Proteins and Enzymes

ID #2176

I'm currently taking MCB 450 along with MCB 150 and I was told in MCB 450 that cysteine was considered to be a polar group. I know in MCB 150 it is considered non-polar because S and H have relatively similar electronegativities, but is one more right than the other?

It is true that cysteine can be considered both hydrophobic and polar depending on the approach you take. Cysteine sits on the border on a hydrophobicity scale being slightly more toward the hydrophobic end than the hydrophilic end. 

In a protein, cysteine tends to act more non-polar. In other (less-physiological) conditions like a micelle (which you will see an example of when we cover lipids), cysteine may act more polar. Both are equally correct and it makes sense that you would learn both in these two classes. MCB150 is a biology class, and you were taught what you would see in biological conditions (such as in a protein). MCB450 is a biochemistry class, and in that class you are looking more at the chemistry of the amino acid. 

I hope this helps. Just apply what you learn in each class to that specific class.

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