THE -R GROUPS DETERMINE THE PROPERTIES OF AMINO ACIDS
Since glycine, the smallest amino acid, can be accommodated in places inaccessible to other amino acids, it often occurs where peptides bend sharply. The hydrophobic R groups of
alanine, valine, leucine, and isoleucine and the aromatic R groups of phenylalanine, tyrosine, and tryptophan typically occur primarily in the interior of cytosolic proteins. The charged R
groups of basic and acidic amino acids stabilize specific protein conformations via ionic interactions or salt bridges. These interactions also function in "charge relay" systems during
enzymatic catalysis and electron transport in respiring mitochondria. Histidine plays unique roles in enzymatic catalysis. The pKa of its imidazole proton permits histidine to function at
neutral pH as either a base or an acid catalyst without the need for any environmentally induced shift. The primary alcohol group of serine and the primary thioalcohol (—SH) group of
cysteine are excellent nucleophiles, and can function as such during enzymatic catalysis. However, the secondary alcohol group of threonine, while a good nucleophile, is not known to
fulfill an analogous role in catalysis. The —OH groups of serine, tyrosine, and threonine also participate in regulation of the activity of enzymes whose catalytic activity depends on the
phosphorylation state of these residues.