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MS C
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Amino

acids and their derivatives participate in cellular functions
such as nerve transmission and biosynthesis of porphyrins,
purines, pyrimidines and urea
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3




In nature there are over 300 naturally occuring amino acids
Only 20 of these occur in proteins(std amino acids)



Some proteins contain amino acid derivatives that are
generated after incorporation of the amino acid into the protein
molecule



Several free L-α-amino acids fulfill important roles in metabolic
processes
...


4





Nineteen of these are α-amino acids with a primary amino
group (–NH3+) and a carboxylic acid (carboxyl; –COOH)
group attached to a central carbon atom
...
Also attached to the C atom is a hydrogen
atom and a variable side-chain or ‘R’ group
...


5

Amino acids may have positive, negative or zero
charge (Zwitterions)
...
This carbon
atom is called α- carbon
because its adjacent to
the carboxyl (acidic)
group

COOH

H

C

COO

–

R

R
Un-ionized form of an
amino acid

Dipolar ion (Zwitterion)
form of an amino acid

**Amino acids in solution at physiologic pH (pH 7
...
00**
...


•

This is known as an asymmetric center or chiral center and
has the property of chirality
...


•

Since they are non-superimposable mirror images of each
other, the two forms represent a class of stereoisomers called
enantiomers

7

•

Enantiomers have physically and chemically indistinguishable
properties by most techniques; but can be distinguished on the
basis of their different optical rotation of plane-polarized light
...


•

Only the L-amino acids are found in proteins
...


8



D-Amino acids that occur naturally include free:
-D-serine and D-aspartate in brain tissue,
-D-alanine and D-glutamate in the cell walls of gram-positive
bacteria, and

- D-amino acids in some non-mammalian peptides and certain
antibiotics
...

Are subdivided into smaller groupings on the basis of
similarities in the properties of their side-chains
...




Some have small side-chains, others large, bulky side-chains
...




Some confer conformational inflexibility
...

**Some are chemically reactive
...




High amount of energy required to disrupt the ionic forces
that stabilize the crystal lattice account for the high melting
points of amino acids (> 200 °C)
...
However, tyrosine, phenylalanine, and especially
tryptophan absorb high-wavelength (250–290nm) ultraviolet
light
...




11

Aliphatic side chains
H CH

Glycine

Alanine

COO

Smallest & simplest structured amino
acid owing to H-atom-does not exist
exhibit chirality
...


–

+
3

NH

CH3 CH

–

COO

+
3

NH

CH3

Valine

Leucine

–

C H CH
CH3

COO
+
3

NH

CH3
CH
CH3

CH2 CH

–

COO

+

NH3

CH3

Isoleucine

Chemically unreactive and
have hydrophobic side
chain-have aversion to
water and like to cluster
...


CH2
CH
CH3

CH

COO

–

+

NH3

12

Amino acids containing hydroxyl (OH) side chains
Serine

Threonine

CH2

CH

OH

NH +
3

CH3

Hydroxylated
...
More hydrophilic and
reactive than valine

**The –OH groups of these amino acids can participate in enzyme regulation**

Side chains containing sulfur atoms

Cysteine

Methionine

CH2

CH

SH

NH3+

CH2

CH2

S CH3

–

COO

CH
+
3

NH

Contains a sulfhydryl group (-SH)
...
-SH is highly reactive
capable of reacting with another cysteine to
form a disulfide bond
...
Is Hydrophobic

13

Side chains containing acidic groups on their amides
– OOC

Aspartic acid

CH2

–

CO O

CH
+

Side chain nearly always negatively
charged at physiological pH
...


–

CH

CO O
+

NH3

Uncharged derivatives of glutamate and aspartate are glutamine and
asparagine respectively, which contain a terminal amide group in place of a
carboxylate

Are therefore polar and de-protonated amino acids

The charged R groups stabilize specific protein conformation
via ionic interactions, or salt bonds
14

Side chains containing basic groups
Arginine

H

N

CH2

CH2

CH2

COO
+
3

+

C

–

CH
NH

NH2

NH2

Lysine

CH2

CH2

CH2

CH2

–

CH

COO

Very polar side chains
which render them highly
hydrophilic
...


