spaced H-bonds formed at the pp backbone between
NH group of one Aa & C=O group of other Aa
- H-bonds DO NOT involve R groups.
- α-helix and β-pleated sheet.
-helix
- extended spiral spring.
- stabilized by hydrogen
bonds, between all (C=O) & (–
NH) groups.
-nth + 4 residue, (1 turn 3.6 Aa)
-- H bonds parallel to the main
axis
- R groups project
perpendicular to main axis.
- R groups influence α-helix;s
interaction
-sheet
- extended zigzag, sheet-like
conformation.
- H bonds between C=O and –NH
groups of one region pp & the C=O
and –NH groups of adjacent
regions of the same chain /
different chains.
- intrachain sheet
- interchain sheets
- R group steric hindrance, thus aa
in β-pleated sheet usually have
small R groups.
- ANTI-PARALLEL –
- PARALLEL
30 & 40 structure
-Interactions between R groups only
- 4 main types of interactions
-Maintains tertiary structure
-Similar bonds allow for interaction
with other pp mols
-Give rise to quat structure
Fibrous proteins
Globular proteins
Repetitive regular sequence of
amino acids
Fixed specific sequence of amino
acids; non-repetitive
Amino acid sequence may vary
slightly between two examples of the
same fibrous protein
Amino acid sequence never varies
between two examples of the same
globular protein
Usually elongated polypeptide
chains wrapped around to form
multimolecular parallel filaments to
strands
Much more compact structures
owing to highly contorted pattern of
folding, bending and twisting along
polypeptide chain to form a spherical
shape.
Has a specific three-dimensional
structure.
Length of polypeptide chain may
vary in two examples of the same
fibrous protein
Length of polypeptide always
identical in two examples of the
same globular protein
Stable structure due to the numerous
intra- and inter-molecular hydrogen
and disulphide bonds
Relatively unstable structure. This is
due to the numerous intra- and intermolecular non-covalent bonds, such
as hydrogen, ionic bonds
Generally insoluble in water
Generally more soluble than fibrous
proteins
Perform structural functions.
Examples include keratin, fibroin and
collagen.
Perform metabolic functions.
Examples include enzymes,
hormones, antibodies and
haemoglobin.
Usually tertiary and quaternary level
of protein organisation
http://www.wiley.com/college/fob/quiz/quiz07/7-7.html
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http://www.wiley.com/college/fob/quiz/quiz06/6-8.html
Aquaporin
3.6 amino acid
residues / turn