Amino acids Flashcards
What are amino acids?
The monomers of proteins - large polymers of amino acids
They come together in a condensation reaction (catalysed by RNA) forming a peptide bond (amide linkage)
What are the ‘standard’ amino acids?
There are 20 natural amino acids
Common are alpha-amino acids (a-amino acids)
Describe the general structure of amino acids?
Carbon with 4 functional groups: Amino group (NH2) Carboxyl group (COOH) Hydrogen atom Variable group (R)
Large variety due to R group
The R group leads to different properties e.g. hydrophobic/hydrophillic
State the hydrophobic, nonpolar, uncharged amino acids?
Glycine Alanine Valine Leucine Isoleucine Methionine Proline Phenylalanine Tryptophan
State the hydrophillic, polar, uncharged amino acids?
Serine Threonine Asparagine Glutamine Tyrosine Cysteine
State the positively charged amino acids?
Lysine
Arginine
Histidine
State the negatively charged amino acids?
Aspartic acid
Glutamic acid
What is a property of amino acids?
Amphoteric - can act as an acid and/or base
At physiological pH - 7.4
Amino group is protonated
Carboxyl group is in the conjugate base form COO-
When amino acids bear different charged groups what are they known as?
Zwitterions
This is due to the ionisable properties of the groups
What are the names of polymers of proteins?
Dipeptide
Tripeptide
Oligopeptide
Polypeptide
What is significant about cysteine?
It has a thiol group that can form a disulfide bond (covalent) with another cysteine via oxidation to form cystine
What are the pK values of amino acids?
A measure of the strength of an acid on a logarithmic scale
The ‘charge’/pK value depends on the pH present
What is significant about aspartic acid and glutamic acid?
In their ionised state they are often referred to as Apartate and Glutamate
Asparagine and Glutamine are the amides of aspartic acid and glutamic acid
Glx = either Glu or Gln Asx = either Asp or Asn
What is the sterochemistry of amino acids?
All natural amino acids (except for glycine) are chiral and therefore optically active
They are asymmetric non-superimposable on their mirror image
They will rotate the plane of polarised light - the direction and angle of rotation can be measured using a polarimeter
What do chiral centers give rise to?
Enantiomers - they are physically and chemically indistinguishable by most techniques
In a polarimeter they rotate the plane in opposite directions but to the same degree
How do we describe the asymmetry of amino acids and other chiral molecules?
Fischer convention
L - signifys rotation of polarised light to the left
D - signifys rotation of polarised light to the right
Fischer projections - different bonds to show the direction within which plane
What fischer configuration are amino acids?
All amino acids derived from proteins have the L stereochemical configuration
Bacteria can use some D-amino acids
However, this doesn’t indicate its ability to rotate the plane of polarized light. Many L‐amino acids are dextrorotatory
Why is stereochemistry important in living systems?
A racemic mixture contains both enantiomers
Most drugs only have one enatiomer with biological activity e.g. ibuprofen
Thalidomide - the inactive enantiomer causes severe birth defects
How are non-standard amino acids produced?
Post-translational modifications
12 amino acids can be modified via:
hydroxylation, methylation, acetylation, carboxylation, and phosphorylation
Give some examples of PTM amino acids?
GABA - glutamate
Dopamine - tyrosine
Histamine - histidine
Thyroxine - tryosine
What is GFP?
Green Fluorescent protein
This chromophore arises from modification of serine, tyrosine and glycine
It is green due to delocalised electrons - it can absorb photons
What is an isopeptide bond?
An amide bond formed between residues that involved side chains