+

+

NH

NH3

3

CH2

Histidine
HN

N

–

COO

CH

+
3

NH

Equally polar and hydrophilic
but may be positively charged
at neutral pH depending on
conditions
...
0

The charged R groups stabilize specific protein conformation
via ionic interactions, or salt bonds
15

Side chains containing aromatic rings
Phenylalanine

CH2

CH

COO

Phenyl ring attached to a
methylene group
...
OH is also reactive,
paticipates in enzyme regulation
...

Hydrophobic

H

Occur primarily in the interior of cytosolic proteins The aromatic rings contain
delocalised electron clouds which enable them to interact other systems and
transfer electrons
...


Imino acid
Proline

+
N
H2

Has an aliphatic side chain bonded to both the
COO nitrogen and α carbon atoms
...
Often
found on bends of folded proteins
...

17

Essential amino acids must be supplied in the diet while non
essential ones can be synthesized by the human body
...

also found in collagen, a fibrous protein of connective tissues
...

b)carboxyglutamate, found in the blood clotting protein
prothrombin and in certain other proteins that bind calcium ions as
part of their biological function
...

d)Selenocysteine is a special case
...
It contains selenium rather than the
sulfur of cysteine
...
Is a constituent of
just a few known proteins
...

They have a variety of functions but are not constituents of
proteins
...
g
...


20

Amino Acids Can Act as Acids and Bases
When an amino acid is dissolved in water, it exists in solution as
the dipolar ion, or zwitterion (German for ―hybrid ion
...




A simple monoamino monocarboxylic α-amino acid, such as
alanine, is a diprotic acid when fully protonated—it has two
groups, the -COOH group and the -NH3 group, that yields
protons:

Owing to this nature amino acids have characteristic
titration curves
23

WHAT IS ACID-BASE TITRATION?
•

Acid-base titration involves the gradual addition or removal of protons
...

-At very low pH, glycine is in fully protonated
form
...

-At the midpoint of any titration, a point of
inflection is reached where the pH is equal to
the pKa of the protonated group being
titrated- pH at the midpoint is 2
...
34
...

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IMPORTANT INFORMATION
...
pKa of any functional group is greatly
affected by its chemical environment-exploited
in the active sites of enzymes to promote
exquisitely adapted reaction mechanisms
...
Glycine has two regions of buffering power
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4
...
Predict the electric charge of an amino acids;
ie is the r/ship between its net electric charge
and the pH of the solution
...
97, glycine is present predominantly
as its dipolar form, fully ionized but with no net
electric charge -this characteristic pH at which
the net electric charge is zero is called the
isoelectric point or isoelectric pH,
designated pI:
CALCULATION OF pI:

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IMPORTANT INFORMATION CONTINUED…
...
Glycine has a net negative charge at any
pH above its pI and will thus move toward the
positive electrode (the anode) when placed in
an electric field
...
At any pH below its pI, glycine has a net
positive charge and will move toward the
negative electrode (the cathode)
...


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MORE COMPLEX TITRATION CURVES
•
•

Involves amino acids with an ionizable R group
...
Consider (a)Glutamate, Glu, E
...
22, lower than that of G,due
to the presence of two carboxyl groups,
which at the average of their pKa values
(3
...


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SUMMARY OF AMINO ACIDS•
20

amino acids commonly found as residues in proteins-contain
an α-carboxyl group, an α-amino group, and a distinctive R group
...
Only the
L stereoisomers are found in proteins
...
[1]as constituents of
proteins (through modification of common amino acid residues
after protein synthesis) [2]as free metabolites
...




Amino acids vary in their acid-base properties and have
characteristic titration curves
...

Amino acids with ionizable R groups have additional ionic
species, depending on the pH of the medium and the pKa of
the R group

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