Question about proline

[thechairman]

why can't proline engage in hydrogen bonding? the mcat books say that proline breaks the alpha helix structure of proteins because it cannot engage in hydrogen bonding. but the structure of proline clearly does have a hydrogen, and in the zwitterion form (at pH 7) there are two hydrogens attached to the amine side of proline.

can anyone help?

http://en.wikipedia.org/wiki/Proline

---

Members don't see this ad.

 

[theman120]

i'm not so sure about the H bonding, but the reason that proline can't be in the alpha helix is that its geometry doesn't fit in the helix, its too flexible creating kinks in the helix.

 

[Foghorn]

why can't proline engage in hydrogen bonding? the mcat books say that proline breaks the alpha helix structure of proteins because it cannot engage in hydrogen bonding. but the structure of proline clearly does have a hydrogen, and in the zwitterion form (at pH 7) there are two hydrogens attached to the amine side of proline.

can anyone help?

http://en.wikipedia.org/wiki/Proline

First figure out the difference between an amine and an amide. Once you've done that, draw an oligopeptide containing Pro somewhere in the middle and you'll see why H-bonding is not possible with Pro.

 

[bigman225]

That answer is a copout. Proline disrupts secondary structure because of its restricted phi-psi angles.

 

[Foghorn]

That answer is a copout. Proline disrupts secondary structure because of its restricted phi-psi angles.

Please read the OP's question for comprehension and take the time to differentiate between Pro as a free amino acid (amine form) and Pro as part of a peptide (amide form). That way you'll be able to visually distinguish as to why one form can participate in H-bonding and why one form cannot. Your cooperation in this matter will be appreciated. Thank you 🙂

 

[bigkdawg161]

That answer is a copout. Proline disrupts secondary structure because of its restricted phi-psi angles.

learn something new everyday!

 

[bigman225]

Foghorn,
I wasn't referring to your response as a copout, but the book's answer involving secondary structure.

I didn't intend to respond to the poster's question as much as the book's inadequate explanation. The fact that Proline's amide group cannot function as a hydrogen bond donor isn't the reason that it disrupts alpha helices. If this were the case, the rest of the intrahelical H-bonds between every 5th peptide group would be maintained and the loss in weak interactions unappreciable. The loss of 1 H bond won't cause an alpha helix to be disrupted.

Pratt and Cornelly's Essential Biochemistry:
"The amino group of Pro is linked to its side chain, which limits the conformational flexibility of a peptide bond involving the amino group. The GEOMETRY of this peptide bond is incompatible with bond angles required for a polypeptide to form an alpha-helix"

 

[artaxerxes]

Foghorn,
I wasn't referring to your response as a copout, but the book's answer involving secondary structure.

I didn't intend to respond to the poster's question as much as the book's inadequate explanation. The fact that Proline's amide group cannot function as a hydrogen bond donor isn't the reason that it disrupts alpha helices. If this were the case, the rest of the intrahelical H-bonds between every 5th peptide group would be maintained and the loss in weak interactions unappreciable. The loss of 1 H bond won't cause an alpha helix to be disrupted.

Pratt and Cornelly's Essential Biochemistry:
"The amino group of Pro is linked to its side chain, which limits the conformational flexibility of a peptide bond involving the amino group. The GEOMETRY of this peptide bond is incompatible with bond angles required for a polypeptide to form an alpha-helix"

correct sir,

Geometry is incompatible with the alpha-helix's strict structure, Proline is too bent to fit into the helix and participate in the H-bonding that stabilizes the helix.

 

[Foghorn]

why can't proline engage in hydrogen bonding? the mcat books say that proline breaks the alpha helix structure of proteins because it cannot engage in hydrogen bonding. but the structure of proline clearly does have a hydrogen, and in the zwitterion form (at pH 7) there are two hydrogens attached to the amine side of proline.

can anyone help?

http://en.wikipedia.org/wiki/Proline

Pro doesn't participate in intramolecular H-bonding in a peptide, unless Pro is the N-terminal amino acid, because the hydrogen (the one on Pro's N-atom) isn't present. Pro can definitely H-bond ias a free amino acid. But in it's amide form, the H atoms aren't there and Pro's N-atom is directly bonded to three carbon atoms.