So, I am trying to come up with forms of alien biochemistry that are different but not so different that these life forms are drinking battery acid or breathing methane. One idea I read about on Orion’s Arm was the use of starches as an alternative to certain proteins, but OA is a big project and understandably didn’t go into much detail. So, what I would like to know, is how this biochemistry would work in an Earth-like environment, (nitrogen-oxygen atmosphere, earthly temperatures, water etc), and more specifically what proteins could starch replace? (Myosin? Enzymes? ???)
$\begingroup$ As a source of food energy? Or as material to build their bodies? Starch and protein simply are not interchangeable for the latter. Some organisms might be able to synthesize amino acids given sufficient calories. $\endgroup$– Boba FitJan 4 at 19:34
2$\begingroup$ You may want to replace the word "starches" with "polysaccharides". $\endgroup$– AlexPJan 4 at 21:06
I found the offending article... Baryos:
... with starches doing many functions performed by proteins in terrestrial life.
Not one of their finer entries (and baryos itself seems super dubious, but there you go).
It might be possible to directly synthesize some kinds of interesting heteropolysaccharide in a similar way to how proteins are synthesized in ribosomes, but I'm not sure I'd call such things "starches" because that term has some particular meanings that aren't really compatible with components of general purpose cellular biochemistry.
Lets assume that polysaccharides in general are what you (and maybe the author of the baryos article) really meant. Whether or not you could make the full gamut of cellular machinery out of these kinds of polymers rather than proteins I'm not sure, and possibly the question is too complex to get a useful answer even from an expert. That does at least give you some wiggle room to use such a biochemistry in your own setting. Just don't call it starch.
how this biochemistry would work in an Earth-like environment
what proteins could starch replace?
Again, assuming "polysaccharides" rather than "starch"... however and whatever you like, so long as you didn't go into any detail. Declare key parts of the biochemistry to be non-protein-based and move on... conserve your effort and detail for where they are required, and save yourself a lot of effort and avoid the prospect of getting stuff obviously wrong.
Inventing a whole biochemistry for you ex nihilo is beyond anyone on this site, I suspect, but given that we have polysaccharides used for stuctural purposes (eg. cellulose and chitin) and for energy storage purposes (eg. starches or mannans) and for protective purposes (eg. bacterial capsules), you can see that at least some kinds of polysaccharide chemistry work just fine here.
Short answer: Probably not but also kind of yes?
The long answer: It depends on what you want to do:
Let's start with the basics: As has been pointed out, proteins are chains of amino acids whose order is denoted in the genetic material (so DNA or RNA). These amino acids have a specific base structure (an amino and a carboxylic terminal group) and a lot of different stuff in between those (functional groups). The different stuff can do great things (make the protein polar, apolar, form sulphuric bonds and so forth). When the protein is synthesised it undergoes complex folding (via a few different steps) to expose, hide and or crosslink all the functional groups. The way this folds highly affects the function of the protein.
Now what's starch? Starch is a chain of glucose molecules (monosaccharides) which form a polymer (a so-called polysaccharide). Starch alone can't really do much, as it just has a few hydrogen atoms which are free to enter chemical bonds. However you can add oxygen to get some energy out of the reaction, which is what our bodies do with carbohydrates...
Now to your question:
Could polysaccharides (which is a little more accurate than "starch") exist which have similar functions as proteins in life on earth? To the extent of my biochemistry knowledge... maybe? However you would need sets of different saccharides with different functional groups in order to get the functional versatility a protein achieves. I am not entirely sure about about the stability of the bonds and some other details as I am not a chemist, however I am pretty convinced that you could built protein analogues out of something else than amino acids. Life on earth actually uses polysaccharides for some things and there are even "hybrids" which combine proteins and polysaccharides for their function (so called glycoproteins which are important in many cellular pathways)...
So in summary: I am rather positive you could build protein analogues out of different compounds than amino acids (some insight from a (bio)chemist would be really interesting on this!), but you can't replace proteins with starch...
Edit: Actually DNA/RNA molecules have a sugar based backbone, as they are built out of modified ribose. And such molecules can perform cellular functions beyond information storage. Especially RNAs have many functions in controlling gene expression and fold in quite complicated patterns with certain properties. So there is even an example in our tree of life of functional molecules which are not based on amino acids
A Protein is anything that can be created by unzipping a DNA molecule and letting things form on the open ends and then fall off. There are many types of DNA molecules and many types of proteins. Enough to make up every part of your body.
A Starch Molecule is just a chain of glucoses attached end-to-end. There is only one type of starch.
It does not make sense to replace proteins with starch. That is like trying to replace plants with cats.
Don't read Orion's Arm.
3$\begingroup$ Sturgeon's law is very much in evidence there. $\endgroup$ Jan 4 at 20:30
1$\begingroup$ there are actually many kinds of starch, using different types, ratios, and configurations of amylose and amylopectin, and there are huge number of carbohydrates. What you describe and call a "starch" is just amylose one component of a starch. $\endgroup$– JohnJan 4 at 21:42