Comment by asdff
2 hours ago
That makes sense that the chirality can affect downstream polymer assembly or even folding in the higher order structures.
Likely we are all left handed on earth because our left handed ancestor outcompeted the right handed organisms in the primordial soup. Or the right handed organisms just didn't evolve in the first place here on earth and there was nothing to outcompete. There might still be some higher order advantages to shifting chirality one way or another. Certain molecules, such as methamphetamine, have differing bioactivity based on chirality. Maybe this can be regulated in some way such as to control the rate of some other downstream process. In an abstracted sense, chemists here on earth are already this organism as they refine reactions to produce desired chirality and reduce expenditure on undesired chirality.
ET could be using different amino acids, or more or fewer. I would hazard to guess there is immense selection to reduce the amino acid set to its most necessary components. This pressure has gotten to the point here on earth where even these necessary components might not all be produced endogenously by the organism who needs them, but consumed from the environment saving energy spent on synthesis. But this requires your neighbor to be producing these AAs, such that you consume them, and you having sufficient feedback mechanisms to not immediately consume all of your neighbor's species and put your own insufficient lineage to extinction.
The living beings use much more amino acids than those that compose proteins.
The relatively low number of amino acids that are used in proteins appears to be caused by the difficulty of modifying the genetic code by adding not yet encoded amino acids to the set of encoded amino acids.
Variations of the genetic code are known at various living beings, but nonetheless they are very rare, because a change in the genetic code requires a lot of other coordinated changes. A new kind of transfer RNA must be encoded in the genome (the only likely origin of such a new tRNA is a mutation in one of the existing) and that RNA molecule must be able to bind preferentially to the codons that are repurposed to encode a new amino acid, and also to molecules of that amino acid, which requires a lot of favorable change is the molecular structure of that RNA.
It seems that in the earliest form of genetic code, there were only 4 distinct symbols, i.e. of the 3 nucleobases of a codon only the central one was meaningful and the 2 peripheral nucleobases did not encode information.
The 4 original symbols selected between 4 major kinds of amino acids: the special amino acid glycine, an acid amino acid, a hydrophobic amino acid and an amino acid with intermediate behavior, like alanine or proline.
These variations would have been enough to build proteins with specific conformations.
The fact that a codon had 3 nucleobases, presumably to ensure the binding to transfer RNA molecules, even if only one of them encoded information, appears to have been a great luck, because this allowed later the expansion of the genetic code, because 3 bases give 64 combinations allowing the encoding of up to 64 symbols.
Most of the possible codons have remained ambiguous until today, but the number of encoded amino acids has increased slowly in time, up to 21, the most recent additions to the encoded set being those of the sulfur-containing amino acids, aromatic amino acids and selenium-containing amino acids.
As you say, there are disadvantages in using many kinds of amino acids, but there are also advantages, by allowing the creation of proteins with properties that are not achievable with a smaller set of amino acids.
The balance between advantages and disadvantages appears to have slowed down continuously the rate of adding new amino acids to the set encoded in the genetic code, so that the majority of the living beings of today have not added any new amino acid since several billion years ago.
Most of the expansions of the genetic code happened before the last common ancestor of all living beings of today, so that today there are very few living beings with more recent modifications in the genetic code.
Life can even use something other than amino acids. They are really inconvenient when you think about it. Fixed nitrogen is extremely rare, and there are no nitrogen-containing minerals other than some exotic exceptions.
Amino acids are useful because they can be easily joined together and split apart (via the C-N bond). But there are other types of "molecular glues" that are viable, like sulfur or phosphorus.
Amino acids are much more likely to be involved in the appearance of life anywhere than other molecules.
For instance it would be less surprising if an alien life form used another kind of polymer to store information, instead of nucleic acids, than if it would not use amino acids.
Some of the simple amino acids are very easy to be synthesized in abiotic conditions, which is why they are ubiquitous in many celestial bodies.
The advantage of amino acids is that they do not contain only one end that can be attached to other molecules, but that they contain two such ends. A molecule with only one connector would attach to another, forming a dimer, after which no further reaction is possible.
A molecule with two connectors, like an amino acid that has both a carboxyl end and an amine end, can be daisy chained into a polymer of arbitrary length. This allows building complex structures.
There are other molecules with two connectors, but they are much more unlikely to appear in abiotic conditions.
Thioesters, i.e. a kind of organic molecules that are bound by a sulfur bridge, like you mention, appear to have been much more important when life has appeared on Earth than today, but such molecules were important as intermediates in metabolic reactions, not as structural blocks, like amino acids, and there are no known naturally-produced molecules with sulfur that could be used as easily as amino acids to make molecules with arbitrary complex shapes.