My biochemistry book says that beta-pleated sheets is a form of secondary structure of proteins, and it is formed between two or more polypeptide chains.

I wonder why the sentence in bold doesn't belong to the quaternary structure instead?!

  • $\begingroup$ Beta strands can form both intra- and interchain sheets. I wouldn't get too caught up in the latter case when deciding what level of protein structure beta sheets fit into. $\endgroup$ – canadianer Oct 27 '17 at 21:41

Quarternary structure is between separate protein chains or proteins themselves. Beta sheets are a form of secondary structure which is within a single polypeptide.

Sure, some of the structures in quarternary complexes will be between “two or more peptide chains”, but the stipulation that these not be part of the same polypeptide chain would be necessary.

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  • $\begingroup$ You say that beta sheet is a structure within a "single polypeptide", but my text book says that it is between "two or more extended polypeptide chains". Is the book wrong?!! $\endgroup$ – Asmaa Oct 28 '17 at 19:19
  • $\begingroup$ No its just a symantic issue. A beta sheet requires 2 stretches of peptide to run parallel to one another. Those 2 stretches of peptide are part of one continuous chain at the secondary structure level. Whether it counts as Beta sheet when 2 stretches polypeptides from totally separate proteins are in close proximity enough to hydrogen bond, I'm not 100% sure. But generally it refers to secondary structure which must be within a single continuous peptide. $\endgroup$ – Joe Healey Oct 28 '17 at 19:30

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