The Structure of Amino Acids
How life formed on planet Earth is not known by science. The fossil record shows the existence of Archaea and Bacteria in rocks that are very old. It also suggests that there was a “prebiotic world” when the Earth was just rocks. Of course, the evidence is thin, and it may not be so. But let us assume that it is.
The question then is, “How did those first life forms come into existence? The study of this assumed bursting forth of life is called Abiogenesis, and it focuses a great deal on amino acids, which are the necessary building blocks needed to create proteins.
What is an Amino Acid?
The diagram we have provided gives you a picture of its chemical nature. Amino acids are molecules that comprise a central Carbon atom to which are attached an Amino Group (NH2) a Hydrogen atom (H) a Carboxyl group (CO2H) and what is generally called a side chain (R group) of other atoms, which may be complex or fairly simple and may include atoms of carbon, hydrogen, oxygen, nitrogen, and sulfur.
There are hundreds of different Amino acids known to man (currently about 500), but only 20 of them are of particular interest. That’s because only 20 of them (referred to as the standard 20) participate in living cells. To be specific, all living cells have these twenty Amino acids.
By name, they are: Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Proline, Serine, Threonine, Tryptophan, Tyrosine and Valine. Note that there are two other Amino Acids, Selenocysteine and Pyrolysine, that are included in proteins only in translation.
These Amino acids are versatile – the R group conferring different properties on them. Some are hydrophobic (water-repelling), some are hydrophilic (water-loving), some are acidic, some are basic, some are aliphatic (the R Group is a chain of carbon and hydrogen atoms), some contain ring structures, some are polar, some are non-polar, and so on. Working together, they are the fundamental “Lego set” for building useful proteins.
So in all the billions of years since the time of the earliest life forms, only those 20 (or 22) Amino acids have been employed by life on Earth. This is generally regarded as a “choice” that life made and has stuck with.
Amino Acids from Outer Space
If you didn;’t know it might surprise you to discover that amino acids are sometimes found in meteorites. The Murchison Meteorite, which fell in Australia in 1969, provides a good example. It contained a diverse array of amino acids, including some not found in terrestrial organisms. A martian meteorite (the Nakhla Meteorite) which fell in Egypt in 1911, also contained amino acids – demonstrating that there are amino acids on Mars.
The Language Parallel
An interesting question is: “How many proteins can you make with the full standard Amino acid Lego set?
Well, the human genome contains the instructions for roughly 20,000 to 30,000 protein-coding genes. And that’s a curiously interesting number. If you think of the standard Amino acids as an alphabet (of 20 or 22 letters), then the language it creates, has in the region of 20,000 to 30,000 words and that corresponds to the general English vocabulary of an educated adult.
In truth, there are well over 100,000 proteins known to Man – far beyond the human genome protein count. But then again, there are well over 100,000 actual words in the English language. (Indeed, Google puts it more in the region of a million.)