fsurman-amino acids

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Transcript fsurman-amino acids

Faculty of Chemistry, VUT Brno, 2nd March 2006
Presentation about
Amino acids
František SURMAN
 Description – general properties
• Amino acids are derivatives of carboxylic acids formed by
substitution of -hydrogen for amino functional group.
• majority of amino acids has amphoteric character –
functional group –COOH is the reason of acidity and –NH2
group causes basic properties.
• in basic environment AA dissociate proton to form carboxyl
anion –COO-. Basic surround defends –NH2 against
• in acidic environment AA accept proton to form amonium
cation –NH3+. Acidic environment defends –COOH against
• Zwitterionic structure is neutral and its value of pH is called
isoelectric point.
• AA are optically active molecules and asymmetry of their
mirror images is not superimposable (except in the case of
glycine where the R-group is hydrogen)
• according new UIPAC
nomenclature L- D- forms
were replaced for (S)- and (R)system
• Ordinary synthesis of asymmetric molecules produces
racemic mixtures. To obtain the naturally ocurring (S-) AA
we must revolve the racemic form. Fortunately we know a
variety of ways.
• Biosynthesis of substances of AA having asymmetric
centers almost produce pure stereoisomers. Using this
criteria, examination of amino acids in practice always
shows racemic mixture.
• Physical propeties – AA are colourless crystalline
substances soluble in water and insoluble in organic
solvents with high melting point.
 Separation of amino acids
• Position of amino group − -amino acids exist in two
enantiomeric forms. Only L-(S-) acids are found in nature.
• Proteinaceous AA − proteins consist of 20 AA.
• Essential (unexpendable) − organism is not able to
synthesize these AA but accept from food.
• Nonessential (expendable) − organism produced
from essential AA by transamination.
• According R- functional group
• Nonpolar (hydrophobic)
• Polar (hydrophilic) – better soluble in water
• Basic – contains more atoms of nitrogen
• Acidic – contains more carboxyles
 Summary of 20 proteinaceous AA
Essential AA
• Valin
• Leucin
• Isoleucine
• Threonine
• Lysin
• Methionin
• Phenylalanine
• Tryptophan
• Arginine
• Histidine
Nonessential AA
• Alanine
• Asparagine
• Aspartate
• Cysteine
• Glutamate
• Glutamine
• Glycine
• Proline
• Serine
• Tyrosine
• Besides basic 20
proteinaceous AA
more AA exist
found only in
some organisms.
• Essential AA are
different for
variety of animal
species and
plants are able to
synthesize all 20
 Reactions of amino acids
Polymerization – form peptides, proteins and enzymes
• A condensation reaction between the carboxyl of one
amino acid and the amino group of another forms a peptide
• Peptides
• Oligopeptides − condensation of 2 – 10 AA units
• Polypeptides − condensation of 11 – 100 AA units
• Proteins − more than 100 AA units
• Disulfide linkage – conversion of cysteine to cystine is like a
conversion of thiols to disulfides by mild oxidizing agents.
This conversion can be reversed by mild reducing agents.
• Disulfide bonds stabilize protein structure by providing
 Synthesis of -amino acids
• A variety of methods have been developed
• Important in industry due to the commercial relevance
 bodybuilding supplements (for big musles)
 sources of vitamins not only for human also for animals
 The Gabriel synthesis (from potassium phthalimide)
 The Strecker synthesis
 Enantioselective synthesis produces only or predominantly
pure AA form. (More info Solomons & Fryhle p.1175 - 1177)
 Metabolism of -amino acids