Why do humans not become what they eat?
The same reason why no organism becomes literally what they eat. In the process of “eating”, different organisms end up doing the same process, just at different scales. They reduce the what they eat to their most basic components, and while there are some proteins that can be found in animals or plants that cannot be found in humans, in the end, they get broken down into more basic structures, which all organisms share.
Why are there only 20 natural amino acids?
The short answer is that there aren’t. We know of other molecules that would qualify as amino acids, some of which found in meteorites. The thing is that these 20 are the ones shared by all life on Earth. There are some extra amino acids present in some organisms, but these are universally shared. There is no concrete proof as to why, but there are theories. Originally the “Frozen accident” theory was one of them, stating that these were chosen, but any other could have worked all the same.
But in the end it could just be a matter of evolutionary convenience as seen here and here. From their physicochemical properties, to the energy levels needed for its synthesis, it seemed that once these 20 amino acids were selected, there simply was no need for more . After all, all proteins on the planet are made from these same building blocks. And in fact, with a lower number of amino acids, some protein structures can still be assembled, albeit with worse properties than the natural ones.
Where did amino acids come from before proteins made them?
There are several theories as to the origin of amino acids. Some say that they were the original organic molecules created on Earth, from different elements present in the atmosphere. The famous Miller-Urey experiment shows this to be a feasible possibility. An alternative, but also feasible possibility, is that of the RNA world, which states that self-replicating RNA was the original precursor. However, in recent years, a theory posing that co-evolution took place has been gaining momentum. All this is to say, we do not currently know, but there are still many things to uncover.
Protein:
RESILIN
I selected this protein due to its unique physical properties. Not only is the most efficiently elastic protein known, it is also an integral part of insect exoskeletons, along with other proteins and polymers. This showcases the versatility of the protein in its different forms, and marks it as a very interesting compound to study and apply in medical and material research, something that has already been done.
The version of the protein I selected (from Anthophora retusa) has 127 amino acids.
The top 3 most common amino acids present ins the sequence are Glycin (18 times), Prolin (12 times), and Threonine (12 times), which are small or medium-sized amino acids, and 2/3 are hydrophobic. These could already provide a reason behind the protein’s properties.
A simple protein BLAST shows us 100 different protein homologues within the resilin family:
Using the rest of the NIH tools allow us to produce a phylogenetic tree of all the sequences:
With this collection of sequences (which can be downloaded as a single text file after the BLAST), when using the ClustalOmega tool, we can get results for multiple alignments, which can also showcase the differences between different tools, and how results can vary depending on which one is used.
The multiple alignments in this case look like this:
And the phylogenetic tree in this case looks much different than the first one:
