Amino acids overview Flashcards
Amino acid:
a compound that contains
both an amino group and a carboxyl
group
(a)- amino group, (a)-carboxyl group, alpha (a) carbon, Side chain
(a)-Amino acid:
an amino acid in which the amino group
is on the carbon adjacent to the carboxyl group
Side chain:
Varies between different amino acids
Amino Acids-Nomenclature:
- 1,1- = (a) amino acid
Example: glycine.
-1,2- = (B) amino acid
Example: β-alanine.
- 1,3- = γ amino acid
Example: γ-aminobutyric acid
(GABA)
Amino acids are the
building…
blocks of proteins
- alphabet of 20 α-amino acids
all proteins in bacteria to humans are
constructed…
from this same set of 20 amino
acids which is over 2 billion years old
amino acids vary in their…
properties of size,
shape, charge, hydrogen-bonding capacity
and chemical reactivity
-amino acids also have many common
properties
The Protein Alphabet: (A-L)
A - Ala - alanine
C - Cys - cysteine
D - Asp - aspartate
E - Glu - glutamate
F - Phe - phenylalanine
G - Gly - glycine
H - His - histidine
I - ile - isoleucine
K - Lys - lysine
L - Leu - leucine
The Protein Alphabet: (M-Y)
M - Met - methionine
N - Asn - asparagine
P - Pro - proline
Q - Gln - glutamine
R - Arg - arginine
S - Ser - serine
T - The - threonine
V - Val - valine
W - Trp - trytophan
Y - Tyr - tyrosine
The language of proteins
-Amino acids
(the alphabet)
- Proteins
(words) - Metabolic pathways
(sentences)
-Biological processes (chapters)
-The cell
(the story)
19 of the 20 common amino acids have
a chiral
α-carbon atom (glycine does not)
Threonine and isoleucine have
2 chiral carbons
each (4 possible stereoisomers each)
Mirror image pairs of amino acids are
are
designated L (levo) and D (dextro)
-Proteins are assembled from L-amino acids
(a few D-amino acids occur in nature)
-The L-amino acids are found in biological systems.
Configuration
- An L-Amino acid has the H2N- on the left hand side
when drawn in this FISCHER projection. - By contrast, most carbohydrates are D-configured
-In the R,S (priority) system, 19 of the vital 20 (a)-amino
acids are S, and one (cysteine) is R.
D/L -configurations
D)
-the form of glyceraldehyde whose asymmetrical carbon atom has a hydroxyl group projecting to the right
L)
-whose asymmetrical carbon atom has a hydroxyl group projecting to the left
Amino Acids
- Colorless, crystalline, water soluble
substances - R group gives individuality
- R group varies from a simple H to a
complex phenyl ring - Classification reflects structural features
or properties of R groups
Important Properties of Amino Acids:
- They have the capacity to polymerize
- They have interesting acid base properties
- They have varied structure and chemical
functionality - They are chiral
Amino acids are…
Ampholytes.
-They can act as either an acid or a base
-They are zwitterions or molecules that
have both a positive and a negative
charge
- Because of their ionic nature they have
extremely high melting temperatures
Although (a)-amino acids are commonly
written in the…
unionized form, they are
more properly written in the zwitterion
(internal salt) form
Most amino acids are…
(charge rates)
zwitterions.
- Amino acids are zwitterions at neutral pH
(contain negative and positive charged groups)
In acid solution the basic amino group is…
protonated
In neutral solution the
zwitterion is formed by
an internal acid-base
reaction.
In basic solution the carboxylic acid…
group
is ionized
Zwitterion Definition:
The form HA , with zero net charge,
Isoelectric Point Definition:
The pH at which the zwitterion is the predominant
form
The average pKa of an -carboxyl group is…
2.19,
which makes it considerably stronger acids than
acetic acid (pKa
4.76)
– The greater acidity is accounted for by the
electron-withdrawing inductive effect of the
adjacent -NH3
+ group
Due to the electron-withdrawing inductive effect of the…
(a)-NH3
+ group, side chain -CO2H groups are also
stronger than acetic acid
– The effect decreases with distance from the -NH3
+ group.
Compare:
(a)-CO2H group of alanine (pKa
2.35) R = CH3
(B)-CO2H group of aspartic acid (pKa
3.86) R = HO2CCH2
(y)-CO2H group of glutamic acid (pKa
4.07) R = HO2CCH2CH2
The average value of pKa
for an (a)-NH3
+ group is…
9.47, compared with a value of 10.60 for a primary
alkylammonium ion
The isoelectric point, pI…
of an amino acid is the pH at
which the majority of its molecules in solution have no
net charge
– the pI for glycine, for example, falls between the pKa
values
for the carboxyl and amino groups
pI equation:
pI = (pK1 + pK2) / 2
Why does the same carboxyl group
have different pKa
values?
the dissociation constant of a
carboxyl group is different when it is in a different chemical
environment.
Make pg 41 flashcard
Protein modification
Further protein diversity is achieved by post-synthesis (AKA posttranslational) modification of proteins. Some are:
- Acetylation of N-terminal
- Proline and lysine oxidised to hydroxyproline
and hydroxylysine - Carboxylation of glutamate residues
- Glycosylation
- Phosphorylation (Ser, Thr, Tyr residues)
- Farnesylation
- Post-synthesis cleavage
- Glycation (diabetes)
post-synthesis modifications to protein structure add…
precision
to the regulation of protein activity inside and outside the cell
Post-Translational Modification
The repertoire of side chain functionality is embellished by the ability of
some amino acids to be chemically modified after translation:
Non-standard amino acids
not found in proteins
Ornithine and citrulline are intermediates in biosynthesis
Amino Acid Derivatives:
- Visible symptoms of allergies are caused by the release of histamine
in mast cells - Serotonin, which is derived from tryptophan, functions as a
neurotransmitter and regulator.
Essential Amino Acids
(EAAs)
- Amino acids that the body cannot metabolize
or make on its own in sufficient quantities - Therefore EAAs must be consumed in the diet
- Usually considered to be from 8-10 amino
acids
–developing children may need more of certain
amino acids and therefore they may be
considered essential to their diet
Essential Amino Acids:
- 10 amino acids not synthesized by the body
- arg, his, ile, leu, lys, met, phe, thr, trp, val
- Must obtain from the diet
- All in dairy products
- 1 or more missing in grains and vegetables
Other required amino acids
- Special cases where more of certain amino acids are needed
- Newborn children
– need more growth promoting aa’s -
arginine, taurine and cystine
- Disease states (eg. alcoholics)
Slide 80
