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Some years ago, in a 1000~ level biology course we learned that the DNA essentially encodes formulas for creating proteins from amino acids. While the human body can synthesize many many amino acids, some we are unable to synthesize and we dub these as essential amino acids because we must obtain them from our diet. However, surely something must be capable of synthesizing the amino acids, otherwise they would not make it up the food chain to us, which opens the door to the possibility that animals may have varying capabilities in regards to amino acid synthesis. Presumably, this is why cats have to obtain a high protein diet, as they have high need for protein but a poor ability to synthesize them?

Moreover, in doing basic research into nutrition to plan a regular workout schedule there is some need to distinguish protein sources as not all protein sources are considered to be complete proteins because they do not provide all amino acids, or otherwise do not provide them in the right balance. Particularly, many fruits and vegetables tend to be poor sources of protein.

For plants which are incomplete protein sources, they would seem to lack the ability to synthesize some amino acids. They also would not be able to obtain these through diet, as they're plants. Consequently, does this mean that some plants may not encode for a specific amino acid throughout their entire DNA?

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  • $\begingroup$ You seem to miss that the concept of essential amino acids is human-centric. Why should a plant encode all the amino acids needed by a human? The plant doesn't need them. $\endgroup$
    – rumtscho
    Jun 13, 2015 at 8:33
  • $\begingroup$ They also would not be able to obtain these through diet, as they're plants. Plant just synthesize carbohydrates by photosynthesis. Plants cannot fix nitrogen and they are dependent on bacteria for that. Though non-parasitic plants can synthesize all amino acids they need, they may not be sufficient for humans as a diet. $\endgroup$
    – WYSIWYG
    Jun 13, 2015 at 8:58
  • $\begingroup$ Apologies, as my (limited) study was focused entirely on human biology. At the time of study, I had assumed that DNA across species was encoded using the same set of amino acids, and what differentiated them was simply what proteins they made with that given set. But later I considered that this couldn't be correct as a nutritionally incomplete protein source would suggest that a specific amino acid was not being utilized. Broadly, my question was whether the set of utilized amino acids also differs between species. $\endgroup$
    – msg45f
    Jun 13, 2015 at 9:07

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With a few exceptions among some bacteria, all species on the planet make protein from the same 21 amino acids, and the relative abundances of amino acids is very similar in proteins from plants, animals, fungi and even prokaryotes. See for example this article (available as PDF here). So protein from pretty much any food provides the same amino acids.

The idea that plant proteins are "incomplete" and lack some amino acids seems to be a myth: see for example this correspondence in the American Heart Association's journal Circulation. Plants generally contain less protein than meats per weight, but there is nothing wrong with the protein they contain. Most people in the world obtain the majority of their protein from plants. There are probably some specific parts of some plants that are not nutritionally optimal --- if you for example lived exclusively on peanuts for a year, you might experience some problems ... But any commonsense, varied plant diet will provide all amino acids. A detailed report (and some mythbusting) is here.

Regarding essential amino acids, as mentioned in comments this is a species-specific concept. Humans and most animals can synthesize about half of the 21 amino acids, while the remaining are essential. There are some species differences --- arginine is essential for cats, for example, but not for adult humans. In contrast, plants can synthesize all amino acids. But their protein composition is similar in the end.

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  • $\begingroup$ So would the amino acid profiles listed on this site (under the protein quality box), for instance, be mostly bunk? $\endgroup$
    – msg45f
    Jun 13, 2015 at 11:27
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    $\begingroup$ Well, I don't know where that particular data comes from so I can't judge it, but there is certainly a lot of unreliable stuff on various nutrition websites. These companies often want to sell nutrition supplements and tend to have little real data to back up their claims ... There are certainly some differences between foodstuffs, since we don't eat entire plants but only some parts (see ajcn.nutrition.org/content/59/5/1203S.short). But generally, if you eat a varied diet, you will obtain all amino acids you need. $\endgroup$
    – Roland
    Jun 13, 2015 at 11:46
  • $\begingroup$ @WYSIWYG Right, I guess that was not very precise. I just meant that they are primary producers (obviously). Will edit. $\endgroup$
    – Roland
    Jun 13, 2015 at 14:07
  • $\begingroup$ I changed the 20 to 21 amino acids since most species also code for selenocysteine. $\endgroup$
    – terdon
    Jun 14, 2015 at 12:59
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    $\begingroup$ @Roland I admit I'm biased since I did my PhD on selenoproteins but it's not that rare. There are 22-25 proteins (depending on how you count them) in the human genome and some animals have many more. They are also essential, those animals that have them can't really survive without them. But yes, I know I'm being pedantic, it's just that it's my field and everyone forgets about poor Sec :) $\endgroup$
    – terdon
    Jun 14, 2015 at 13:51

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