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What I want to know is if polypeptides (those which are not considered as proteins) can also "exhibit" a tertiary structure.

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  • $\begingroup$ Welcome to BiologySE! I don't think I'm following your question... can you be more specific? Polypeptides are proteins... what polypeptides "are not considered as proteins"? $\endgroup$ – Vance L Albaugh Jun 13 '16 at 23:08
  • $\begingroup$ @VanceLAlbaugh I think my problem comes from that direction. I understand that proteins are a specific case of polypeptides. Is this wrong? $\endgroup$ – PiQ Jun 13 '16 at 23:45
  • $\begingroup$ a chain of amino acids connected through peptide bonds is a polypeptide - that chain is primary structure of a protein... there are some semantic differences... there is a nice overview of the basics of protein structure on wikipedia - basically what is covered in a basic biochemistry text...en.wikipedia.org/wiki/Protein_structure#Tertiary_structure $\endgroup$ – Vance L Albaugh Jun 14 '16 at 2:00
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I believe the OP is referring to an old and outdated system of classification, in which all proteins are polypeptides but not all polypeptides are proteins: some books put a lower limit of 50 AA residues, others talked about 60 or 70 AA for a polypeptide being considered a protein. Once again, this system is no longer used (just check Lehninger, "Principles of Biochemistry").

But let's pretend that this system is valid and use it just to answer this question: can a small polypeptide that is not a protein, ranging from 2 to 50 AA lenght, have a tertiary structure?

To answer that, we have to remember that the tertiary structure is not (as some defines) simply the "3D structure of a polypeptide". After all, a single amino acid has a 3D structure, as any molecule indeed. The tertiary structure is the specific 3D format of the polypeptide, but due to interactions of its amino acid side chains.

So, the question is: can a small polypeptide fold over itself and have a tertiary structure? The answer is yes. Apparently, peptides with more than 10 AA can fold: http://www.ncbi.nlm.nih.gov/pubmed/12729764

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  • $\begingroup$ Just one more thing. Does the current system of classification have a name? Or can you summarize the idea? Thank you in advance. $\endgroup$ – PiQ Jun 14 '16 at 22:24
  • $\begingroup$ I don't think so. I know the "old system" because I started giving classes in the early 90s, and some books used it. But today we don't set a limit of AA to define a protein. $\endgroup$ – user24284 Jun 14 '16 at 22:40
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Your instinct is valid, there are other molecules which can be defined as having tertiary structure.

The most common example is RNA, which can fold into a vast array of conformations. These can be part of ribonucleoprotein complexes like telomerases, spliceosomes, or ribosomes or they can function on their own like tRNAs and many ribozymes such as RNase P. An example of a common tertiary structural motif in RNA is the pseudoknot; a haripin loop in which some of the bases in the loop have base paired to another portion of the RNA molecule.

https://s3.amazonaws.com/classconnection/411/flashcards/6689411/png/pseudoknot-14B4284ABC1512AC32A.png

It is also possible for DNA to possess tertiary structure, though the term is used less often when referring to DNA. Usually DNA tertiary structure describes topological structures formed by supercoiling, protein scaffolding, etc. However, DNA can also form cruciform motifs similar to RNA hairpin loops (though it is less common). If multiple cruciform motifs and other structures were to interact in three dimensions, this could be another form of DNA tertiary structure.

http://www.siumed.edu/~bbartholomew/images/Lehninger/img037.gif

You may also be interested in DNA origami, in which 3D building blocks made from DNA are assembled in desired ways by using sequence complementarity (generally).

I think that polysaccharides and oligosaccharides are also sometimes referred to as having tertiary structures, but I don't know as much about those. I encourage you to look into pectin, amylopectin, branched oligosaccharides, etc in the context of tertiary structures.

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  • $\begingroup$ All this information is very useful. Thank you. $\endgroup$ – PiQ Jun 14 '16 at 5:20

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