Amino acids, peptides, and proteins

Amino acids are the basic monomeric building blocks of all proteins. Proteins are complex macromolecules that perform important functions in an organism’s metabolism.

Functions of proteins:

  1. Component of muscle tissue, movement
  2. Enzymes; function of enzymes as catalysts of biochemical reactions
  3. Hormones (regulation)
  4. Receptors in the cell membrane or intracellular receptors
  5. Ion channels
  6. Functions in the blood, defense, coagulation, etc.

The term "peptide" is often used as a synonym for protein. Although the term "peptide" generally refers only to smaller proteins with molecular weights up to 10,000, this convention is not always followed.

For example, the term "peptide hormone" is used in anti-doping regulations, which includes, among others, growth hormone and erythropoietin (EPO)—molecules with molecular weights well over 20,000.

Fig. 1: Basic structure of all proteinogenic amino acids (left); Fig. 2: Glycine (right)
Fig. 3 L-alanine (left) and D-alanine (right)

There are a total of 20 amino acids, all of which share a common structure (Fig. 1) and differ only in the nature of their side chain R. The simplest amino acid is glycine, with R = H (Fig. 2).

Fig. 1 Basic structure of all proteinogenic amino acids (left); Fig. 2 Glycine (right)

With the exception of glycine, all other amino acids found in proteins have four different groups (substituents) attached to the alpha-carbon atom. Carbon atoms with four different substituents are optically active, can rotate polarized light, and are referred to as chiral centers.

Therefore, there are two forms of each amino acid that are mirror images of each other and are designated as L- and D-forms (Fig. 3). The L- and D-forms are not identical due to their spatial arrangement. All proteinogenic (naturally occurring) amino acids possess the L-form.

Fig. 3 L-alanine (left) and D-alanine (right)

Amino Acids - Abbreviations, Symbols, Molecular Weights, and Chemical Formulas

NameAbbrev.SymbolMolecular weight 
g/mol		Chemical 
formula
AlanineAla  A  89.0935C3H7NO2
ArginineArg  R174.2017C6H14N4O2
AsparagineAsn  N132.1184C4H8N2O3
Aspartic acidAsp  D133.1032C4H7NO4
CysteineCys  C121.1590C3H7NO2S
GlutamineGln  Q146.1451C5H10N2O3
Glutamic acidGlu  E147.1299C5H9NO4
GlycineGly  G  75.0669C2H5NO2
HistidineHis  H155.1552C6H9N3O2
IsoleucineIle  I131.1736C6H13NO2
LeucineLeu  L131.1736C6H13NO2
LysineLys  K146.1882C6H14N2O2
MethionineMet  M149.2124C5H11NO2S