Amino acids are molecules that, from a chemical standpoint, are characterized
by the presence of an amine group and a carboxylic group (acid), and hence its
name: amino acid. Its main function is to serve as monomers for the synthesis
of peptides and proteins. Of all the amino acids in nature, there is a set of
20 amino acids which are designated by standard amino acids, and that are used
as building blocks for most of the proteins produced by any living being. These
amino acids are widely studied and have been the central elements in my last
posts. Just to remember, the standard amino acids are:
- glycine
- alanine
- proline
- valine
- leucine
- isoleucine
- methionine
- phenylalanine
- tyrosine
- tryptophan
- serine
- threonine
- cysteine
- asparagine
- glutamine
- lysine
- arginine
- histidine
- aspartate
- glutamate
However, besides the standard amino acids, there are many others that are found
in some proteins and are called non-standard amino acids. The idea of using
these non-standard amino acids is simple to understand. By having a composition
different from the standard amino acids, they present different physicochemical
properties. Therefore, when it is necessary to introduce in a protein a local
with certain properties, if they cannot be provided by the standard amino acid,
it is incorporated in the sequence a non-standard amino acid. With regard to
translation, in these cases, at these sites are introduced standard amino
acids, which suffer post-translation covalent modifications to give amino acids
with other features. I would like to highlight something that I think it is
important. How you will notice below, several of the non-standard amino acids
are found in the extracellular matrix proteins. As the extracellular matrix is
a very complex structure, establishing numerous interactions with many
different molecules (extracellular and cellular molecules), it is necessary
that the proteins that make up the matrix may present a high versatility in the
interactions that they establish, hence the need to specifically include some
amino acids that have different characteristics. Some non-standard amino acid
examples include:
- Cystine, desmosine and isodesmosine, which are amino acids found in
extracellular matrix proteins such as elastin;
- Hydroxyproline and hydroxylysine, found in the most abundant protein of the extracellular matrix – collagen;
- Gamma-carboxyglutamate, found in osteocalcin which is an extracellular matrix
protein of bone, but also in the pro-thrombin, which is important for the
coagulation cascade;
- Phosphoserine, phosphothreonine and phosphotyrosine, which are found in many
different proteins, as protein phosphorylation is the most common
post-translational modification, and always involves amino acids with hydroxyl
groups in their side chains;
- N-acetillysine, which is fundamental to the structure of histones:
- Methyllyisine, which is found in myosin, a motor protein of our cytoskeleton,
more specifically of actin filaments.