polarity of cysterin question.

[Smooth Operater]

How come cysteine is polar, but hydrophobic? I thought polar molecules are always hydrophilic? thanks!

---

Members don't see this ad.

 

[Zoom-Zoom]

How come cysteine is polar, but hydrophobic? I thought polar molecules are always hydrophilic? thanks!

I thought it had something to do with disulfide bonds, although apparently free cysteine is even more hydrophobic than disulfide-bonded cysteine.

Where's Q when you need her?

 

[astrife]

last time I posted a question regarding classwork I was subsequently raped by the mods.... we'll see how this thread works out

 

[Zoom-Zoom]

last time I posted a question regarding classwork I was subsequently raped by the mods.... we'll see how this thread works out

You can always sneakily post in the MCAT discussion forum...

 

[Smooth Operater]

mmm.. I just noticed that Tyrosine is polar but yet hydrophobic..And Glycine is non-polar and yet hydrophilic.. What's wrong??? it seems all the rules I learn in basic chemistry do not make sense at all now.

 

[FutureNeuroStud]

mmm.. I just noticed that Tyrosine is polar but yet hydrophobic..And Glycine is non-polar and yet hydrophilic.. What's wrong??? it seems all the rules I learn in basic chemistry do not make sense at all now.

Polarity has less to do with hydrophobicity/hydrophilicity than you think. You have to consider many variables when talking about biochemicals and even chemicals in general.

Glycine is a relatively small molecule with two acidic protons (all amino acids have at least two). Tyrosine is a phenol. The phenyl group makes it less hydrophilic because of the amount of carbon.

Cysteine has a thiol group, which makes it polar. The sulfur makes it less hydrophilic. Cystine (disulfide-bonded cysteine) has 4 acidic protons (two of them being hydroxyl); thus, it is more hydrophilic than cysteine, but because of the disulfide bond, it is less polar.

Hope that helps...

 

[63768]

glycine is variable. some textbooks will label it as hydrophilic and others will label it as hydrophobic. because it has a side chain consisting of just a hydrogen, it can exhibit both properties.

tyrosine as you know has that huge phenyl group with a lone hydroxy at the distal end. so that makes it hydrophobic even though it is way more soluble than phenylalanine is.

cysteine is polar but hydrophobic because sulfur does not hydrogen bond as readily as oxygen does. same thing with methionine. that and the fact that cysteine associates in disulfide bridges makes it more likely to be buried in the middle of a protein than on its surface.

hydrophilicity and -phobicity depends on its ability to interact with water in hydrogen bonding, not necessarily its polarity.

 

[Smooth Operater]

thanks yourmom,

I think I get it. So, hydrophilicity and hydrophobicity can be translated to how well the functionalities can form hydrogen bonds with the water molecules? Thanks!