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Thread: Evolutionary benefit of our 20 amino acids: why not more?

  1. #1 Evolutionary benefit of our 20 amino acids: why not more? 
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    Our protein translational system can, theoretically, code for 64 different peptides(63 if you discount the stop codon), so why don't we see more amino acids being utilized in translation? While I can understand not utilizing new, "bizarre," amino acids that would require their own metabolic pathways for synthesis and degredation, there are quite a few intermediary amino acids which we regularly synthesize already, or even utilize in roles outside protein synthesis. For instance, homocysteine is made form methionine and can converts serine into cysteine, and ornathine is used to make arginine and is integral to the urea cycle. Other possible amino acids could be 2-amino-3-keto butyrate and Glutamic gamma semialdehide, and others.

    Especially with the similarities between some of these to their counterparts, it would seem plausible for a mutant tRNA, or mutant tRNA synthase, to be able to couple tRNAs with some of these amino acids. Even the one carbon difference between ornithine and lysine would allow it to act alter a proteins functionality, and as such it would seem strange that evolution hasn't favored utilizing these available intermediates.

    Thoughts?


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    Forum Radioactive Isotope skeptic's Avatar
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    The simplest explanation is simply that it is one of the results of the fact that evolution is an imperfect process.

    There are lots of things evolution might have generated, if it were perfect. For example : what about a four legged flying vertebrate? Four legs are better than two, but no winged vertebrate with four legs exists. The evolutionary reason is clear.

    Perhaps, in the same way, potentially advantageous new amino acids were simply overlooked by evolution.


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  4. #3  
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    Quote Originally Posted by skeptic
    The simplest explanation is simply that it is one of the results of the fact that evolution is an imperfect process.

    There are lots of things evolution might have generated, if it were perfect. For example : what about a four legged flying vertebrate? Four legs are better than two, but no winged vertebrate with four legs exists. The evolutionary reason is clear.

    Perhaps, in the same way, potentially advantageous new amino acids were simply overlooked by evolution.
    I had considered that, however it seems unlikely that there would be no examples of ornithine or homocysteine usage if they were viable. The modifications necessary for their usage would be fairly minimal. Yes, evolution is imperfect and random, but a billion years of randomness would undoubtedly give us at least some examples of lifeforms which use these amino acids.
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    Quote Originally Posted by TheBiologista
    Awesome! Thanks!
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  7. #6  
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    You also have to remember that more than one codon is needed to code for one amino acid. This is necessary to compensate for random point mutations which occur during DNA replication (I think one in a 10 million so).

    For example:
    TCT, TCA, TCC and TCG all code for Serine. Thus even if one base in DNA is changed to another due to mutation it will still code for the same amino acid and therefore protein function isn't affected at all. This is called silent mutation.

    Even if two bases are changed due to mutation, this will often lead to different amino acid....but the one with similar properties. For example:
    Codons NUN where N is any nucleotide tend to code for hydrophobic amino acids and so on...

    If all possible codons would code for one specific amino acid, DNA would be very prone to point mutations which could prove to be deadly for organisms in the long run.
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  8. #7  
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    Quote Originally Posted by Postovka
    You also have to remember that more than one codon is needed to code for one amino acid. This is necessary to compensate for random point mutations which occur during DNA replication (I think one in a 10 million so).

    For example:
    TCT, TCA, TCC and TCG all code for Serine. Thus even if one base in DNA is changed to another due to mutation it will still code for the same amino acid and therefore protein function isn't affected at all. This is called silent mutation.

    Even if two bases are changed due to mutation, this will often lead to different amino acid....but the one with similar properties. For example:
    Codons NUN where N is any nucleotide tend to code for hydrophobic amino acids and so on...

    If all possible codons would code for one specific amino acid, DNA would be very prone to point mutations which could prove to be deadly for organisms in the long run.
    While that is true, having say, 30 amino acids, would still be very reasonable. As you said, most mutations will lead to similar amino acids being inserted, and if homocysteine had a tRNA, I'd imagine mutations would have it replace cysteine, and the same can be said for Ornathine and lysine. Though there might be instances where this could be detrimental, generally speaking, these mutations would likely lead to silent AA changes. I don't think the possible negative effects of mutations so minor would outweigh the benefit of added flexibility in making proteins, in fact, I think that this would have an evolutionary benefit in terms of allowing more mutations to occur, with more possibilities for favorable mutations to occur. If, say for instance, there is a situation where a homocysteine would be better than cysteine, this would allow a more viable protein to be formed and spread. If it is not functional, it won't go anywhere, and if it's less functional than it will likely disappear over a some generations(if it spreads at all), or it will simply not have any significant effect. Also, if you were to replace isoleucine with either cysteine or homocysteine, I don't imagine the two would produce very different effects.
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