The origin of life and the overall evolutionary process is quite complex yet extremely interesting to learn about. It is an extraordinary knowledge hub which gives the entire account of how and where did the early development of organic molecules start. Though man has succeeded in unravelling many nature mysteries, there are some unsolved issues still troubling scientists. One of them being the mystery of why only l amino acids are considered important for living organisms and specially for human beings and not d amino acids.
The answer seems to be hidden in the evolutionary process, which we will be trying to resolve in this article.
What Are Amino Acids?
Amino acids are the building blocks of proteins and are the key elements when it comes to perform important human body functions. Of about more than 500 naturally found amino acids, only 20 amino acids are genetically coded and hence are the most important for human beings and other living organisms.
Alanine, Arginine, Asparagine, Aspartic Acid, Cysteine, Glutamine, Glutamic Acid, Glycine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Proline, Serine, Threonine, Tryptophan, Tyrosine and Valine are the 20 amino acids, and all of them are able to occur into isomeric forms due to the specific structural ability they have.
Structure of Amino Acids
The basic structure of amino acids contains two important groups Amino group (NH2) and Carboxylic group(-COOH). Both these groups are linked together with a carbon atom, known as alpha carbon. They are the organic compounds that contain amine and carboxyl functional groups, along with a side chain specific to each amino acid.
The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, although other elements are found in the side chains of certain amino acids. The central carbon atom is the one to whom all other chains and structures are attached.
This is the alpha carbon and is also known as chiral carbon, as the carbon atom has four groups bound to it. Amino, carboxylic, hydrogen and R group are the four groups bound to this carbon atom.
R is just to refer the side chain attached to these amino acids. Every amino acid is differentiated and identified with its unique R group.
As seen above all amino acid are able to occur in two isomeric forms, because of the possibility of forming two different stereoisomers, or enantiomers around the central chiral carbon atom. The so formed isomers are classified into two as per their left-handed and right-handed configurations called as L and D amino acids.
The only exception is Glycine, for which the R group is just a hydrogen atom. Which is why there is this duplication of hydrogen atoms. And so, glycine is the only amino acid that does not have a chiral carbon.
What Is L Amino Acids?
L-amino acids are the stereoisomers of a particular amino acid whose amino group is on the left side in its Fisher projection. They are the type of stereoisomers that produce proteins with the help of cells. Also they are vital for all types of functional and structural works within a cell.
They take form of an enzyme when catalyzing any biochemical reactions, or regulate the biological processes when work as hormones. They are also the type of amino acids that are produced by lightning reactions.
Naturally occurring amino acid L, find their source in all kinds of proteins produced by either animals or plants. They are the only amino acids that are manufactured within the cells and transformed into proteins.
Important fact to be noted down is that amino acids are only the L amino acids which participate in the protein synthesis process. Since proteins are vital for cell maintenance, L amino acids find their origin within the evolution of life. Amino acids occur in both L and D types, but only the L forms are used by cells.
The origin of left-handed stereoisomers of amino acids dominate in nature even though their synthesis normally results in an equal mixture of the right- and left-handed molecular forms is yet unknown. But there are several theories that can be considered as the basis of the concept.
One of the theories being the asymmetric Earth rotation that causes polarized light opposite polarity and reversed temperature profiles in the oceans. Destruction of the d-isomer and the storage of the daily l-isomers in excess and in protection from radiation by nightfall initiated an l-amino-acid excess embodied in early marine forms.
What Is the Role of L Amino Acids in Evolution?
As seen in the previous sections, except one amino acid glycine, all other amino acids have two forms D and L. D-amino acid is a mirror image of L-amino acid. Isomers that are mirror images of each other are called enantiomers and L- and D-amino acids are not super-imposable like right and left hands.
What is more interesting is the fact that only L-amino acids are building blocks of proteins. Hence as proteins are essential to maintain the cell, the origin of L-amino acids is directly linked with the evolution of life.
It is established that amino acids are generated in the prebiotic earth from simple molecules such as methane, water, ammonia, and hydrogen by so called chemical evolution. Ordinary chemical synthesis of amino acids always generates a racemic mixture, which is a mixture of the same amounts of L- and D-amino acids, as enantiomers have identical chemical properties.
The question is how L-amino acid is generated and how did the quantity of L-amino acid become more abundant than that of D-amino acid in nature.
A mixture of racemic amino acids caused spontaneous and effective resolution by itself during re-crystallization in the presence of excess racemic asparagine, even if asymmetric synthesis of a single amino acid had never occurred without the aid of an asymmetric molecule.
This implied that L-amino acid generation is the result of the racemic amino acids including D & L-asparagine, which were formed by chemical evolution in the hot prebiotic sea, which gradually cooled and the solubility of the sea decreased. Then D-asparagine incidentally crystallized at first followed by crystallization of other D-amino acids on the crystals of D-asparagine, leaving the sea only with l isomers, from which the cells developed. This story indicates the important role of the L-amino acids in the overall evolution process.
Though there are many more theories explaining the concept, this theory is the most accepted one among the scientists all over the world. amino acids have been an important part of human life in some or the other forms and the great bonding between the two continues till date.