Quote Originally Posted by jvkohl
From Wikpedia: (because

even I had to check my assumptions)
"a peptide is an amino acid molecule without secondary structure; on gaining

defined structure, it is a protein." Thus the same molecule can be either a peptide or a protein depending on its

environment, though there are peptides that cannot be

proteins."

JVK
Don't trust Wikipedia too much. What we're

talking about here are polypeptides, not peptides. A peptide is a single amino acid residue (the amino acid as it is

once it's part of a chain), and a polypetide is two or more amino acids linked together.

Oxytocin is a

9-unit polypeptide.

All proteins are polypeptides, but the primary distinguishing factor between

polypeptides and proteins is whether they have tertiary structure, and this is determined mainly by the number of

peptides. The smallest human proteins which have been identified are about 45 peptides in length.

Proteins

have three structures:

- primary, which is the sequence of peptides (amino acids) in the chain.
-

secondary, which is the linkage between peptides in one part of the chain with peptides in the other part of the

chain (usually through the same hydrogen bonds that keep water liquid at room temperature).
- tertiary, which is

the complete structure of a protein, including all the folding (beta sheets, alpha helixes, etc.)

Oxytocin is

only 9 units long. This is too short to have ANY tertiary structure. So oxytocin is NOT a protein.

But it

does have a secondary structure, as can be seen in this paper:

secondary structure of part of

oxytocin. The dotted lines are hydrogen bonds.