Concept Summary

Amino Acids, Peptides, and Proteins

The α-carbon    of  an  amino   acid    is  attached    to  four    groups: an  amino   group,  a   carboxyl    group,  a

hydrogen    atom,   and an  R   group.  It  is  a   chiral  stereocenter    in  all amino   acids   except  glycine.

All amino   acids   in  eukaryotes  are L-amino acids.  They    all have    (S) stereochemistry except

cysteine,   which   is  (R).

Amino   acids   are amphoteric, meaning they    can act as  acids   or  bases.

Amino   acids   get their   acidic  characteristics from    carboxylic  acids   and their   basic   characteristics

from    amino   groups.

In  neutral solution,   amino   acids   tend    to  exist   as  zwitterions (dipolar    ions).

Amino   acids   can be  classified  by  their   R   groups.

Nonpolar    nonaromatic amino   acids   include alanine,    valine, leucine,    isoleucine, glycine,

proline,    and methionine.

Aromatic    amino   acids   include tryptophan, phenylalanine,  and tyrosine.   Both    nonpolar

nonaromatic and aromatic    amino   acids   tend    to  be  hydrophobic and reside  in  the interior    of

proteins.

Polar   amino   acids   include serine, threonine,  asparagine, glutamine,  and cysteine.

Negatively  charged amino   acids   contain carboxylic  acids   in  their   R   groups  and include

aspartic    acid    and glutamic    acid.

Positively  charged amino   acids   contain amines  in  their   R   groups  and include arginine,   lysine,

and histidine.

Nonpolar    nonaromatic and aromatic    amino   acids   tend    to  be  hydrophobic and reside  in  the

interior    of  proteins.

Polar,  negatively  charged (acidic),   and positively  charged (basic) amino   acids   tend    to  be

hydrophilic and reside  on  the surface of  proteins,   making  hydrogen    bonds   with    the aqueous

environment.