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Amino acids

Met dank aan Dr. Detke
Most amino acids are chiral
All naturally occuring amino acids
except Gly are the L isomer
 L & D amino acids are stereoisomers

Most amino acids are
zwitterions

amino acids
are
zwitterions
at neutral pH
(contains
negative and
positive
charged
groups)
Small aliphatic amino acids

Glycine
 non-chiral
 smallest
 most
flexible in
polypeptides
 less hydrophobic

Alanine
 chiral
 next
smallest
 less hydrophobic
Other aliphatic amino acids

Val, Leu, Ile
 Highly
hydrophobic
 branched side-chains
Yet another aliphatic amino acid

Proline
 imino
acid
 rigid ring structure
 puts kinks in
polypeptides
Aromatic amino acids
Phenylalanine
•Absorbs UV
•Very hydrophobic
Tyrosine
Tryptophan
•Absorbs UV
•Really absorbs UV!
•less hydrophobic
•less hydrophobic
•H-bonds
•H-bonds
•OH can ionize
•OH can be modified
Sulfur-containing amino acids

Methionine
 highly
hydrophobic
 initiator amino acid

Cysteine
 less
hydrophobic
 forms disulfide bonds
 SH group can ionize
Oxidation of cysteine to cystine


Disulfide bond
formation is
reversible
Disulfide bonds
form in proteins
and help
stabilize their
structures
Alcoholic amino acids

Ser, Thr
 less
hydrophobic
 H-bonding potential
 can be modified
 Thr is an isostere of
Val
Acidic amino acids



Asp, Glu
highly
hydrophilic
negatively
charged at
neutral pH
Basic amino acids



Highly hydrophilic
Lys and Arg are positively charged at neutral pH
His is partially charged at neutral pH
Amide-containing amino acids

Asn, Gln
 highly
hydrophilic
 H-bonding potential
 isosteres of Asp and
Glu
Peptide bond formation
The peptide bond is planar
Fig. 4.6
Aspartame – a dipeptide
200X Sweeter than Sucrose !!!
aspartylphenylalanine methyl ester