Tyrosine is nonpolar?

[balopathic45]

Just missed this question on a kaplan section practice test. I've seen conflicting information regarding the polarity of tyrosine. Would the AAMC ever expect us to make this distinction?

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[Sauna]

Tyrosine has a single polar hydroxyl group that partly offsets the very non-polar benzene ring, but overall the side chain structure is be considered moderately non-polar.

 

[frodo25]

I think on the test thought, Tyrosine should be thought of as polar

 

[BerkReviewTeach]

According to pretty much every biochemistry book around and the BR books, tyrosine is definitely polar.

 

[GoljansRightBicep]

I will also second that my professor and my textbook (Garrett & Grisham, 5th ed.) list tyrosine as polar.

 

[novak123]

If it can form H-bonds it's polar and thus will at least have some dipole dipole interactions also.

 

[7331poas]

If it can form H-bonds it's polar and thus will at least have some dipole dipole interactions also.

Thats not necessarily true. The larger the hydrocarbon the less polar the compound, even if there is a hydroxyl group somewhere.

Just because something can form OH bonds, doesn't necessarily mean it has a larger electric dipole moment.

Thats the point they are making with tyrosine.

 

[novak123]

Thats not necessarily true. The larger the hydrocarbon the less polar the compound, even if there is a hydroxyl group somewhere.

Just because something can form OH bonds, doesn't necessarily mean it has a larger electric dipole moment.

Thats the point they are making with tyrosine.

Duly noted, but the entire molecule doesn't have to be polar to be soluble in water. Think amphipathic molecules like phospholipids. In this case it would indeed mean that having an OH group bequeaths it with a larger dipole moment because there is no symmetry to effectively cancel it out. Furthermore, non-polar molecules are defined as those that primarily only have van der waals interactions.

 

[BerkReviewTeach]

Keep in mind that the terms used in biochemistry are their jargon. For instance, Glutamic acid is called an acidic amino acid despite existing as a conjugate base anion at pH = 7. They name it for what it does when added to water (lose a proton from its neutral state).

In the case of tyrosine, biochemists (and every biochemistry text book I checked) refer to it as polar for the exact reason novak stated. It's side chain can form a hydrogen bond, which makes it protic. They categorize it as a polar/protic amino acid. That OH group will not sit in a non polar cavity, because it will bind H2O molecules and the polar moiety will not be found in a hydrophobic pocket.

 

[balopathic45]

Keep in mind that the terms used in biochemistry are their jargon. For instance, Glutamic acid is called an acidic amino acid despite existing as a conjugate base anion at pH = 7. They name it for what it does when added to water (lose a proton from its neutral state).

In the case of tyrosine, biochemists (and every biochemistry text book I checked) refer to it as polar for the exact reason novak stated. It's side chain can form a hydrogen bond, which makes it protic. They categorize it as a polar/protic amino acid. That OH group will not sit in a non polar cavity, because it will bind H2O molecules and the polar moiety will not be found in a hydrophobic pocket.

So basically my answer was correct?

 

[Doctor Dream]

I was wondering about this question too.... I understand the whole polar/non-polar debate going on here about tyrosine, but looking back at the question... Is it a biological fact that tyrosine is found most often on the interior, or is Kaplan assuming it is because they claim it to be non-polar? If the former, does anyone know why?

 

[BerkReviewTeach]

I was wondering about this question too.... I understand the whole polar/non-polar debate going on here about tyrosine, but looking back at the question... Is it a biological fact that tyrosine is found most often on the interior, or is Kaplan assuming it is because they claim it to be non-polar? If the former, does anyone know why?

Good question. For a globular protein, there is the theory proposed by Pauling and Mirsky that hydrogen bonding holds the protein together (particularly the secondary structure) and that water molecules incorporate into the structure to facilitate H-bonding. However, in the Kauzmann hypothesis, the driving force behind tertiary structure is hydrophobic interactions and entropic in nature (free water has greater entropy). Kauzmann proposes that hydrophilic R-groups (such as tyrosine) will fold into the core of the protein to hydrogen-bond with other protic side chains and in doing so release the previously bound water molecules to the surrounding solution. The enthalpy is essentially unchanged, but there is an increase in water's entropy that drives this folding theory. So by the Kauzmann hypothesis, you can support choice D listed above. However, the Kauzmann hypothesis suggests that all protic/polar side chains can exist in the core, which is just not the case. It also fails to fully address the entropy change when the protein folds away from water (reducing the protein's entropy). This likely explains why the two opposing theories exist.

Given that this particular topic is brand new to the MCAT (was introduced with MR5), you can bet it is a great candidate to be best tested.

 

[Doctor Dream]

Good question. For a globular protein, there is the theory proposed by Pauling and Mirsky that hydrogen bonding holds the protein together (particularly the secondary structure) and that water molecules incorporate into the structure to facilitate H-bonding. However, in the Kauzmann hypothesis, the driving force behind tertiary structure is hydrophobic interactions and entropic in nature (free water has greater entropy). Kauzmann proposes that hydrophilic R-groups (such as tyrosine) will fold into the core of the protein to hydrogen-bond with other protic side chains and in doing so release the previously bound water molecules to the surrounding solution. The enthalpy is essentially unchanged, but there is an increase in water's entropy that drives this folding theory. So by the Kauzmann hypothesis, you can support choice D listed above. However, the Kauzmann hypothesis suggests that all protic/polar side chains can exist in the core, which is just not the case. It also fails to fully address the entropy change when the protein folds away from water (reducing the protein's entropy). This likely explains why the two opposing theories exist.

Given that this particular topic is brand new to the MCAT (was introduced with MR5), you can bet it is a great candidate to be best tested.

Thanks for the in-depth explanation! I think I remember those theories being presented in my biochem class now that you mention them. How does that relate to other polar amino acids like serine or threonine, are they more likely to be found on the interior as well? And is it still accurate to say that acidic/basic amino acids are more likely to be found on the exterior?