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Molecular Biology
2.4 Proteins
 Made
of C, H, O and N
 Proteins
are large molecules constructed of
many amino acids
 Most
cells
abundant organic compound found in living
Most important macromolecule in the body…
Proteins Rule Everything!
Functions?






Structural parts
 nails, hair, cell membrane,
and cartilage
Pigments
 skin, eyes, and chlorophyll
Hormones and receptors
Muscle contractions
Immunity-Antibodies
Enzymes






Cytoskeleton
Blood clotting
Transport of nutrients/gases
Cell adhesion
Cell membrane transport
Packing of DNA
 Total
energy gain is 4 Calories/gram.
(however, energy gain is not their main
function).
 Amino
acids are linked together by
condensation reactions to form polypeptides.
 The
building blocks of proteins are amino
acids.
Amino Acids
Centre carbon atom
Bonded to:
 Hydrogen (H)
 Amine group (-NH2)
 Carboxyl group (-COOH)
 R- group (different in each amino acid)

 Amino
acids are linked together by
condensation reactions to form polypeptides.
 The
amino acids are bonded together by
peptide bonds to form proteins.
 The
bond that is formed from the acid group
(COOH) and the amino group (NH2)
 Water is eliminated = condensation reaction
 Proteins are frequently called polypeptides
(many peptide bonds).
 The
smallest polypeptide consists of 20
amino acids (less than that called an
“oligopeptide)
 Insulin
contains 2 polypeptides: 21 amino acids
and 30 amino acids
 Human titin has 34 350 amino acids
 There
are 20 different amino acids used by
ribosomes to make polypeptides
 Draw
molecular diagrams to show the formation
of a peptide bond:



Locate the Amine, Carboxyl, and R-groups
Show the formation of a peptide bond between 2 amino acid
Try to draw an oligopeptide with all 4
Alanine
Serine
Glutamic acid
Glycine
Did you notice?
 The amine and carboxyl are used in the peptide bonds
 Chain of atoms linked with single bonds form backbone
 H attached to N by single bond
 O attached to C by double bond
 The R-groups remain
Alanine
Serine
Glutamic acid
Glycine
 The
amine and carboxyl group are used in
the condensation reaction
 The R-groups give the polypeptide its
character
 The R-groups provide an amazing range of
proteins
 Some
proteins contain amino acids not
contained in the list of 20.
 This is due to modification after a polypeptide
has been made. Example: hydroxyproline in
collagen
 Patterns,
trends, discrepancies: most but not
all organisms assemble polypeptides from the
same amino acids.
 We can exclude the possibility that this trend
is due to chance…
 What reasons would account for almost all
organisms using the same 20 amino acids?
 These were the ones produced before life
 These are the ideal
 All life evolved from a single ancestral
species
 Amino
acids can be linked together in any
sequence giving a huge range of possible
polypeptides
 Calculate
the possibilities
Number of
Number of possible sequences
Amino Acids in
the Chain
1
201
2
202
3
400
8000
4
64 000 000
 Shapes
–
 primary
(linear)
 secondary (β-pleated sheets and α-helix)
 tertiary (bent-coiled)
 quaternary (compact with a specific structure).
 You
can unfold a protein (de-nature) by
exposing the protein to heat, radiation or a
change in pH. (i.e. frying an egg, baking a
cake, UV exposure, x-rays).
Questions
1.
2.
3.
4.
5.
6.
What 2 functional groups do all amino acids
have in common?
Draw a generic formation of a peptide bond.
Identify the amino terminus, carboxyl terminus,
and peptide bond.
Define primary structure.
Name the 2 types of secondary structure.
What type of bond stabilizes this structure?
Distinguish between polypeptide and protein.
Why do proteins have more diverse functional
roles than carbohydrates?