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4.4.1. Protein structure

Protein structure

The proteins are formed by one or more polypeptide chains and although they are only composed of twenty amino acids different in each cell there are thousands of different proteins with characteristic functions.

The proteins acquire a structure which sometimes is very complex, since they are non - linear molecules, but fold into the space acquiring a characteristic shape (tertiary structure) that determines their biological function.

The structure of a protein is the arrangement of polypeptide chains in space.

Due to their complexity, there are four structural levels known as:

Fundamental ideas about the structure of proteins

The structure of a protein is the arrangement of polypeptide chains in space. They are distinguished:

  • Primary structure: it is a linear sequence of amino acids linked bypeptide bonds  .
  • Secondary structure: the peptide chain is arranged in space according to the number of hydrogen bonds that are formed.
    • α helix: the amino acid chain is wound in a clockwise turn. Hydrogen bonds form between the O of the  –C=O of an amino acid and the H of NH of the fourth amino acid that follows.

In the case of the collagen helix, it is wound in a left-handed way and there are three amino acids per turn.

    • Β conformation: A helix is ​​not formed, but the peptide chain adopts a zigzag shape, without the presence of hydrogen bonds between amino acids of the same chain. If this peptide chain folds, hydrogen bonds can appear between amino acids, forming a folded sheet.
  • Tertiary structure. It is the disposition that the secondary structure adopts in space when it folds over itself and originates a globular conformation
  • Quaternary structure. Structure that occurs when the protein is made up of two or more polypeptide chains, whether they have a tertiary structure or not.

Variations in temperature and pH can cause denaturation of proteins, breaking the bonds that originate the tertiary and quaternary structure, and even, on occasions, the secondary structure. If denaturation occurs, the protein acquires a filamentous conformation and precipitation of said protein occurs.