In the sentence: "Bacteriophage (viral) polymerases are typically monomeric polypeptides".

I know that polypeptides are chains of amino acids monomers. But what is a "monomeric polypeptide" and what other kinds of polypeptides are there?

  • $\begingroup$ You're right to highlight this confusing statement. As David's answer points out, this is a case of tautology. The term "monomer" or "single polypeptide chain" would be more appropriate I believe. It's a very minor point though and the sentence does technically make sense. $\endgroup$ – James May 13 '16 at 9:46

Monomer in this context means the protein has only one polypeptide chain. In some proteins, after the individual chains have folded into their 3D or tertiary structure, they associate (generally non-covalently) into a higher order or quaternary structure. A protein with a quaternary structure containing two copies of the same chain would be called a homo-dimer. One with one copy of each of two different chains is a hetero-dimer.

Haemoglobin is a well-known example of a protein with quaternary structure — it has two copies of the alpha- and two of the beta- chain. Further elementary treatment can be found in Berg et al. and explanation of the nomenclature in this Wikipedia page.

Monomeric polypeptide is somewhat misleading in the sentence quoted. Monomeric protein would have probably been better as you wouldn't be likely to say ‘dimeric polypeptide’, given that polypeptide implies a single chain.

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    $\begingroup$ A small doubt: What would you call mature insulin? It has two polypeptide chains covalently linked via disulphide bonds. It is a monomeric protein, however. $\endgroup$ – WYSIWYG May 13 '16 at 8:51
  • $\begingroup$ @WYSIWYG I wouldn't use my first sentence as a definition of monomer, just as an explanation of what the writer meant. In talking about insulin I personally would say that although it has two polypeptide chains it consists of a single globular subunit and therefore cannot be said to have quaternary structure (as I would define it). This is easy to justify since we now know that the general tertiary structure of insulin is formed by the folding of a single polypeptide chain, proinsulin, which is cleaved to activate it (with a small but significant change in tertiary structure). $\endgroup$ – David May 13 '16 at 11:20

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