Faculty of Chemistry, VUT Brno, 2nd March 2006

Presentation about

Amino acidsAmino acids

byby

František SURMANFrantišek SURMAN

Description – general properties • Amino acids are derivatives of carboxylic acids formed by Amino acids are derivatives of carboxylic acids formed by

substitution of substitution of -hydrogen for amino functional group.-hydrogen for amino functional group.

• majority of amino acids has amphoteric character – majority of amino acids has amphoteric character – functional group –COOH is the reason of acidity and –NHfunctional group –COOH is the reason of acidity and –NH2 2

group causes basic properties.group causes basic properties.• in basic environment AA dissociate proton to form carboxyl in basic environment AA dissociate proton to form carboxyl

anion –COOanion –COO--. Basic surround defends –NH. Basic surround defends –NH2 2 against against

dissociation.dissociation.• in acidic environment AA accept proton to form amonium in acidic environment AA accept proton to form amonium

cation –NHcation –NH33++. Acidic environment defends –COOH against . Acidic environment defends –COOH against

dissociation.dissociation.

• Zwitterionic structure is neutral and its value of pH is called Zwitterionic structure is neutral and its value of pH is called isoelectric point.isoelectric point.

• AA are optically active AA are optically active moleculesmolecules and and asymmetry asymmetry of theirof their mirror imagesmirror images is is not superimposable not superimposable ( (except in the case of except in the case of glycine where the R-group is hydrogenglycine where the R-group is hydrogen))

• according new UIPAC according new UIPAC nomenclature Lnomenclature L- D- forms - D- forms were were replaced for replaced for (S)- and (R)- system(S)- and (R)- system

• Ordinary synthesis of Ordinary synthesis of asymmetricasymmetric molecules molecules produces produces racemic mixtures. racemic mixtures. To obtain the naturally ocurring (S-) AA To obtain the naturally ocurring (S-) AA we must revolve the racemic form. Fortunately we know a we must revolve the racemic form. Fortunately we know a variety of ways.variety of ways.

• Biosynthesis of substancesBiosynthesis of substances of AA of AA having asymmetric having asymmetric centers almost producecenters almost produce pure stereoisomers.pure stereoisomers. Using this Using this criteria, examination of aminocriteria, examination of amino acids inacids in practice practice always always showshowss racemic racemic mixture. mixture.

• Physical propeties – AA are colourless crystalline Physical propeties – AA are colourless crystalline substances soluble in water and insoluble in organic substances soluble in water and insoluble in organic solvents with high melting point.solvents with high melting point.

ValineArginineArginine

Alanine LysineLysine

• Position of amino groupPosition of amino group − − -amino acids exist in two -amino acids exist in two enantiomeric forms. Only L-(S-) acids are found in nature.enantiomeric forms. Only L-(S-) acids are found in nature.

• Proteinaceous AAProteinaceous AA −− proteins consist of 20 AA. proteins consist of 20 AA.• Essential Essential (unexpendable) (unexpendable) −− organism is not able to organism is not able to

synthesize these AA but accept from food.synthesize these AA but accept from food.• Nonessential Nonessential (expendable) (expendable) − − organism produced organism produced

from essential AA by transamination.from essential AA by transamination.• According R- functional groupAccording R- functional group

• Nonpolar (hydrophobic)Nonpolar (hydrophobic)• Polar (hydrophilic) – better soluble in waterPolar (hydrophilic) – better soluble in water• Basic – contains more atoms of nitrogenBasic – contains more atoms of nitrogen• Acidic – contains more carboxylesAcidic – contains more carboxyles

Separation of amino acids

Summary of 20 proteinaceous AAEssential AAEssential AA

• Valin Valin

• LeucinLeucin

• IsoleucineIsoleucine

• ThreonineThreonine

• LysinLysin

• MethioninMethionin

• PhenylalaninePhenylalanine

• TryptophanTryptophan

• ArginineArginine

• HistidineHistidine

Nonessential AANonessential AA

• Alanine Alanine

• Asparagine Asparagine

• AspartateAspartate

• Cysteine Cysteine

• Glutamate Glutamate

• Glutamine Glutamine

• Glycine Glycine

• Proline Proline

• Serine Serine

• TyrosineTyrosine

• 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 AA.

PolymerizationPolymerization – form peptides, proteins and enzymes – form peptides, proteins and enzymes• A condensation reaction between the carboxyl of one A condensation reaction between the carboxyl of one

amino acid and the amino group of another forms a peptide amino acid and the amino group of another forms a peptide bond.bond.

• PeptidesPeptides• Oligopeptides Oligopeptides − − condensation ofcondensation of 2 – 10 AA units 2 – 10 AA units • Polypeptides Polypeptides − − condensation of 11 – 100 AA unitscondensation of 11 – 100 AA units

• ProteinsProteins −− more than 100 AA units more than 100 AA units

Reactions of amino acids

• Disulfide linkageDisulfide linkage – conversion of cysteine to cystine is like a – conversion of cysteine to cystine is like a conversion of thiols to disulfides by mild oxidizing agents. conversion of thiols to disulfides by mild oxidizing agents. This conversion can be reversed by mild reducing agents.This conversion can be reversed by mild reducing agents.

• Disulfide bonds stabilize protein structure by providing Disulfide bonds stabilize protein structure by providing cross-link.cross-link.

• A variety of methods have been developedA variety of methods have been developed• Important in industry due to the commercial relevanceImportant in industry due to the commercial relevance

bodybuilding supplements (for big musles)bodybuilding supplements (for big musles) sources of vitamins not only for human also for animalssources of vitamins not only for human also for animals

Synthesis of -amino acids

The Gabriel synthesis (from potassium phthalimide)The Gabriel synthesis (from potassium phthalimide) The Strecker synthesisThe Strecker synthesis

Enantioselective synthesis produces only or predominantly Enantioselective synthesis produces only or predominantly pure AA form. (More info Solomons pure AA form. (More info Solomons && FrFryhle p.1175 - yhle p.1175 - 1177)1177)

Metabolism of -amino acids

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