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.

Esophagus (Anatomy and Histology)

Inervación del esófago


It is an organ that is part of the digestive system, it is characterized as a muscular cylindrical duct that carries food from the oropharynx to the stomach, this being its main function.


Transport food from the oral cavity to the stomach.



Anatomically adult esophagus is characterized by having a length between 25 to 30 cm

It has a cylinder shape and has three narrowings:

  • Cricoid, corresponds to the cricopharyngeal muscle, with a light of 1.5 cm, is the narrowest region of the entire esophagus.
  • Thoracic (Broncho-aortic), this is formed when the anterior and left lateral walls of the esophagus are pushed towards the esophagus’s light as they are crossed by the left main bronchus and the aortic arch. (Its diameter is around 1.6 cm).
  • Frenic (Diaphragmatic), This is due to the mechanism of the gastroesophageal sphincter. (The diameter of this zone varies depending on the degree of distension of the esophagus to the passage of food, but values between 1.6 and 1.9 cm have been obtained)

And it is divided into 3 Regions or portions (Image 1)

  • Cervical region: 5 cm long
  • Thoracic region: 16-20 cm in length
  • Abdominal region 3 cm long
  • And the passage through the diaphragm the esophagus measures 1 cm

Image 1

Its origin is located at the lower edge of the cricopharyngeus muscle (portion of the inferior constrictor muscle of the pharynx), located at the lower edge of the cricoid cartilage, at the level of C6 and C7. (Image 2)

Image 2

Then the esophagus enters the thorax, locating anterior to the vertebral bodies and posterior to the trachea; at the level of T4 begins to locate posterior to the pericardium and anterior to the thoracic descending aorta, until reaching the diaphragm which crosses through the esophageal hiatus, this last portion (Abdominal portion) of the esophagus that faces to the left leading to the cardia of the stomach as seen in Image 1

Cervical Region:

It is the portion that lies between C6 and T2.

  • On the anterior side, it is in contact with the trachea, which it passes slightly to the left from C7.
  • On the posterior side we find the spinal column (prevertebral leaf of the cervical fascia)
  • On the right lateral face, it is related to the right recurrent Laryngeal nerve, inferior thyroid vein and part of the right common carotid artery.
  • On the lateral side Left: The left common carotid artery is closer to the esophageal border.

Thoracic portion:

It is the portion that lies between T2 to the Diaphragm.

At its entrance to the thorax, the esophagus is always retrotracheal, passing between the two pre-domes.

  • On the anterior side, above T4, the trachea is located and below it is the posterior face of the pericardium and the left atrium.
  • On the posterior side, above T4, the spinal column and part of the thoracic duct are located and below the T4, the thoracic descending aorta is located.
  • On the right lateral face, we find the azygos vein.
  • On the left lateral aspect, it is related to the thoracic descending aorta in most of its course (Image 3).

Image 3

Abdominal portion

It is the last portion of the esophagus that is just below the diaphragm before connecting with the stomach (Image 1), here it is mainly related in all its faces to the peritoneum.


In the cervical region the esophagus is irrigated mainly by the upper esophageal arteries that come from the inferior thyroid arteries; The thoracic region is the middle esophageal arteries that come directly from the aorta artery, the brochial and intercostal arteries; The abdominal region the lower esophageal arteries come from the posterior gastric and the lower phrenic arteries (Image 4).

Image 4

Drainage system

The drainage, as well as the subjects previously seen in the esophagus, are classified according to their region. (Image 5)

  • Cervical region, blood drains to the lower thyroid veins.
  • Thoracic region, blood drains to the superior phrenic veins, bronchial, pericardial reaching the azygos vein which drains into the superior cava system.
  • Abdominal region, the drainage is towards the left gastric vein through its gastroesophageal collaterals, tributary of the territory of the hepatic portal vein.

Image 5


The esophagus has a motor innervation (Sympathetic – Parasympathetic). (Image 6)

  • Sympathetic system adopts the pathway of the vascular nerves and reaches the esophagus with the arteries.
  • The parasympathetic system reaches the esophagus by the left recurrent laryngeal nerve branch of the vagus nerve.

Image 6


The esophagus as well as the entire digestive tract is made up of four histological layers (mucosa, submucosa, muscular and adventitia) (Image 7).

  • The Mucous is the innermost layer of the esophagus and is made up of three parts
  • Submucous: It is relatively lax with many elastic fibers, which allows considerable distension during the passage of the food bolus. You can also find small seromucous glands that allow lubrication of the mucosa.
  • Muscular: It is made up of two layers.
  • Adventicia: Conformed by loose connective tissue.

Image 7


(1) Boya Vegue J. Atlas de Histología y Organografía Microscópica. 3a ed. Madrid, España: Editorial Medica Panamericana; 2011.

(2) Frank H. Netter M. Atlas de Anatomía Humana. 4.a ed. Barcelona, España: Elsevier Masson; 2007.

(3) Latarjet M, Ruiz Liard A. Esófago. 4a ed. Buenos Aires, Argentina: Editorial medica panamericana; 2007.