Difference Between Cysteine and Cystine

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Difference Between Cysteine and Cystine

Postby ArielM » Fri Nov 03, 2017 6:22 pm

I was just reading the old NS Bio/Biochem book about amino acids on page 251-252. Near the end, it briefly discusses that cysteine resudies spontaneously form covalent disulfide bonds with thiol groups of other cysteine residues to create the relatively insoluble amino acid cystine.

The names of these two amino acids are so similar that it is easy to mistake the two amino acids to be the same thing, especially by glazing over and reading too fast or by skimming- it is easy to overlook that the two amino acids, cysteine and cystine are not the same thing.

So what is the difference between these two amino acids?

The book doesn't just come out and tell me plainly what the difference is, which puts me in a position to only be able to make an educated guess... and to infer that the difference in the two amino acids could be that two or more cysteines make a cystine? I don't know if that is true or not, that is just my guess. Could anyone provide more information about what the true differences between cysteine and cystine actually are?
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Re: Difference Between Cysteine and Cystine

Postby NS_Tutor_Andrew » Fri Nov 03, 2017 10:18 pm

Hi ArielM,

Thanks for the question! Cystine (with an i) is a dimer made up of two molecules of cysteine (with an ei). As is surprisingly often the case, the Wikipedia article is pretty helpful if you want more detail, but for the most part, for the MCAT it will suffice to just know that 2 cysteines = 1 cystine (through a disulfide bond). Upon review, that wording could definitely be more informative, and we will make a point of improving it for the next edition of our Biochemistry book.

Hope this clarifies things, & best of luck!
Andrew D.
Content Manager, Next Step Test Prep.

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