Basic information on proteins and amino acids

What are proteins?

They are macromolecules that are characterized by being formed by polymer chains of amino acids with specific sequences that fulfill different functions within the human body. During this article we will talk about the generalities, functions and structures of proteins.

What are amino acids?

They are molecules that are characterized by being composed of a central carbon atom (α-Carbon) linked to a hydrogen atom (H), a carboxyl group (COO), an amino group (HN) and a specific side chain (R ). they are characterized as the structural unit of proteins.


Side Chains

Son las propiedades químicas específicas de los aminoácidos que determinan los papeles de su estructura y función proteica.

What is the classification of amino acids?

The amino acids are classified into 4 groups, each group has a specific property in common that is defined by the side chains (R), two of these groups are hydrophilic and two are hydrophobic.

  • Non-polar amino acids
  • Polar amino acids
  • Basic Amino Acids
  • Acid Amino Acids


Non-polar amino acids

There are 10 non-polar amino acids (glycine, alanine, valine, leucine, isoleucine, proline cysteine, methionine, phenylalanine and tryptophan) that are characterized by not having an interaction with water.

  • Glycine (Gly) G: It is the simplest amino acid, with a side chain consisting of a single hydrogen atom.

  • Alanine (Ala) A; Valina (Val) V; Leucine (Leu) L; Isoleucine (Ile) I: These amino acids are characterized by having hydrocarbon side chains (R) composed of up to four carbon atoms (C).

  • Proline (Pro) P: It is the only amino acid that contains a side chain linked to the Nitrogen of the amino group and to the α coal forming a cyclic structure.
  • Cysteine (Cys) C; Methionine (Met) M: These two amino acids are characterized by having sulfur atoms; in addition, methionine is characterized by being more hydrophobic compared to cysteine because it contains a sulfhydryl (SH) group in its structure.
  • Phenylalanine (Phe) F; Tryptophan (Trp) W: These amino acids are characterized by having side chains that contain very hydrophobic aromatic rings.

Polar Amino Acids

This group is composed of 5 amino acids (Serine, Threonine, Tyrosine, Asparagine, Glutamine) that have no charge and can form bonds with Hydrogen and water; they are still hydrophobic molecules.

  • Serine (Ser) S; Threonine (Thr) T; Tyrosine (Try) Y: These amino acids have hydroxyl groups in each of their side chains.

  • Asparagine or Aspargin (Asn) N; Glutamine (Gln) Q: These amino acids are characterized by having amide groups (O = C-NH) in their side chain.


Acid Amino Acids

This group is made up of 2 amino acids (Aspartic acid and glutamic acid) that are characterized by having a carboxyl group (COO).

  • Aspartic acid (Asp) D; Glutamic Acid (Glu) E: They are negatively charged within the cell and is therefore often referred to as aspartate and glutamate.

Basic Amino Acids

In this group is made up of 3 amino acids (Lysine, Arginine and Histidine) that are characterized by having side chains with basic groups loaded, are hydrophilic this means that these amino acids are in contact with water.

  • Lysine (Lys) k; Arginine (Arg) R: They are very basic amino acids and their side chains are positively charged inside the cell.
  • Histidine (His) H: In its side chain is an aromatic ring, in addition to which it can be loaded or without positive charge at physiological pH.

How do amino acids join?

They are linked by means of peptide bonds in the amino group of one amino acid and the α-carboxyl group of the other amino acid as can be seen in the following image.

What are polypeptides?

They are linear chains of amino acids (aa) usually hundreds or thousands of them in their length; each polypeptide chain has two ends, the first is the amino terminus or N terminating and the second is the α carboxyl or C terminal end.

How is the structure of proteins?

Proteins adopt characteristic three-dimensional configurations, which are crucial for their functions; 4 levels can be described.

Primary Structure: It is the sequence of amino acids linked by peptide bonds.

Secondary Structure: It is the regular ordering of the amino acids within the polypeptide chains. There are two common types:

  • Α Helix: It is formed when a region of a polypeptide chain is wound on itself generating hydrogen bonds between the CO groups and the NH group of two different peptide bonds.
  • Β sheet: It can be formed between several polypeptide chains and can be oriented in parallel or antiparallel.

Tertiary Structure: It is the folding of the polypeptide chain as a result of the interactions between the side chains of amino acids that are in different regions of the primary sequence, this means that it is the combination of the primary and secondary structures; they fold into structures called domains that are the basic units of tertiary structures.

A crucial determinant of tertiary structure is the location of hydrophilic amino acids on the surface, where they interact with water and hydrophobic in the interior.

Quaternary Structure: Consists of the interaction of different polypeptide chains in compound proteins, this means that these proteins are structured by 2 or more polypeptide chains


1. Geoffrey M. Cooper & Robert E. Hausman. 2010. 2 Composición de las Células. In: Marbán Libros, S.L

ASM Press, eds. La Célula. Whashington: pp. 52-58.