Biological
Building
Blocks III PROTEINS Amino
Acids
• Building
blocks
of
Proteins
• Composed
of
two
parts:
– Backbone
– Side-‐chain
• Biology
employs
20
amino
acids
– Some
can
be
modified
for
addiIonal
variability
• About
half
of
the
amino
acids
are
"essenIal"
meaning
that
they
cannot
be
made
by
metabolic
conversion
from
other
molecules
and
thus
need
to
be
eaten
– For
the
ten
essenIal
amino
acids:
• Threonine,
Tryptophan,
Valine,
Arginine,
HisIdine,
Lysine,
Phenylalanine,
Leucine,
Isoleucine,
Methionine
– Remember
this
phrase:
• These
Ten
Valuable
Amino
Acids
Have
Long
Preserved
Life
In
Man
– This
means
our
body
has
the
ability
to
make
the
rest
of
the
20
amino
acids
from
simpler
building
blocks
Structures
Side
chains
come
in
4
“flavors”
Amino
Acids
can
someImes
be
Modified
in
interesIng
ways
• Amino
acids
side
groups
may
be
further
modified
to
have
sugars,
fats,
other
modificaIons
• OYen
these
modificaIons
are
very
important
in
changing
the
way
the
amino
acid
behaves
Amino
Acids
are
joined
together
using
PepIde
Bonds
to
make
Proteins
• Biologists
calls
this
process
translaIon,
and
it
happens
when
a
messenger
RNA
is
“read”
by
a
ribosome.
• PepIde
bonds
link
amino
acids
together
through
-‐NH2
on
one
amino
acid
and
–COOH
on
another
PepIde
Bond
is
Planar
and
rigid
* The peptide plane consists of six atoms, Cα1, C, O, N, H, Cα2
* H-N-Cα bond angle is 121o, not 109.5o. ProperIes
of
PepIde
Bond
• Barrier
to
RotaIon
about
C-‐N
bond
is
20
kcal
• PepIde
bond
is
planar
with
Cα
groups
typically
trans
to
the
pepIde
bond
(be^er
accommodate
R
groups)
• C
and
N
are
planar
(2-‐D)
and
not
able
to
rotate
• Cα
groups
are
tetrahedral
(3-‐D)
and
able
to
rotate
though
some
angles
are
unfavorable
due
to
steric
repulsion
with
other
atoms.
• Cα
Naturally
occurring
amino
acids
are
ALMOST
exclusively
the
L
stereoisomer.
Biological Building Blocks III
of 11
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